The human penis lost the sensitive ridges common to many animals more than 800,000 years ago in an evolutionary move that promoted longer sex and monogamy, U.S. researchers said.
Men are missing a sequence of DNA that indirectly leads to the development of penile spines in animals including macaques, chimpanzees and mice, researchers from Pennsylvania State University said today in the journal Nature. The missing DNA is one of 510 such deletions that distinguish humans from other animals, the scientists said. The spines in primates are made of keratin, the same substance in fingernails, feathers and hair.
A spineless penis means more time is needed for copulation, which may be why humans have sex for longer periods than related primates like chimpanzees that have the spines. Longer sex increases the chance of humans bonding in pairs to raise children, the researchers said. Chimpanzees are more promiscuous in the race to impregnate females, and having ridged penises may increase the odds of success by enhancing rapid copulation, the scientists said.
“We often think of brain size and bipedalism as key characteristics of what makes us human,” said Philip Reno, an assistant professor at Penn State in University Park, Pennsylvania, who was one of the study’s three lead authors, in a statement. “But another difference is our sexual behavior.”
The absence of the DNA sequence was noted in Neanderthals, suggesting that the human penis lost its spine somewhere between 7 million and 800,000 years ago, according to the statement. The so-called silent DNA may have affected the nearby human androgen receptor gene, which helps produce male sex characteristics. While silent DNA, also referred to as junk DNA because its purpose isn’t always clear, doesn’t code proteins, it can change how a gene is expressed.