The absence of a bone in the human penis, known as the baculum, has long been a subject of curiosity, as this structure is present in the vast majority of other mammals. The baculum, or os penis, is an extra-skeletal bone that provides rigidity and support during copulation, but it is entirely missing in Homo sapiens. This anatomical difference is a significant evolutionary deviation from our closest primate relatives, such as chimpanzees and gorillas, who possess this bone. Understanding why humans lost the baculum requires examining its function in the animal kingdom and tracing the changes in human reproductive behavior.
Defining the Baculum in the Animal Kingdom
The baculum is a slender, extra-skeletal bone that develops within the soft tissue of the penis, providing a structural element not connected to the rest of the skeleton. This bone is highly diverse, with its size, shape, and complexity varying dramatically across different species. The baculum is found in most placental mammals, including rodents, bats, carnivores, and many non-human primates.
The size of the baculum can range from a tiny sliver, only a few millimeters long, to an impressive structure over 60 centimeters in length, such as the one found in the walrus. Even among primates, the baculum is not uniform; the great apes, including chimpanzees and gorillas, possess a bone that is quite small. In the absence of a baculum, the human erection relies entirely on hydraulic pressure created by blood filling the spongy tissues known as the corpora cavernosa.
The Functional Purpose of the Baculum
The primary function of the baculum is to provide a mechanical advantage during copulation, offering structural support independent of blood pressure. This skeletal rigidity is important for species that engage in prolonged intromission, defined as penetration lasting longer than three minutes. The bone allows the male to maintain penetration for extended periods, which plays a role in reproductive competition.
Prolonged copulation increases the chance of successful fertilization while preventing the female from immediately mating with a rival male. This post-copulatory competition is intense in species where females mate with multiple partners. Studies show a correlation between the length of the baculum in primates and carnivores and the duration of copulation. The bone acts to maximize a male’s paternity in highly competitive mating systems.
Evolutionary Hypotheses for Human Absence
The scientific consensus suggests the human baculum was lost because the evolutionary pressure to retain it disappeared. This loss is estimated to have occurred in the hominin lineage around 1.9 million years ago, coinciding with a significant shift in mating systems. The most prominent theory links the loss of the baculum to the transition toward a predominantly monogamous or low-polygyny reproductive strategy in ancestral humans.
In a mating system where a male remains in close contact with a single female partner, the need for prolonged intromission to guard the female from competitors is reduced. Human copulation is significantly shorter than the duration observed in species with prominent bacula, typically lasting only a few minutes. Once the duration of intromission dropped below the three-minute threshold required for the bone’s structural benefit, the baculum became redundant.
Another hypothesis proposes that the hydraulic erection system in humans became a form of “honest signaling” of male quality. Achieving and maintaining an erection without skeletal support requires a healthy cardiovascular system and a low level of stress. Females may have unconsciously selected males capable of achieving this blood-pressure-dependent erection, as it served as a reliable indicator of overall physical fitness.
The already diminished size of the baculum in the ancestors shared with chimpanzees and bonobos—whose bones are only about 6 to 8 millimeters long—made its complete loss easier once the selection pressure for prolonged mating disappeared. Ultimately, the change in human social structure and mating behavior eliminated the functional necessity of the bone. The trait was then lost through genetic drift or weak negative selection.