"Humans have evolved a more monogamous long-term bonding system, which involves a whole series of anatomical changes," lead author David Kingsley told Discovery News.
"Spines are no longer present on the human penis, intercourse is longer, and females are sexually receptive for an extended period of time rather than just around ovulation," he added.
Kingsley, principal investigator at The Kingsley Lab at the Stanford University School of Medicine, and his colleagues studied such changes on a genetic level. The researchers identified 37,251 ancestral primate gene sequences lost in humans and Neanderthals. Probing even further, the scientists then focused on molecular events that likely led to significant anatomical changes in humans.
The scientists found complete deletion of 510 such gene sequences in humans, most of which are non-coding and are near genes associated with nerve function and steroid regulation. One of these deletions removed penile spines in human males.
These spines, still present on chimpanzee males, "have been proposed to do many different things, including increasing stimulation in males, increasing stimulation in females, removing copulatory plugs left by other males or even inflicting minor damage during mating so that females are less receptive to sexual intercourse with other males."