This paper is really a tour de force of analytical chemistry, because it does something that I didn’t think was possible. The team is looking at a rather ancient creature, Dickinsonia. In fact, you could argue that it’s the ancient creature, since it’s one of the Ediacaran organisms that are part of the first explosion of complex living forms in the entire fossil record. This is well before the (already ancient) Cambrian explosion, the one that produced the strange Burgess Shale fauna. The Ediacaran community (PDF), at first glance, appears to be about as close as we’re going to get for a while to extraterrestrial life, because, as far as can be told, it was totally wiped out at some point for reasons that remain controversial. None of the Ediacarans have left any obvious, or even non-obvious descendants; it’s as if a giant reset button was hit on macroscopic life. Did the Cambrian organisms arise and displace (or even just eat) the Ediacaran ones? Did ocean conditions change too much or too quickly for them? All of the above?
You may have noticed that I’m tiptoeing around the words “animal” and “plant”, and that’s because no one has really been sure if those describe Ediacaran organisms or not, which is a convincing demonstration of how odd they are. Dickinsonia fossils are found in a number of places around the world (the Flinders range in Australia, near Arkhangelsk in Russia, in Ukraine, etc.), and they look pretty much like the one shown at right. They have bilateral symmetry – well, sort of, because those segments alternate, so technically it’s a glide symmetry. And they have one end rounder than another, and if you’d like to call one of those the head and one the tail, be my guest.
They appear to have lived on the bottom of the seas of the time, feeding on microbial mats, although they certainly don’t have mouths (way too early for anything as advanced as a mouth to be showing up, apparently). The best guess is that they might have anchored themselves to the bottom with some sort of sticky gunk and secreted enzymes to absorb bacterial food through their ventral surfaces (they’re often found with microbial mat impressions around them). But who knows? They appear to have competed with each other – you never really find them overlaying each other in a particular fossil, and some of those fossils really look like one Dickinsonia is trying not to be too close to another one. They range in size from a few inches to several feet across, and if you’d like to assign the larger ones to a different species (Dickinsonia rex), you might be right. Or not. Who knows?
The belief is that they were a sort of thick jellyfish consistency, a flat inflated bag (or collection of bags), which brings up a very reasonable question: how come they left such excellent fossils? Something was clearly different about Ediacaran conditions – maybe more silica in the water, maybe different sorts of bacteria that allowed fossilization to proceed in a different manner, who knows. But we have surprisingly nice fossils of Ediacarans, the better to confuse us.
This new paper seems to have settled the “Is it an animal?” question. Analysis of the fossils themselves show cholesterol-like steroids associated with them, and these are only found in animals (as opposed to plants, protozoans, or fungi). There are always a lot of stigmasteroids in these fossil layers, an ancient biochemical pathway associated with green algae. The next most common class are ergosteroids (generally from fungi), and the least common are the cholesteroid steroids. But in the organic matter of the Dickinsonia fossils you switch to over 90% cholesteroids. In the zone immediately above and below the organisms, the sterane distribution is also skewed, in a way that suggests decay bacteria at work on the steroids of the dead Dickinsonia themselves. The same team had reported, earlier this year, a similar analysis on another Ediacaran organism (Beltanelliformis) that established it as large round colonies of cyanobacteria, so the “molecular fossil” approach is really working out.
The idea of these compounds surviving for this long in analyzable, interpretable condition is quite striking. It’s been increasingly clear that more recent fossils (dinosaur bones, etc.) can harbor bacterial communities of their own, modern organisms moving in to consume this (ancient but still useful) food. That complicates things, if decomposition is still an active process after seventy million years, but in this case, there seems no doubt about the animal interpretation. If you want to claim intact proteins or nucleotides, though, that’s going to be a very different question. Those things are a lot more fragile and a lot more subject to bacterial confusion.
What this new paper tells us about the Ediacarans is that my remark above about “close to extraterrestrial” is completely wrong. These things are animals, biochemically like today’s animals in their steroid content, and are not some weirdo branch of large lichens or something (a theory that’s been hard to disprove until now!) The Ediacaran creatures, as odd as they look, are indeed our multicellular macroscopic forerunners and not a start at large organisms that’s off in its own world as compared to the Cambrian explosion. There are connections between the two, and the organisms in each can be classified as plants, animals, fungi, protozoans, cyanobacteria and so on. These categories have been with us for a very long time indeed.