Atavism

An atavism is the phenomenon by which an phenotypic trait reappears in a organism after a period of absence. An atavism refers to a trait that is not present in an individual’s immediate predecessors, although it is present in their ancestors.

This is possible because of the way genes behave throughout the evolutionary process. When phenotypic traits are lost during the process of natural selection the information genetics in the DNA that codes for them is not necessarily lost and may instead be inactive. This means that genes can be reactivated, even after remaining inactive for many generations.

Atavisms differ from the vestigial structures . These are phenotypic traits that are present in all members of an species which in the evolutionary past of the organism have been necessary for survival, but which are no longer used or functional in modern individuals.

Examples of atavisms

The human tail

There are several scientific incidences of human babies being born with a tail that contains cartilage and vertebrae which is referred to as the “caudal appendage”.

All human babies grow in the womb with a small prenatal tail, a “throwback” feature of our evolutionary roots as arboreal primates. The genes that control tail growth are normally switched off as a result of gene regulation and the glue is reabsorbed in the tissues of the embryo . However, there are cases where this genetic regulation malfunctions and the tail continues to grow beyond the embryonic stage of development.

The image shows a human embryo in the womb. The tail is visible. Although genes, which control development, are usually switched off by genetic regulation, sometimes this fails and a human is born with a tail.

Chickens with Teeth

By inspecting the beaks of unhatched embryos, scientists have discovered several occurrences of chick embryos exhibiting evidence of tooth formation, and even embryonic chicks with full sets of developed teeth! To make matters stranger, the developing teeth are similar to those found in their closest living relatives … crocodiles!

Bird lost its ability to make teeth some 80 million years ago, yet modern chickens still possess the gene responsible for causing the odontogenesis (tooth formation), although the gene has long been switched off. Scientists have named this gene Talpid2.

Even though Talpid2 is lethal, which means that gene expression causes the chicks to never hatch successfully, the discovery has supported atavism. To test the hypothesis, the biologist Matthew Harris designed a virus which could mimic the molecular signals of the mutation genetic mutation. When implanted in normal healthy chicks, they briefly developed the same cone-shaped crocodile teeth, although they were rapidly resorbed in the tissue of the beak.

Dolphins with legs

In 2006, a dolphin was captured near Japan that, in addition to possessing the two front flippers that all dolphins have, had a pair of small, symmetrical pelvic fins closer to the tail.

This is an example of atavism; the flippers are reminiscent of the hind legs, so they appear to be a phenotype backward” phenotype of the ancestor of dolphins that could walk on land. As mammals, the evolutionary lineage of cetaceans goes back to land mammals such as hippopotamuses, and the discovery of the four-limbed dolphin suggests that the genes coding for limb growth are still present.