Friday, May 13, 2005

Designing sperm

Chimp, human divide may have made us prone to cancer | Science Blog:
Chimpanzees and humans share a common ancestor, and even today 99 percent of the two species' DNA is identical. But since the paths of man and chimp diverged 5 million years ago, that one percent of genetic difference appears to have changed humans in an unexpected way: It could have made people more prone to cancer.

A comparative genetic study led by Cornell University researchers suggest that some mutations in human sperm cells might allow them to avoid early death and reproduce, creating an advantage that ensures more sperm cells carry this trait. But this same positive selection could also have made it easier for human cancer cells to survive.
The more we learn about our evolution, the clearer it is that evolution has not designed us intelligently. Chimps can get away with being cancer prone in old age because they don't live so long, but humans do. So chimps should have the long-lived sperm and the cancer, while we should be content with the weaker sperm but no cancer.

Humans don't go into an easily noticed estrus, though. When a female chimp is ready to mate, she shows it. Humans don't work that way, for reasons also best explained by evolution. That means mating has to be constant. If fertility is unpredictable, the male in a pair-bond might not be around on exactly the right day, so males whose sperm survives longer will have more offspring.

The other factor is polyandry in human ancestors. Since multiple partners are so common in chimps, I'll set that aside. Clearly, this isn't an adaptation to sperm competition alone.

So our unusual mating system and produced selective pressure on sperm to be longer lived. That had the unfortunate side effect of reducing all cells resistance to cancer.

Oops. Not such an intelligent design. But just what you'd expect of evolution.