Science Alert reports on an evolutionary study that has found our butts (from which everybody, as the children’s book tells us, poops) were originally a little bit more risque. The earliest anus apparently developed from an aperture originally used to distribute male sex cells:
Researchers from the University of Bergen in Norway investigated the genetics of xenacoelomorphs; distant relatives of flatworms that have a cul-de-sac for a gut. Despite this lack of a dedicated poop-hole, xenacoelomorphs use some of the same genes we use to turn our digestive system into a tube, only to create a genital opening known as a gonadopore instead.
“Once a hole is there, you can use it for other things,” zoologist Andreas Hejnol told Michael Le Page at New Scientist.
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Developmental biologist Carmen Andrikou and team found that when the xenacoelomorph inverts its outer skin to develop a gonadopore, it uses some of the exact same genes other animals use to make their butt holes.
A number of animals today, including birds and platypus, also have a joint hole for both reproductive and digestive functions – a cloaca.
“The presence of cloaca within animals as well as the gonopore-oral fusion witnessed in species of [flatworm], suggests that a connection between the digestive and the reproductive system is either easy to evolve convergently or shares a common ancestry,” Andrikou and colleagues write in their paper, which is still awaiting peer review.
This all suggests that our anuses evolved after a male’s sperm chute merged with the digestive tract to form a second opening, the researchers explain – implying animals didn’t evolve anuses until after our own branch of the family tree parted ways with xenacoelomorphs’ ancestors.