As a rather recent invention, I'm afraid you
are. The suggestion was first made by a Frenchman, Etienne Geoffroy St. Hilaire, back in 1822. He had been observing the way embryos develop, noticing that arthropods (insects, lobsters etc.) have their spinal cords down the belly,
while you and I and Spot, our dog, wear our spinal cords down the back. It all goes back rather a long time, to some 530 million years ago, the time of the Cambrian explosion of experimental species. Many patterns of
organisation of anatomy were tried out then, most of them failing and becoming extinct. Among your ancestors was a hypothetical creature known as RFW, the Roundish Flat Worm, much older than Jewish Adam. But it did resemble him in
some important ways. It had a front end with a mouth, a rear end with an anus, a middle bit divided into segments, a dorsal surface and a ventral one. The front and the rear ends were connected by a nerve cord running along the
belly.
The sea egg and the jellyfish have different arrangements. There's no need to be snobbish about them. There were even some Cambrian specimens looking rather like an eiderdown, with their surfaces for absorbing food on the
outside, outestines rather than intestines. That wasn't such a good idea.
The RFW didn't survive till now, so we can't experiment with him. However, the common fruit fly is still with us and is a useful example of an
insect. The mouse is a convenient example of a vertebrate, and we are getting to know a lot about the genes of both species.
If you haven't met many genes before you will be surprised (and perhaps annoyed) by the names they have
been given. They are sometimes rather weakly humorous, often German or made-up words, and you should remember that in reality each gene consists of hundreds or even thousands of letters in the DNA alphabet.
The eyeless
gene is the one for making fruitfly eyes. Other genes are called B4, gurken, windbeutel, sonic hedgehog, knirps, fushi tarazu, oskar, decapentaplegic, even DMP4.
You see what I mean! It's a new language, like the one you learned when you started with your computer.
What the horrible scientists do in their heartless laboratories is to knock out a gene, see what happens, and try to replace
it with a different gene, perhaps from another animal. The results can be amazing. What is equally amazing is how that blob of glup called an egg manages to turn into a proper fly, a real mouse.
First of all the egg develops some directions, head/tail and front/back.
In his new book, Genome, Matt Ridley elegantly describes the next step, looking at the matter from the viewpoint of a single cell: "Each cell
can, as it were, taste the ((cellular)) soup inside itself, feed the information into its hand-held GPS ((Global Positioning System)) microcomputer and get out a reading ' you are in the rear half of the body, close to the
underside.' Very nice to know where you are."
So what do you do when you know where you are? Make a kidney? A liver? Some special "homeotic" genes will tell you. The eyeless gene for making fruitfly eyes is recognisably
the same as the gene for making mouse eyes, where it is called pax-6.
In flies there are two genes concerned with back/front. Decapentaplegic is dorsalising, makes backs. Short gastrulation makes
bellies. But wait for it, kiddo! In mice and men the gene BMP4 is almost exactly the same as decapentaplegic. But BMP4 makes bellies. And the mouse gene which so closely resembles short gastrulation,
but is called chordin, makes mousey backs.
So that makes you and me into upside-down arthropods, topsy-turvy fruitflies. And, you know, I don't really care! I just like to learn about the family.
