Brown bear penis bones. (Museum of Toulouse, Creative Commons via Wikimedia)

The baculum, also called the os penis or penis bone, is a puzzling thing. It sits in the tip of the organ, not connected to any larger skeletal structure. Your pet cat has one if it is a he, as does your male dog. Many male mammals do — chimpanzees, gorillas, weasels and bears. The walrus has a particularly impressive baculum, up to 22 inches in length. The bone was even larger in the past. A fossilized, 4.5-foot os penis of an extinct walrus species fetched $8,000 at auction in 2007.

But humans, curiously, do not have penis bones. One reading of Genesis offered an explanation for the disappearing bone by way of creation myth. It was the penis bone, not a rib bone, a pair of biblical scholars argued in 2015, that God removed to fashion Eve from Adam. (This interpretation went over about as well as one might expect.)

As to why humans lack the bones, a study published on Wednesday in the Proceedings of the Royal Society B offered a possible explanation. By the standards of primate reproduction, humans do not need to do the deed for a long enough time to warrant an os penis. Plus, our breeding habits are, in the context of our great ape cousins, fairly low-pressure.

A pair of researchers at the University College London examined several sexual characteristics of primates and mammal carnivores, including features like polygamy, testes mass, seasonal mating and intromission time (how long an act of penetration lasts). For primates, the best predictor for whether the male had a penis bone was if intromission lasted three minutes or longer. There was also a correlation between long intromission and length of the bone for both primates and carnivores.

Study author Matilda Brindle wrote at the Conversation that “humans don’t quite make it into the ‘prolonged intromission’ category. The average duration from penetration to ejaculation for human males is less than two minutes.” These long bouts of primate intromission are not exactly romantic. The end goal is gestation, not gesture. They are insurance to a male mammal, Brindle pointed out, that a female does not mate “with anyone else before his sperm have had a chance to work their magic.”

The bones exist for a few reasons, wrote Durham University anthropologist Lauren Reid in 2012. They offer speed, obviating the need to wait for a penis to fill with blood. They also “help males maintain an erection long enough to penetrate a female’s reproductive tract and deliver sperm.” (There are other hypotheses. A mouse baculum could “stimulate the female reproductive tract,” Australian researchers argued in 2013, according to National Geographic. This was to improve odds that eggs became mice.)

Penis bones appear and vanish in mammalian evolutionary history with some frequency. “If you asked a mammalogist,” Matthew Dean, a biologist at University of Southern California, told The Washington Post in June, “they would look up to the sky, think for a bit, then tell you that it must have evolved multiple times.” The authors of the new study traced the penis bone to a first appearance between 145 million and 95 million years ago.

The three-minute-plus intromission explanation does not quite work for great apes like chimpanzees, which copulate on the order of seconds. (Nor is the chimp penis bone large — in fact it can be as a short as a quarter of an inch long.) Instead, the driving factor at play seemed to be competition, the University College London scientists found. Chimpanzees mate in high-pressure conditions: They are polygamous, and competition to breed among males is intense.

As monogamous animals capable of mating year-round, the scientists argued, ancient humans gave up the need for a bone. “After the human lineage split from chimpanzees and bonobos and our mating system shifted towards monogamy, probably after 2 [million years ago], the evolutionary pressures retaining the baculum likely disappeared,” University College London anthropologist and study author Kit Opie said in a news release. “This may have been the final nail in the coffin for the already diminished baculum, which was then lost in ancestral humans.”

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