Interesting news. Scientists have potentially discovered the hallmarks of the earliest ever life on earth. It takes the form of tiny particles of 12carbon-rich graphite in a crystal of zircon. The crystal was taken from Jack Hills in Australia, famous for the age of its rocks, and is only 70 or so microns across. Of 10,000 crystals studied this was the only one to contain such inclusions. The crystal is estimated to be 4.1 billion years old. That is really, really old. The earth itself is only about 4.5 billion years old so this is right at the beginning of when the earth would have been even vaguely stable enough to support life, even life as primitive as this.
So what on earth makes them think that this is the signature of a living organism? That’d be the 12carbon. All life on earth that we know of has a particular fondness for this isotope of carbon, preferentially sequestering it over other carbon isotopes like 13carbon and 14carbon; this is because it is lighter and easier to manipulate by our various enzymes. Any organic matter, or even fossils, that you care to analyse will have a higher ratio of 12carbon to 13carbon than any material of non-organic origin. This is precisely the signature found in the crystal.
The high density of 12carbon combined with the extreme age of the zircon is what makes this such a potentially significant finding. The earliest fossils we have only go back as far as 3.5 billion years ago (b.y.a.), bacteria just don’t fossilise well, they’re tiny and squishy. We also have chemofossils; these are a type of fossil that are only detectable as the chemical residue left by the breakdown of organic matter. We have some of these dating back to about 3.8 b.y.a. Before this it gets even more difficult as there just aren’t sedimentary rocks on the earth in tact from that long ago; they have been tossed and turned and melted and generally messed about in such a way that there is no chance of identifying any fossils in them. This means it’s necessary to get creative. What we can see is that the oldest in tact rocks contain flakes and crystals that must have been created by the destruction of an even earlier rock formation. That is what this crystal is.
This open access publication from The Proceedings of the National Academy of Science then, pushes back the previous oldest evidence of life on earth another 300 million years or so. There seems to be a general, and valid, supposition that the form of life that produced the graphite would have been some kind of prokaryote, a bacterium. For me all this talk of the earliest life on earth once again brings to mind the question: what is life? When you talk about the origins of life can something as simple as a single, self-replicating molecule qualify? And if not, how complex does that molecule have to become before it does qualify? Food for thought.