The search for modern micro-organisms which resemble the earliest microbes on the ancient Earth has turned up some incredible bacteria. Most micro-organisms even extremo-philes live in community in their ecosystem with other microbes.
However, almost 2.8 km down beneath the surface of the earth, in a gold mine in South Africa one species of bacteria has been found which appears to be the only known organism that lives on its own. It lives by metabolising sulphur, hydrogen (produced by radioactive decay) and carbon dioxide dissolved in an ancient water source. It lives at temperatures of 61c and if it ever had oxygen tolerance it has lost it.
The environment that this creature lives in is in many ways like the early earth which had little oxygen.
Desulforudis audaxviator is of course almost certainly not actually an early life form itself. It may be just as likely that it is a relatively modern organism which millions of years ago found itself in and adapted to this subsurface rock environment. But it does help to point to the the type of genetic make up that early organisms must have had to cope with these food sources.
Desulforudis gets the last part of its name from a Jule verne reference to a "bold traveller" heading for the centre of the Earth. This bold traveller is probably not our long lost bacterial grandfather, or an actual representative of earths oldest lifeforms. It may live like grandpa did and like the same food but it just aint grandpa - probably.
There are estimates available for the amount of sub-surface microbial life and some estimates suggest that there is an amazing amount of microbial life beneath our feet. None of them so far seem to live such a lonely existence though as Desulforudis audaxviator.