Sunday, June 19, 2011
Sunday Spinelessness - The first animals (fossils)
I see that Prime is playing a documentary by David Attenborough on the origin of the animals. One of my favourite people talking about one of my favourite topics is motivation enough to dust off one of the many posts in The Atavisms massive "drafts" folder....
You and I are connected. Trace our lineages back long enough and we are guaranteed to find a shared ancestor. In fact, we are each connected to all of life on earth. Every cell in your body can trace its existence back through an unbroken chain of cell division and DNA replication to the origin of life. The same is true of every creature on earth, so, in a very real sense you are connected to all life on this planet.
Animals are a bit weird though. Every piece of DNA in your body is the latest link in chain that goes back to the origin of life, but most of your cells can't continue that chain. Very early on in our development, there is a fundamental distinction between the cells that make our germline (and will go on to make sperms and eggs) and those that make our bodies (the so called somatic cells). It seems natural to us for some of our cells to have no chance of leaving descendants that outlive us. But how the transition from singled celled organisms, each with a chance of reproducing, to multi-celled creatures in which most cells on reproduce 'by proxy' happened is one of the most fascinating questions in biology. So, how did animals evolve?
What is an animal?
Before we can ask how animals came about, we have to know just what makes animals different from other types of life. If you studied biology at school you may remember that animals are "heterotrophs" - that is they eat food rather than make their own. Lots of singled-celled animals are heterotrophs, but almost all animals are multicellular (some very strange single-celled parasites called Myxozoa appear to have evolved from multicellular ancestors). All animals are capable of moving, at some stage in their lives. There are plenty of animals that spend their whole adult life on one spot, but they all have larval forms that can get about under their own steam. Finally, animals are the only group of organisms to go through a "blastula" stage in development, and the only creatures to use collagen to hold themselves together. Since these characters are all unique to animals, and found in all branches of the animal family tree, we'd expect the common ancestor that unites all animals to share them.
So how to we reconstruct this ur-animal? This is one of those exciting fields of science where a whole suite of different tools and methods need be used to try and arrive at a clear picture The events that we are talking about happened around 600 million years ago, there are lots of different ways to try an peer back to that time, but as we'll see, each of them has their own strengths and weaknesses. Over the next couple of weeks I'm going to look at evidence for different methods and see if we can't pull together a consensus view of what the first animals might have been like.
If you want to know what animals looked like 600 million years ago, you'd think the obvious place to look was 600 million year old rocks. There used to be a real problem here. Until the 1950s the fossil record seemed to have a very abrupt start in rocks from the Cambrian period (around 540 million years ago). Cambrian rocks had plenty of fully developed animals, while earlier formations seemed to have no fossils at all. The problem of life seeming to arrive fully-formed in the Cambrian period has come to be known as Darwin's Dilemma. In Chapter 10 of The Origin (p308) Darwin talks about it (in a characteristic style that has been abused by creationists who seemed to think Darwin didn't believe his one theories). First he acknowledges the problem, and grants that it could be used to argue against the evolutionary origin of life
To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer... The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained.
So Darwin was happy to admit, at least at the time that he was writing, that there was no clear reason why pre-Cambrian rocks had no fossils and this lack of fossils was a mark against his theory. But, he goes on to offer a particular explanation for why pre-Cambrian rocks might be fossil free even if their were many pre-Cambrian creatures (which, as far as I can tell turned out to be wrong) and a more general reason to be skeptical about claims that rested on a lack of fossil evidence. Darwin argued that the evidence geologists had so far uncovered was only a thin slice of the full history of earth, in fact it was like:
A history of the world imperfectly kept and written in a changing dialect. Of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved, and of each page, only here and there a few lines. Each word of the slowly-changing language, more or less different in the successive chapters, may represent the forms of life, which are entombed in our consecutive formations, and which falsely appear to have been abruptly introduced. On this view the difficulties above discussed are greatly diminished or even disappear.
When we view fossils not as a complete record of life on earth, but as a few snapshots of different periods, the apparently sudden appearance of animals in the fossil record becomes less of a problem.
But Darwin was right, simply saying the fossil record was incomplete might be perfectly reasonable, but it wasn't the satisfactory answer he would have wanted. Darwin's Dilemma lacked that answer until the 1950s and the discovery of Charnia A frond-like impression in a rock discovered by a school boy in England was the first fossil to confirmed to exist in pre-Cambrian rocks.
People had found things that looked like fossils in old rocks before, but they were dismissed or ignored either because the impression were presumed to be of an non-organic origin or because the dates of the rocks were disputed. Charina was different, the impression is clearly biological, and the rocks from which it was collected had been dated by the British Geological Survey. Once Charnia has been established as something real and something pre-Cambrian, people got a bit more serious aobut older fossils. In time an entire community of large creatures were discovered, what we now called the Ediacaran bioata
Though the Ediacarans were morphologically diverse, but there were made up from repeats of some pretty simple patterns. Charnia had is fronds growing from a rib Ediacaria left disc-like fossils and Dickinsonia was a flat oval made of tubular sections (which probably inflated like an air mattress)
Above: A Dickinsonia fossil. Below A reconstruction of an Ediacaran ocean (with way too much light!)
It's hard to say quite what the Ediacarans were. The rocks they are preserved in were formed at the bottom of deep oceans, so it's clear they were heterotrophs (because photosynthesis wouldn't work down there). The fronds like Charnia must have been filter feeders (or "absorbers" feeding by osmosis) while the surface spreaders like Dickinsonia also left trace fossils, suggesting they dragged themselves across the sea floor. It's not clear if there were any predators in these systems (leading some to call the time the "Garden of Ediacara"). So, the Ediacarans were heterotrophs, they could move and they were obviously multicellular - it seems like they must be animals. But placing them into the tree of life we've created by looking at modern animals has proved extremely difficult. When it was discovered Charnia was thought to be like a modern sea pen (a cniderian) but recent evidence evidence suggests that interpretation was wrong. Some researchers, most notably Adolf Seilacher, have argued that the Edicarans don't fit easily within modern animals because most of them aren't very closely related to them. In this view, most of the Ediacaran biota form a distinct group, which seperatley arrived at the idea of being large, multicellular heterotrophs. Others have argued that particular fossils fit into existing groups; Kimberella as a mollusc, or, more sketchily Vernanimalcula as an early Bilaterian.
It's very hard to come down on one side or another here. If the Ediacarans really are early animals, then when we look at them we are looking at the first twigs of what would go on to form the mighty branching tree of animal life. We shouldn't expect those first twigs, all those years ago, to contain the traits that would go on to define animal groups. On the other hand, the fossil record is so fragmentary, and gap between out time an theirs so great, placing any particular early fossil into a modern takes a bit of leap. It seems likely some of the creatures preserved in Ediacaran rocks are closely related to animals, and some others are experiments in multicellular life that burnt out. (For what it's worth, this guy thinks the are all lichens). Whatever the Ediacarans were, they left the scene, replaced rapidly in the fossil record by the Cambrian fauna. The amazing diversity that arose in the so called Cambrian explosion is worth more than blog post by itself, but it's not really relevant to the question at hand, reconstructing the first animals.
I should include a little discussion on some of the claims to "first animal fossil" you might have read in headlines. "Firsts" are important in paleontology, and they're a sure fire way to get your work into a good journal, but the further back we go the more circumstantial the evidence becomes. Some very old rocks have biological chemicals that are as far as we know only produced by modern animals. That is circumstational evidence for the presence of animals in those periods, but when the rocks are hundreds of millions of years older than the oldest known animals, it seems like good evidence that some animal-ancestors could make those chemicals too. Similarly, there are very old trace fossils (tracks left behind by crawling creatures), but we know modern single-celled eukaryotes are capable of producing similar tracks, so they don't provide water tight evidence for the presence of animals in the sediments that have been preserved.
So, fossils seem to leave us with as many questions as answers. The patchiness of the record means we can't be sure the Ediacarans were early animals, the earlier "animal" fossils are sketchy at best and the Cambrian fauna (wonderful as it is) is just too late to tell us about the origins of animals. Next week, I'll see if modern organisms can't help up understand how ancient animals might have got their start, but now I'm going to tune in to David Attenborough and see how much we agree with each other... (8:30, Prime TV)