How long did it take for life to get started on Earth? The planet formed roughly 4.5 billion years ago, although it was uninhabitable for a while afterward. By 2.7 billion years ago, there was unambiguous evidence of complex biological communities in the form of stromatolites, microbial biofilms that can structure sediments in coastal environments. Back in 2006, a paper described evidence that these complex communities have been present as far back as 3.5 billion years ago, based on rocks at the Strelley Pool Formation in Australia.

Now, a new study of rocks from Strelley Pool provides further evidence that these formations were laid down by biological activity. The isotopes of sulfur in the organic material in these rocks show a pattern similar to what we see in material with a known biological origin.

Atomic isotopes are chemically identical (they can undergo the same reactions) but differ in terms of mass. In the fast-paced and energy-sensitive reaction environment inside a cell, that slight difference in mass means that lighter isotopes are more readily incorporated into biological molecules. Over time, even a small difference in the use of lighter isotopes can build up, leading to a significant divergence from the isotope ratio found in non-biological material.

Sulfur has four stable isotopes (out of a total of 25!), with 32S being the most common. Although 33S and 34S are also present in measurable amounts, biological organisms have a strong preference for 32S. For organisms that metabolize organic material by transferring its hydrogen to a sulfur atom (converting sulfate to H 2 S, the smell of rotten eggs), 32S is greatly preferred. As a result, some of the 34S found in the sulfate in the environment ends up being depleted in the hydrogen sulfide produced by the bacteria.

But there are also some natural processes that produce a bias in isotope ratios, which means that simply getting all the sulfur out of a rock and looking at the overall levels won't tell you much about the source. Instead, the typical hallmark of biological deposits is variability—some layers may simply be trapped environmental materials, while others have a large dose of biological molecules, and should show a skewed isotope ratio. The trick is to look layer-by-layer and search for signs of variability.

That's exactly what the authors of the new paper did, taking thin slices of rock and scanning across them with an ion beam and analyzing the sulfur blasted from the surface. The result was a layer-by-layer measurement of the differences in 34S, with different sampling areas only microns apart.

The isotope ratios ranged widely (from a difference of -17.4 parts-per-thousand to 36.6 ppt). Many of the differences were associated with inclusions of organic material within the rock. Similar values were found in iron-sulfur compounds that had precipitated out. Information from the levels of 33S also suggested the original source of the sulfur was from the atmosphere, consistent with what we know of its likely composition during the Archean era.

As the authors note, this isn't a conclusive demonstration that the layered deposits in these rocks were once biofilms. But it adds to the evidence suggesting they were. And it suggests that, even at this early date, they were using a metabolic pathway that we know is still in use today.

And that evidence suggests that complex biological communities existed as far back as 3.5 billion years ago—which is pretty impressive given that the Late Heavy Bombardment only ended about 3.8 billion years ago. That also stretches out the gap between the obvious presence of life and the origin of multicellular animals, suggesting it might have taken nearly 2 billion years for organisms on Earth to make that leap.

PNAS, 2012. DOI: 10.1073/pnas.1207491109 (About DOIs).

Thanks to authors Tomaso Botognali and Alex Sessions for some helpful clarifications on this piece.