For some reason I missed this paper published last November in Proc. Nat. Acad. Sci. USA by Elizabeth Bell et al. , and it doesn’t seem to have been given a lot of attention by the press. That may be because its conclusions are questionable, and based on a very small sample. But if they’re right, it’s a pretty amazing result, for the authors report the presence of what may be biogenic carbon—that is, carbon derived from living organisms—from the Jack Hills of western Australia, and that carbon was dated at 4.1 billion years old. Since the Earth is about 4.54 billion years old, and the zircons of the Jack Hills are the oldest known material of terrestrial origin on our planet (4.4 billion years is the oldest sample), the finding of biogenic life in zircons dated at 4.1 billion years means that life may have originated very, very soon after the Earth formed. But these findings are preliminary.
The oldest widely accepted evidence of life on Earth are 3.4 billion year old microfossils from the cratons of the Strelley Pool formation, also from Western Australia. (Old, stable parts of the Earth are called “cratons.”) To get older evidence than that, you have to date and do isotopic analysis of flecks of graphite that may be derived from organisms. The oldest carbon generally accepted as being of biological origin is about 3.8 billion years old.
The dating is done by radiometrically dating the minerals containing graphite (carbon) flecks (usually zircon derived from melting earlier “mud rocks” that presumably contained organismal remains), and the biogenic origin is studied by looking at the amounts of carbon 13 versus carbon 12 in the flecks. Non-organismal carbon has a relatively higher amount of carbon 13 than does biogenic carbon.(The different ratios come from the fact that organisms absorb atmospheric carbon into their bodies, which is higher in carbon-12 than inorganic carbon). The values of these isotopes are transformed into a statistic called δ13C; δ13C values of 24 or lower are generally assumed to be signatures of carbon derived from organisms.
At any rate, Bell et al. dated zircons found in the Jack Hills. One of them contained carbon flecks (and was crack-free, so the graphite didn’t insinuate itself after the zircon was formed); and for that sample they determined the average δ13C of the flecks using spectral analysis.
Here’s the prepared zircon with the flecks inside. The bar is 30 microns long, or about a thousandth of an inch.
And here’s the average value of δ13C (triangle) for the Jack Hills sample, compared with the ratios for 3.8-billion-year old graphite that is widely accepted as being organic in origin. The average value of the Jack Hills carbon was -24, so it’s within the range of organic carbon; i.e. life might have been around by 4.1 billon years ago. That would extend the origin of life back another 300 million years beyond what we know, so that life may have originated no more than 500 million years after the Earth formed.
Now the authors note that there are other processes that could produce low values of δ13C, including the Fischer-Tropsch chemical process, carbon derived from meteorites, isotope fractionation by diffusion, and so on, but they claim that a biogenic origin is “at least as plausible” as these (not a strong statement!).
I won’t go write further, as I only wanted to call your attention to some tantalizing evidence that life may have been around a lot earlier than we thought. Whether this becomes widely accepted will take a while—and much more work. After all, this paper is based on just a single sliver of zircon. If you want to see the entire paper, and can’t get it from the link below, just ask.
h/t: Latha Menon
Bell, E. A., P. Boehnke, T. M. Harrison, and W. L. Mao. 2015. Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon. Proceedings of the National Academy of Sciences 112:14518-14521.