The Origin of Spines
BY ALPER ÖZKAN (MSN/PhD I)
d_ozkan@ug.bilkent.edu.tr
Greetings and salutations! It is, no doubt much to your chagrin, the beginning of a new semester (I for one am not looking forward to the massive lines during lunch), and I happen to be a columnist responsible for writing on history, mythology and biology; so if you enjoy any of these three subjects, it might be wise to read this column regularly. Do know that I'm not very good at it, though. In fact, if there's anything I can be said to be exceedingly competent at, it's forgetting about promises until the last possible moment to fulfill them - as I have confirmed yet again today, for it's Tuesday, and I had promised to get a column done by Tuesday. I wonder whether I don't have some forgotten promises from seven years ago that will come back to haunt me when I least expect it. Ah well, enough of the inane nonsense, the column isn't going to write itself. This week will be about the origin of chordates, a comparatively small but nonetheless important group that includes us. Or rather, I'll pretend this week will be about how chordates came to be, but talk about an interesting theory that was almost certainly wrong about that.
Chordates are one of those groups that seem to suddenly pop into existence during the Cambrian, presumably just for the sake of confusing and irritating paleontologists. This period features the oldest animal that we can definitely conclude to be one of us: Named Pikaia after Mount Pika near Burgess (where the animal was first discovered), this small, rare fossil was once thought to be an annelid worm because it had a segmented body. Later, it was noticed that those segments were actually muscle bands and that the animal had a notochord, and since any animal that has a notochord is called a chordate, it was yanked out of the annelid assembly (which is by the way getting pretty crowded nowadays, with the addition of peanut worms, beardworms and spoonworms into the group) and thrown to our side. Now we know that Pikaia looked rather like the modern lancelet, equal parts worm and fish in appearance but definitely closer to the fish side when it comes to anatomy.
This is the main reason that guessing vertebrate ancestry is somewhat difficult: the Cambrian was some 530 million years ago, and the era immediately before it, the Ediacaran, doesn't help matters because it was overrun with animals very much unlike anything anyone has ever seen. Nonetheless, the leading theory is that we find our closest relatives in hemichordates and, surprisingly enough, echinoderms (both of these are extant groups, so molecular techniques have been employed to confirm the connection). Echinodermata, as you no doubt know, is the phylum containing some of the least chordate-like animals, including sea cucumbers, sea urchins, starfish and sea lilies, united by fivefold radial symmetry (among various other features such as ambulacral grooves and an armor of calcite plates). But this was not always so: While this arrangement was so successful that virtually every extant echinoderm features it in one way or another, those animals begin their lives as perfectly normal (to our standards, at least) bilateral creatures and fossils show that there existed echinoderms not too alien to us. Or perhaps not alien to us at all, because there's a theory that claims we're derived from them!
The theory in question is called… Nah, never mind. I'm approaching the word limit, and it wouldn't do for you to just read the theory on Wikipedia and not bother with the column (well, I suspect no one would do that, but still…) Tune in next week to find out about bilaterally symmetrical echinoderms, potential ancestors that completely did away with symmetry and look like video game mutants; how the starfish acquired its arms; and how close we really are to a group of animals you usually notice only when you step on a sea urchin on the beach. (As a final note, sea urchins can see with the assistance of their spines. I don't have the space to write about that, so just use Google to learn about it from someone who can write far better than I.)