The big science news last week was a report announcing the world’s oldest microfossils found in rocks near Hudson’s Bay in Canada. These aren’t any old rocks, these are THE old rocks, with an estimated age of between 3.77 and 4.32 BILLION years. Some of the oldest rocks on the planet. Really almost unimaginably old. The Earth is 4.65 billion years old, and for those first 500 million years our lovely planet was one rough neighborhood, with planet scouring, ocean vaporizing meteorites and asteroids being frequent visitors. So if the estimates are right, these fossil-bearing rocks probably formed some time during, or shortly after, liquid water and the oceans became a permanent feature of Earth.
What was exciting for me was that these fossils were made out of iron, indicating the bacteria that could have made them might have been growing on iron. These are the kind of bacteria I study in modern environments, so it’s something I know a lot about. In fact, I was contacted by three journalists who gave me an advance copy of the research article so I could comment on it, and I ended up with a quote in The Guardian newspaper, so that was cool.
These kinds of discoveries are not without controversy, often quite a bit of controversy, since figuring out fact from fiction can be very difficult. The facts in this case are that we know bacteria that grow on iron produce lots of thin filaments of iron oxides as a by-product of their growth. Sometimes these can be hollow tubes, sometimes they are twisted hair-like filaments. My colleagues and I have published papers on this, check this one out for some beautiful images.
We know these structures can be preserved as fossils in the rock record, and geologists are pretty good at dating how old things are. There are examples of 400-500 million year old micro-fossils that nearly exactly resemble modern-day iron-oxidizing bacteria, and there are even fossils from around a billion years old that are very similar to modern day microbes. Older than that, and most of the samples I’ve seen are either in the ‘could be, but not for sure’, or ‘boy, that’s really stretching it’ categories.
The problem is there are purely chemical reactions that can produce similar structures, so if the preservation is not excellent it becomes harder and harder to tell, biological from nonbiological. The structures these authors show are in the definite shoulder shrugging category. They are consistent with biology, but not inconsistent with purely chemical deposition too. How’s that for mealy mouth decisiveness. The authors provide some other evidence for life in the carbon that is associated with the filaments, and I’m not enough of an expert to say much about the meaning of that, other than if they hadn’t found it that would have been troubling. One thing I liked about this study was they did show groups of fossilized filaments including some even lined up together in the same orientation. We know in modern environments these bacteria always grow in tight groups and often the filaments they produce line up together as they respond to queues in their environment. So that’s a positive. On the other hand, the filaments they show are about five times bigger than modern bacteria, which is disconcerting, since if anything one would expect cells to get larger and not smaller over time.
So where do I stand? Of course, I’d like to believe what they’ve found is true. Who doesn’t want the organisms they’ve spent the better part of a scientific lifetime studying, to be among the most important on Earth (okay, I already knew that), but the oldest as well, that would extra special. Based on the evidence about which I’m most knowledgeable, all I can say is: intriguing but not proof.
Will there ever be proof? That’s a tough question. If you could find very well preserved iron micro-fossils, and definitively prove the age of the rocks that would be pretty convincing. If we could find some trace of an organic molecule that might be indicative of iron metabolism that would make the case even stronger. Right now we don’t have such a molecule, although I have some colleagues who have a hint of one. So, of course, like any good scientist, my answer is we need more study!
To put this in perspective, there’s another group of ancient fossil-making microbes called cyanobacteria that were the first organisms to make oxygen. Even though they don’t make nearly as nice fossils as the iron-oxidizing bacteria, they’ve been studied a lot more, so we know more about them, and how they impact the fossil record. In fact, until this recent discovery, cyanobacteria were the oldest fossils on Earth. By contrast, there are only a few labs that study the fossil-making implications of the iron-metabolizing bacteria, so there is still much to be learned.
So what’s the significance. First, of course, it provides new evidence for the antiquity of life and makes us appreciate how life and our planet have evolved together. If the results as presented are true it shakes up what we thought we knew about environmental conditions on ancient Earth, for one thing there may have been more oxygen around than previously thought. It also would make us re-examine the evolution of life and metabolism, for example, most of the modern bacteria that live on iron are old, but still don’t look like they came along particularly early in the evolution of life. Which brings us to the origins of life. These results suggest life originated shortly after Earth became habitable, which speeds up the timeline for life by at least a few hundred million years. It also continues the discussion of the idea that life originated elsewhere and landed on Earth, all ready to go — the idea of panspermia. Of course, all these factors make the search for life on other worlds all the more interesting, especially with the also very recent news of a group of new planets that are a circling a not so distant star.
Exciting times in the life of iron and the ironies of life!