Under the United States Constitution, the President of the United States is the head of stateand head of government of the United States. As chief of the executive branch and head of the federal government as a whole, the presidency is the highest political office in the United States by influence and recognition. The president is also the commander-in-chief of theUnited States Armed Forces. The president is indirectly elected to a four-year term by anElectoral College (or by the House of Representatives should the Electoral College fail to award an absolute majority of votes to any person). Since the ratification of the Twenty-second Amendment to the United States Constitution in 1951, no person may be elected President more than twice, and no one who has served more than two years of a term to which someone else was elected may be elected more than once.[1] Upon the death, resignation, or removal from office of an incumbent President, the Vice President assumes the office. The President must be at least 35 years of age and a “natural born” citizen of the United States.

This list includes only those persons who were sworn into office as president following the ratification of the United States Constitution, which took effect on March 4, 1789. For American leaders before this ratification, see President of the Continental Congress.[2] The list does not include any Acting Presidents under the Twenty-fifth Amendment to the United States Constitution.

There have been 43 people sworn into office, and 44 presidencies, as Grover Cleveland served two non-consecutive terms and is counted chronologically as both the 22nd and 24th president. Of the individuals elected as president, four died in office of natural causes (William Henry Harrison,[3] Zachary Taylor,[4] Warren G. Harding,[5] and Franklin D. Roosevelt), four were assassinated (Abraham Lincoln,[6] James A. Garfield,[6][7] William McKinley,[8] and John F. Kennedy) and one resigned (Richard Nixon).[9]

George Washington, the first president, was inaugurated in 1789 after a unanimous vote of the Electoral College. William Henry Harrison spent the shortest time in office with 32 days in 1841, and Franklin D. Roosevelt spent the longest with over twelve years, but died shortly into his fourth term in 1945. He is the only president to serve more than two terms and a constitutional amendment was passed to prevent that from reoccurring. Andrew Jackson, the seventh president, was the first to be elected by men of all classes in 1828 after most laws barring non-land-owners from voting were repealed. Warren Harding was the first elected after women gained voting rights in 1920. Lyndon Johnson, in 1964, was elected after the Civil Rights Era reforms eliminated laws that suppressed minority votes. History records four presidents – John Q Adams, Rutherford B. Hayes, Benjamin Harrison and George W Bush – who lost the popular vote but won in the electoral college and assumed office. John F. Kennedy has been the only president of Roman Catholic faith, and the current president, Barack Obama, is the first president who is an African American.[10]


      No party       Federalist       Democratic-Republican       Democratic       Whig       Republican

[n 1]
President Took office Left office Party Term
[n 1]
Vice President
1 Gilbert Stuart, George Washington (Lansdowne portrait, 1796).jpg George Washington
April 30, 1789 March 4, 1797 no party 1(1789)   John Adams
2 Adamstrumbull.jpg John Adams
March 4, 1797 March 4, 1801 Federalist 3(1796)   Thomas Jefferson
3 Thomas Jefferson by Rembrandt Peale, 1800.jpg Thomas Jefferson
March 4, 1801 March 4, 1809 Democratic-
4(1800)   Aaron Burr
5(1804)   George Clinton
4 James Madison.jpg James Madison
March 4, 1809 March 4, 1817 Democratic-
6(1808)   George Clinton[n 2]
March 4, 1809 – April 20, 1812
vacant[n 3]
April 20, 1812 – March 4, 1813
7(1812)   Elbridge Gerry[n 2]
March 4, 1813 – November 23, 1814
vacant[n 3]
November 23, 1814 – March 4, 1817
5 Jm5.gif James Monroe
March 4, 1817 March 4, 1825 Democratic-
8(1816)   Daniel D. Tompkins
6 John Quincy Adams by GPA Healy, 1858.jpg John Quincy Adams
March 4, 1825 March 4, 1829 Democratic-
10(1824)   John C. Calhoun
7 Andrew jackson head.jpg Andrew Jackson
March 4, 1829 March 4, 1837 Democratic 11(1828)   John C. Calhoun[n 4]
March 4, 1829 – December 28, 1832
vacant[n 3]
December 28, 1832 – March 4, 1833
12(1832)   Martin Van Buren
8 Mb8.gif Martin Van Buren
March 4, 1837 March 4, 1841 Democratic 13(1836)   Richard Mentor Johnson
9 William Henry Harrison by James Reid Lambdin, 1835.jpg William Henry Harrison
March 4, 1841 April 4, 1841
[n 2]
Whig 14(1840)   John Tyler
[n 5]
WHOportTyler.jpg John Tyler
April 4, 1841 March 4, 1845 Whig
April 4, 1841 – September 13, 1841
vacant[n 3]
no party[n 6]
September 13, 1841 – March 4, 1845
11 James Knox Polk by GPA Healy, 1858.jpg James K. Polk
March 4, 1845 March 4, 1849 Democratic 15(1844)   George M. Dallas
12 Zachary Taylor by Joseph Henry Bush, c1848.jpg Zachary Taylor
March 4, 1849 July 9, 1850
[n 2]
Whig 16(1848)   Millard Fillmore
13 Millard Fillmore by George PA Healy, 1857.jpg Millard Fillmore
July 9, 1850 March 4, 1853 Whig vacant[n 3]
14 Franklin Pierce by GPA Healy, 1858.jpg Franklin Pierce
March 4, 1853 March 4, 1857 Democratic 17(1852)   William R. King[n 2]
March 4, 1853 – April 18, 1853
vacant[n 3]
April 18, 1853 – March 4, 1857
15 JamesBuchanan crop.jpg James Buchanan
March 4, 1857 March 4, 1861 Democratic 18(1856)   John C. Breckinridge
16 Lincoln by George H Story c1915.jpg Abraham Lincoln
March 4, 1861 April 15, 1865
[n 7]
Republican 19(1860)   Hannibal Hamlin
National Union[n 8]
20(1864)   Andrew Johnson
17 Aj17.gif Andrew Johnson
April 15, 1865 March 4, 1869 Democratic
National Union;[n 8]
no party[n 9]
[n 3]
18 Ug18.gif Ulysses S. Grant
March 4, 1869 March 4, 1877 Republican 21(1868)   Schuyler Colfax
22(1872)   Henry Wilson[n 2]
March 4, 1873 – November 22, 1875
vacant[n 3]
November 22, 1875 – March 4, 1877
19 Rhayes.png Rutherford B. Hayes
March 4, 1877 March 4, 1881 Republican 23(1876)   William A. Wheeler
20 James Garfield portrait.jpg James A. Garfield
March 4, 1881 September 19, 1881
[n 7]
Republican 24(1880)   Chester A. Arthur
21 Chester A Arthur by Daniel Huntington crop.jpeg Chester A. Arthur
September 19, 1881 March 4, 1885 Republican vacant[n 3]
22 Grover Cleveland portrait2.jpg Grover Cleveland
March 4, 1885 March 4, 1889 Democratic 25(1884)   Thomas A. Hendricks[n 2]
March 4, 1885 – November 25, 1885
vacant[n 3]
November 25, 1885 – March 4, 1889
23 Benjamin Harrison by Eastman Johnson (1895).jpg Benjamin Harrison
March 4, 1889 March 4, 1893 Republican 26(1888)   Levi P. Morton
24 Grover Cleveland portrait2.jpg Grover Cleveland
March 4, 1893 March 4, 1897 Democratic 27(1892)   Adlai Stevenson I
25 Official White House portrait of William McKinley.jpg William McKinley
March 4, 1897 September 14, 1901
[n 7]
Republican 28(1896)   Garret Hobart[n 2]
March 4, 1897 – November 21, 1899
vacant[n 3]
November 21, 1899 – March 4, 1901
29(1900)   Theodore Roosevelt
26 TRSargent.jpg Theodore Roosevelt
September 14, 1901 March 4, 1909 Republican vacant[n 3]
30(1904)   Charles W. Fairbanks
27 TaftOfficial Portrait.jpg William Howard Taft
March 4, 1909 March 4, 1913 Republican 31(1908)   James S. Sherman[n 2]
March 4, 1909 – October 30, 1912
vacant[n 3]
October 30, 1912 – March 4, 1913
28 Ww28.gif Woodrow Wilson
March 4, 1913 March 4, 1921 Democratic 32(1912)   Thomas R. Marshall
29 Wh29.gif Warren G. Harding
March 4, 1921 August 2, 1923
[n 2]
Republican 34(1920)   Calvin Coolidge
30 Calvin Coolidge.jpg Calvin Coolidge
August 2, 1923 March 4, 1929 Republican vacant[n 3]
35(1924)   Charles G. Dawes
31 Herbert Clark Hoover by Greene, 1956.jpg Herbert Hoover
March 4, 1929 March 4, 1933 Republican 36(1928)   Charles Curtis
32 Franklin Roosevelt - Presidential portrait.jpg Franklin D. Roosevelt
March 4, 1933 April 12, 1945
[n 2]
Democratic 37(1932)
[n 10]
  John Nance Garner
39(1940)   Henry A. Wallace
40(1944)   Harry S. Truman
33 HarryTruman.jpg Harry S. Truman
April 12, 1945 January 20, 1953 Democratic vacant[n 3]
41(1948)   Alben W. Barkley

1949 – 1953

34 Dwight D. Eisenhower, official Presidential portrait.jpg Dwight D. Eisenhower
January 20, 1953 January 20, 1961
[n 11]
Republican 42(1952)   Richard Nixon
35 John F Kennedy Official Portrait.jpg John F. Kennedy
January 20, 1961 November 22, 1963
[n 7]
Democratic 44(1960)   Lyndon B. Johnson
36 Lyndon B. Johnson - portrait.png Lyndon B. Johnson
November 22, 1963 January 20, 1969 Democratic vacant[n 3]
45(1964)   Hubert Humphrey
January 20, 1965 – January 20, 1969
37 Richard Nixon - Presidential portrait.jpg Richard Nixon
January 20, 1969 August 9, 1974
[n 4]
Republican 46(1968)   Spiro Agnew[n 4]
January 20, 1969 – October 10, 1973
vacant[n 3]
October 10, 1973 – December 6, 1973
  Gerald Ford
December 6, 1973 – August 9, 1974
38 Gerald Ford - Presidential portrait.jpg Gerald Ford
August 9, 1974 January 20, 1977 Republican vacant[n 3]
August 9, 1974 – December 19, 1974
  Nelson Rockefeller
December 19, 1974 – January 20, 1977
39 James E. Carter - portrait.gif Jimmy Carter
(1924– )
January 20, 1977 January 20, 1981 Democratic 48(1976)   Walter Mondale
40 Ronald Reagan - Presidential portrait.jpg Ronald Reagan
January 20, 1981 January 20, 1989 Republican 49(1980)   George H. W. Bush
41 George H. W. Bush - portrait by Herbert Abrams (1994).jpg George H. W. Bush
(1924– )
January 20, 1989 January 20, 1993 Republican 51(1988)   Dan Quayle
42 Clinton.jpg Bill Clinton
(1946– )
January 20, 1993 January 20, 2001 Democratic 52(1992)   Al Gore
43 President George W Bush portrait.jpg George W. Bush
(1946– )
January 20, 2001 January 20, 2009 Republican 54(2000)   Dick Cheney
44 Official portrait of Barack Obama.jpg Barack Obama
(1961– )
January 20, 2009 Incumbent Democratic 56(2008)   Joe Biden

Living former presidents

As of July 2012, there are four living former presidents:

President Term of office Date of birth
Jimmy Carter 1977–1981 October 1, 1924 (age 87)
George H. W. Bush 1989–1993 June 12, 1924 (age 88)
Bill Clinton 1993–2001 August 19, 1946 (age 66)
George W. Bush 2001–2009 July 6, 1946 (age 66)

The most recent death of a former president was that of Gerald Ford (1974–1977) on December 26, 2006.

history of the sea


Map of the Gulf Stream by Benjamin Franklin, 1769-1770. Courtesy of the NOAAPhoto Library.

Humans first acquired knowledge of the waves and currents of the seas and oceans in pre-historic times. Observations on tides were recorded by Aristotle and Strabo. Early modern exploration of the oceans was primarily for cartography and mainly limited to its surfaces and of the creatures that fishermen brought up in nets, though depth soundings by lead line were taken.

Although Juan Ponce de León in 1513 first identified the Gulf Stream, and the current was well-known to mariners, Benjamin Franklin made the first scientific study of it and gave it its name. Franklin measured water temperatures during several Atlantic crossings and correctly explained the Gulf Stream’s cause. Franklin and Timothy Folger printed the first map of the Gulf Stream in 1769-1770.[1][2]

When Louis Antoine de Bougainville, who voyaged between 1766 and 1769, and James Cook, who voyaged from 1768 to 1779, carried out their explorations in the South Pacific, information on the oceans themselves formed part of the reports. James Rennell wrote the first scientific textbooks about currents in the Atlantic and Indian oceans during the late 18th and at the beginning of 19th century. Sir James Clark Ross took the first modern sounding in deep sea in 1840, and Charles Darwinpublished a paper on reefs and the formation of atolls as a result of the second voyage of HMS Beagle in 1831-6. Robert FitzRoypublished a report in four volumes of the three voyages of the Beagle. In 1841–1842 Edward Forbes undertook dredging in the Aegean Sea that founded marine ecology.

As first superintendent of the United States Naval Observatory (1842–1861) Matthew Fontaine Maury devoted his time to the study of marine meteorology, navigation, and charting prevailing winds and currents. His Physical Geography of the Sea, 1855 was the first textbook of oceanography. Many nations sent oceanographic observations to Maury at the Naval Observatory, where he and his colleagues evaluated the information and gave the results worldwide distribution.[3]

The steep slope beyond the continental shelves was discovered in 1849. The first successful laying of transatlantic telegraph cable in August 1858 confirmed the presence of an underwater “telegraphic plateau” mid-ocean ridge. After the middle of the 19th century, scientific societies were processing a flood of new terrestrial botanical and zoological information.

In 1871, under the recommendations of the Royal Society of London, the British government sponsored an expedition to explore world’s oceans and conduct scientific investigations. Under that sponsorship the Scots Charles Wyville Thompson and Sir John Murraylaunched the Challenger expedition (1872–1876). The results of this were published in 50 volumes covering biological, physical and geological aspects. 4417 new species were discovered.

Other European and American nations also sent out scientific expeditions (as did private individuals and institutions). The first purpose built oceanographic ship, the “Albatros” was built in 1882. The four-month 1910 North Atlantic expedition headed by Sir John Murrayand Johan Hjort was at that time the most ambitious research oceanographic and marine zoological project ever, and led to the classic 1912 book The Depths of the Ocean.

Oceanographic institutes dedicated to the study of oceanography were founded. In the United States, these included the Scripps Institution of Oceanography in 1892, Woods Hole Oceanographic Institution in 1930, Virginia Institute of Marine Science in 1938,Lamont-Doherty Earth Observatory at Columbia University, and the School of Oceanography at University of Washington. In Britain, there is a major research institution: National Oceanography Centre, Southampton which is the successor to the Institute of Oceanography. In AustraliaCSIRO Marine and Atmospheric Research, known as CMAR, is a leading center. In 1921 the International Hydrographic Bureau (IHB) was formed in Monaco.

In 1893, Fridtjof Nansen allowed his ship “Fram” to be frozen in the Arctic ice. As a result he was able to obtain oceanographic data as well as meteorological and astronomical data. The first international organization of oceanography was created in 1902 as the International Council for the Exploration of the Sea.

The first acoustic measurement of sea depth was made in 1914. Between 1925 and 1927 the “Meteor” expedition gathered 70,000 ocean depth measurements using an echo sounder, surveying the Mid atlantic ridge. The Great Global Rift, running along the Mid Atlantic Ridge, was discovered by Maurice Ewing and Bruce Heezen in 1953 while the mountain range under the Arctic was found in 1954 by the Arctic Institute of the USSR. The theory of seafloor spreading was developed in 1960 by Harry Hammond Hess. The Ocean Drilling Project started in 1966. Deep sea vents were discovered in 1977 by John Corlis andRobert Ballard in the submersible “Alvin“.

In 1939 the Carnegie Institution assigned Dr. Charles S. Piggot the mission of exploring the sea bed for Radium deposits. For this deep water mission the Western Union Cable vessel the Lord Kelvin was converted to the ability to lower a cable down several miles, which had a unit attached that fired a hollow dart into the sea bed floor which was then raised with the seabed sample for analysis. [4]

In the 1950s, Auguste Piccard invented the bathyscaphe and used the “Trieste” to investigate the ocean’s depths. The nuclear submarine Nautilus made the first journey under the ice to the North Pole in 1958. In 1962 there was the first deployment of FLIP (Floating Instrument Platform), a 355 foot spar buoy.

Then, in 1966, the U.S. Congress created a National Council for Marine Resources and Engineering DevelopmentNOAA was put in charge of exploring and studying all aspects of Oceanography in the USA. It also enabled the National Science Foundation to awardSea Grant College funding to multi-disciplinary researchers in the field of oceanography.[5]

From the 1970s, there has been much emphasis on the application of large scale computers to oceanography to allow numerical predictions of ocean conditions and as a part of overall environmental change prediction. An oceanographic buoy array was established in the Pacific to allow prediction of El Niño events.

1990 saw the start of the World Ocean Circulation Experiment (WOCE) which continued until 2002. Geosat seafloor mapping data became available in 1995.

In 1942, Sverdrup and Fleming published “The Ocean” which was a major landmark. “The Sea” (in three volumes covering physical oceanography, seawater and geology) edited by M.N. Hill was published in 1962 while the “Encyclopedia of Oceanography” by Rhodes Fairbridge was published in 1966.


Making Sustainable Choices for Our Environment. It Starts at Home.

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The Green Turning Point

The effects of global warming and diminishing reserves of fossil fuels and other primary resources presents an unprecedented challenge to us all to make good choices for our environment. Conserving our dwindling resources and reducing the impact that we are having on the environment begins with how we use them and individuals can have a positive influence by making sustainable choices in their own homes.

There are many things common to most households which can have their impact on the environment reduced by developing environmentally friendly habits and making more informed choices. Often adopting these strategies is also beneficial in other ways as well; and making sustainable choices can often save you money and may even be a step towards a healthier lifestyle. Making sustainable choices for everyday things like where we get our energy, how far the food that we eat has travelled and how we use our vehicles…

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History of the Earth

The age of Earth is approximately one-third of theage of the universe. An immense amount of biological and geologicalchange has occurred in that time span.

Earth formed around 4.54 billion years ago by accretion from the solar nebula. Volcanic outgassing likely created the primordial atmosphere, but it contained almost no oxygen and would have been toxic to humans and most modern life. Much of the Earth was molten because of extreme volcanism and frequent collisions with other bodies. One very large collision is thought to have been responsible for tilting the Earth at an angle and forming the Moon. Over time, such cosmic bombardments ceased, allowing the planet to cool and form a solidcrust. Water that was brought here by comets and asteroidscondensed into clouds and the oceans took shape. Earth was finally hospitable to life, and the earliest forms that arose enriched the atmosphere with oxygen. Life on Earth remained small and microscopic for at least one billion years. About 580 million years ago, complex multicellular life arose, and during the Cambrian period it experienced a rapid diversification into most major phyla. Around six million years ago, the primate lineage that would lead tochimpanzees (the closest relatives of humans) diverged from the lineage that would lead to modern humans.

Biological and geological change has been constantly occurring on our planet since the time of its formation. Organisms continuouslyevolve, taking on new forms or going extinct in response to an ever-changing planet. The process of plate tectonics has played a major role in the shaping of Earth’s oceans and continents, as well as the life they harbor. The biosphere, in turn, has had a significant effect on the atmosphere and other abiotic conditions on the planet, such as the formation of the ozone layer, the proliferation of oxygen, and the creation of soil. Though humans are unable to perceive it due to their relatively brief life spans, this change is ongoing and will continue for the next few billion years.

Geologic time scale

Main article: Geologic time scale

The history of the Earth is organized chronologically in a table known as the Geologic Time Scale, which is split into intervals based onstratigraphic analysis.[2][3] A full time scale can be found at the main article.

Millions of Years

[edit]Solar System formation

An artist’s rendering of a protoplanetary disk

The standard model for the formation of the Solar System (including the Earth) is the solar nebula hypothesis.[4] In this model, the Solar system formed from a large, rotating cloud of interstellar dust and gas called the solar nebula. It was composed of hydrogen and heliumcreated shortly after the Big Bang 13.7 Ga (billion years ago) and heavier elements ejected by supernovae. About 4.5 Ga, the nebula began a contraction that may have been triggered by the shock wave of a nearby supernova.[5] A shock wave would have also made the nebula rotate. As the cloud began to accelerate, its angular momentumgravity and inertia flattened it into a protoplanetary disk perpendicular to its axis of rotation. Small perturbations due to collisions and the angular momentum of other large debris created the means by which kilometer-sized protoplanets began to form, orbiting the nebular center.[6]

The center of the nebula, not having much angular momentum, collapsed rapidly, the compression heating it until nuclear fusion of hydrogen into helium began. After more contraction, a T Tauri star ignited and evolved into the Sun. Meanwhile, in the outer part of the nebula gravity caused matter to condense around density perturbations and dust particles, and the rest of the protoplanetary disk began separating into rings. In a process known as runaway accretion, successively larger fragments of dust and debris clumped together to form planets.[6] Earth formed in this manner about 4.54 billion years ago (with an uncertainty of 1%)[7][8][9][10] and was largely completed within 10–20 million years.[11] The solar wind of the newly formed T Tauri star cleared out most of the material in the disk that had not already condensed into larger bodies. The same process is expected to produce accretion disks around virtually all newly forming stars in the universe, some of which yield planets.[12]

The proto-Earth grew by accretion until its interior was hot enough to melt the heavy, siderophile metals. Having higher densities than the silicates, the metals sank. This iron catastrophe resulted in the separation of a primitive mantle and a (metallic) core only 10 million years after the Earth began to form, producing the layered structure of Earth and setting up the formation of Earth’s magnetic field.[13]Earth’s first atmosphere, captured from the solar nebula, was composed of light (atmophile) elements from the solar nebula, mostly hydrogen and helium. A combination of the solar wind and Earth’s heat would have driven off this atmosphere, as a result of which the atmosphere is now depleted in these elements compared to cosmic abundances.[14]

[edit]Hadean and Archean Eons

Main articles: Hadean and Archean

The first eon in Earth’s history, the Hadean, begins with the Earth’s formation and is followed by the Archean eon at 3.8 Ga.[2]:145 The oldest rocks found on Earth date to about 4.0 Ga, and the oldest detrital zircon crystals in rocks to about 4.4 Ga,[15][16][17] soon after the formation of the Earth’s crust and the Earth itself. The giant impact hypothesis for the Moon’s formation states that shortly after formation of an initial crust, the proto-Earth was impacted by a smaller protoplanet, which ejected part of the mantle and crust into space and created the Moon.[18][19][20]

From crater counts on other celestial bodies it is inferred that a period of intense meteorite impacts, called the Late Heavy Bombardment, began about 4.1 Ga, and concluded around 3.8 Ga, at the end of the Hadean.[21] In addition, volcanism was severe due to the large heat flow and geothermal gradient.[22] Nevertheless, detrital zircon crystals dated to 4.4 Ga show evidence of having undergone contact with liquid water, suggesting that the planet already had oceans or seas at that time.[15]

By the beginning of the Archean, the Earth had cooled significantly. Most present life forms could not have survived in the Archean atmosphere, which lacked oxygen and an ozone layer. Nevertheless it is believed that primordial life began to evolve by the early Archean, with candidate fossils dated to around 3.5 Ga.[23] Some scientists even speculate that life could have begun during the early Hadean, as far back as 4.4 Ga, surviving the possible Late Heavy Bombardment period in hydrothermal vents below the Earth’s surface.[24]

[edit]Formation of the Moon

Main articles: Moon and Giant impact hypothesis

Artist’s impression of the enormous collision that likely formed the Moon

Earth’s only natural satellite, the Moon, is larger relative to its planet than any other satellite in the solar system.[nb 1] During the Apollo program, rocks from the Moon’s surface were brought to Earth. Radiometric dating of these rocks has shown the Moon to be 4.53 ± .01 billion years old,[27] at least 30 million years after the solar system was formed.[28] New evidence suggests the Moon formed even later, 4.48 ± 0.02 Ga, or 70–110 million years after the start of the Solar System.[29]

Theories for the formation of the Moon must explain its late formation as well as the following facts. First, the Moon has a low density (3.3 times that of water, compared to 5.5 for the earth[30]) and a small metallic core. Second, there is virtually no water or other volatiles on the moon. Third, the Earth and Moon have the same oxygen isotopic signature (relative abundance of the oxygen isotopes). Of the theories that have been proposed to account for these phenomena, only one is widely accepted: The giant impact hypothesis proposes that the Moon originated after a body the size of Mars struck the proto-Earth a glancing blow.[1]:256[31][32]

The collision between the impactor, sometimes named Theia,[28] and the Earth released about 100 million times more energy than the impact that caused the extinction of the dinosaurs. This was enough to vaporize some of the Earth’s outer layers and melt both bodies.[31][1]:256 A portion of the mantle material was ejected into orbit around the Earth. The giant impact hypothesis predicts that the Moon was depleted of metallic material,[33] explaining its abnormal composition.[34] The ejecta in orbit around the Earth could have condensed into a single body within a couple of weeks. Under the influence of its own gravity, the ejected material became a more spherical body: the Moon.[35]

[edit]First continents

Animation showing the movement of Earth’s continents throughout history, starting from the Cambrian period

Mantle convection, the process that drives plate tectonics today, is a result of heat flow from the Earth’s interior to the Earth’s surface.[36]:2 It involves the creation of rigid tectonic platesat mid-oceanic ridges. These plates are destroyed by subduction into the mantle atsubduction zones. During the early Archean (about 3.0 Ga) the mantle was much hotter than today, probably around 1600 °C,[37]:82 so convection in the mantle was faster. While a process similar to present day plate tectonics did occur, this would have gone faster too. It is likely that during the Hadean and Archean, subduction zones were more common, and therefore tectonic plates were smaller.[1]:258

The initial crust, formed when the Earth’s surface first solidified, totally disappeared from a combination of this fast Hadean plate tectonics and the intense impacts of the Late Heavy Bombardment. However, it is thought that it was basaltic in composition, like today’s oceanic crust, because little crustal differentiation had yet taken place.[1]:258 The first larger pieces of continental crust, which is a product of differentiation of lighter elements duringpartial melting in the lower crust, appeared at the end of the Hadean, about 4.0 Ga. What is left of these first small continents are calledcratons. These pieces of late Hadean and early Archean crust form the cores around which today’s continents grew.[38]

The oldest rocks on Earth are found in the North American craton of Canada. They are tonalites from about 4.0 Ga. They show traces ofmetamorphism by high temperature, but also sedimentary grains that have been rounded by erosion during transport by water, showing rivers and seas existed then.[39] Cratons consist primarily of two alternating types of terranes. The first are so-called greenstone belts, consisting of low grade metamorphosed sedimentary rocks. These “greenstones” are similar to the sediments today found in oceanic trenches, above subduction zones. For this reason, greenstones are sometimes seen as evidence for subduction during the Archean. The second type is a complex of felsic magmatic rocks. These rocks are mostly tonalitetrondhjemite or granodiorite, types of rock similar in composition to granite (hence such terranes are called TTG-terranes). TTG-complexes are seen as the relicts of the first continental crust, formed by partial melting in basalt.[40]:Chapter 5

[edit]Oceans and atmosphere

Graph showing range of estimated partial pressure of atmospheric oxygen through geologic time [41]

Earth is often described as having had three atmospheres. The first atmosphere, captured from the solar nebula, was composed of light (atmophile) elements from the solar nebula, mostly hydrogen and helium. A combination of the solar wind and Earth’s heat would have driven off this atmosphere, as a result of which the atmosphere is now depleted in these elements compared to cosmic abundances.[14] After the impact, the molten Earth released volatile gases; and later more gases were released by volcanoes, completing a second atmosphere rich in greenhouse gases but poor in oxygen. [1]:256 Finally, the third atmosphere, rich in oxygen, emerged when bacteria began to produce oxygen about 2.8 Ga.[42]:83–84,116–117

In early models for the formation of the atmosphere and ocean, the second atmosphere was formed by outgassing of volatiles from the Earth’s interior. Now it is considered likely that many of the volatiles were delivered during accretion by a process known as impact degassing in which incoming bodies vaporize on impact. The ocean and atmosphere would therefore have started to form even as the Earth formed.[43] The new atmosphere probably contained water vapor, carbon dioxide, nitrogen, and smaller amounts of other gases.[44]

Planetesimals at a distance of 1 astronomical unit (AU), the distance of the Earth from the Sun, probably did not contribute any water to the Earth because the solar nebula was too hot for ice to form and the hydration of rocks by water vapor would have taken too long.[43][45] The water must have been supplied by meteorites from the outer asteroid belt and some large planetary embryos from beyond 2.5 AU.[43][46] Comets may also have contributed. Though most comets are today in orbits farther away from the Sun thanNeptune, computer simulations show they were originally far more common in the inner parts of the solar system.[39]:130-132

As the planet cooled, clouds formed. Rain created the oceans. Recent evidence suggests the oceans may have begun forming as early as 4.4 Ga.[15] By the start of the Archean eon they already covered the Earth. This early formation has been difficult to explain because of a problem known as the faint young Sun paradox. Stars are known to get brighter as they age, and at the time of its formation the Sun would have been emitting only 70% of its current power. Many models predict that the Earth would have been covered in ice.[47][43]A likely solution is that there was enough carbon dioxide and methane to produce a greenhouse effect. The carbon dioxide would have been produced by volcanoes and the methane by early microbes. Another greenhouse gas, ammonium would have been ejected by volcanos but quickly destroyed by ultraviolet radiation.[42]:83

[edit]Origin of life

One of the reasons for interest in the early atmosphere and ocean is that they form the conditions under which life first arose. There are a lot of models, but little consensus, on how life emerged from non-living chemicals; chemical systems that have been created in the laboratory still fall well short of the minimum complexity for a living organism.[48][49]

The first step in the emergence of life may have been chemical reactions that produced many of the simpler organic compounds, including nucleobases and amino acids, that are the building blocks of life. An experiment in 1953 by Stanley Miller and Harold Ureyshowed that such molecules could form in an atmosphere of water, methane, ammonia and hydrogen with the aid of sparks to mimic the effect of lightning.[50] Although the atmospheric composition was likely different from the composition used by Miller and Urey, later experiments with more realistic compositions also managed to synthesize organic molecules.[51] Recent computer simulations have even shown that extraterrestrial organic molecules could have formed in the protoplanetary disk before the formation of the Earth.[52]

The next stage of complexity could have been reached from at least three possible starting points: self-replication, an organism’s ability to produce offspring that are very similar to itself; metabolism, its ability to feed and repair itself; and external cell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances.[53]

[edit]Replication first: RNA world

The replicator in virtually all known life is deoxyribonucleic acid. DNA is far more complex than the original replicator and its replication systems are highly elaborate.

Main article: RNA world

Even the simplest members of the three modern domains of life use DNA to record their “recipes” and a complex array of RNA and protein molecules to “read” these instructions and use them for growth, maintenance and self-replication.

The discovery that a kind of RNA molecule called a ribozyme can catalyze both its own replication and the construction of proteins led to the hypothesis that earlier life-forms were based entirely on RNA.[54] They could have formed an RNA world in which there were individuals but no species, asmutations and horizontal gene transfers would have meant that the offspring in each generation were quite likely to have different genomes from those that their parents started with.[55] RNA would later have been replaced by DNA, which is more stable and therefore can build longer genomes, expanding the range of capabilities a single organism can have.[56] Ribozymes remain as the main components of ribosomes, the “protein factories” of modern cells.[57]

Although short, self-replicating RNA molecules have been artificially produced in laboratories,[58]doubts have been raised about whether natural non-biological synthesis of RNA is possible.[59][60][61] The earliest ribozymes may have been formed of simpler nucleic acids such asPNATNA or GNA, which would have been replaced later by RNA.[62][63] Other pre-RNA replicatorshave been posited, including crystals[64]:150 and even quantum systems.[65]

In 2003 it was proposed that porous metal sulfide precipitates would assist RNA synthesis at about100 °C (212 °F) and ocean-bottom pressures near hydrothermal vents. In this hypothesis, lipid membranes would be the last major cell components to appear and until they did the proto-cells would be confined to the pores.[66]

[edit]Metabolism first: Iron-sulfur world

Another long-standing hypothesis is that the first life was composed of protein molecules. Amino acids, the building blocks of proteins, are easily synthesized in plausible prebiotic conditions, as are small peptides (polymers of amino acids) that make good catalysts.[67]:295–297 A series of experiments starting in 1997 showed that amino acids and peptides could form in the presence ofcarbon monoxide and hydrogen sulfide with iron sulfide and nickel sulfide as catalysts. Most of the steps in their assembly required temperatures of about 100 °C (212 °F) and moderate pressures, although one stage required 250 °C (482 °F) and a pressure equivalent to that found under 7 kilometres (4.3 mi) of rock. Hence self-sustaining synthesis of proteins could have occurred near hydrothermal vents.[68]

A difficulty with the metabolism-first scenario is finding a way for organisms to evolve. Without the ability to replicate as individuals, aggregates of molecules would have “compositional genomes” (counts of molecular species in the aggregate) as the target of natural selection. However, a recent model shows that such a system is unable to evolve in response to natural selection.[69]

[edit]Membranes first: Lipid world

Cross-section through aliposome.

It has been suggested that double-walled “bubbles” of lipids like those that form the external membranes of cells may have been an essential first step.[70] Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously form liposomes, double-walled “bubbles”, and then reproduce themselves. Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to natural selection for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within the liposomes than they would have outside.[71]

[edit]The clay theory

Some clays, notably montmorillonite, have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their crystalline pattern, are subject to an analog of natural selection (as the clay “species” that grows fastest in a particular environment rapidly becomes dominant), and can catalyze the formation of RNA molecules.[72] Although this idea has not become the scientific consensus, it still has active supporters.[73]:150–158[64]

Research in 2003 reported that montmorillonite could also accelerate the conversion of fatty acids into “bubbles”, and that the bubbles could encapsulate RNA attached to the clay. Bubbles can then grow by absorbing additional lipids and dividing. The formation of the earliest cells may have been aided by similar processes.[74]

A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotideslipids and amino acids.[75]

Last common ancestor

It is believed that of this multiplicity of protocells, only one line survived. Current phylogenetic evidence suggests that the last universal common ancestor (LUCA) lived during the early Archean eon, perhaps 3.5 Ga or earlier.[76][77] This LUCA cell is the ancestor of all life on Earth today. It was probably a prokaryote, possessing a cell membrane and probably ribosomes, but lacking a nucleus or membrane-bound organelles such as mitochondria or chloroplasts. Like all modern cells, it used DNA as its genetic code, RNA for information transfer and protein synthesis, and enzymes to catalyze reactions. Some scientists believe that instead of a single organism being the last universal common ancestor, there were populations of organisms exchanging genes by lateral gene transfer.[76]

[edit]Proterozoic Eon

Main article: Proterozoic

The Proterozoic eon lasted from 2.5 Ga to 542 Ma (million years ago).[2]:130 In this time span, cratons grew into continents with modern sizes. The change to an oxygen-rich atmosphere was a crucial development. Life developed from prokaryotes into eukaryotes and multicellular forms. The Proterozoic saw a couple of severe ice ages called snowball Earths. After the last Snowball Earth about 600 Ma, the evolution of life on Earth accelerated. About 580 Ma, the Ediacara biota formed the prelude for the Cambrian Explosion.

[edit]Oxygen revolution

See also: Ozone layer

Lithified stromatolites on the shores ofLake ThetisWestern Australia. Archean stromatolites are the first direct fossil traces of life on Earth.

banded iron formation from the 3.15 Ga Moories Group, Barberton Greenstone Belt,South Africa. Red layers represent the times when oxygen was available, gray layers were formed in anoxic circumstances.

The earliest cells absorbed energy and food from the environment around them. They usedfermentation, the breakdown of more complex compounds into less complex compounds with less energy, and used the energy so liberated to grow and reproduce. Fermentation can only occur in an anaerobic (oxygen-free) environment. The evolution of photosynthesis made it possible for cells to manufacture their own food.[78]:377

Most of the life that covers the surface of the Earth depends directly or indirectly onphotosynthesis. The most common form, oxygenic photosynthesis, turns carbon dioxide, water and sunlight into food. It captures the energy of sunlight in energy-rich molecules such as ATP, which then provide the energy to make sugars. To supply the electrons in the circuit, hydrogen is stripped from water, leaving oxygen as a waste product.[79] Some organisms, including purple bacteria and green sulfur bacteria, use an anoxygenic form of photosynthesis that use alternatives to hydrogen stripped from water as electron donors; examples are hydrogen sulfide, sulfur and iron. Such organisms are mainly restricted to extreme environments such as hot springs and hydrothermal vents.[78]:379–382[80]

The simpler anoxygenic form arose about 3.8 Ga, not long after the appearance of life. The timing of oxygenic photosynthesis is more controversial; it had certainly appeared by about 2.4 Ga, but some researchers put it back as far as 3.2 Ga.[79] The latter “probably increased global productivity by at least two or three orders of magnitude.”[81][82] Among the oldest remnants of oxygen-producing lifeforms are fossil stromatolites.[81][82][41]

At first, the released oxygen was bound up with limestoneiron, and other minerals. The oxidized iron appears as red layers in geological strata called banded iron formations that formed in abundance during the Siderian period (between 2500 Ma and 2300 Ma).[2]:133When most of the exposed readily reacting minerals were oxidized, oxygen finally began to accumulate in the atmosphere. Though each cell only produced a minute amount of oxygen, the combined metabolism of many cells over a vast time transformed Earth’s atmosphere to its current state. This was Earth’s third atmosphere.[83]:50–51[42]:83–84,116–117

Some of the oxygen was stimulated by incoming ultraviolet radiation to form ozone, which collected in a layer near the upper part of the atmosphere. The ozone layer absorbed, and still absorbs, a significant amount of the ultraviolet radiation that once had passed through the atmosphere. It allowed cells to colonize the surface of the ocean and eventually the land: without the ozone layer, ultraviolet radiation bombarding land and sea would have caused unsustainable levels of mutation in exposed cells.[84][39]:219–220

Photosynthesis had another major impact. Oxygen was toxic; much life on Earth probably died out as its levels rose in what is known as the oxygen catastrophe. Resistant forms survived and thrived, and some developed the ability to use oxygen to increase their metabolism and obtain more energy from the same food.[84]

[edit]Snowball Earth

Main article: Snowball Earth

The natural evolution of the Sun made it progressively more luminous during the Archean and Proterozoic eons; the Sun’s luminosity increases 6% every billion years.[39]:165 As a result, the Earth began to receive more heat from the Sun in the Proterozoic eon. However, the Earth did not get warmer. Instead, the geological record seems to suggest it cooled dramatically during the early Proterozoic. Glacial deposits found in South Africa date back to 2.2 Ga, at which time paleomagnetic evidence puts them near the equator. Thus, this glaciation, known as the Makganyene glaciation, may have been global. Some scientists suggest this and following Proterozoic ice ages were so severe that the planet was totally frozen over from the poles to the equator, a hypothesis called Snowball Earth.[85]

The ice age around 2.3 Ga could have been directly caused by the increased oxygen concentration in the atmosphere, which caused the decrease of methane (CH4) in the atmosphere. Methane is a strong greenhouse gas, but with oxygen it reacts to form CO2, a less effective greenhouse gas.[39]:172 When free oxygen became available in the atmosphere, the concentration of methane could have decreased dramatically, enough to counter the effect of the increasing heat flow from the Sun.[86]

[edit]Emergence of eukaryotes

Chloroplasts in the cells of a moss

Modern taxonomy classifies life into three domains. The time of the origin of these domains is uncertain. The Bacteria domain probably first split off from the other forms of life (sometimes called Neomura), but this supposition is controversial. Soon after this, by 2 Ga,[87] the Neomura split into the Archaea and the Eukarya. Eukaryotic cells (Eukarya) are larger and more complex than prokaryotic cells (Bacteria and Archaea), and the origin of that complexity is only now becoming known.

Around this time, the first proto-mitochondrion was formed. A bacterial cell related to today’sRickettsia,[88] which had evolved to metabolize oxygen, entered a larger prokaryotic cell, which lacked that capability. Perhaps the large cell attempted to digest the smaller one but failed (possibly due to the evolution of prey defenses). The smaller cell may have tried toparasitize the larger one. In any case, the smaller cell survived inside the larger cell. Usingoxygen, it metabolized the larger cell’s waste products and derived more energy. Part of this excess energy was returned to the host. The smaller cell replicated inside the larger one. Soon, a stable symbiosis developed between the large cell and the smaller cells inside it. Over time, the host cell acquired some of the genes of the smaller cells, and the two kinds became dependent on each other: the larger cell could not survive without the energy produced by the smaller ones, and these in turn could not survive without the raw materials provided by the larger cell. The whole cell is now considered a single organism, and the smaller cells are classified as organelles called mitochondria.[89]

A similar event occurred with photosynthetic cyanobacteria[90] entering large heterotrophic cells and becomingchloroplasts.[83]:60–61[91]:536–539 Probably as a result of these changes, a line of cells capable of photosynthesis split off from the other eukaryotes more than 1 billion years ago. There were probably several such inclusion events. Besides the well-establishedendosymbiotic theory of the cellular origin of mitochondria and chloroplasts, there are theories that cells led to peroxisomes,spirochetes led to cilia and flagella, and that perhaps a DNA virus led to the cell nucleus,[92],[93] though none of them is widely accepted.[94]

Archaeans, bacteria, and eukaryotes continued to diversify and to become more complex and better adapted to their environments. Each domain repeatedly split into multiple lineages, although little is known about the history of the archaea and bacteria. Around 1.1 Ga, the supercontinent Rodinia was assembling.[95][96] The plantanimal, and fungi lines had split, though they still existed as solitary cells. Some of these lived in colonies, and gradually a division of labor began to take place; for instance, cells on the periphery might have started to assume different roles from those in the interior. Although the division between a colony with specialized cells and a multicellular organism is not always clear, around 1 billion years ago[97] the first multicellular plants emerged, probably green algae.[98]Possibly by around 900 Ma[91]:488 true multicellularity had also evolved in animals.

At first it probably resembled today’s sponges, which have totipotent cells that allow a disrupted organism to reassemble itself.[91]:483-487 As the division of labor was completed in all lines of multicellular organisms, cells became more specialized and more dependent on each other; isolated cells would die.

Supercontinents in the Proterozoic

Main article: Supercontinent cycle

A reconstruction of Pannotia (550 Ma).

Reconstructions of tectonic plate movement in the past 250 million years (the Cenozoic and Mesozoic eras) can be made reliably using fitting of continental margins, ocean floor magnetic anomalies and paleomagnetic poles. No ocean crust dates back further than that, so earlier reconstructions are more difficult. Paleomagnetic poles are supplemented by geologic evidence such as orogenic belts, which mark the edges of ancient plates, and past distributions of flora and fauna. The further back in time, the scarcer and harder to interpret the data get and the more diverse the reconstructions.[99]:370

Throughout the history of the Earth, there have been times when continents collided and formed a supercontinent, which later broke up into new continents. About 1000 to 830 Ma, most continental mass was united in the supercontinent Rodinia.[99]:370[100] Rodinia may have been preceded by Early-Middle Proterozoic continents called Nuna and Columbia.[99]:374[101][102]

After the break-up of Rodinia about 800 Ma, the continents may have formed another short-lived supercontinent, Pannotia, around 550 Ma. The hypothetical supercontinent is sometimes referred to as Pannotia or Vendia.[103]:321–322 The evidence for it is a phase of continental collision known as the Pan-African orogeny, which joined the continental masses of current-day Africa, South-America, Antarctica and Australia. The existence of Pannotia depends on the timing of the rifting between Gondwana (which included most of the landmass now in the Southern Hemisphere, as well as the Arabian Peninsula and the Indian subcontinent) and Laurentia (roughly equivalent to current-day North America).[99]:374 It is at least certain that by the end of the Proterozoic eon, most of the continental mass lay united in a position around the south pole.[104]

[edit]Late Proterozoic climate and life

A 580 million year old fossil of Spriggina floundensi, an animal from the Ediacaranperiod. Such life forms could have been ancestors to the many new forms that originated in the Cambrian Explosion.

The end of the Proterozoic saw at least two Snowball Earths, so severe that the surface of the oceans may have been completely frozen. This happened about 710 and 640 Ma, in theCryogenian period.[105] These severe glaciations are less easy to explain than the early Proterozoic Snowball Earth. Most paleoclimatologists think the cold episodes were linked to the formation of the supercontinent Rodinia. Because Rodinia was centered on the equator, rates of chemical weathering increased and carbon dioxide (CO2) was taken from the atmosphere. Because CO2 is an important greenhouse gas, climates cooled globally. In the same way, during the Snowball Earths most of the continental surface was covered withpermafrost, which decreased chemical weathering again, leading to the end of the glaciations. An alternative hypothesis is that enough carbon dioxide escaped through volcanic outgassing that the resulting greenhouse effect raised global temperatures.[106]Increased volcanic activity resulted from the break-up of Rodinia at about the same time.

The Cryogenian period was followed by the Ediacaran period, which was characterized by a rapid development of new multicellular lifeforms.[107] Whether there is a connection between the end of the severe ice ages and the increase in diversity of life is not clear, but it does not seem coincidental. The new forms of life, called Ediacara biota, were larger and more diverse than ever. Though the taxonomy of most Ediacaran life forms is unclear, some were ancestors of groups of modern life.[108] Important developments were the origin of muscular and neural cells. None of the Ediacaran fossils had hard body parts like skeletons. These first appear after the boundary between the Proterozoic and Phanerozoic eons or Ediacaran and Cambrian periods.

[edit]Phanerozoic Eon

Main article: Phanerozoic

The Phanerozoic is the major eon of life on Earth. It consists of three eras: The PaleozoicMesozoic, and Cenozoic,[3] and is the time when multi-cellular life greatly diversified into almost all of the organisms known today.[109]

[edit]Paleozoic Era

Main article: Paleozoic

The Paleozoic era (meaning: era of old life forms) was the first and longest era of the Phanerozoic eon, lasting from 542 to 251 Ma.[3]During the Paleozoic, many modern groups of life came into existence. Life colonized the land, first plants, then animals. Life usually evolved slowly. At times, however, there are sudden radiations of new species or mass extinctions. These bursts of evolution were often caused by unexpected changes in the environment resulting from natural disasters such as volcanic activitymeteorite impacts orclimate changes.

The continents formed at the break-up of Pannotia and Rodinia at the end of the Proterozoic would slowly move together again during the Paleozoic. This would eventually result in phases of mountain building that created the supercontinent Pangaea in the late Paleozoic.

[edit]Cambrian explosion

Main article: Cambrian explosion

Trilobites first appeared during the Cambrian period and were among the most widespread and diverse groups of Paleozoic organisms.

The rate of the evolution of life as recorded by fossils accelerated in the Cambrian period (542–488 Ma).[3] The sudden emergence of many new species, phyla, and forms in this period is called the Cambrian Explosion. The biological fomenting in the Cambrian Explosion was unpreceded before and since that time.[39]:229 Whereas the Ediacaran life forms appear yet primitive and not easy to put in any modern group, at the end of the Cambrian most modern phyla were already present. The development of hard body parts such as shells,skeletons or exoskeletons in animals like molluscsechinodermscrinoids and arthropods (a well-known group of arthropods from the lower Paleozoic are the trilobites) made the preservation and fossilization of such life forms easier than those of their Proterozoic ancestors. For this reason, much more is known about life in and after the Cambrian than about that of older periods. Some of these Cambrian groups appear complex but are quite different from modern life; examples are Anomalocaris and Haikouichthys.

During the Cambrian, the first vertebrate animals, among them the first fishes, had appeared.[91]:357 A creature that could have been the ancestor of the fishes, or was probably closely related to it, was Pikaia. It had a primitive notochord, a structure that could have developed into a vertebral column later. The first fishes with jaws (Gnathostomata) appeared during the Ordovician. The colonisation of new niches resulted in massive body sizes. In this way, fishes with increasing sizes evolved during the early Paleozoic, such as the titanic placoderm Dunkleosteus, which could grow 7 meters long.

The diversity of life forms did not increase greatly because of a series of mass extinctions that define widespread biostratigraphic units called biomeres.[110] After each extinction pulse, the shelf regions were repopulated by similar life forms that may have been evolving slowly elsewhere.[111] By the late Cambrian, the trilobites had reached their greatest diversity and dominated nearly all fossil assemblages.[112]:34 The boundary between the Cambrian and Ordovician (the following period, 488 to 444 million years ago) is not associated with a recognized major extinction.[113]:3

[edit]Paleozoic tectonics, paleogeography and climate

Pangaea was a supercontinent that existed from about 300 to 180 Ma. The outlines of the modern continents and other landmasses are indicated on this map.

At the end of the Proterozoic, the supercontinent Pannotia had broken apart in the smaller continents LaurentiaBalticaSiberia and Gondwana.[114] During periods when continents move apart, more oceanic crust is formed by volcanic activity. Because young volcanic crust is relatively hotter and less dense than old oceanic crust, the ocean floors will rise during such periods. This causes the sea level to rise. Therefore, in the first half of the Paleozoic, large areas of the continents were below sea level.

Early Paleozoic climates were warmer than today, but the end of the Ordovician saw a shortice age during which glaciers covered the south pole, where the huge continent Gondwana was situated. Traces of glaciation from this period are only found on former Gondwana. During the Late Ordovician ice age, a few mass extinctions took place, in which manybrachiopods, trilobites, Bryozoa and corals disappeared. These marine species could probably not contend with the decreasing temperature of the sea water.[115] After the extinctions new species evolved, more diverse and better adapted. They would fill the niches left by the extinct species.

The continents Laurentia and Baltica collided between 450 and 400 Ma, during theCaledonian Orogeny, to form Laurussia (also known as Euramerica).[116] Traces of the mountain belt which resulted from this collision can be found in ScandinaviaScotland and the northern Appalachians. In the Devonian period (416–359 Ma)[3] Gondwana and Siberia began to move towards Laurussia. The collision of Siberia with Laurussia caused the Uralian Orogeny, the collision of Gondwana with Laurussia is called the Variscan or Hercynian Orogeny in Europe or the Alleghenian Orogeny in North America. The latter phase took place during the Carboniferous period (359–299 Ma)[3] and resulted in the formation of the last supercontinent, Pangaea.[40]

[edit]Colonization of land

Artist’s conception of Devonian flora

Oxygen accumulation from photosynthesis resulted in the formation of an ozone layer that absorbed much of the Sun’s ultraviolet radiation, meaning unicellular organisms that reached land were less likely to die, and prokaryotes began to multiply and become better adapted to survival out of the water. Prokaryote lineages[117] had probably colonized the land as early as 2.6 Ga[118] even before the origin of the eukaryotes. For a long time, the land remained barren of multicellular organisms. The supercontinent Pannotia formed around 600 Ma and then broke apart a short 50 million years later.[119] Fish, the earliest vertebrates, evolved in the oceans around 530 Ma.[91]:354 A major CambrianOrdovician, plants (probably resembling algae) and fungi started growing at the edges of the water, and then out of it.[120]:138–140 The oldest fossils of land fungi and plants date to 480–460 Ma, though molecular evidence suggests the fungi may have colonized the land as early as 1000 Ma and the plants 700 Ma.[121] Initially remaining close to the water’s edge, mutations and variations resulted in further colonization of this new environment. The timing of the first animals to leave the oceans is not precisely known: the oldest clear evidence is of arthropods on land around 450 Ma,[122] perhaps thriving and becoming better adapted due to the vast food source provided by the terrestrial plants. There is also unconfirmed evidence that arthropods may have appeared on land as early as 530 Ma.[123]

[edit]Evolution of tetrapods

Further information: Evolution of tetrapods

Tiktaalik, a fish with limb-like fins and a predecessor of tetrapods. Reconstruction from fossils about 375 million years old.

At the end of the Ordovician period, 443 Ma,[3] additional extinction events occurred, perhaps due to a concurrent ice age.[115] Around 380 to 375 Ma, the first tetrapods evolved from fish.[124] It is thought that perhaps fins evolved to become limbs which allowed the first tetrapods to lift their heads out of the water to breathe air. This would allow them to live in oxygen-poor water or pursue small prey in shallow water.[124] They may have later ventured on land for brief periods. Eventually, some of them became so well adapted to terrestrial life that they spent their adult lives on land, although they hatched in the water and returned to lay their eggs. This was the origin of the amphibians. About 365 Ma, another period of extinction occurred, perhaps as a result of global cooling.[125] Plants evolved seeds, which dramatically accelerated their spread on land, around this time (by approximately 360 Ma).[126][127]

About 20 million years later (340 Ma[91]:293–296), the amniotic egg evolved, which could be laid on land, giving a survival advantage totetrapod embryos. This resulted in the divergence of amniotes from amphibians. Another 30 million years (310 Ma[91]:254–256) saw the divergence of the synapsids (including mammals) from the sauropsids (including birds and reptiles). Other groups of organisms continued to evolve, and lines diverged—in fish, insects, bacteria, and so on—but less is known of the details.

[edit]Mesozoic Era

Main article: Mesozoic

Dinosaurs were the dominant terrestrial vertebrates throughout most of theMesozoic

The Mesozoic (“middle life”) era lasted from 251 Ma to 65.5 Ma.[3] It is subdivided into theTriassicJurassic, and Cretaceous periods. The era began with the Permian–Triassic extinction event, the most severe extinction event in the fossil record; 95% of the species on Earth died out.[128] It ended with the Cretaceous–Paleogene extinction event that wiped out the dinosaurs. The Permian-Triassic event was possibly caused by some combination of theSiberian Traps volcanic event, an asteroid impact, methane hydrate gasification, sea level fluctuations, and a major anoxic event. Either the proposed Wilkes Land crater[129] in Antarctica or Bedout structure off the northwest coast of Australia may indicate an impact connection with the Permian-Triassic extinction. But it remains uncertain whether either these or other proposed Permian-Triassic boundary craters are either real impact craters or even contemporaneous with the Permian-Triassic extinction event. Life persevered, and around 230 Ma, dinosaurs split off from their reptilian ancestors.[130] The Triassic–Jurassic extinction event at 200 Ma spared many of the dinosaurs,[3][131] and they soon became dominant among the vertebrates. Though some of the mammalian lines began to separate during this period, existing mammals were probably small animals resembling shrews.[91]:169

By 180 Ma, Pangaea broke up into Laurasia and Gondwana. The boundary between avian and non-avian dinosaurs is not clear, butArchaeopteryx, traditionally considered one of the first birds, lived around 150 Ma.[132] The earliest evidence for the angiosperms evolving flowers is during the Cretaceous period, some 20 million years later (132 Ma).[133] Competition with birds drove many pterosaurs to extinction and the dinosaurs were probably already in decline when, 65 Ma, a 10-kilometre (6.2 mi) asteroid struck Earth just off theYucatán Peninsula where the Chicxulub crater is today. This ejected vast quantities of particulate matter and vapor into the air that occluded sunlight, inhibiting photosynthesis. Most large animals, including the non-avian dinosaurs, became extinct,[134] marking the end of the Cretaceous period and Mesozoic era.

[edit]Cenozoic Era

Main article: Cenozoic

The Cenozoic era began at 65.6 Ma,[3] and is subdivided into the Paleogene and Neogene periods. Mammals and birds were able to survive the Cretaceous–Paleogene extinction event which killed off the dinosaurs and many other forms of life, and this is the era in which they diversified into their modern forms.

[edit]Diversification of mammals

Mammals have existed since the late Triassic, but prior to the Cretaceous–Paleogene extinction event they remained small and generalized. During the Cenozoic, mammals rapidly diversified to fill the niches that the dinosaurs and other extinct animals had left behind, becoming the dominant vertebrates and creating many of the modern orders. With many marine reptiles extinct, some mammals began living in the oceans and became cetaceans. Others became felids and canids, swift and agile land predators. The dryer global climate of the Cenozoic lead to the expansion of grasslands and the evolution of grazing and hoofed mammals such asequids and bovids. Other mammals adapted to arboreal living and became the primates, of which one lineage would lead to modern humans.

[edit]Human evolution

Reconstruction of Australopithecusafricanus, an early hominid.

Main article: Human evolution

A small African ape living around 6 Ma was the last animal whose descendants would include both modern humans and their closest relatives, the chimpanzees.[91]:100–101 Only two branches of its family tree have surviving descendants. Very soon after the split, for reasons that are still unclear, apes in one branch developed the ability to walk upright.[91]:95–99 Brain size increased rapidly, and by 2 Ma, the first animals classified in the genus Homo had appeared.[120]:300 Of course, the line between different species or even genera is somewhat arbitrary as organisms continuously change over generations. Around the same time, the other branch split into the ancestors of the common chimpanzee and the ancestors of the bonobo as evolution continued simultaneously in all life forms.[91]:100–101

The ability to control fire probably began in Homo erectus (or Homo ergaster), probably at least 790,000 years ago[135] but perhaps as early as 1.5 Ma.[91]:67 The use and discovery of controlled fire may even predate Homo erectus. Fire was possibly used by the early Lower Paleolithic (Oldowan) hominid Homo habilis or strong australopithecines such asParanthropus.[136]

It is more difficult to establish the origin of language; it is unclear whether Homo erectus could speak or if that capability had not begun until Homo sapiens.[91]:67 As brain size increased, babies were born earlier, before their heads grew too large to pass through thepelvis. As a result, they exhibited more plasticity, and thus possessed an increased capacity to learn and required a longer period of dependence. Social skills became more complex, language became more sophisticated, and tools became more elaborate. This contributed to further cooperation and intellectual development.[137]:7 Modern humans (Homo sapiens) are believed to have originated around 200,000 years ago or earlier in Africa; the oldest fossils date back to around 160,000 years ago.[138]

The first humans to show signs of spirituality are the Neanderthals (usually classified as a separate species with no surviving descendants); they buried their dead, often with no sign of food or tools.[139]:17 However, evidence of more sophisticated beliefs, such as the early Cro-Magnon cave paintings (probably with magical or religious significance)[139]:17–19 did not appear until 32,000 years ago.[140] Cro-Magnons also left behind stone figurines such as Venus of Willendorf, probably also signifying religious belief.[139]:17–19By 11,000 years ago, Homo sapiens had reached the southern tip of South America, the last of the uninhabited continents (except for Antarctica, which remained undiscovered until 1820 AD).[141] Tool use and communication continued to improve, and interpersonal relationships became more intricate.


Main article: History of the world

Vitruvian Man by Leonardo da Vinciepitomizes the advances in art and science seen during the Renaissance.

Throughout more than 90% of its history, Homo sapiens lived in small bands as nomadichunter-gatherers.[137]:8 As language became more complex, the ability to remember and communicate information resulted in a new replicator: the meme.[142] Ideas could be exchanged quickly and passed down the generations. Cultural evolution quickly outpacedbiological evolution, and history proper began. Between 8500 and 7000 BC, humans in theFertile Crescent in Middle East began the systematic husbandry of plants and animals:agriculture.[143] This spread to neighboring regions, and developed independently elsewhere, until most Homo sapiens lived sedentary lives in permanent settlements as farmers. Not all societies abandoned nomadism, especially those in isolated areas of the globe poor indomesticable plant species, such as Australia.[144] However, among those civilizations that did adopt agriculture, the relative stability and increased productivity provided by farming allowed the population to expand.

Agriculture had a major impact; humans began to affect the environment as never before. Surplus food allowed a priestly or governing class to arise, followed by increasing division of labor. This led to Earth’s first civilization at Sumer in the Middle East, between 4000 and 3000 BC.[137]:15 Additional civilizations quickly arose in ancient Egypt, at the Indus River valley and in China. The invention of writing enabled complex societies to arise: record-keeping and libraries served as a storehouse of knowledge and increased the cultural transmission of information. Humans no longer had to spend all their time working for survival—curiosity and education drove the pursuit of knowledge and wisdom.

Various disciplines, including science (in a primitive form), arose. New civilizations sprang up, traded with one another, and fought for territory and resources. Empires soon began to develop. By around 500 BC, there were advanced civilizations in the Middle East, Iran, India, China, and Greece, at times expanding, at times entering into decline.[137]:3 The fundamentals of the Western world were largely shaped by the ancient Greco-Roman culture. The Roman Empire was Christianized by Emperor Constantine in the early fourth century and declined by the end of the fifth. Beginning with the seventh century, Christianization of Europe begin. In 1054 CE the Great Schismbetween the Roman Catholic Church and the Eastern Orthodox Church led to the prominent cultural differences between Western andEastern Europe.

In the fourteenth century, the Renaissance began in Italy with advances in religion, art, and science.[137]:317–319 At that time the Christian Church as a political entity lost much of its power. European civilization began to change beginning in 1500, leading to thescientific and industrial revolutions. That continent began to exert political and cultural dominance over human societies around the planet, a time known as the Colonial era (also see Age of Discovery).[137]:295–299 In the eighteenth century a cultural movement known as the Age of Enlightenment further shaped the mentality of Europe and contributed to its secularization. From 1914 to 1918 and 1939 to 1945, nations around the world were embroiled in world wars. Established following World War I, the League of Nations was a first step in establishing international institutions to settle disputes peacefully. After failing to prevent World War II, it was replaced by theUnited Nations. In 1992, several European nations joined in the European Union. As transportation and communication improved, the economies and political affairs of nations around the world have become increasingly intertwined. This globalization has often produced both conflict and cooperation.

[edit]Recent events

Main article: Modern era
See also: Modernity and Future

Astronaut Bruce McCandless II outside of the space shuttle Challenger in 1984

Change has continued at a rapid pace from the mid-1940s to today. Technological developments include nuclear weaponscomputersgenetic engineering, andnanotechnology. Economic globalization spurred by advances in communication and transportation technology has influenced everyday life in many parts of the world. Cultural and institutional forms such as democracycapitalism, and environmentalism have increased influence. Major concerns and problems such as diseasewarpoverty, violent radicalism, and recently, human-caused climate change have risen as the world population increases.

In 1957, the Soviet Union launched the first artificial satellite into orbit and, soon afterward,Yuri Gagarin became the first human in space. Neil Armstrong, an American, was the first to set foot on another astronomical object, the Moon. Unmanned probes have been sent to all the known planets in the solar system, with some (such as Voyager) having left the solar system. The Soviet Union and the United States were the earliest leaders in space exploration in the 20th century. Five space agencies, representing over fifteen countries,[145]have worked together to build the International Space Station. Aboard it, there has been a continuous human presence in space since 2000.[146] The World Wide Web was developed in the 1990s and since then has proved to be an indispensable source of information in the developed world.

[edit]See also

Animals…..MUST READ!!!!!

IUCN Red List

IUCN Red List refers to a specific category of threatened species, and may include critically endangered species. The IUCN Red List uses the term endangered species as a specific category of imperilment, rather than as a general term. Under the IUCN Categories and Criteria, endangered species is between critically endangered and vulnerable. Also critically endangered species may also be counted as endangered species and fill all the criteria

The more general term used by the IUCN for species at risk of extinction is threatened species, which also includes the less-at-risk category of vulnerable species together with endangered and critically endangered.

IUCN categories, and some animals in those categories, include:

NASA’s Curiosity Update…

The extraordinary feat was among several astonishing achievements by US space agency’s £1.6 billion Curiosity rover, which landed on the surface of the Red Planet earlier this month.

The Black Eyed Peas rapper’s song, titled Reach for the Stars, was beamed more than 300 million miles back to Nasa’s Jet Propulsion Laboratory (JPL) in Pasadena, California.

The first music broadcast from another planet came after the planetary explorer beamed back incredible high-resolution, colour portrait images from Mars.

Nasa staff clapped their hands and held their arms in the air, smiling and swaying to the rhythm during the slightly less scientific use of the rover’s hi-tech equipment and communications ability.

The achievement also gave great delight to dozens of students who gathered at the laboratory to listen.

“It seems surreal,” said, who is also an actor.

He explained how Charles Bolden, the Nasa administrator, had called him to suggest beaming a song back from Mars as part of educational outreach efforts by the US space agency.

The song, which includes lyrics “I know that Mars might be far, but baby it ain’t really that far”, involved a 40-piece orchestra including French horns, rather than a more modern electronically-generated sound.

The 37-year-old, whose real name William James Adams, told a student audience that he didn’t “want to do a song that was done on a computer,” given that it was going to be the first piece of music broadcast back to the Earth from Mars.

“I wanted to show human collaboration and have an orchestra there and something that would be timeless, and translated in different cultures, not have like a hip hop beat or a dance beat,” he said.

“A lot of times … people in my field aren’t supposed to try to execute something classical, or orchestral, so I wanted to break that stigma.”

The aim was to inspire young people like those at the Nasa event, including some from Boyle Heights in East Los Angeles where the musician grew up, to take a greater interest in science.

The musician, who promotes science and mathematics education, was among more than a dozen celebrities who were invited to JPL to watch Curiosity’s landing earlier this month. Others included Wil Wheaton, Seth Green and Morgan Freeman.

Nasa Administrator Charles Bolden addressed the crowd in a video message encouraging students to study science, technology, engineering and maths.

“Mars has always fascinated us, and the things Curiosity tells us about it will help us learn about whether or not life was possible there,” he said.

“And what future human explorers can expect. has provided the first song on our playlist of Mars exploration.”

Nasa experts this week released more pictures taken by the rover, which landed at Gale Crater on the Red Planet on August 6.

One showed a panorama, in pin-sharp resolution showing individual rocks, of the landscape visible from the rover, including Mount Sharp, the slopes of which Curiosity plans to drive toward in the coming weeks and months.

In 2008, Nasa beamed the Beatles’ “Across the Universe” into the cosmos to commemorate the 40th anniversary of the song.

Enceladus: home of alien lifeforms?

Enceladus is little bigger than a lump of rock and has appeared, until recently, as a mere pinprick of light in astronomers’ telescopes. Yet Saturn’s tiny moon has suddenly become a major attraction for scientists. Many now believe it offers the best hope we have of discovering life on another world inside our solar system.

The idea that a moon a mere 310 miles in diameter, orbiting in deep, cold space, 1bn miles from the sun, could provide a home for alien lifeforms may seem extraordinary. Nevertheless, a growing number of researchers consider this is a real prospect and argue that Enceladus should be rated a top priority for future space missions.

This point is endorsed by astrobiologist Professor Charles Cockell of Edinburgh University. “If someone gave me several billion dollars to build whatever space probe I wanted, I would have no hesitation,” he says. “I would construct one that could fly to Saturn and collect samples from Enceladus. I would go there rather than Mars or the icy moons of Jupiter, such as Europa, despite encouraging signs that they could support life. Primitive, bacteria-like lifeforms may indeed exist on these worlds but they are probably buried deep below their surfaces and will be difficult to access. On Enceladus, if there are lifeforms, they will be easy to pick up. They will be pouring into space.”

The cause of this unexpected interest in Enceladus – first observed by William Herschel in 1789 and named after one of the children of the Earth goddess Gaia – stems from a discovery made by the robot spacecraft Cassini, which has been in orbit of Saturn for the past eight years. The $3bn probe has shown that the little moon not only has an atmosphere, but that geysers of water are erupting from its surface into space. Even more astonishing has been its most recent discovery, which has shown that these geysers contain complex organic compounds, including propane, ethane, and acetylene.

“It just about ticks every box you have when it comes to looking for life on another world,” says Nasa astrobiologist Chris McKay. “It has got liquid water, organic material and a source of heat. It is hard to think of anything more enticing short of receiving a radio signal from aliens on Enceladus telling us to come and get them.”

Cassini’s observations suggest Enceladus possesses a subterranean ocean that is kept liquid by the moon’s internal heat. “We are not sure where that energy is coming from,” McKay admits. “The source is producing around 16 gigawatts of power and looks very like the geothermal energy sources we have on Earth – like the deep vents we see in our ocean beds and which bubble up hot gases.”

At the moon’s south pole, Enceladus’s underground ocean appears to rise close to the surface. At a few sites, cracks have developed and water is bubbling to the surface before being vented into space, along with complex organic chemicals that also appear to have built up in its sea.

Equally remarkable is the impact of this water on Saturn. The planet is famed for its complex system of rings, made of bands of small particles in orbit round the planet. There are seven main rings: A, B, C, D, E, F and G, and the giant E-ring is linked directly with Enceladus. The water the moon vents into space turns into ice crystals and these feed the planet’s E-ring. “If you turned off the geysers of Enceladus, the great E-ring of Saturn would disappear within a few years,” says McKay. “For a little moon, Enceladus has quite an impact.”

Yet the discovery of Enceladus’s strange geology was a fairly tentative affair, says Professor Michele Dougherty of Imperial College London. She was the principal investigator for Cassini’s magnetometer instrument. “Cassini had been in orbit round Saturn for more than six months when it passed relatively close to Enceladus. Our results indicated that Saturn’s magnetic field was being dragged round Enceladus in a way that suggested it had an atmosphere.”

So Dougherty and her colleagues asked the Cassini management to direct the probe to take a much closer look. This was agreed and in July 2005 Cassini moved in for a close-up study. “I didn’t sleep for two nights before that,” says Dougherty. “If Cassini found nothing we would have looked stupid and the management team might not have listened to us again.”

Her fears were groundless. Cassini swept over Enceladus at a height of 173km and showed that it did indeed possess an atmosphere, albeit a thin one consisting of water vapour, carbon dioxide, methane and nitrogen. “It was wonderful,” says Dougherty. “I just thought: wow!”

Subsequent sweeps over the moon then revealed those plumes of water. The only other body in the solar system, apart from Earth, possessing liquid water on its surface had been revealed. Finally came the discovery of organics, and the little moon went from being merely an interesting world to one that was utterly fascinating.

“Those plumes do not represent a torrent,” cautions McKay. “This is not the Mississippi pouring into space. The output is roughly equivalent to that of the Old Faithful geyser in Yellowstone national park. On the other hand, it would be enough to create a river that you could kayak down.

“The fact that this water is being vented into space and is mixed with organic material is truly remarkable, however. It is an open invitation to go there. The place may as well have a big sign hanging over it saying: ‘Free sample: take one now’.”

Plumes spurting ice particles, water vapor and trace organic compounds from the surface of Saturn's moon Enceladus.Plumes spurting ice particles, water vapor and trace organic compounds from the surface of Saturn’s moon Enceladus.

Collecting that sample will not be easy, however. At a distance of 1bn miles, Saturn and its moons are a difficult target. Cassini took almost seven years to get there after its launch from Cape Canaveral in 1997.

“A mission to Enceladus would take a similar time,” says McKay. Once there, several years would be needed to make several sweeps over Enceladus to collect samples of water and organics. “Then we would need a further seven years to get those samples back to Earth.”

Such a mission would therefore involve almost 20 years of space flight – on top of the decade needed to plan it and to construct and launch the probe. “That’s 30 years in all, a large chunk of any scientist’s professional life,” says McKay.

McKay and a group of other Nasa scientists based at the Jet Propulsion Laboratory in Pasadena are undaunted, however. They are now finalising plans for an Enceladus Sample Return mission, which would involve putting a probe in orbit round Saturn. It would then use the gravity of the planet’s biggest moon, Titan, to make sweeps over Enceladus. Plume samples would then be stored in a canister that would eventually be fired back to Earth on a seven-year return journey.

Crucially, McKay and his colleagues believe such a mission could be carried out at a relatively modest cost – as part of Nasa’s Discovery programme, which funds low-budget missions to explore the solar system. Previous probes have included Lunar Prospector, which studied the moon’s geology; Stardust, which returned a sample of material scooped from a comet’s tail; and Mars Pathfinder, which deployed a tiny motorised robot vehicle on the Red Planet in 1997.

“The criteria for inclusion in the Discovery programme demand that any mission must cost less than $500m, though that does not include the price of launch,” says McKay. “We think we can adapt the technology that was developed on the Stardust mission to build an Enceladus Sample Return. If so, we can keep the cost below $500m. We are finalising plans and will announce our proposals in autumn.”

Such a mission is backed by Dougherty. “I think Enceladus is one of the best bets we now have for finding life on another world in our solar system. It is certainly worth visiting but it is not the only hope we have. The icy moons of Jupiter – such as Ganymede, Callisto and Europa – still look a very good prospect as well.”

And there is one problematic issue concerning Enceladus: time. “Conditions for life there are good at present but we do not know how long they have been in existence,” says McKay. “They might be recent or ancient. For life to have evolved, we need the latter to have been the case. At present, we have no idea about their duration, though geologists I have spoken to suggest that water and organics may have been there for a good while. The only way we will find out is to go there.”

The late entry of Enceladus in the race to find extraterrestrial life adds an intriguing new destination for astrobiologists in their hunt for aliens. Before its geysers were discovered, two main targets dominated their research: Mars and the icy moons of Jupiter. The former is the easiest to get to and has already received visits from dozens of probes. On 6 August, the $2.5bn robot rover Curiosity is set to land there and continue the hunt for life on the Red Planet. “For life to evolve you need liquid water, and although it is clear it once flowed on Mars, its continued existence there is debatable,” says Cockell. “By contrast, you can see water pouring off Enceladus along with those organics.”

Many scientists argue that water could exist deep below the Martian surface, supporting bacteria-like lifeforms. However, these reservoirs could be many metres, if not kilometres, below Mars’s surface and it could take decades to find them. Similarly, the oceans under the thick ice that covers Europa – and two other moons of Jupiter, Ganymede and Callisto – could also support life. But again, it will be extremely difficult for a robot probe to drill through the kilometres of ice that cover the oceans of these worlds.

Enceladus, by these standards, is an easy destination – but a distant one that will take a long time to reach. “No matter where we look, it appears it will take two or three decades to get answers to our questions about the existence of life on other worlds in the solar system,” says Cockell. “By that time, telescopes may have spotted signs of life on planets elsewhere in the galaxy. Our studies of extra-solar planets are getting more sophisticated, after all, and one day we may spot the presence of oxygen and water in our spectrographic studies of these distant worlds – an unambiguous indication that living entities exist there.

However, telescopic studies of extra-solar planets won’t reveal the nature of those lifeforms. Only by taking samples from planets in our solar system and returning them to laboratories on Earth, where we can study them, will we be able to reveal their exact nature and mode of replication – if they exist, of course. The little world of Enceladus could then have a lot to teach us.


NASA’s Curiosity Update                             

Mars looks remarkably like the California desert in a new photo beamed home by NASA’s Curiosity rover, researchers said.

In the new black-and-white image, Curiosity’s Gale Crater landing site bears a striking resemblance to the desert landscape a hundred miles or so east of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., where the rover was built, scientists said.