Map of life expectancy at birth from Global Education Project.

Sunday, September 10, 2006

The Tree of Life

Not Ygdrassil, but the conventional representation of evolutionary relationships.

For those concerned about me, the results of my examination were entirely reassuring. I will offer my sociological observations anon. Meanwhile, for reasons which will soon become apparent, this seems a good occasion for the next installment on evolution.

The major groups of metazoa -- the multicellular animals -- are all present in the fossil record going back to about 600 million years ago. We presume that animals with similar patterns of embryonic development are related to each other, but we can only conjecture about which may have come first and whether the more complex ones are necessarily descended from the simpler ones.

Sponge Bob is unique among his kind in that he is mobile and has a nervous system. Sponges in general (phylum Porifera) have no specialized organs, and cannot move. They may not really be multicellular organisms at all, but rather a highly evolved form of colonial living by individual cells. But the rest of the metazoa are classified as:

Diploblastic: Having two embryonic cell layers. These are the cnidaria (jellyfish, anenomes, and so on) and the comb jellies.

Triploblastic: Having three embryonic cell layers. Triploblastic organisms are thought to be more closely related to each other than to the diploblasts, but they come in several varieties. Those lacking a true internal body cavity (that is not the gut, but a fluid filled cavity between the gut and the body wall) are called acoelomates or pseudocoelomates, because the body cavity in Latin is called the coelem. In coelomate animals, those having a true body cavity -- and that includes you -- there are two distinct ways in which the body is formed during embryonic development. In the protostomes -- which include the molluscs, annelids (earthworms etc.), arthropods (insects, crustaceans, and what not), and various lesser known phyla -- the first opening which appears in the embryo becomes the mouth. In the deuterostomes -- which include you, and Sponge Bob's friend Patrick the starfish and all the other echinoderms -- that opening becomes the anus, and the mouth forms later. So, it appears that the closest relatives of the vertebrates are that very different looking phylum, the pentagonally symmetrical sea stars, sand dollars, and sea urchins.

You may have noticed that one thing that all of the triploblastic organisms have in common is a gut, a hollow tube running through the body. The food goes in one end, gets serially disassembled and the useful components absorbed, and the waste goes out the other end.

That is a major innovation, and for those contemplating possible options for reincarnation, it's a good reason to request not to come back as a diploblastic jellyfish. (Thanks to Jan Pechenik for this observation.) The food comes in, and the waste goes out, through the same opening. Life without an anus is clearly inferior. Not only is it impossible to take a meal until the last one has been discharged, but movement involves physical distortion of the digestive cavity and expulsion of much of what is in there. So, you can't digest and swim at the same time. Finally, you have to discharge your gametes and embryos through the same opening. Yuck.

So, let's hear it for one of our most important body parts, that gets very little respect.

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