Thursday, May 7, 2009

A Kick up the Blastopore

"When I view all beings not as special creations, but as the lineal descendants of some few beings which have lived long before the first bed of the Cambrian system was deposited, they seem to me to become ennobled."
-Charles Darwin,
The Origin of Species

In the popular imagination, evolution is an ape with incrementally improving posture. Human beings came from monkeys. Before that, we crawled out of the ocean. But both these primal ancestors are relatively recent arrivals on the earth. Vertebrates are only 400 to 500 million years old. But our ancestors have been around since life first evolved. What did they look like before they developed bony skeletons?

If you trace the evolutionary line back far enough, we are descended from invertebrate animals. So far, I haven't come across any theories on what our invertebrate forebears looked like. Perhaps this is because no one knows. Invertebrates make poor fossils because of their soft, fragile body parts. However, human beings are more closely related to some groups living invertebrates than to others. Perhaps this offers clues as to what our invertebrate ancestors looked like. Try to guess who our closest invertebrate relatives are. Do see any familiar faces below?

What about the molluscs? I hear a lot of stories around the zoology department about octopi in laboratories who wait until their researchers go home, then climb out of their tanks and switch off all the lights in the lab. I'm not sure if these stories are true (perhaps this can be the subject of future blogs), but look at that big brain! Surely the octopus is kin. I'm not too sure about the bivalve, though.

Or what about the platyhelminths? No one really wants to be closely related to a tapeworm, but they can't be discounted.

How about the nematodes? Is that primate-level intelligence brewing behind those unassuming piercing mouthparts?

Or the echinoderms? Sea cucumbers certainly look like a certain part of the primate anatomy, but is this evidence of family ties?

How about the cnidarians, with their very economical use of the same orifice for multiple bodily functions? (see a previous blog). Surely they're not our nearest invertebrate cousins?

The annelids are also contenders. Do you have an inner bloodsucking leech?

My personal favourite would have been the arthropods. There is something familiar about insect faces. You can at least look them in their two eyes, which are located above a mouth, and see something not too different from a basic vertebrate face. Or less different than a blind tube sporting a bunch of piercing hooks, anyway.

But the answer is surprising.

The winner is: ECHINODERMS. Starfish, sea cucumbers, anemones and their ilk are our nearest invertebrate relatives. This does not necessarily mean that our invertebrate ancestors looked like starfish (although part of me really wants to believe they did), but that echinoderms and vertebrates had a more recent common ancestor than vertebrates and any other invertebrate group.

One important piece of evidence for this relationship is as follows. At a very early stage in our lives, all of us (you, me, echinoderms, tapeworms) looked a bit like this:

Before our body plans formed, we were all big, fat, amorphous balls of cells. Those were the days! Our balls of cells each had a little hole called a blastopore in them. In all other groups of animals, the blastopore developed into the mouth of the animal. In vertebrates and echinoderms, it formed the anus. Our guts run in reverse. The thing that sets vertebrates and echinoderms apart is that, compared to other animals, we are all talking out of our arses.

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