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.
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