Life On Earth: 3.95 Billion Years Ago

Scott K. Johnson:

You could be forgiven for thinking that the remnants of the Earth’s first life don’t want to be found. Between geology and happenstance, the earliest life has certainly covered its tracks well. While paleontologists studying dinosaurs can sometimes bring an unambiguously gigantic femur home, those who study the origins of life are usually left arguing over the significance of microscopic motes of rock.

A new discovery in northernmost Labrador, made by a team led by Takayuki Tashiro of the University of Tokyo, fits into that latter category. But don’t let its abstract smallness of the evidence dull your excitement. The researchers argue they have uncovered evidence that there was life on Earth more than 3.95 billion years ago—on a planet that isn’t much more than 4.5 billion years old itself.

Counting carbon

Some of the evidence for early life is in the form of fossilized microorganisms. It can be difficult to rule out bacterium-shaped mineral bits that can form in other ways, but research published earlier this year identified microscopic structures that seem to fit the bill in 3.7 billion-year-old rocks that were once part of seafloor hydrothermal vents.

Microbial life similar to modern blue-green algae (actually a bacterium) has long built signature mounds of sediment in shallow waters, and these can also be preserved in the geologic record. A study last year purported to push the appearance of that type of evidence back to 3.7 billion years, as well.

But before either of those discoveries, the title of “oldest evidence for life” had belonged to carbon isotope signatures in Greenland rocks that are 3.7 to 3.8 billion years old. This title was extremely contentious, though. The idea behind that date is relatively straightforward: biology has a chemical preference for carbon-12 over carbon-13, so biological carbon tends to have less carbon-13 in it than average. These ancient, metamorphosed rocks contain carbon in the form of the mineral graphite. And that carbon had a biological bent—light on carbon-13.

Not everyone thought that life was the best explanation for this signature, though. Just as microbial fossils can have mineral lookalikes, there are non-biological ways to make graphite lose some of its carbon-13 in metamorphic rocks. These non-biological means require specific conditions. In some cases, those conditions seemed to fit the sample’s history, providing a disappointing explanation for the carbon isotopes sans critters. But in other rock samples, some researchers argued that the alternative explanations couldn’t work, so that at least some of the graphite still pointed to a biological source.