Occasionally an individual crops up with an anomaly that looks like the reappearance of an ancestral trait. A horse can be born with extra toes, a human baby with a tail. These sporadically expressed remnants of ancestral features are called atavisms, from the Latin atavus, or "ancestor."

They differ from vestigial traits because they occur only occasionally rather than in every individual.

True atavisms must recapitulate an ancestral trait, and in a fairly exact way. They aren't simply monstrosities. A human born with an extra leg, for example, is not an atavism because none of our ancestors had five limbs. The most famous genuine atavisms are probably the legs of whales. We've already learned that some species of whales retain vestigial pelvises and rear leg bones, but about one whale in 500 is actually born with a rear leg that protrudes outside the body wall. These limbs show all degrees of refinement, with many of them clearly containing the major leg bones of terrestrial mammals—the femur, tibia, and fibula. Some even have feet and toes!

Why do atavisms like this occur at all? Our best hypothesis is that they come from the re-expression of genes that were functional in ancestors but were silenced by natural selection when they were no longer needed. Yet these dormant genes can sometimes be reawakened when something goes awry in development. Whales still contain some genetic information for making legs—not perfect legs, since the information has degraded during the millions of years that it resided unused in the genome—but legs nonetheless. And that information is there because whales descended from four-legged ancestors. Like the ubiquitous whale pelvis, the rare whale leg is evidence for evolution.

Modern horses, which descend from smaller, five-toed ancestors, show similar atavisms. The fossil record documents the gradual loss of toes over time, so that in modern horses only the middle one—the hoof—remains. It turns out that horse embryos begin development with three toes, which grow at equal rates. Later, however, the middle toe begins to grow faster than the other two, which at birth are left as thin "splint bones" along either side of the leg. (Splint bones are true vestigial features. When they become inflamed, a horse gets "the splints.") On rare occasions, though, the extra digits continue developing until they become true extra toes, complete with hoofs. Often these atavistic toes don't touch the ground unless the horse is running. This is exactly what the ancient horse Merychippus looked like fifteen million years ago. Extra-toed horses were once considered supernatural wonders: both Julius Caesar and Alexander the Great were said to have ridden them. And they are wonders of a sort—wonders of evolution—for they clearly show genetic kinship between ancient and modern horses.

The most striking atavism in our own species is called the "coccygeal projection," better known as the human tail. As we'll learn shortly, early in development human embryos have a sizable fish-like tail, which begins to disappear about seven weeks into development (its bones and tissues are simply reabsorbed by the body). Rarely, however, it doesn't regress completely, and a baby is born with a tail projecting from the base of its spine (figure 14). The tails vary tremendously: some are "soft," without bone, while others contain vertebrae—the same vertebrae normally fused together in our tailbone. Some tails are an inch long, others nearly a foot. And they aren't just simple flaps of skin, but can have hair, muscles, blood vessels, and nerves. Some can even wiggle! Fortunately, these awkward protrusions are easily removed by surgeons.

What could this mean, other than that we still carry a developmental program for making tails? Indeed, recent genetic work has shown that we carry exactly the same genes that make tails in animals like mice, but these genes are normally deactivated in human fetuses. Tails appear to be true atavisms.

Some atavisms can be produced in the laboratory. The most amazing of these is that paragon of rarity, hen's teeth. In 1980, E. J. Kollar and C. Fisher at the University of Connecticut combined the tissues of two species, grafting the tissue lining the mouth of a chicken embryo on top of tissue from the jaw of a developing mouse. Amazingly, the chicken tissue eventually produced tooth-like structures, some with distinct roots and crowns! Since the underlying mouse tissue alone could not produce teeth, Kollar and Fisher inferred that molecules from the mouse reawakened a dormant developmental program for making teeth in chickens. This meant that chickens had all the right genes for making teeth, but were missing a spark that the mouse tissue was able to provide. Twenty years later, scientists unraveled the molecular biology and showed that Kollar and Fisher's suggestion was right: birds do indeed have genetic pathways for producing teeth, but don't make them because a single crucial protein is missing. When that protein is supplied, tooth-like structures form on the bill. You'll remember that birds evolved from toothed reptiles. They lost those teeth more than sixty million years ago, but clearly still carry some genes for making them—genes that are remnants of their reptilian ancestry.

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