Elizabeth Bell from the University of California at Los Angeles and co-authors have discovered graphite inclusions in zircons from western Australia that date back 4.1 billion years, to a time period known as the Hadean. They suggest that the light carbon isotope ratio in the graphite, which was trapped in the zircon when it formed 4.1 billion years ago, is consistent with a biological origin and may be evidence that a terrestrial biosphere had emerged by this time. Light carbon isotope ratios are considered a biosignature, because life prefers lighter versions of carbon with fewer neutrons, because it takes less work to process. (A warning, though: there also are non-biological chemical pathways to form lighter isotopic variations of carbon.)
The finding from Bell et al. is consistent with another discovery by John Valley of the University of Wisconsin and colleagues about a year ago, of a zircon nearly 4.4 billion years old. Their study indicated that Earth’s early magma ocean must have cooled enough already by that point that the planet was able to form a rocky crust. Once temperatures cooled down enough to allow liquid water, life could have originated on Earth.
The new report by Bell and her co-authors suggest that this might have happened as early as 4.1 billion years ago. It probably wasn’t much earlier, because the researchers found that only one in more than 10,000 zircons included these graphite inclusions. Previous estimates of the first appearance of life on Earth ranged from 3.5 billion to 3.8 billion years ago (based on fossil evidence) to about 4.0 billion years ago (based on isotopic evidence from some of the oldest rocks on Earth, found in Greenland). Earth formed about 4.55 billion years ago, which means that life must have originated rather quickly after its formation.
Determining the date when life first appeared on Earth is of extreme importance, not only to understanding our own planet, but also to understanding our neighboring planets Venus and Mars, which are thought to have had liquid water oceans on their surface during the Hadean. If life existed on Earth that early, all three planets would likely have harbored a biosphere, as there was a great deal of meteorite exchange between these worlds early in the Solar System’s history.