The answer that one typically gets is that our tail became vestigal because "we do not need it." Why don't we? The vast majority of bipedal animals have massive tails in order to keep balance during walking and running. There has never been a tailless biped except for hominids, precisely because it is such a poor design. My uninformed thinking has been that our lack of tail is one of those tragic evolution stories: the tail was lost by our quadripedal ape ancestor. In retrospect, it could've been very useful to us bipeds, but once lost, it has been lost for good. However, it seems that that is not quite the story. The answer might be that we evolved something better than tails, something that is better for endurance running: our behinds.
Estimated dates of innovations: Loss of tail: 23 Mya. Rotation of thumb: 18 Mya. Stable elbow: 15 Mya. Upright walking: 3.7 Mya. Broad sacrum: 3.3 Mya. First stone tools: 2.5 Mya. 'Human' knee joint and foot: 1.8 Mya. High forehead: 0.1 Mya
http://www.idlex.freeserve.co.uk/idle/evolution/human/early/golden.html...Human embryos have a tail that measures about 1/6th of the size of the embryo itself. As the embryo develops into a fetus, the tail is absorbed by the growing body, but some traces remain even in adults, as coccyx. Occasionally, a child is born with a "soft tail," described by one embryologist as containing "no vertebrae, but blood vessels, muscles, and nerves, of the same consistency as the short tail of the Barbary ape."
http://www.straightdope.com/classics/a1_077.html...humans lack one of the most obvious features of animals adapted for serious speed: a tail. In creatures that cover ground bipedally, such as kangaroos, kangaroo rats, and roadrunners, the tail is the major balance organ. In the whole history of vertebrates on Earth, humans are the only striding biped that's a runner that's tailless... Without the balancing help of a tail, how do we avoid falling over when we run? The butt, it turns out, is crucial among traits that make us uniquely human. Chimps and other primates have little buns. Our own rear ends are huge; the upper part of the gluteus maximus is greatly expanded. Although few scholars have studied its role in running, the butt is, according to Bramble, "basically a substitute for a tail."
http://discovermagazine.com/2006/may/tramps-like-ushttp://www.eurekalert.org/pub_releases/2004-11/hu-erm111504.phphttp://www.hno.harvard.edu/gazette/2004/11.18/01-running.html...The human gluteus maximus is a distinctive muscle in terms of size, anatomy and function compared to apes and other non-human primates. We tested the hypothesis that the human bottom plays a more important role in running than walking. The results indicate that the gluteus maximus is mostly quiescent with low levels of activity during level and uphill walking, but increases substantially in activity and alters its timing with respect to speed during running. The major functions of the gluteus maximus during running are to control flexion of the trunk on the stance-side and to decelerate the swing leg; contractions of the stance-side gluteus maximus may also help to control flexion of the hip and to extend the thigh. Evidence for when the gluteus maximus became enlarged in human evolution is equivocal, but the muscle's minimal functional role during walking supports the hypothesis that enlargement of the gluteus maximus was likely important in the evolution of hominid running capabilities.
http://jeb.biologists.org/cgi/content/abstract/209/11/2143 (J. Exp. Biol. 209, 2143 (2006)). Also,
http://www3.interscience.wiley.com/cgi-bin/abstract/110482977One often hears that it was our brain that made us human. Perhaps, but first it was our butt. That is where the winding road to
H. sapience originates.
Why don't we have tails
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