Researchers from Harvard University have recorded and documented the evolution of human genitalia along with those in other male animals.

Male reptiles such as snakes and lizards have two penises that evolved from hind legs of these animals and are located near their limbs unlike humans. Closer to humans are the avian species or birds. In birds, their single male reproductive organs developed from tissues that were once tail buds found on the lower area of their bodies.

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Different animal species vary when it comes to the location of their penises that come in unique shapes and early origins. But of course, these sexual organs have similar purposes.

The evolution of the cloaca, the posterior opening in intestinal and urinary tracts, also coincides with the evolution of external genitals. Researchers have discovered the location of the cloaca when the fetus is developing determines what cells are used for the creation of genitals.  

For humans, birds and other mammals, the cloaca is found near the tail bud that is bound to grow into single sex organs. During its early development in reptiles such as lizards and snakes, the cloaca sent out chemical signals that also reached tissues in the hind legs known as the lateral plate mesoderm that eventually forms a pair of penises.

During the course of the study, researchers used chicken fetuses implanted with cloaca to further observe the evolution of genitalia. Some developing chickens were given cloaca in the normal location while some were given it near their hind legs.

Chickens with misplaced cloaca in the hind legs started forming sexual organs in that area, similar to snakes and lizards.

Despite mammalian and reptilian genitals not having the same evolutionary origins, they still share the same genetic program derived from ancestral molecular signals, said Clifford Tabin, chair of the department of genetics at Harvard Medical School.

Other animals such as sea slugs use their penises in a different manner. The slugs, which are hermaphrodites, have penises that look and function like a needle. While mating, the male slug latches onto and stabs the female slug with its needle-like penis.

Turtles, on the other hand, have huge penises that can measure up to half their body length. A prime example is the leatherback turtle that has a penis up to four feet long. Echidnas are the only known animal with four heads on their male genitals. The female has two vaginal openings.

This study about the evolution of male genitalia among different animal species was published in the journal, Nature.

TagsEvolution of the Male Genitalia: Why Lizards Have Two and Humans Only One, evolution of male genitalia, penises, Harvard University, evolution of human genitalia