An atavism can mean an organism that is a real or supposed evolutionary throwback; the unexpected appearance of primitive traits; or a reversion to or reappearance of a trait that had been present in a lineage in the past, but which had been absent in intervening generations.

Real atavism in modern biology

There are many examples of atavisms, some of which are:

(1) rare formation of extra toes (2nd and 4th digits) in horses, similar to what is seen in the archaic horses Mesohippus and Merychippus.

(2) atavistic thigh muscles in Passeriform birds and sparrows.

(3) hyoid muscles in dogs.

(4) wings in earwigs (normally wingless).

(5) atavistic fibulae in birds (the fibulae are normally extremely reduced).

(6) extra toes in guinea pigs and salamanders.

(6) the atavistic dew claw in many dog breeds.

(7) sometimes whales have been found with hind legs, which have been found in baleen whales, humpback whales, and in many specimens of sperm whales. Most of these are of whales with femurs, tibia, and fibulae; however, some even include feet with complete digits.

There are also many atavisms in humans. One such example is when human babies are born with true tails, also called "coccygeal process", "coccygeal projection", "caudal appendage", and "vestigial tail". True human tail is characterized by a complex arrangement of adipose and connective tissue, central bundles of longitudinally arranged striated muscle in the core, blood vessels, nerve fibres, nerve ganglion cells, and specialized pressure sensing nerve organs (Vater-Pacini corpuscles). It is covered by normal skin, replete with hair follicles, sweat glands, and sebaceous glands. True human tails range in length from about one inch to over 5 inches long (on a newborn baby), and they can move via voluntary striped muscle contractions in response to various emotional states.

Although human tails usually lack skeletal structures, several human tails have also been found with cartilage and up to five, well-developed, articulating vertebrae.

Atavism in History

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 1870’s. He attempted to identify physical characteristics common to criminals and labelled those he found as atavistic, ‘throwback’ traits that determined 'primitive' criminal behaviour. His statistical evidence and the notion that physical traits determine inevitable criminality (an idea closely related to the concepts of eugenics) have long since been debunked, but the concept that physical traits may affect the likelihood of criminal behaviour in the individual remains popular in some circles. For instance, the presence of an XYY chromosomal abnormality was widely touted as a predictor of violent behaviour among male prison inmates in past decades. Cf. also the White supremacist belief that Blacks are ethnically inferior to Whites, and that this, rather than economics and a demonstrably racially-skewed justice system, accounts for the disproportionate African-American prison inmate population. Despite the number of studies done on the subject, no conclusive evidence exists of any statistical link between physical traits and individual criminality.

Ideas that the genetic clock could somehow be reversed led to the selective breeding of cattle with selected primitive traits, in hopes of reviving the extinct aurochs.

Cultural references to atavism

The term atavism is sometimes also applied in the discussion of culture. Some social scientists describe the return of older, "more primitive" tendencies (e.g., warlike attitudes, "clan identity," etc. -- anything suggesting the social and political atmosphere of thousands of years ago) as "atavistic." "Resurgent Atavism" is a common name for the belief that people in the modern era are beginning to revert to ways of thinking and acting that are throwbacks to a former time. This is especially used by sociologists in reference to violence.

The neo-pagan subculture also uses this same terminology ("atavism" or "resurgent atavism") to describe how modern, Western countries are experiencing both the decline of Christianity and the rise of religious movements inspired by the pagan religions of centuries past. Some cite the rise of environmentalism, scientific inquiry, and liberalization of society as contributing to an increasingly secular society, one in which religious sentiments are more frequently tied with an appreciation of the physical world rather than set against it. Occasionally, the use of these terms in reference to "alternative" spirituality or in an occult context implies the use of violence to assert these changing religious views--for example, a rash of church burnings across Scandinavia has been described as a part of this trend because many of the perpetrators were self-described "pagans" seeking to overthrow what they deemed to be centuries of religious oppression by Christianity. It should be noted, however, that most adherents to modern pagan religions and related belief systems do not support these actions.