What does Atavism Mean in Biology?

Example of atavism in biology
In biology, the term 'atavism' refers to the reappearance of an ancestral trait in an individual, which is not present in the immediate predecessors of that individual. In other words, it can be called a reversal of evolutionary changes or a reversion to ancestral type. This Buzzle post explores and explains this concept.
Contrary to popular belief, the condition of hypertrichosis, which causes excessive growth of facial hair in humans, is not an example of atavism. This is because the facial area of our simian ancestors was clear of hair. It would have been a true case of atavism only if humans had evolved from werewolf-like ancestors.
The term 'atavism' is derived from the Latin word 'atavus' which means great-great-great-grandfather or an ancestor. It is called an evolutionary throwback due to the characteristic reappearance of ancestral traits in modern-day individuals. These traits could reappear via various developmental incidents.

In social science, this term is used to refer to the cultural tendency of people to revert to the ways of thinking and behavior from a former time. In criminal psychology, it refers to the belief that all criminals bear ancestral atavistic facial features that serve as a premonition of a primitive criminal behavior. Although the idea of atavism gained popularity by this claim made by Cesare Lombroso in the 1870s, the claim itself was abandoned shortly after it was declared.
Atavism in Biology
It may arise due to genetic and/or physical factors. The most common mechanism of this phenomenon would be genetic, where the genes responsible for a previously occurring, ancestral phenotypic feature are found to be expressed in the cell, either due to a loss-of-function mutation in its repressing gene, or due to a gain-of-function mutation in that gene itself. Such an expression of a latent gene, would produce the ancestral phenotype in the individual. The same result may also be seen if the period of fetal development of an individual is cut short or prolonged.

The genetic mechanism is possible due to the fact that despite the non-expression of a gene, the sequence is retained in the DNA, although in an inactive form. Hence even if the trait is not exhibited in a species, there are high chances that the organism still retains a copy of the gene in its genome.

The other method of altering the developmental period of the organism also achieves a similar effect. This is possible due to the fact that in the very early stages of development, the fetus looks more or less the same across all higher mammals. This can be explained by reasoning that all animals share at least a few ancestors along the evolutionary timeline. As the time progresses, these similar fetuses start developing and undergo rapid modifications to evolve into similar organisms as their parent organisms (Ernst Haeckel's theory of recapitulation). Hence if an animal's development is halted at a certain stage, it may still exhibit ancestral traits. A prolongation, in theory, would afford more time for the fetus to stay in the womb and help it in developing an ability.

Both these explanations are still speculative, as a definite mechanism explaining this phenomenon has not yet been fully worked out.
Examples of atavism in animals
╬ Hind limb on whales

╬ Hind fins on dolphins

╬ Hind limbs on snakes

╬ Extra toe on horses

╬ Reemergence of sexual reproduction in Hieracium pilosella and the Crotoniidae family of mites

╬ Teeth-like structures on the beaks of chickens

╬ Hatchling feathers on newborn birds
Examples of atavism in humans
╬ Coccygeal projection in the form of a non-functional tail

╬ Accessory breasts, i.e. presence of extra nipples

╬ Large and long canine teeth

╬ Snake heart, i.e. myocardial structure is altered such that it resembles a reptilian heart

╬ Movable pinna
It was briefly believed that atavisms could be induced and accumulated in order to revive an extinct ancestral species by carrying out intentional selection in breeding and breeding back. This theory was used to attempt the revival of the extinct species of aurochs, by breeding ancient land-races with primitive traits. The attempt resulted in the formation of a new breed of cattle, called the Heck cattle, but was unable to revive the ancestral breed.