Chimps Have Evolved Too
As one astute commenter pointed out, chimps have been evolving too. We didn’t evolve from chimps, but from a common ancestor that evolved into chimps as well as humans. On the path leading to chimps, it appears the Y chromosome has shrunk and degenerated to almost nothing. It’s something I wrote about for my previous column, Carnal Knowledge.
For a time, a popular theory was bandied about that the Y chromosome in humans was also degenerating fast and would disappear in 125,000 years. Some even speculated that human males would go extinct. What would happen to females? It doesn’t much matter since that now looks to be wrong. When scientists sequenced the complete genome of the chimpanzee in 2005, they found that the human Y chromosome has stabilized while the chimp version continues to degenerate.
Most of what’s left of the chimp Y is devoted to genes controlling the production of sperm – something chimps do much more copiously than do humans. Chimps also have testicles more than twice the size of human ones. The reason our paths diverged in this department has to do with differences in our mating systems. Chimp females show little interest in sex unless they’re in estrus, during which they often try to mate with multiple males in quick succession. Chimp females reportedly can go through eight males in 15 minutes.
That sets in motion something called sperm competition. He who produces the most sperm will most successfully pass on his genes to the next generation, even if other chimps are smarter or better looking. So over the course of chimp evolution, any good sperm-making genes tended to proliferate. Many of these genes are on the Y chromosome, where defective version of more general genes could ride along with good sperm-making ones.
A shrinking Y chromosome is unlikely to matter to the future of chimps, since they’re being driven toward extinction by deforestation, hunting and disease. Scientists estimate only 100,000 to 200,000 are left in the world.