The fact is that there is no good reason that sperm development has to work best at lower temperatures. It’s just a fluke, an example of poor design. If nature had an intelligent designer, he or she would have a lot to answer for. But since natural selection and other evolutionary forces are the true designers of our bodies, there is no one to question about this. We must interrogate ourselves: Why are we like this?
The so-called “argument from poor design” goes back to Darwin himself. Prior to evolutionary theory, most people, scientists included, considered the world and everything in it to be the flawless creation of a perfect God. Of course, the rampant imperfections we can all easily spot called out for explanation and usually invoked a response along the lines of a “fall from grace” or some other such hand-waving. Now that we know that evolution is the creative force of life, we can be free from the expectation of perfection.
But we’re not. Far too often we repeat refrains like “Well, it must do something important or natural selection would have eliminated it,” or “Living things are perfectly suited for their habitats,” or “Evolution doesn’t tolerate inefficiency.” We haven’t really moved on from the creationist mindset that expects to see perfection in nature.