Zircon crystals — little time capsules preserving bits of Earth's ancient past — have in our recent past given evolutionary scientists a whole new view of conditions on the early Earth. Previously thought to have been a Hadean hell of molten magma for a very long time, the early Earth is now seen in the light of zircon-trapped atoms as a place that cooled enough for water to condense within 160 million years of being spit out of a solar nebula and coalescing as a fiery ball. Now the discovery of flecks of graphite in an Australian zircon crystal has raised the possibility that life evolved very quickly in that ancient world, 300 million years earlier than previously thought.
Carbon is contained in all living things and of course in inorganic molecules like carbon dioxide. Graphite is a form of pure carbon that forms under hot conditions. Graphite in rock dated as old as 3.8 billion years has previously been taken as the earliest physical residue of life on Earth. Like the latest graphite-in-zircon discovery reported in Proceedings of the National Academy of Sciences, the proportion of carbon isotopes found in that graphite from Greenland was consistent with biogenic carbon—carbon produced by a living organism through a process like photosynthesis.
"Twenty years ago, this would have been heretical; finding evidence of life 3.8 billion years ago was shocking," explains UCLA geochemist and study coauthor Mark Harrison. "Life on Earth may have started almost instantaneously," he adds. "With the right ingredients, life seems to form very quickly."