Human genitalia evolved through development which has been chronicled by a team of researchers from Harvard University.
Male snakes and lizards each possess a pair of penises, which evolved from tissues that form hind legs in other species. Their genitalia are located near their limb structures, unlike our own species. The single male reproductive organs in humans and birds developed from tissues once utilized for tail buds, and are located lower on the body.
Penises in different animals may be placed in different locations, and have unique shapes and origins. However, the organs are used for similar purposes, and express similar genes throughout much of the animal kingdom.
Embryonic cloaca develop into gut and urinary tracks, along with forming external genitalia. Researchers found that the location of the cloaca in the developing fetus determines which cells are utilized to create the organ. In humans, birds, and mammals, cloaca are located near the tail bud, creating single organs. In developing lizards and snakes, chemical signals from the cloaca reach a variety of hind leg tissues known as the lateral plate mesoderm, first, resulting in paired penises.
Chicken fetuses were implanted with cloaca as part of the study. Some of the developing birds were provided with cloaca in the normal location, while some were placed near hind legs. External reproductive organs, similar to those of snakes and lizards, started to form in the chickens with misplaced cloaca.
"While mammal and reptile genitalia are not homologous [having the same evolutionary origin] in that they are derived from different tissue, they do share a 'deep homology' in that they are derived from the same genetic program and induced by the same ancestral set of molecular signals," Clifford Tabin, chair of the department of genetics at Harvard Medical School, said.
Sea slugs are hermaphroditic, but possess a penis that looks - and functions - like a hypodermic needle. Mating is accomplished by grabbing onto - and stabbing - another member of the species, often causing severe damage.
Turtles have enormous male sexual organs, measuring up to half their body length. A leatherback sea turtle six feet in length could sport a penis up to four feet long.
The Echidna, an egg-laying mammal similar to the platypus, is the only animal known to have genitalia with four heads. Females of the species have two vaginal openings, so biologists question why this adaptation developed.
Limbs and genitalia use similar chemical signals during fetal development, which could help explain the findings related to the location of the cloaca.
Study of the formation of male genitalia in different species was detailed in the journal Nature.