Back story: signs of life.

Direct evidence of Earth's earliest life is hard to find. So scientists look to biomarkers, chemicals and structures thought to be produced by living things and then preserved. But evidence from many biomarkers remains contentious.

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Fossil stromatolite (one shown above) suggest the early activity of cyanobacteria, but some argue the structures could also have abiotic origins.

13C: 12C

In nature, carbon-13 is much rarer than carbon-12. Since living things concentrate C-12 during metabolism, the ratio of the isotopes in rocks can be used as evidence of life.

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Hopanes are a kind of molecular fossil, formed as part of the cell membranes of living things. But discerning between ancient hopanes and modern contaminants can pose problems.