<P> Family - tree relationships may also help to narrow down the date when lineages first appeared . For instance, if fossils of B or C date to X million years ago and the calculated "family tree" says A was an ancestor of B and C, then A must have evolved more than X million years ago . </P> <P> It is also possible to estimate how long ago two living clades diverged--i.e. approximately how long ago their last common ancestor must have lived--by assuming that DNA mutations accumulate at a constant rate . These "molecular clocks", however, are fallible, and provide only a very approximate timing: for example, they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved, and estimates produced by different techniques may vary by a factor of two . </P> <P> The evolutionary history of life stretches back to over 3,000 million years ago, possibly as far as 3,800 million years ago . Earth formed about 4,570 million years ago and, after a collision that formed the Moon about 40 million years later, may have cooled quickly enough to have oceans and an atmosphere about 4,440 million years ago . However, there is evidence on the Moon of a Late Heavy Bombardment from 4,000 to 3,800 million years ago . If, as seems likely, such a bombardment struck Earth at the same time, the first atmosphere and oceans may have been stripped away . The oldest clear evidence of life on Earth dates to 3,000 million years ago, although there have been reports, often disputed, of fossil bacteria from 3,400 million years ago and of geochemical evidence for the presence of life 3,800 million years ago . Some scientists have proposed that life on Earth was "seeded" from elsewhere, but most research concentrates on various explanations of how life could have arisen independently on Earth . </P> <P> For about 2,000 million years microbial mats, multi-layered colonies of different types of bacteria, were the dominant life on Earth . The evolution of oxygenic photosynthesis enabled them to play the major role in the oxygenation of the atmosphere from about 2,400 million years ago . This change in the atmosphere increased their effectiveness as nurseries of evolution . While eukaryotes, cells with complex internal structures, may have been present earlier, their evolution speeded up when they acquired the ability to transform oxygen from a poison to a powerful source of energy in their metabolism . This innovation may have come from primitive eukaryotes capturing oxygen - powered bacteria as endosymbionts and transforming them into organelles called mitochondria . The earliest evidence of complex eukaryotes with organelles such as mitochondria, dates from 1,850 million years ago . </P>

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