Within the first billion years of Earth’s history, life appeared in the oceans and began to affect the Earth’s atmosphere and surface, leading to the proliferation of anaerobic and, later, aerobic organisms. Some geological evidence indicates that life may have arisen as early as 4. 1 billion years ago. Since then, the combination of Earth’s distance from the Sun, physical properties and geological history have allowed life to evolve and thrive. In the history of life on Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species that ever lived on Earth are extinct. Estimates of the number of species on Earth today vary widely; most species have not been described. Over 7. 7 billion humans live on Earth and depend on its biosphere and natural resources for their survival. Humans have developed diverse societies and cultures; politically, the world has around 200 sovereign states.
The existence of ample amounts of hydrogen in the subsurface ocean of Enceladus indicates that microbes--if any exist there--could use it to obtain energy by mixing with carbon dioxide dissolved in water. This particular chemical reaction, termed methanogenesis, because it manufactures methane as a byproduct, may have been of critical importance in the emergence of life on our planet. Discovered on March 31, 2005, by a team of planetary scientists led by Dr. Michael E. Brown of the California Institute of Technology (Caltech) in Pasadena, Makemake was initially dubbed 2005 FY 9, when Dr. Brown and his colleagues, announced its discovery on July 29, 2005. The team of astronomers had used Caltech's Palomar Observatory near San Diego to make their discovery of this icy dwarf planet, that was later given the minor-planet number of 136472. Makemake was classified as a dwarf planet by the International Astronomical Union (IAU) in July 2008. Dr. Brown's team of astronomers had originally planned to delay announcing their discoveries of the bright, icy denizens of the Kuiper Belt--Makemake and its sister world Eris--until additional calculations and observations were complete. However, they went on to announce them both on July 29, 2005, when the discovery of Haumea--another large icy denizen of the outer limits of our Solar System that they had been watching--was announced amidst considerable controversy on July 27, 2005, by a different team of planetary scientists from Spain. But small moons like Methone are usually geologically inactive and bereft of an atmosphere. Therefore, they are usually unable to smooth away the scars. Dr. Peter Thomas of Cornell University in Ithaca, New York, explained it this way in the May 17, 2013 New Scientist: "When we look at objects less than 200 kilometers in radius, they are all like potatoes. They have lumps, grooves, craters." This makes Methone's smooth surface a mystery. Dr. Thomas is a Cassini team member.