Atavisms: Medical, Genetic, and Evolutionary Implications

Also see previous post: Occipital Knob on Modern Skull / A deformity, or something else?   

Nenad Tomić, Victor Benno Meyer-Rochow  (click to access PDF)

“That a being should be born resembling in certain characters an ancestor removed by two or three, and in some cases by hundreds or even thousands of generations, is assuredly a wonderful  fact. . . . If . . . we suppose . . . that many characters lie dormant in both parents during a long succession of generations, the foregoing facts are intelligible.”  Charles Darwin, 1859


Traits expected to be lost in the evolutionary history of a species occasionally reappear apparently out of the blue. Such traits as extra nipples or tails in humans, hind limbs in whales, teeth in birds, or wings in wingless stick insects remind us that certain genetic information is not completely lost, but can be reactivated. Atavisms seem to violate one of the central evolutionary principles, known as Dollo’s law, that “an organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors.” Although it is still not clear what triggers and controls the reactivation of dormant traits, atavisms are a challenge to evolutionary biologists and geneticists. This article presents some of the more striking examples of atavisms, discusses some of the currently controversial issues like human quadrupedalism, and reviews the progress made in explaining some of the mechanisms that can lead to atavistic features. 

Problem Outline

The original usage of the term atavismus was derogatory: Cesare Lombroso, one of the founders of modern-era criminology, proposed in his 1876 book L’uomo delinquent (reprinted in 2006 as The Criminal Man) that criminality is a throw-back to the uncivilized behavior of the pre-Homo sapiens era. In subsequent editions of The Criminal Man, Lombroso refined his understanding of atavism, declaring it insufficient to explain multiple inadequacies found in criminals and instead choosing to lump together the notions of epilepsy, atavism, and moral insanity as causes of inborn criminality. It took some time and efforts to refine Darwin’s evolutionary theory before the notion of atavism was rescued from its positivist connotations in jurisprudence. In modern biology, the term atavism refers to the reappearance of a “lost character” (either morphological or behavioral) that was typical of remote ancestors. In order to be distinguished from closely related instances of character homology, atavisms have to fulfill several conceptual, comparative anatomical, and developmental biological criteria: (1) a high degree of phenotypic resemblance to a predecessor from distant lineages (i.e.,atavism is not found in parents or immediate ancestors); (2) appearing as a fully developed character in adult life stages; (3) markedly infrequent occurrence across populations, usually in one or a few individuals; and (4) no organism can display an atavistic structure that was not previously found in its ancestry (Hall 1995).

Each of the four criteria has their conceptual and biological counterparts: (1´) if the reversional character with a high degree of similarity was lost somewhere in phylogeny, and not the identical structure (Hall 1995). Each of the four criteria hast heir conceptual and biological counterparts: (1´) if the reversional character with a high degree of similarity is found in the last common ancestor, then it more readily a homologue (or a homologous structure); (2´) if a recurring structure is an incompletely developed version of a character found in ancestors and occurs in adult stages, it is classified as a vestige, but if the partially formed feature occurs in embryonic stages it is called a rudiment (Renshaw 1923); (3´) when atavistic traits occur across entire phyla they are referred to as “phylogenetic character reversals” (Stiassny 1992); (4´) it should be possible to identify sets of atavisms that are permitted (and those that are not) for any given species down the phylogenetic tree.

The design of the reversion is “faithful” to the original in purpose, origin and patterning, which allows us to deem it a throwback even if it is not an exact replica in a strict sense. Both these caveats appear self-evident, but they are often overlooked, thus causing considerable confusion when considering the contentious aspects of homology in the binary context of phylogenetic irreversibility versus evolutionary reversion. For reasons of clarity (and following Hall 1995), we shall postulate that atavisms come to the fore in three basic manners: (1) as naturally occurring phenomena, in both individual organisms and all members of the species; (2) in selective breeding of laboratory animals; and (3) as experimentally induced atavisms following embryonic graft manipulation as well as generation of transgenic animals. All three will briefly be discussed in the subsequent paragraphs.

First, the rate of naturally occurring atavisms is most probably underestimated. The reason is two fold: an atavism will be recognized as such only in those organisms for which a compelling fossil record has enabled thorough understanding of morphological evolution, and deviation from the standard is more readily recognized if it is obvious—in other words, external. Although atavisms may occasionally occur in the same ancestral lineage, they mostly occur in nonrelated taxa, as is the case with the Taiji bottlenose dolphin, whose hind limbs are evidential of the dolphin’s land-dwelling mammalian ancestry. Such rare atavistic anomalies appearing in individual organisms are conspicuous even to lay people precisely due to their singular character, and they will henceforth be referred to as “spontaneous atavisms,” to distinguish them from the more systemic appearance of taxic (phylogenetic) atavisms. Phylogenetic reversional events, which are found in all organisms of a particular clade are in fact just as illuminating with respect to such underlying developmental processes as spontaneous atavisms. However, their identification entails comprehensive knowledge of the phylum’s morphology and well-characterized phylogenetic history, which is hardly always the case. Go to PDF for “the rest of the story”


Read more at Buzzle:


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. (Can the results of atavism and of epigenetics be confused? What about genetic introgression? This topic may have serious implications for how western ideas about “diversity” may be inadequate.)

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. (It seems that there may be alternate, duplicate or conflicting explanations for the phenomenon, according to the discipline that is doing the explaining. Medical doctors may label “variations” in human form as “defects” that need repaired. See previous post: Occipital Knob on Modern Skull / A deformity, or something else?   

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;  Movable pinna (muscles that move ears)
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.





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