Atavism

Atavism is the tendency to revert to ancestral type. In biology, an atavism is an evolutionary throwback, such as traits reappearing which had disappeared generations before. Atavisms can occur in several ways. One way is when genes for previously existing phenotypical features are preserved in DNA, and these become expressed through a mutation that either knock out the overriding genes for the new traits or make the old traits override the new one. A number of traits can vary as a result of shortening of the fetal development of a trait (neoteny) or by prolongation of the same. In such a case, a shift in the time a trait is allowed to develop before it is fixed can bring forth an ancestral phenotype.

In the social sciences, atavism is a cultural tendency—for example, people in the modern era reverting to the ways of thinking and acting of a former time. The word atavism is derived from the Latin atavus. An atavus is a great-great-great-grandfather or, more generally, an ancestor.

During the interval between the acceptance of evolution and the rise of modern understanding of genetics, atavism was used to account for the reappearance in an individual of a trait after several generations of absence. Such an individual was sometimes called a “throwback”. The term is often used in connection with the unexpected reappearance of primitive traits in organisms.The notion of atavism was used frequently by social Darwinists, who claimed that inferior races displayed atavistic traits, and represented more primitive traits than their own race. Both the notion of atavism, and Haeckel’s recapitulation theory, are saturated with notions of evolution as progress, as a march towards greater complexity and superior ability.

In addition, the concept of atavism as part of an individualistic explanation of the causes of criminal deviance was popularised by the Italian criminologist Cesare Lombroso in the 1870s. He attempted to identify physical characteristics common to criminals and labeled those he found as atavistic, ‘throwback’ traits that determined ‘primitive’ criminal behavior. His statistical evidence and the closely related idea of eugenics have long since been discredited, but the concept that physical traits may affect the likelihood of criminal or unethical behavior in the individual strangely still has some scientific support.

The notion that somehow, atavisms could be made to accumulate by selective breeding, or breeding back, led to breeds such as the Heck cattle. This had been bred from ancient landraces with selected primitive traits, in an attempt of “reviving” the extinct aurochs.

Evolutionarily, traits that have disappeared phenotypically do not necessarily disappear from an organism’s DNA. The gene sequence often remains, but is inactive. Mathematically, such an unused gene has a reasonable probability of remaining in the genome in a functional state for around 6 million years, but after 10 million years it is almost certain that the gene will no longer function. As long as the gene remains intact, a fault in the genetic control suppressing the gene can lead to it being expressed again. Sometimes, dormant genes can be induced to be expressed by supplying the stimuli artificially.Examples observed include:

• Hind legs on whales or snakes
• Hind fins on dolphins
• Extra toes on horses, as in archaic horses
• Re-emergence of sexual reproduction in the flowering plant Hieracium pilosella and the Crotoniidae family of mites.
• Teeth in chickens

Atavisms have been observed in humans as well. Babies have been born with a vestigial tail, called “coccygeal process”, “coccygeal projection”, and “caudal appendage”. It can also be evidenced in humans who possess large teeth, like those of other primates. In addition, a case of “Snake Heart”, the presence of “coronary circulation and myocardial architecture [which resemble] those of the reptilian heart”, has also been reported in medical literature.

• Humans with extra nipples (Accessory breast)
• Humans with large canine teeth