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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.
Evolutionarily, traits that have disappeared phenotypically do not necessarily disappear from an organism's DNA. The gene sequence often remains, but is inactive. Such an unused gene may remain in the genome for many generations. 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 artificial stimulation.
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 chicken 
Examples in humans 
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.
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 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 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.
Cultural atavism 
The term atavism is sometimes also applied in the discussion of culture. Some social scientists[who?] describe the return of older, "more primitive" tendencies (e.g. warlike attitudes, "clan identity", 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, in the book Lords of Chaos 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.
Atavism is a key term in Joseph Schumpeter's explanation of World War I in 20th century liberal Europe. He defends a liberal view of international relations that an international society built on commerce will avoid war because of war's destructiveness and comparative cost. His reason for WWI is termed "atavism", in which he claims the vestigial governments in Europe (the German Empire, Russian Empire, Ottoman Empire, and Austro-Hungarian Empire) pulled the liberal Europe into war, and that the liberal structure of the continent did not cause it. He used this idea to say that liberalism and commerce would continue to have a soothing effect in international relations, and that war would not arise in nations who are built on commercial ties.
Hunter S. Thompson used the phrase "atavistic endeavor" in many of the pieces he wrote and the phrase is still strongly associated with him.
See also 
- "Multi-cell Organisms". Universe-review.ca. Retrieved 2011-09-29.
- TalkOrigins Archive. "29+ Evidences for Macroevolution: Part 2". Archived from the original on 29 October 2006. Retrieved 2006-11-08.
- Held, L. (2009). Quirks of Human Anatomy, an Evo-Devo Look at the Human Body. Cambridge University Press. ISBN 978-0-521-73233-8.
- Collin, R.; Cipriani, R. (2003). "Dollo's law and the re-evolution of shell coiling". Proceedings of the Royal Society B 270 (1533): 2551–2555. doi:10.1098/rspb.2003.2517. PMC 1691546. PMID 14728776.
- Tyson, R.; Graham, J. P.; Colahan, P. T.; Berry, C. R. (2004). "Skeletal atavism in a miniature horse". Veterinary radiology & ultrasound : the official journal of the American College of Veterinary Radiology and the International Veterinary Radiology Association 45 (4): 315–317. PMID 15373256.
- Domes, K.; Norton, R. A.; Maraun, M.; Scheu, S. (2007). "Reevolution of sexuality breaks Dollo's law". Proceedings of the National Academy of Sciences 104 (17): 7139–7144. doi:10.1073/pnas.0700034104. PMC 1855408. PMID 17438282.
- Gould, S.J. "Hen's Teeth and Horses' Toes" In: Hen's Teeth and Horse's Toes, New York: W. W. Norton, ISBN 978-0-393-01716-8
- "What our tails tell us". Los Angeles Times. 2007-02-15. Archived from the original on 12 March 2009. Retrieved 2009-03-08
- Walia, I.; Arora, H. S.; Barker, E. A.; Delgado Rm, 3.; Frazier, O. H. (2010). "Snake Heart: A Case of Atavism in a Human Being". Texas Heart Institute journal / from the Texas Heart Institute of St. Luke's Episcopal Hospital, Texas Children's Hospital 37 (6): 687–690. PMC 3014134. PMID 21224948.
- "http://www.criminology.fsu.edu/crimtheory/lombroso.htm" Retrieved on 26 July 2012.
- Haselhuhn, M. P.; Wong, E. M. (2011). "Bad to the bone: Facial structure predicts unethical behaviour". Proceedings of the Royal Society B: Biological Sciences 279 (1728): 571–6. doi:10.1098/rspb.2011.1193. PMID 21733897.
- An example of this usage of the term can be found in Friedrich A. Hayek (1978). "The Atavism of Social Justice". New Studies in Philosophy, Politics, Economics and the History of Ideas. Chicago: University of Chicago Press.
- Joseph Schumpeter (1969). "Imperialism and Capitalism". Imperialism and Social Classes. Cleveland: The World Publishing Company.
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