A heterosexual couple trying to copulate with a spiky male member would be painful enough, but don’t even force me to have a mental picture of two men going at it when one of them as a thorned wang. Ugh, too late. But while others in the animal kingdom have horny penises, humans do not. How come?
It has long been believed that humans evolved smooth penises as a result of adopting a more monogamous reproductive strategy than their early human ancestors. Those ancestors may have used penile spines to remove the sperm of competitors when they mated with females. However, exactly how this change came about is not known. The researchers did not set out to study penile spines. Rather, they were looking for chunks of DNA that had been lost from the human genome but not the chimp genome, so they could then try to pinpoint what those chunks did. The approach differs from that in most studies, explain Bejerano and Kingsley, in looking at what has been deleted from the human genome rather than what is present. “In the case of our study, had you started from the human genome, there would be nothing there to see,” says Bejerano.
They first systematically identified 510 DNA sequences missing in humans and present in chimps, finding that those sequences were almost exclusively from the non-coding regions of the genome, between genes. They then homed in on two sequences whose absence in humans they thought might be interesting — one from near the androgen receptor (AR) gene and one from near a gene involved in tumour suppression (GADD45G).
Inserting the chimpanzee sequences into mouse embryos revealed that the former sequence produced both the hard penile spines and sensory whiskers present in some animals. The latter sequence acted as a kind of brake on the growth of specific brain regions — with the removal of its function appearing to have paved the way for the evolution of the larger human brain. “The goal of the project was to find molecular lesions [losses] that underlie human evolutionary traits, with the examples illustrating different aspects of the principle,” says Kingsley. “Until we looked at where the DNA was expressed, we had no idea which switch — if any — it would actually control,” adds Bejerano.
As for why your penis might bend to the left, or curve downwards, or rotate ninety degrees counter-clockwise when erect? I think god just wanted us to have more variety :)