Think you have a rough sex life? Try being a female chimp.
The male common ancestor of humans and chimpanzees had spines on their members that likely increased stimulation during mating, according to a new study in the journal Nature. Human males (and Neanderthals) dropped this trait, while chimps kept the spine.
The penis spines, while improving stimulation, can also inflict damage on females during intercourse.
The discovery, made after a detailed comparative analysis of human, Neanderthal and chimpanzee genomes, reveals that both humans and Neanderthals went on a separate evolutionary path from chimpanzees and other primates after humans often paired up into couples.
The finding also bolsters the theory that humans and Neanderthals would have been sexually compatible and likely mated.
"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."
Another major genome deletion led to an expansion of specific brain regions in both humans and Neanderthals.
Kingsley explained that genome deletions can lead to anatomical losses or gains, depending on how the sequence is laid out and evolves.
Most of the losses and gains identified by the researchers are also found in Neanderthals, suggesting that the changes evolved after the split of the human and chimp lineages about six million years ago, but before the split between Neanderthals and modern humans occurred roughly 500,000 years ago.
"Neanderthals are already known to have cranial capacities that were as large or larger than humans, so it is not surprising that molecular changes related to brain expansion are found in both Neanderthals and modern humans," Kingsley said.
"Our study shows that Neanderthals also likely resembled modern humans in lacking penile spines," he added. "This is consistent with the idea that interbreeding could have occurred between the two forms, an idea that is already strongly supported by the detailed analysis of the Neanderthal genome published last spring by Svante Paabo's group."
Paabo, director of the Department of Genetics at the Max Planck Institute for Evolutionary Anthropology, told Discovery News that it was smart of Kingsley and his team to have "identified a set of features lost in humans that are likely to have functional consequences."
"This is a clever thing to do and," he added, "as so often is the case with good ideas, seems almost obvious in hindsight."
"Since two of the almost 500 conserved features they identified turn out to be very interesting, I am sure that several other ones on their list will turn out to be so too." Paabo concluded.