Life on Earth may have started hundreds of millions of years earlier than previously believed.
Scientists from the University of California, Los Angeles and Stanford University say life on Earth may have popped up "almost instantaneously" after our planet formed 4.5 billion years ago – some 300 million years earlier than previous studies indicated.
"With the right ingredients, life seems to form very quickly," said Mark Harrison, a co-author of a study in the current issue of the Proceedings of the National Academy of Sciences, in a news release.
For decades, geologists thought life developed after the end of millions of years of heavy bombardment, during which Earth and its neighboring planets got pummeled by a steady stream of massive meteor impacts.
But what if life had begun before the heavy bombardment?
"If all life on Earth died during this bombardment, which some scientists have argued, then life must have restarted quickly," said Patrick Boehnke, a co-author of the research.
During the heavy bombardment, scientists theorized that the frequent impacts left the Earth battered, dry, and desolate. The era is known as the Hadean, named for the Greek underworld.
But now, tiny zircons suggest otherwise.
"Evidence from Hadean zircons has provided a new picture of a much more hospitable planet more like the Earth of today in many respects," said Elizabeth Bell, the lead author of the study, to the Royal Society of Chemistry. "For instance, the zircons' low crystallization temperatures and enrichment in oxygen-18 suggest the presence of liquid water."
Dr. Bell and her colleagues studied more than 10,000 zircon crystals that formed in magma 4.1 billion years ago, found in what is now Western Australia.
Zircons are heavy, durable minerals that can capture and protect tiny snapshots of the environment in which they formed, enabling researchers to use them as time capsules. Bell and Boehnke have pioneered chemical and mineralogical tests for ancient zircons, which they were using to search for dark flecks of carbon, the signature of life.
Of the 10,000 samples, 656 zircons included suspicious dark specks. The geochemists closely analyzed the 79 most promising zircons, and found one containing graphite, a pure form of carbon that is usually associated with living things.
"Nobody has offered a plausible alternative explanation for graphite of non-biological origin into a zircon," said Dr. Harrison.
The zircon was 4.1 billion years old, according to its ratio of uranium to lead, but the graphite inclusion was even older – meaning life potentially began soon after earth formed more than 4.5 billion years ago.
Not only does the graphite point to life, but its specific ratio of carbon-12 to carbon-13 is the fingerprint of photosynthesis, say the researchers, because some enzymes "prefer" carbon-12.
"We need to think differently about the early Earth," says Bell.