Pinpointing the beginning of life on Earth is tricky. The fossil record only goes so far. While geologists have uncovered hints of life dating back 3.8 billion years, a controversial new study claims to have discovered evidence for the building blocks of life as old as 4.1 billion years. If true, this find suggests that organic compounds formed while the planet was still in its infancy.
While scientists know that our planet first formed about 4.5 billion years ago, the oldest evidence of life are fossil traces of 3.8 billion-year-old microbes called Archaea, Colin Barras writes for The New Scientist.
The intervening years between our planet's conception and evidence of Archea is coined the Hadean, after Hades, the Greek god of the underworld. During this time, the Earth’s surface was likely molten. So scientists' only clues about this period are hidden in tiny crystals called zircons, nearly indestructible baubles that form in magma, Julia Rosen writes for Science Magazine.
In the new study, published in the Proceedings of the National Academy of Sciences, scientists inspected 10,000 zircon crystals for clues about Earth’s earliest days like insects sealed in amber. But they weren't looking for bugs, they were looking for other rocks, which is exactly what they found: One single 4.1 billion-year-old crystal containing graphite.
Graphite is made up entirely of carbon, and the isotopic pattern of this particular graphite resembled modern organic matter. “On Earth today, if you were looking at this carbon, you would say it was biogenic,” lead author Elizabeth Bell tells Rosen. “Of course, that’s more controversial for the Hadean.”
Finding evidence of organics from the Hadean is a big deal, but it’s a far cry from saying that Bell and her team discovered an ancient microbe. While a growing body of evidence suggests that early Earth wasn’t as sterile as scientists once thought (and could have even had liquid water), some researchers are skeptical that this single piece of evidence is enough to suggest life existed during the Hadean.
“I know a lot of people want to use such data as evidence of life, but this is governed more by what they want the outcome to be rather than scientific principles,” NASA astrobiologist Thomas McCollom tells Barras.
This skepticism in part comes from a study published in 2008 that claimed to have similarly found microscopic diamonds embedded in 4.25 billion-year-old zircon crystals. After their results were questioned, the scientists discovered the diamonds were merely contamination from grit used to polish the crystals.
While Bell and her team were careful to prevent similar issues, other researchers remain wary that graphite could have formed during the Hadean. Some suggest the graphite could have even been encapsulated at a later date by zircon melting and recrystallization.
“That one negative experience doesn’t mean nobody should try again,” California Institute of Technology geologist John Eiler tells Rosen. “But let’s just say, I’m cautious.”
While Bell and her colleagues are excited by their find, they aren’t ruling out non-biological explanations for the graphite either. In the meantime, the best support for their theory would be replication—whether it is from other graphite-containing Hadean zircons or ancient Martain life, which has rocks even older than the Earth, Rosen writes.
“Hopefully we didn’t just chance on the one freak zircon that had graphite in it,” Bell tells Rosen. “Hopefully there is actually a fair amount of it.”