Preserved in time: microfossils offer clues to biosphere’s birthday. Don’t get the candles out.
Updated: Jun 3, 2020
Miraculous preservation of 3.4 billion-year-old microfossils reveals new clues about the earliest forms of life
Dinosaurs are hatchlings next to these Archean treasures. Deep within ancient sedimentary rock (the Strelley Pool Formation, Western Australia), scientists have found the most credible traces of what’s believed to be the oldest forms of life. The organic microfossils are 3.4 billion years old. Before getting onto the interesting stuff, can we just pause for a moment and unpack that information?
We can’t fathom “3.4 billion years ago”, but I’m going to cleverly trick you into thinking you can. Imagine that the birth of the solar system and planet earth is at your shoulder (4.6 billion years ago); present day 2018 is at the very tip of your middle fingernail. Humans have been around since 1 mm in from the tip of that fingernail (for 0.0002 billion years), and dinosaurs are at your knuckles (0.066 billion years ago). These 3.4 billion-year-old fossils are about as old as your armpit, not ‘too long’ after our moon acquired its characteristic craters (3.9 billion years ago).
Still, no one really knows when life and the planet’s ecosystems (i.e., the biosphere) came into being. The current estimate is 4 billion years ago, evolving into a ‘sophisticated’ state 3.7 billion years ago. But the highly metamorphosed nature of the oldest sedimentary rocks means that there’s few reliable traces of life from which to predict the age of the biosphere.
3.4 billion years ago, the earth was a profoundly different place. No oxygen, no ozone layer, no animals or plants; high temperatures, shorter days, a much closer moon, and frequent tidal waves. Extreme conditions and the degradation that microorganisms experience during fossilisation mean that the biogenecity of the Strelley Pool rock has been dubious. The lack of authentic organic microfossils with identifiable molecular structures that are indicative of ‘life’ means that the scientific community has had a hard time coming to any kind of agreement about how old the biosphere is. If only we had unequivocal evidence… oh hi, here’s what this article’s about.
The most astonishing thing about this new research is how well preserved the microfossils are, despite being exposed to extremely hostile conditions. “It’s remarkable that these microfossils have survived the extreme conditions they’ve experienced over the last 3.4 billion years,” says Dr Julien Alleon (MIT, Department of Earth, Atmospheric and Planetary Sciences). “We know from their molecular structure that they have been exposed to temperatures of up to 300°C for long periods. And yet we are still able to see signs of their original chemistry.” Until now, these temperature conditions have been believed to totally destroy molecular information.
The study, published as a Letter last week in Geochemical Perspectives, reports the miraculous molecular preservation of these armpit-old organic microfossils. The study was carried out by researchers at the CNRS, Lille University, and Nagoya University, and is part of the European Commission’s PaleoNanoLife project.
The study used high-resolution spectroscopic tools that perform chemical analyses at the submicrometer scale. These data provide strong support for the authenticity of these traces as fossilised life forms.
“We measured individual Archean microfossils using this technique and realised that their original chemistry was not as degraded as it was previously believed,” says Dr Sylvain Bernard at the Institut de minéralogie, de physique des matériaux et de cosmochimie. “Basically, the way our samples absorb the X-ray beam give us some insights about their chemical nature.” This degree of molecular preservation highlights the palaeobiological potential of the Earth’s oldest geological record, and also gives us new insights into the early biosphere.
The authors compared spectroscopy data from the 3.4 billion-year old Strelley Pool Formation fossils with more recent ones (1.9 billion years old, from the shores of Lake Superior, Ontario, Canada) and with modern bacteria. All three samples showed similar absorption features – this means that that their residual chemicals were initially made from the same building blocks. In short, “[O]ur results support the growing body of evidence advocating their biogenicity” says Dr Bernard.
And the 3.4 billion-dollar question… what were these organisms? What did these first forms of life look like, how did they contribute to the biosphere, how did they function, survive, and evolve?
“We do not know exactly what kind of microorganisms the Strelley Pool microfossils were, but we demonstrate in this study that their original chemical nature was not totally destroyed during the geological history that they experienced,” says Dr Bernard, “This means that we may soon precisely know the chemical nature of the earliest life on Earth, and that we will eventually be able to place constraints on the timing, tempo and trajectory of its evolution.”