



Abiogenesis is the natural process of life arising from non-living matter, such as simple organic compounds. The age of the Earth is about 4.54 billion years old. The earliest life on Earth arose at least 3.5 billion years ago, during the Eoarchean Era when sufficient crust had solidified following the molten Hadean Eon. The earliest physical evidence of life on Earth is biogenic graphite from 3.7 billion-year-old metasedimentary rocks found in Western Greenland and microbial mat fossils in 3.48 billion-year-old sandstone from in Western Australia. Some theories, such as the Late Heavy Bombardment theory, suggest that life on Earth may have started even earlier, and may have begun as early as 4.25 billion years ago according to one study, and even earlier yet, 4.4 billion years ago, according to another. The mechanism by which life began on Earth is unknown, although many hypotheses have been formulated. Since emerging, life has evolved into a variety of forms, which have been classified into a hierarchy of taxa. Life can survive and thrive in a wide range of conditions. Nonetheless, more than 99 percent of all species, amounting to over five billion species, that ever lived on Earth are estimated to be extinct. Estimates on the number of Earth’s current species range from 10 million to 14 million, of which about 1.2 million have been documented and over 86 percent have not yet been described.