Extra Toes in Horses and other hoof-bearing animals

Horses are sometimes born with three-toed feet instead of a single hoof. Even when this does not happen, extra toe bones are often present in addition to the one supporting the hoof. In contrast, most fossil eqids possess three-toed feet. Similar phenomena are observed in other ungulates such as cows, deer, and pigs.

Vestigial snake limbs

Snakes, especially boas, sometimes posses disconnected thigh-bones inside their bodies. Some snakes occasionally develop leg nubs which may aid in copulation. This suggests that snakes are descended from legged reptiles. Conversely, certain lizards such as the glass "snake" are legless.

Crab Tails

Crabs possess small tails which are usually (but not always) folded under their abdomens. Many supposed fossil crabs have tails which trail behind them like lobsters and crayfish.

Toenails on the fins of Manatees

Most manatees have vestigial toenails on their fins. These features serve no apparent purpose, but they suggest that ancesteral manatees could walk on land.

Ancestral insect wing configurations which reappear

Flies and roaches sometimes develop extra pairs of wings like their fossilized precursors.

Flightless Wings

All flightless birds have undersized wings. Often these wings are unused, or they are used only for display. Notable exceptions are penguins and other diving birds: They use their wings as fins while swimming.

Striped Horses

Mating of different solid-colored equids such as horses and donkeys sometimes produces offspring with stripes similar to zebras'. This effect suggests vestigal stripe-producing genes in equids which only surface under special circumstances.

Hipbones in Whales

Some whales have hipbones which are attached to no limbs. These bones serve little purpose, but they may sometimes serve as an attachment point for muscles which support the sex organs in either sex.

Wisdom Teeth

These late-errupting teeth are the bane of many people because their jaws are too small to accept them. Some lucky people never develop them.

Bird Teeth Genes

Unlike modern birds, which are all toothless, most bird fossils from the Mesozoic Era possess teeth. In an experiment, chicken embryo jaw tissue was grown beside mouse jaw tissue inside a mouse's eye. The chicken jaw produced conical teeth similar to the bird fossils'. This result indicates vestigal tooth-producing genes in chickens.

The Human Appendix

In the human gastrointestinal system, there is a small organ known as the appendix. Although unnecessary and generally useless, it does contain a cluster of antibody-producing cells similar to cells found in the lymphatic system and tonsils. Like the tonsils, it can be safely removed.

Tails Which Reappear in Tailless Primates

Some humans (and other tailless primates) are sometimes born with vestigial tails. Like bears' tails, they are too small to be useful for anything.

Note to noders: Please attach any examples we have missed! Thanks!

Loren I. Petrich (lip@s1.gov)
Almost every single one of these features is an Atavism. An Atavism is the apparent reversion of an animal back to an ancient form. They normally occur when dormant genes are turned on to when a mutation in a gene causes the animal to develop the gene coding of an ancient relative.

There have been many incidents of atavisms recorded throughout history, all the way back to the time of Caesar. They have long been considered unimportant oddities. To some they were seen as a reminder that previous imperfections lay just below the surface.

Stephen J. Gould claims something else entirely. He contends that they teach an important lesson the potential result of small genetic changes. Atavisms support the belief that phenotypes can change rapidly with one or two small changes unlike the standard gradualist model of evolution where many small changes are needed over generations. Genetic systems contain extensive, hidden components and capacities for expressing small changes as large effects. That these components exist is the only way for atavisms to exist. He feels that these components, which have possibly been hidden for millions of years, would allow for rapid evolutionary changes and adaptation.

Another example of an atavism could be some Flightless Beetle’s wings. The wings are fully developed, but the cover on the backs of the beetles became fused to its shell at some point in history, rendering it flightless.

Atavism is a theory presented by, among others, Gould. It does not explain the unlikelihood of 1/3 of horses in a later German test study having the ancient gene, and the other 2/3 that had 3 toes having the 3rd toe duplicated. Also, as melknia says, the probability that the same gene mutates everytime these fairly common atavisms come up is fairly low.

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