“Rudimentary life may have existed on Earth 3.95 billion years ago, a time when our infant planet was being bombarded by comets and had hardly any oxygen, researchers said Wednesday.
A team presented what they say is the oldest-known fossil evidence for life on the Blue Planet—grains of graphite, a form of carbon, wedged into ancient sedimentary rocks in Labrador, Canada.
The previous most ancient life traces were reported in March, from a site in Quebec estimated at between 3.8 billion and 4.3 billion years old, though an author of the new study called that dating process “highly controversial.”
“This is the oldest evidence,” Tsuyoshi Komiya of The University of Tokyo insisted in an email exchange with AFP.”
Read more at: https://phys.org/news/2017-09-life-earth-date-bn-years.html
https://phys.org/news/2017-09-life-earth-date-bn-years.html
Lars Fosdal 
There is also a theory Earth may have had life of some kind before it collided with the object that formed the Earth/Moon pair. But, that one is a LOT harder to prove.
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David Hess If there was microbial life, it was probably incinerated in that event.
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Lars Fosdal I would tend to agree. But, some new observations indicate the energy of the impact was not evenly distributed. Isotope ratios from different areas of our crust indicate some are large chunks of primordial Earth. If you’re into the panspermia theory, that may have allowed some simple life to survive. It might make for some good scifi, at least. 😀
phys.org – Scientists may have identified echoes of ancient Earth
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