But in the decades since, we realized something: even though the fossils themselves may no longer be discernible to us today, the remnants of organic matter leave a particular signature in the form of carbon. You may be used to “carbon dating” in the form of measuring the carbon-14 to carbon-12 ratio in organisms, since both forms of carbon are absorbed into organic matter, with carbon-14 being created in the upper atmosphere by cosmic rays and decaying with a half-life of around 5,700 years. As long as you’re alive, you breathe in and ingest both forms of carbon; when you decompose, the carbon-14 decays and isn’t replaced by any new carbon-14. Hence, if you can measure the carbon-14 to carbon-12 ratio (carbon dating), you can know roughly, with an error of a few thousand years, how long ago a particular organism died.
This can only take you back around a hundred thousand years or so before the carbon-14 content gets too low to be effective. But there’s another form of carbon we don’t talk about in the same breath: carbon-13, which, like carbon-12, is stable, and which is about 1.1% as abundant as the other forms of carbon.
Living organisms – as far as we’ve been able to biologically observe – seem to prefer to uptake carbon-12 to carbon-13, due to metabolic enzymes reacting with carbon-12 more efficiently. If you find an ancient source of carbon and it’s enhanced with carbon-12 as opposed to carbon-13, that’s a good indicator that it’s the remnants of an organic life-form. By looking for graphite, a form of pure carbon, deposited in otherwise highly metamorphosed rocks (things like zircons), we’ve been able to push back well beyond that 1-2 billion year barrier, and had placed the emergence of Earth-life all the way back to 3.8 billion years ago, or just some 750 million years after Earth formed. But as of 2015, we’ve done even better.