The study, published recently in the Proceedings of the National Academy of Sciences, presents evidence in the form of zircon crystals. These minerals (which are related to the cubic zirconium made to mimic diamonds) incorporate and preserve materials from their environment as they form. The tiny crystals are basically indestructible, so they're left behind as time capsules of sorts as other minerals are lost to time.
An examination of over 10,000 zircons from the early days of the planet yielded one fleck of crystal with pure carbon in the form of graphite. The zircon was dated as 4.1 billion years old, and analysis of the carbon suggested it could have been formed by organic processes -- by a photosynthetic creature, for example.
“On Earth today, if you were looking at this carbon, you would say it was biogenic,” lead author Elizabeth Bell, a geochemist at the University of California, told Science. “Of course, that’s more controversial for the Hadean.”
The study authors suggest that perhaps early Earth was more hospitable than previously thought. Recent studies have indicated as much, showing that there was liquid water even during our eon named for Hades. And if their findings are correct, they point out, and simple life arose almost instantaneously on Earth, then it could mean that simple lifeforms are common throughout the universe.
But even the researchers involved in the study admit that something other than a photosynthesizing organism could have created this particular cocktail of carbon. It's an intriguing finding, but it's far from proof that life definitely existed at that time. To really show that life thrived in the Hadean, they'll need a lot more than one tiny crystal.