<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> The evolutionary history of life on Earth traces the processes by which both living organisms and fossil organisms evolved since life emerged on the planet, until the present . Earth formed about 4.5 billion years (Ga) ago and evidence suggests life emerged prior to 3.7 Ga . Although there is some evidence to suggest that life appeared as early as 4.1 to 4.28 Ga this evidence remains controversial due to the non-biological mechanisms that may have formed these potential signatures of past life . The similarities among all present - day organisms indicate the presence of a common ancestor from which all known species have diverged through the process of evolution . 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 1.9 million are estimated to have been named and 1.6 million documented in a central database to date . More recently, in May 2016, scientists reported that 1 trillion species are estimated to be on Earth currently with only one - thousandth of one percent described . </P> <P> The earliest evidences of life on Earth are biogenic carbon signatures and stromatolite fossils discovered in 3.7 billion - year - old metasedimentary rocks discovered in western Greenland . In 2015, "remains of biotic life" were potentially found in 4.1 billion - year - old rocks in Western Australia . In March 2017, researchers reported evidence of possibly the oldest forms of life on Earth . Putative fossilized microorganisms were discovered in hydrothermal vent precipitates in the Nuvvuagittuq Belt of Quebec, Canada, that may have lived as early as 4.280 billion years ago, not long after the oceans formed 4.4 billion years ago, and not long after the formation of the Earth 4.54 billion years ago . According to biologist Stephen Blair Hedges, "If life arose relatively quickly on Earth...then it could be common in the universe ." </P> <P> Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean and many of the major steps in early evolution are thought to have taken place within them . The evolution of photosynthesis, around 3.5 Ga, eventually led to a buildup of its waste product, oxygen, in the atmosphere, leading to the great oxygenation event, beginning around 2.4 Ga . The earliest evidence of eukaryotes (complex cells with organelles) dates from 1.85 Ga, and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism . Later, around 1.7 Ga, multicellular organisms began to appear, with differentiated cells performing specialised functions . Sexual reproduction, which involves the fusion of male and female reproductive cells (gametes) to create a zygote in a process called fertilization is, in contrast to asexual reproduction, the primary method of reproduction for the vast majority of macroscopic organisms, including almost all eukaryotes (which includes animals and plants). However the origin and evolution of sexual reproduction remain a puzzle for biologists though it did evolve from a common ancestor that was a single celled eukaryotic species . Bilateria, animals with a front and a back, appeared by 555 Ma (million years ago). </P>

What are the first living organisms on earth
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