There's an article this week in
Nature (Pilcher, Helen, 'Back to Our Roots',
Nature vol 435, 23 June 2005), on research directed at working out the features of the ancestral metazoan (or 'urmetazoan', as the current usage would have it, but there's a word, really, only an etymologist could love)—this being the first organism to go truly multicellular, circa 600 million years BP.
Find this sort of stuff really interesting, and always have. There are two aspects of this issue I find endlessly engrossing.
First, there's the positively inspiring idea that you can develop a useful picture from contemporary evidence of something from a world so very long vanished. We get excited (or some of us do) when an astronomer announces they've detected another planet in a neighbouring star system—itself an incredible achievement, really, given the distances involved and the technical challenge of coercing elusive photons reflected from its surface or other equally difficult indirect evidence (such as the perturbations of the position of its primary) into a form that actually tells us something. Similarily, I find it remarkable to contemplate that by studying sponges, and looking at the biochemical mechanisms common to those and other extant metazoans, we can get a glimpse into a world from so unimaginably long ago—a world easily as distant, really, as Gliese 876—a world before true multicellularity, when probably the only macroscopic life of any organization whatsoever had arranged itself only into the relatively simple mats of blue-greens making a living as stromatolites.
Love contemplating that world, of course. It's not for nothing, I suppose, that when I got around to writing fiction, I occasionally found myself writing of other worlds. And then there's the matter of how many people like reading that kind of thing—there's something in a lot of us, methinks, that longs to explore distant lands. And the late Precambrian, that's pretty distant.
And what we look for there, this brings me to the second source of fascination: here, we look for the first living thing whose cells were not all quite alike—whose cells differentiated into tissues, and thus made up something new: a multicellular organism, which by dividing the labour involved, made the business of living that much more baroque and interesting.
Metazoans, when you think about it, have a whole new set of interesting problems to solve, versus single-celled organisms. There's the matter of differentiation in the first place—how the single cell from which each organism originally grows gives rise to differing cells within differing tissues. It's a neat trick, this, really, when you consider that in so much of the rest of biology, a single cell with a given set of genes always does, more or less, the same thing. In the metazoans, in contrast the cell with those genes gives rise to all manner of other cells—cells which
mostly have the same genes (there are a few exceptions—a few odd cells like red blood cells have none, and the cells set aside for sexual reproduction, of course, have about half as many as the rest of the body), but which do very different things. From this single cell, we get neurons and epithelial cells, bone marrow cells, and B- and T- lymphocytes. All of which, in the course of differentiation and development, will be organized into tissues, the tissues then organized (in many metazoans like us, anyway), into organs.
Then there's the matter of coordination. It's complicated enough, actually, how single cell organisms respond to stimuli in their environment to go about their business. In organisms of tens of trillions of cells, it can get to be a whole new level of elaborate. So there are hormones and chemical gradients, and, in some of us, nerves (themselves remarkable tissues relying in part on both of the above mechanisms) to pass information around—get the signals from one end of the body to another—so that tens of trillions, in the end, can work together, and function as one.
And somehow, in the end, it all comes together. From that one cell, a body grows, and makes its way in the world. And apart from occasional hiccups like cancerous tumours (in which the normally carefully orchestrated business by which individual cells are coerced to divide or not goes awry), all its descendant cells play their roles in the different parts of that body, and, voila: metazoan life, in all its dynamism and near infinite variety.
Really going on here, I guess. Apologies. But it really is a thing.
The
Nature article, anyway, necessarily doesn't get into that much of the whole business. There's some discussion of the discovery of metazoan-like 'transcription factors' in sponges—proteins critical to controlling differentiation and development—they regulate transcription of the DNA, and thus control which genes are, effectively, 'turned on' in a given cell (or how turned on they are), thus governing, in large part, what sort of cell that particular cell becomes. The interesting conclusion seems to be that the roots of multicellularity may have preceded its actual development, and there was something environmental about the era of 600 million years BP that finally opened up that evolutionary pathway.
As to that environmental feature, oxygen levels are a prime suspect, since geological records do suggest they rose dramatically around that time, and the higher, 'breathable' concentrations would then more easily diffuse through multiple cell layers, making it a bit easier for organisms to become more complex.
Worth a read, though regrettably subscription only. Looks like there's an online version
here.
