One of the simplest yet hardest concepts to grasp about evolution is that it is, well, kind of dumb. That is to say: it’s not intelligent, or sentient, or pointed towards a particular goal. It’s a force, a big dumb unthinking truth about the world, like gravity. There is no intelligent designer to redesign things or correct mistakes.
One of the implications of this is that there are no completely novel large-scale inventions in biology. Every structure or behavior that evolves is based on something that existed previously. Ritualized displays (like fighting or mating displays) are based on and adapted from either preparatory or nervous behaviors. Organs are based on tissues already formed.
But this can lead to some interesting problems.
Consider this analogy. You have a car and live on a deserted island. But suddenly the water level starts rising and pretty soon you’ll need to get the heck out of the place. The car and a small selection of tools is all you get. So what can you do? The only thing you can: Go MacGyver on that car and go sailing.
You survive and make it to another island. This new island is inhabited and your new friends, who have never seen a boat before, all beg you to show them how you did it. Problem is, you only know one way to build a boat, and it requires, of all the odd things, a car. This seems counterintuitive and weird, but it works, and it’s all you know.
The same thing happens in biology. Below is a picture of pharyngeal pouches on a human fetus. These pouches are based on the gill structures in primitive vertebrates, yet grow into your ears and tonsils (among other things). There is no way to grow tonsils without first having the pouches.
This dependency on previous structures explains a lot about what is weird with the human body. Our eyes don’t work quite right (they grew out of the brain), the male penis is used for both waste and reproduction (blame primitive cloacas), female hips are way too narrow for childbirth, etc. If any mad scientist geneticist sits down and decides to make Human v2.0 they’ll have their work cut out for them.
Note: Tomorrow is the last day of my undergraduate education, I shall be graduating on Sunday. Expect more frequent updates starting this summer.
Sources: Picture taken from, ironically enough, creationism.org