Separating the baby from the backwater

Anuses segregate babies from back ends, says Patricia Brennan, Mount Holyoke College assistant professor of biology.

By Keely Sexton 

The anus is an understudied evolutionary marvel. It represents a major evolutionary step in many organisms, separating an organism’s intake orifice from the body’s excretory egress.

The scientific jury is very much still out on the exact origins of anuses, however, and even on when and how often they may have independently evolved across different species. But one common feature they share is to segregate babies from back ends, said Patricia Brennan, Mount Holyoke College assistant professor of biology, in a recent article in the Atlantic.

Brennan studies cloacae — a single opening where digestive, urinary and reproductive tracts all merge — which are common among birds, reptiles and amphibians. Cloacae come with many conveniences, including the ability of a female to eject the sperm of an unsatisfactory mate and tee up for another reproductive chance with little fuss. However, like many one-stop shops, cloacae come with some drawbacks.

“You have all your digestive waste pretty much in direct contact with genitalia,” Brennan told the magazine. 

One of the evolutionary pressures leading to the evolution of a discrete excretory tract might have been to reduce the risk of genital infection.

Read the entire article.

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