This chapter might have left you with the impression that living “designs” are perfect. Not just beautiful but ideally fit for purpose, whether that purpose is seeing, changing colour, running fast to catch prey, running fast to avoid becoming prey, looking exactly like tree bark, looking irresistible to peahens or whatever. If it has, I have to disappoint you, just a little.
Especially if you look under the skin of living things, you’ll see flaws, and they are very revealing. What they reveal is evolutionary history. They are very much not what you’d expect to see if the animals had been intelligently designed. In fact, some are just the opposite.
Various species of fish make their living on the sea floor, and their bodies are flat. There are two ways of being flat. The obvious way is to lie on your belly and flatten the body from the top, so it spreads out sideways. That’s what skates and rays have done. You could think of them as sharks that have fallen victim to a garden roller. But plaice, sole and flounders have done it differently. They lie on one side. Sometimes the left side, sometimes the right. But they never lie on the belly like skates.
It will have occurred to you that there’s a problem with lying on your side if you’re a fish. One of your eyes is against the bottom of the sea and is therefore pretty useless.
That problem doesn’t arise for skates and rays. Their eyes are on top of their flattened heads and both are useful for seeing things.
So, what did the plaice and flounders do about it? They grew a distorted, twisted skull, so that both eyes look upwards instead of one being flat against the sea bottom. And I do mean twisted and distorted. No sensible designer would have produced an arrangement like that. It makes no sense from a design point of view, but it has history written all over its Picasso-like face. Unlike the shark ancestors of skates and rays, the ancestors of these flatfish were shaped like a herring, a vertical blade. T
he left eye looked to the left and the right eye looked to the right. Symmetrically, as a good designer might wish. When they changed their way of life to live on the bottom, they couldn’t go back to the drawing board, in the way that a designer would. Instead they had to modify what was already there. Hence the distorted head.
Here’s another famous example of a revealing flaw: the retina of your eye. It’s back to front.
It’s the same for all vertebrates. I’ve already described the retina as a screen of photocells. The photocells are hooked up to the brain by nerve cells. The sensible way to hook them up is the one used by cephalopods like octopuses. Their “wires” connecting the photocells to the brain leave from the back of the retina in a sensible manner.
Not so the equivalent wires from the vertebrate retina. Here the photocells are wired backwards. Each photocell points away from the light. So how do the wires – the nerve cells – leading from the photocells manage to reach the brain? They travel over the surface of the retina, taking information from the photocells, and converge on a circular patch in the middle of the retina where they dive through and then head back to the brain (see the diagram overleaf). The place where they dive through is called the “blind spot”. Because, not surprisingly, it is blind. What a ridiculous arrangement!
The famous German scientist Hermann von Helmholtz (he was both a medical doctor and a pioneering physicist) once said that if a designer had presented him with the vertebrate eye he would have sent it back. Actually, although he would be perfectly justified in doing so, it works pretty well, as we can all see! The layer of nerve cells running over the surface of the retina is thin, and they are transparent enough to let light through.
My favourite example of bad design is the recurrent laryngeal nerve.
The larynx is the voice box, in the throat. It’s supplied by two nerves from the brain called the laryngeal nerves. One of these, the superior laryngeal, is sensibly wired up directly from the brain to the voice box. The other one, the recurrent laryngeal, is crazy.
It goes down the neck from the brain, shoots straight past the larynx (the place where it is supposed to end up), way down into the chest. There it loops around one of the main arteries attached to the heart, then whizzes straight back up the neck and finally ends up in the larynx, where it should have stopped on the way down. In a giraffe, that’s quite some detour. I saw this vividly when I assisted for a television programme in the dissection of a giraffe which had unfortunately died in a zoo.
Once again, this is obviously bad design, but it makes perfect sense if you look at the history. Our ancestors were fish. Fish have no neck. The fishy equivalent of the recurrent laryngeal nerve is not recurrent. It supplies one of the gills. The most direct route from the brain to the gill is behind the equivalent artery. It’s not a detour at all.
Later in history, when the neck started to lengthen, the nerve needed to make a slight detour. The neck got steadily longer as the generations passed. And the detour too got longer and longer. Even when the detour became absurdly long in the ancestors of the giraffe, because of the way evolutionary change works...it carried on just getting longer rather than changing the route altogether to jump over the artery.
A designer would have taken one look at the nerve, as it passes within inches of the larynx on its way down the long, long neck, and said, “Wait a minute, that’s ridiculous.” Again, a Helmholtz would have sent it back. It’s the same with the tube that carries our sperm from the testes to the penis. Instead of going by the most direct route, it travels up into the abdomen and loops over the tube carrying urine from the kidney to the bladder. Again, the detour makes sense only if you look at the evolutionary history.
Excerpted with permission from Outgrowing God: A Beginner’s Guide, Richard Dawkins, Bantam Press.