March 28, 2008

Human Tails

Stupid Question ™
Sept. 20, 2001
By John Ruch
© 2001

Q: Why don’t humans have tails?
—Mr. Boru

A: Actually, we do have tails, in a vestigial form. As Darwin put it in “The Descent of Man,” “A tail, though not externally visible, is really present in man and the anthropomorphous apes, and is constructed on exactly the same pattern in both.”

He was referring to the coccyx, rightly known in the vernacular as the “tail bone.” This string of (usually) three or four rudimentary vertebrae forms a pointed curve at the base of the spine in humans and apes (such as the chimpanzee and gorilla). The coccyx is of course entirely contained by the body, and cannot be moved like a full animal tail.

Even before Darwin, the coccyx was regarded as one of the prime pieces of evidence for humankind’s relationship to animals. In skeletal, muscular, vascular and nervous structure, it is homologous to the tails of other animals in general and of monkeys in particular.

However, it’s not simply a withered tail. The coccyx serves as an anchor for muscles that support the viscera and close off the back of the pelvis. At least some of these muscles, the ileo-coccygeus sheet, are clearly adapted forms of the muscles that, in monkeys, allow the tail to be moved from side to side.

Human evolutionists are certain that the oldest direct ancestor of humans and apes, probably around 20 million years ago, possessed a tail. We know where it went, but why did it go?

Nobody’s ever gotten farther than Darwin’s general hypothesis that tail modification was “directly connected” with the development of semi-erect postures in apes and a fully erect posture in humans. (The fossil record indicates both changes occurred early on, if not simultaneously.) Monkeys and lemurs, our other close cousins, use their tails to balance and brake themselves while leaping. But the early apes took to swinging arm-over-arm through the trees and spending time on the ground, probably making a tail unnecessary and more useful as an anchor for extra pelvic-support muscles. (A couple species of ground-dwelling monkeys possess only stubs of tails.)

But a body part doesn’t disappear just because it’s outdated, and it’s hard to see extra pelvic support as a significant selective advantage. Almost all monkeys and lemurs sit or stand upright for short periods of time without their guts popping out.

It’s probably a mistake to focus only on the coccyx. The spine has gone through a lot of changes, including the sacral vertebrae fusing into a single bone just above the coccyx. The body is an entire system that can often change in multiple, unpredictable ways when only one part is gaining a selective advantage.

It’s possible the transformation of the tail was a genetic byproduct of the larger pelvic mutations that made upright posture possible, or of a completely different change altogether. The decoding of the human genome may one day tell the tail’s tale.

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