What is an atavism? It's an evolutionary throwback. It's when an ancestral gene that has been "switched off" gets "switched on" again. It's the most bizarre evidence for evolution you've ever seen. One of the tamer examples would be the horses who are born with three toes, just like their ancestors. Even more bizarre are the humans born with true tails. Stranger still is the whale found with legs, feet, and toes. Here is the story of human tails, and how we know that these are atavisms:

"Less than one third of the well-documented cases are what are medically known as "pseudo-tails" (Dao and Netsky 1984; Dubrow et al. 1988). Pseudo-tails are not true tails; they are simply lesions of various types coincidentally found in the caudal region of newborns, often associated with the spinal column, coccyx, and various malformations.

In contrast, the true atavistic tail of humans results from incomplete regression of the most distal end of the normal embryonic tail found in the developing human fetus The true human tail is characterized by a complex arrangement of adipose and connective tissue, central bundles of longitudinally arranged striated muscle in the core, blood vessels, nerve fibres, nerve ganglion cells, and specialized pressure sensing nerve organs. It is covered by normal skin, replete with hair follicles, sweat glands, and sebaceous glands. True human tails range in length from about one inch to over 5 inches long (on a newborn baby), and they can move via voluntary striped muscle contractions in response to various emotional states."

Talk Origins

The Whale with Legs:
 Whales and Dolphins with Legs

Live Science reports that,
"There are many cases where whales have been found with rudimentary hind limbs in the wild, and have been found in baleen whales, humpback whales, and in many specimens of sperm whales. Most of these examples are of whales that had only leg bones, but there were some that included feet with complete digits."

This fits in perfectly with the fossil evidence that whales descended from walking land mammals.

Recently a dolphin was captured which appeared to have hind limbs.

The Bird With Teeth

"Working late in the developmental biology lab one night, Matthew Harris of the University of Wisconsin noticed that the beak of a mutant chicken embryo he was examining had fallen off. Upon closer examination of the snubbed beak, he found tiny bumps and protuberances along its edge that looked like teeth--alligator teeth to be specific. The accidental discovery revealed that chickens retain the ability to grow teeth, even though birds lost this feature long ago."

From a Scientific American Article.

This of course compliments the fossil evidence that birds are descended from reptiles: Archaeopteryx had teeth as well.
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