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" ... 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. ... "
" ... When scientists search for ancient remnants of life, they look for graphite deposited in highly metamorphosed rocks. This method led us to push the emergence of life, based on evidence from Earth-based rocks, back to 3.8 billion years ago, or just 750 million years after Earth formed. But looking at graphite deposits in zircons — some of which are 4.1 billion years old or possibly even older — shows this same carbon-12 enhancement at the expense of carbon-13. ... "