The question of when did life first appear on Earth has been grappled with for thousands of years, well before science became a field of inquiry separate from philosophy. Current estimations, based on geological data, hold that life could only have appeared around 3.8 billion years ago, after the Hadean period, during which our planet was still sterile, lifeless, extremely hot, and constantly bombarded by meteorites.
Now, however, a controversial new study presents potential evidence that life arose as many as 300 million years before that. The clues lie in microscopic flecks of graphite – a carbon mineral – trapped inside a single large crystal of zircon.
These tiny crystals (barely spanning the width of a human hair), composed of silicon, oxygen and zirconium, grow in magmas, and can outlast the rocks in which they initially formed – in fact, although the oldest rocks on Earth date back only 4 billion years, researchers have found zircons up to 4.4 billion years old.
“They are pretty much our only physical samples of what was going on on the Earth before 4 billion years ago,” says Elizabeth Bell, a geochemist at the University of California, Los Angeles (UCLA), and lead author of the new study, published October 19th in the Proceedings of the National Academy of Sciences.
In the study, Bell and her colleagues examined a 4.1 billion year-old piece of zircon found in the Jack Hills in Western Australia, which contains bits of potentially undisturbed graphite that has a low ratio of heavy to light carbon atoms – called isotopes – consistent with the isotopic signature of organic matter.
“On Earth today, if you were looking at this carbon, you would say it was biogenic,” Bell says. “Of course, that’s more controversial for the Hadean.”
Despite competing explanations of how it formed, Bell’s team favours the idea that it started out as organic matter, which was then dragged into the Earth’s mantle during a collision of tectonic plates, and transformed into graphite that eventually found its way into a zircon crystal.
While it’s not yet clear whether this account is true (for instance, no one really knows if organic matter can survive in magma chambers long enough to form graphite), it would fit nicely with the growing body of evidence proposing a more hospitable early Earth that was much milder and more hospitable to life than the current model allows.
“We know there was liquid water,” said Mark van Zuilen, a geomicrobiologist at the Paris Institute of Earth Physics. “There’s nothing that holds us back from assuming life was there.”
In order to prove that the inclusions are really of organic origin, the team will have to make a concerted effort to find more Hadean carbon in Jack Hill zircons, as the graphite might have actually formed during a later bout of metamorphism, or came from a certain type of meteorite, which have light isotopic compositions.
“Hopefully we didn’t just chance on the one freak zircon that had graphite in it,” said Bell. “Hopefully there is actually a fair amount of it.”