The oldest accepted trace of life

Graphite through the Isua metasediments is depleted in carbon-13 by around 20 parts per thousand, the fractionation produced when organisms preferentially take up the lighter isotope. Its distribution along sedimentary horizons — rather than in later veins — argues it was buried as organic matter, not injected afterwards. It is the earliest chemical evidence of life that most researchers accept, though no cell or body fossil survives the metamorphism.