Did life have hellish origins? Carbon with an organic-like signature has been discovered sealed within a crystal that formed during an interval of Earth’s history named after Hades, the underworld of Greek mythology. The find predates other evidence of life by 300 million years.

Our planet formed roughly 4.5 billion years ago, but it’s anyone’s guess exactly when life first appeared. The oldest reliable fossils are about 3.43 billion years old. Chemical signatures in even older rocks suggest life might have been present 3.8 billion years ago.

All of these early fossil signatures belong to the Archaean, which began 4 billion years ago. It is generally thought that conditions on Earth before then were so extreme that life wouldn’t have stood a chance of survival – which is why the pre-Archaean stage of Earth’s history has been dubbed the Hadean.


Signs of life

But Elizabeth Bell and Mark Harrison at the University of California, Los Angeles, and their colleagues think life might have existed in the Hadean after all.

They analysed more than 10,000 zircon crystals smaller than a millimetre in length that date from the Archaean and Hadean. In one Hadean crystal they found tiny flecks, or inclusions, of graphite, which must have been incorporated into the zircon crystal when it formed some 4.1 billion years ago.

The researchers analysed the carbon isotopes in two of the graphite flecks, and found both graphite flecks had a high carbon-12 to carbon-13 isotope ratio, a characteristic feature of organic origins as life preferentially absorbs carbon-12.

It’s not the first time that people have claimed the discovery of potentially organic carbon in Hadean zircons – but the carbon in those earlier claims turned out to be an artefact of the preparation techniques used to study the zircons, says Harrison. “I think there will be little dispute regarding the primary nature of the inclusions,” he says.

What will be up for discussion is whether the isotopic signature is evidence for the presence of life, says Bell. The general chemical make-up of the zircon crystals suggests that the magma they cooled from was generated by the melting of a mud-rich sediment, which is the sort of environment in which organic remains might accumulate.

Lifeless reactions

But the team points out there are also inorganic ways that isotopically light carbon could have accumulated in Hadean environments, for instance, through some of the Fischer-Tropsch chemical reactions that can turn carbon monoxide and hydrogen into liquid hydrocarbons.

Ultimately, carbon isotope data on its own is too ambiguous to decide whether Hadean carbon is evidence of Hadean life, says Thomas McCollom at the University of Colorado, Boulder. “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,” he says.

It’s always going to be tough to convince everyone to accept that Hadean zircons carry proof of life. “Proof is a realm better suited to mathematics than the natural sciences,” says Steve Mojzsis also at the University of Colorado, Boulder.

Even so, he thinks the new study is important. “Harrison and his team have challenged us now to think deeply about just how ancient the biosphere could be and to find new ways to explore for a cryptic record of it.”

Journal reference: PNAS, DOI: 10.1073/pnas.1517557112

Read more: “First life: The search for the first replicator”

Image credit: Richard Bizley/SPL

Correction:This article has been updated to clarify that it is the ratio of different carbon isotopes, not just the presence of those isotopes, that is characteristic of organic origin.