3.4 billion year old fossils recently discovered

Martin Brasier of Oxford and his team have just described some very ancient fossils discovered in rocks dated to 3.4 billion years of age. The article is unfortunately behind a paywall, so I am unable to review it, but Jerry Coyne lays out the evidence neatly:

As the authors note, “determining the biogenicity [biological origin] of putative Archaean microfossils is notoriously difficult.” How do we know that these things are real remnants of bacteria and not just inclusions or artifacts? There are several independent lines of evidence, none conclusive but together building a very solid case:

  • They look like cells, being cell-shaped, cell-sized, and forming chains of spheroids that look like chains of both well-established fossil bacteria and modern bacteria. Some can even be seen “dividing” or expelling their contents after cell damage (see figure above).
  • The variation in size of the bodies is small—smaller than you’d expect if they were abiological inclusions. A uniformity of size, however, is expected if they’re all members of one living species.
  • The cell “walls” of the microfossils, too, are of uniform thickness, unlike that of artifacts like silica grains coated with carbon.
  • The geochemistry of the bacteria and surrounding rock supports the idea that these are true organisms. This involves not only the isotopic nature of the carbon, but the presence of nitrogen, a crucial biomarker, within the cell walls.

This reminds me very much of Margulis’ endosymbiotic theory and its multiple lines of evidence. Of course, that idea was a cornerstone achievement of 20th century biology whereas this recent discovery pushes the known origins of life back about 200 million years, but that does not mean it isn’t important. Braiser’s find goes to demonstrate the nature of early bacteria. Many of the features in these fossils show that ancient bacteria operated in ways similar to certain bacteria today – if you find something that works, keep it up. The find also demonstrates that life can come to be quite quickly. Earth is only 4.54 billion years old and probably was not hospitably cool enough for life for its first half billion years. Given this 3.4 billion year date plus the fact that these fossils are fairly complex, it is not a stretch to say life dates back even further, probably quite soon after it was possible for it to do so on Earth.

Finally, I have to reflect Coyne’s curiosity that this was not published in Nature or Science, but rather Nature Geoscience. It’s still in a prestigious enough journal, but the paper is important enough where it deserves publication that comes with a higher profile. I’m sure the authors attempted to reach the best level possible, so who knows what the deal is.

Only in the light of evolution

Now that finals are over, I can devote more time to my dear, neglected blog. I begin with a series:

I am following a specific chapter in Jerry Coyne’s Why Evolution is True.

The fossil record: We should see fossils in a certain order if evolution is correct. They should go from simple to more complex overall, and the fossils we see in the most recent strata should resemble extant life much more than the fossils we see in old strata.

We should also see changes within lineages. We should be able to observe instances of gradual change in species that eventually leads up to either current species or at least to the time of extinction for these species.

Here’s a simple timeline of life’s history. Click it.

What the evidence shows is gradual change. First we find simple bacteria which survived off energy from the Sun, then we see more complicated cells known as eukaryotes arise. (You are a eukaryote.) Next we see a slew of multi-cellular animals arise. They’re still simple, but much more complex than the original bacteria. A few million years later more complicated life arrives. Early (and simple) plants begin to take hold. Soon the fossil record begins to show more plant complexity with low-lying shrub such as ferns, then conifers, then deciduous trees, and finally flowering plants. Gradual changes occur in the oceans and fresh waters which lead to fish and then tetrapods (Tiktaalik comes to mind).

One of my favorite fossils is trilobites. They’re extremely common due to their hard bodies. In fact, even their eyes are well-preserved because of their hard mineral make-up. I personally recall entering touristy-stores seeing countless fossils of these guys in the mid-west to the west (which, unsurprisingly, was once a shallow sea). This image shows the different lineages of this organism. Studies show that the ‘rib’ count has changed over time in each individual species, often without regard to how the other species changed. Going back further, there is less and less divergence in each species. Eventually, as evolution predicts, they all meet at a common ancestor.

So naturally the next step is to find fossils which show more significant changes. Let’s take birds and reptiles. They hold similarities between each other, both morphologically (certain shapes and structures) and phylogenetically (genetic sequence). A good hypothesis is that they came from one common ancestor. If this is true, the links between birds and its ancestors and reptiles and its ancestors should lead to the same point. They do. Dinosaurs are the ancestors of both. The links between birds and dinosaurs are so incredibly well established that I’d prefer to not go over them in detail. But for starters, some dinosaurs sported feathers and claws and had the same proteins for the feather-making process as extant birds. The links between reptiles and dinosaurs is easier just on intuition, so I’ll leave it at that for now.

Other transitional fossils include the already mentioned Tiktaalik. A view of the history of life can be see here. This shows the change in head and neck structure. Recent research on long-ago discovered Tiktaalik fossils has shown the importance in the gradual bone changes in the neck. These changes – a hallmark of evolution – were important to the ability to turn its head. This is a hallmark because natural selection only modifies what already exists. This is precisely what happened.

Going further with this example, evolution makes predictions as to how early fish evolved to survive on land. If there were lobe-finned fish 390 million years ago and obviously terrestrial organisms 360 million years ago (which is what the fossil record shows), then if scientists are to find transitional fossils, they should date in between that time frame. There should be an animal that shows both features of lobe-finned fish and terrestrial animals. Tiktaalik is that animal. It has fins, scales, and gills, but it also has a flat, salamander-like head with nostrils on top of its nose. This is a good indication that it could breathe air. Its eyes were also placed there, indicating that it swam in shallow waters. Furthermore, it was lobe-finned, but shows bones (which eventually evolved into the arm bones you used to get out of bed today) that were able to support its weight to prop itself up. And of course, it dates to 375 million years ago.

Next, evolution says the fossil record should show recent fossils being more closely related to extant species than are early fossils. This is precisely what happens. Sixty million years ago there were no whales. Fossils resembling modern whales only show up 30 million years ago. So, again, evolution makes a predication: if transitional fossils are to be found, they will be within this gap. And so it is.

We begin with Indohyus. It was an artiodactyl. This is important because extant whales have vestigial bones which indicate that they came from this order: scientists expected to find this because, again, evolution predicted it. It should be of no surprise that this fossil dates to about 48 million years ago, right in the predicted gap. From here there is a gradual evolution shown in the fossil record which leads up to modern whales.