3.5 billion-year-old microbe fossils get another look

Carbon isotopes seem to show ancient samples really are living in origin.

The title of “oldest evidence of life” has been provisionally claimed by a growing and confusing crowd of discoveries recently. At least until the last few years, the crown rested comfortably on a 3.47 billion-year-old rock from Western Australia called the Apex Chert. First described in the early 1990s, this rock contained a variety of microscopic structures that looked for all the world like the fossilized remains of microbial life.

Like other finds in this category, the Apex Chert has seen its fair share of controversy as researchers skeptically poked and prodded. Just two years ago, we covered a study that concluded these microfossils were simply clever lookalikes created by minerals crystallizing near a hydrothermal vent. In that version of events, some carbon (which may or may not have come from living things) stuck to vaguely microbe-shaped mineral crystals.

A recent study led by William Schopf—who discovered the Apex Chert in the first place—brings newer tools to bear on the question. And the researchers believe the results show that these microfossils are not impostors.

Schopf and his team subjected 11 purported fossils from the original sample to an incredibly precise spot-measurement instrument that can determine the mix of carbon isotopes that are present. (It’s the same instrument that we once visited, in fact.)

The first question is simply whether the carbon in the fossils—and the random carbon particles that can be found around them—matches the isotope signature of carbon from living organisms. Biology is somewhat choosy when it comes to isotopes of carbon. The extra neutron in carbon-13 causes organisms to prefer its lighter version; non-biological chemical reactions are typically more indiscriminate. So an unusually low share of carbon-13 is an indicator of biological carbon.

All the carbon in the samples passes this test. And the carbon inside the fossils contained even less carbon-13 than the random bits of carbonaceous stuff outside the fossils.

But the most interesting comparison is between the relevent fossil specimens. In the original study, Schopf identified five different types of fossils in the Apex Chert, which he suggested corresponded to five different species or types of microbial organisms. It turns out they each had distinct carbon isotope signatures. If these fossils were just lookalike mineral crystals coated in carbon, you would expect to see no consistent carbon isotope pattern—they should all be roughly the same. But if these were different types of organisms subsisting on different chemical fuels, it would make sense to see variations in the carbon isotopes.

The isotope signatures can actually hint at what these organisms would have been like. Two of them are within the range of photosynthetic, single-celled life.

The other three would match up with an interesting pair: methane-producing archaea and methane-consuming bacteria. That would be pretty cool, as the existence of these two types of life have been guessed at from carbon isotope measurements of very old rocks but never pinned to specific microbial fossils. Their presence would hint at the diversity of life, even in the early days. Then again, recent studies have claimed to find evidence of life from 3.7 or even 3.95 billion years ago—and that would make 3.47 billion-year-old lifeforms comparative spring chickens.

But as for the truth about the Apex Chert, the authors argue there is simply too much consistent evidence supporting the conclusion that these are real microbial fossils. The multiple lines of evidence make it more difficult to find a plausible non-biological explanation. They’ll now probably keep their place in an exclusive VIP section unless a better objection comes along.