Evolution:mammal perineum appears pre-planned (Introduction)

by David Turell , Sunday, August 27, 2017, 00:30 (2428 days ago) @ David Turell

Embryonal research dates the four layers of the mammalian perineum back to early tetrapods 360 million years ago:

https://www.sciencedaily.com/releases/2017/08/170823090922.htm

Mammalian perineal structure derived from septation of the cloaca is an important evolutionary innovation that allows myriad anatomical configurations, diverse reproductive strategies, and precise excretory control available only to mammals. The researchers were surprised to discover that, despite the perineum's structural complexity, the muscles of the mammalian perineum show a reemergence of a simple pattern of body wall layering that dates back more than 360 million years ago -- during the origin of tetrapods, the first vertebrates to move out of the water onto land.

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with the transition from water to land, the body wall of most tetrapods expanded to four muscles layers in order to stabilize their trunks against gravity and facilitate the more complex movements of the limbs during terrestrial locomotion. Amniotes, vertebrates who can reproduce on land and away from water, have four muscle layers in the thorax and the abdomen, but still retain the original two muscle layers in the pelvis and perineum. Mammals, like other amniotes, have four muscle layers in the thoracic and abdominal body wall. Unlike other amniotes, mammals extended the four muscle layers to the perineum. This allows mammals to control their newly derived structures associated with the external genitalia and the anus.

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Among mammals, Placentalia, the group that includes humans, is the only group of vertebrates to evolve the specialized suite of perineal characteristics that includes the erectile tissues of the penis and the clitoris, the urethra, the distal rectum, the anus, as well as the voluntary muscles that control these structures....Separation of the cloaca into distinct and more complex perineal structures required mammals to develop correspondingly more complex and precise mechanisms of muscular control for these structures.

Researchers often use embryological development to find important clues about how evolutionary processes may have occurred. Modern mammals develop a cloaca embryologically, which subsequently divides into a urogenital half- that continues to form the urethra and the erectile tissues of the penis and clitoris, and an anorectal half- that develops into the distal rectum and the anal canal. The authors discovered evidence for restructuring of the body wall muscle layers during fetal development that support cloacal separation into distinct structures and is retained into adulthood.

This study, conducted by a team at Midwestern University, is the first time that the four serially homologous trunk body wall layers in the mammalian perineum are defined. These muscle layers support a unique mammalian perineal structure derived from separation of the cloaca, an important evolutionary innovation that allows diverse reproductive strategies and precise excretory control available only to mammals. (my bold)

Comment: This is one of the best examples of evolutionary pre-planning I've seen. Note my bold. The preliminary changes go back 360 million years, long before the appearance of mammals and their special needs for reproduction. God at work!