The Origin of the Penis: Where Did It Come From?
By Cerina M. Karr, Biology, 2023
According to fundamental evolutionary principles, traits are likely to get passed onto offspring if they contribute to one’s fitness, i.e., the ability to survive and reproduce. Therefore, it is essential that genitalia are kept intact so that organisms may produce offspring. Why, then, is human male genitalia so vulnerable?
What are the options?
An important distinction to make is that while the penis is an external organ, fertilization itself is an internal process in humans.
Tailless amphibians, such as frogs, fertilize externally. The male frog grips the female in an embrace known as amplexus. The female expels her eggs at the same time males release sperm, and the result is an external gelatinous mass of joined eggs.
Amphibians with tails undergo internal fertilization. The male lays down small masses of sperm called spermatophores. The female collects one through the cloaca, which is the opening for both reproductive and excretory processes in reptiles, amphibians, birds, most fish, and monotremes. In fact, the word “monotreme” comes from the Greek words “monos” and “trema,” which together mean “single hole.”
In reptiles, male genitalia is kept inside the body until intercourse stimulates erectile tissues to bring it to the outside. This keeps the genitals protected until they are truly needed.
After the first animals began living exclusively on land, external fertilization (the frog method) would no longer suffice; the egg would be prone to desiccation (drying out). Consequently, the amniotic egg emerged as a new means of provisioning the embryo with moisture and nutrients. In humans and most other mammals, the fertilized egg is retained within the mother during development.
External genitalia
The evolution of male genitalia in amphibians and reptiles and fertilization methods have been well-established. However, it is still unclear why external male genitalia evolved in mammals. What possible advantage could there be to having genitals so unprotected?
One theory is that there is no room for internal storage of the penis in mammals. Perhaps there is some advantage to using extra abdominal space for the digestive and excretory systems. Space does not seem to be a huge issue, however, since males don’t have to fit a uterus and occasionally an entire human baby inside of them.
Another theory is that external male genitalia provide a means of female selection in wild mammals. Sexual selection occurs in many species of primates. Males have to compete with each other for the fertile females. This usually occurs through fighting, and females typically select males of higher status rather than males with better physical attributes. However, if the appearance of genitalia has some relevance to sexual selection, having external genitalia would be logical. Some primates, such as chimpanzees and bonobos, may have evolved long penises to ejaculate at more advantageous locations within the female reproductive tract. Therefore, sexual selection regarding penis size would make evolutionary sense. Some claim this would not apply to humans, however, due to a propensity to often select mates based on factors such as personality.
An additional theory has to do with female pleasure. Having external genitalia introduces the concept of foreplay, which can increase the chances of female orgasm. Although female orgasm is not necessary for fertilization, it is speculated that the evolutionary purpose of female orgasm is to leave females immobilized after copulation. Studies have shown that remaining horizontal after intercourse could increase sperm retention in the vaginal canal, improving chances of pregnancy. This theory has more to do with the evolutionary purpose of female orgasm, though, and not with external male genitalia.
Perhaps having external genitalia somehow enhances its position in the vaginal canal to improve fertilization. A study headed by Maxi Richmond from the University of Connecticut was conducted on scarab beetles and suggests that the main structural goal of male and female reproductive organs is to allow them to lock tightly together: “The male and female genital morphology in Phyllophaga thus appears to function to enhance the mechanical fit between the sexes as the male hangs from the female.” Of course, it is difficult to apply a beetle study to humans, as there are fundamental differences in fertilization methods.
The mammalian penis is ostensibly a very mysterious organ. Do the benefits described above really compensate for its vulnerability as an external organ? Regardless of its origins, it can be agreed upon that the penis has its reasons for being external, for evolution always does its job.
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