An atavism (from Latin atavus, a great-grandfather’s grandfather) is the appearance of a structure from an ancestral type. Whereas vestigial structures are degenerative or incomplete like flightless wings of the kiwi or the shrunken pelvis found in some whales and snakes, atavisms are ancestral traits that reappear in modern individuals. For a structure to be an atavism, the organism’s parents and recent ancestors can’t possess the trait.
Living whales and dolphins born with legs
Anatomists and paleontologists have predicted for decades that whales evolved from terrestrial ancestors. In the past few decades numerous whale fossils have been found, mostly in Pakistan, that have filled in the fossil record for whale evolution. There are also numerous cases of live whales being found with rudimentary hind limbs. Examples include baleen, humpback and sperm whales. These limbs include femurs, tibia and fibulae. In some cases even feet with complete digits (toes) have been observed. Approximately one whale in 500 is born with a rear leg that protrudes out of it’s body. In one case from 1919, a dissection of a female humpback whale atavistic leg demonstrated a femur, tibia, tarsus, and metatarsal. It was over four feet long. When attached to the whale, the femur was inside the body cavity and attached to the pelvic vestigial rudiments. Origin of whales - multiple lines of evidence
In 2006, Japanese fisherman captured a bottlenose dolphin with atavistic hind limbs. The specimen is at the Taiji Whaling Museum
Horses and toes
The fossil record is well documented demonstrating horse evolution. One trend is the loss of toes with the distant horse ancestor having five toes compared to the modern horse having one. Modern horse embryos begin with three toes but on rare occasions a horse embryo during development fails to suppress the growth of the outer toes and it is born with true extra toes, complete with hoofs. Horses with these extra toes were often thought to have supernatural connections and both Julius Caesar and Alexander the Great were said to have ridden them.
Bird teeth experiments
For many decades it has been known that birds carry the DNA to make rudimentary teeth, especially enamel. No living bird has teeth yet they carry the ancestral genes to produce them. In one experiment in 1980 the embryonic mouth tissues of a chick were grafted onto a developing mouse jaw. The mouse grew different teeth than it normally did; they were conical peg-like teeth similar to those found in the earliest toothed birds from the fossil record and had roots and crowns. The underlying mouse tissue itself could not grow teeth, so it had to be the DNA in the chicken that held the genes to make teeth. Twenty years later, researchers identified a single protein that was missing and this turned off the bird genes to make teeth. If you apply the protein, toothlike structures will form the bill.
What could be better than chickens growing teeth? Horner on chicken atavisms and Jurassic Park in real science?
Atavism Activation and "Chickenosaurus". March, 2011. TED (16 min.)
Dead Genes For Tooth Enamel (Jerry Coyne)
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