On Design Faults
Evolution is becoming a contentious and increasingly-political topic, and it’s becoming one of the places where the world of science intersects with the world of politics. Unfortunately, the cultures of the two worlds are very different, and the intersection can be rather ugly. Science is the realm of facts and politics arguably that of opinion. You see this very clearly in the ‘evidence’ (such as it is) that gets presented in debates on the topic of evolution. All too frequently, these get hung up on the issue of fossils.
Now, fossils are fascinating things, and I don’t suggest otherwise. They have a lot to add to our knowledge and they give us a unique window into the lost world of millions of years past. However, they are not the be-all and end-all of evidence for evolution. Another important source of the evidence is anatomy, both human and not. There are some aspects of physiology that just don’t make any sense outside of an evolutionary context.
Consider, for instance, the coccyx and the fovea.
What are these strange things I speak of?
The coccyx is a small bone located where your spinal column joins your pelvis. You can feel it, as an odd little lump sticking out down there.
The coccyx has no function in modern humans – however, it is a feature we share with the other great apes, including our closest living relatives, Pan Troglodytes. In these other species the coccyx’s purpose is apparent – it is the anchor for a tail. Indeed, to some extent, this can even still be occasionally observed in humans. One of medicine’s more surprising facts is a number of children are born every year, worldwide, with vestigial tails. These are invariably surgically removed, for cosmetic reasons. (Also for matters of practicality – the human tail is essentially useless.) And the place they sprout from is – you’ve guessed it – the coccyx.
Tails were the norm for our distant ancestors. We lost them as we began to walk. Our spines slowly curved in such a way as to make a backside-counterweight unnecessary, while the growing arches of our feet provided extra stability. The balancing-implement that was the tail would have gradually become – being blunt – surplus meat. And the body doesn’t want to spend energy on something it doesn’t really need anymore, so tails disappeared. But they left behind this little bone, a kind of internal ghost. One can’t help but think of a piece of driftwood, left high on a beach by a retreating tide.
My point about the coccyx concerns purpose, or perhaps the lack thereof. Suppose that there had been no previous stages of human development. Suppose that there were no prior forms. Suppose that we were created from thin air 6,000 years ago. Why would the coccyx be there? It serves no purpose, yet it requires energy and nutrients from the body to grow it, energy and nutrients that could go elsewhere. It doesn’t do anything, it carries no load, it is in fact arguably in the way of the spine.
If you were consciously designing a human being, you wouldn’t put it in. It only makes any sense as a left-over from a previous phase of development. The combination of random mutations that would make it disappear haven’t had time to happen, so it’s still there. Also, although it doesn’t do anything, it also doesn’t really do much harm – or at least, not enough to stop you breeding. So there is very little selection pressure for it to ‘go away’. (Maybe it will in a few million years’ time, but I think it’s safe to assume that humans will carry on having coccyxes for the foreseeable future!)
A different but similar case is found in the human eyeball. It’s something called the fovea.
The fovea is an ingenious and amazingly clever solution to a problem that should never have existed in the first place. Like the coccyx, it’s not something you’d ever design in.
The retina is the bit of the human eye that ‘perceives’ light. It’s the bit where the photoreceptor cells are. Crudely, these cells take incoming light and turn it into electrical signals to the brain, where these signals become the image that you actually see. But, the retina is a living thing. It needs water, oxygen, food. It needs a blood supply. And it has a blood supply – a network of capillaries that are sat on top of it.
Yes, that’s right. The retina’s view is partly-blocked by it’s own life-support setup. This is why most of the human field of view has a low resolution – you can’t read out of the corners of your eyes, for instance. There’s only so much that can be done when there are translucent, fluid-filled tubes in the way. The fovea is a partial compensation for this. It’s a region of densely-packed photoreceptors, without any blood vessels in front of it. As nutrients and oxygen diffuse in from outside, it necessarily has to be small. It results in a high-resolution, but small, region of clear vision.
It’s an ingenious adaptation to a problem you shouldn’t have to being with.
Why does this problem still exist? Well, obviously, the more logical structure would be to put the blood vessels behind the retina. However, think of the transitional stage – when the blood vessels are in the retina. At this point, some space that could be taken up by photoreceptors is instead taken up by tubing – a lot of space, as it happens. At this point, the eye will work a lot worse then the original case, and any creature that mutates in this direction will get selected against.
The point I’m trying to make is that these two features only really make sense if we assume that the human organism has been through some sort of previous stage of development. They are things you’d never design in, if you were starting from scratch. Obviously, by themselves they do not ‘prove’ that evolution has occurred, but they do represent one step of the path to it.
This entry was posted on August 17, 2010 at 12:45 pm and is filed under Evolution, Social Concern with tags biology, evidence, evolution, science, skepticism. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.