Evolution: Biology

Basic Ideas: Atavismen

Contents

This article will explain what atavism is and why it is viewed as evidence of macroevolution. It will further be shown why atavism cannot be regarded as proof of macroevolution

The atavism argument

Evaluation of the argument

Criticism of the argument

Summary

The atavism argument

Indications for evolution are derived from teratology. This is the science and study of deformities. Anomalies in individual development can be caused by mutation but also by environmental influences. There are innumerable deformities, but they occur very rarely in single individuals.

Some deformities show certain similarities to structures present in presumed ancestors of the species in question. Cases such as these are frequently known as atavism. An interpretation based on the theory of evolution is contained within this term: “atavism” comes from the Latin atavus = great grandfather, oldest known ancestor. The deformed structures in question are supposedly a reminder of the form found in a phylogenetic ancestor. They are said to be “returns” to earlier phylogenetic stages. In humans, cervical fistulas, unusually profuse body hair, a tail (see Fig. 200) and additional nipples are named as examples of atavism. Cervical fistulas are canals in the area of the throat and outer skin of the neck, and they are seen as gill slits which have remained open (a return to the fish stage). Unusual body hair is supposedly a reminder of ancestors with fur; a tail-like structure in the coccygeal region is likewise seen to be an indication of ancestors with tails. An example of atavism in animals is an additional toe in horses (Fig.201). In this case a structure normally formed once on each foot has been developed twice (superfluously), presumably due to a malfunction.

Evaluation of the argument

Atavism – similarly to vestigial organs (Vestigial organs) – is no direct indication of progressive evolution, although a macroevolutionary change is indirectly inferred when, for example, certain deformities that are seen as atavism are viewed as proof that humans had fish-like ancestors. However, this is basically only comparative biological argumentation from which no conclusive evidence for macroevolution can be gained (compare Similarities in morphology and anatomy). For the mechanisms of the indirectly developed, hypothetical remodelling are not known.

Criticism of the argument

The argumentation that interprets deformities as atavism is inconsistent.

Deformities taken as indications of an assumed phylogeny (that is, interpreted as atavism) if they demonstrate similarities with presumed ancestors of the organism in question, are in fact used very selectively. Almost all deformities, however, cannot be interpreted as evolutionary returns. Deformities such as forked ribs, hare lip, polydactyly, the development of two heads or the presence of a fifth leg, for example, are for a certainty not indications of earlier phylogenetic stages. Interpreting deformities as atavism is thus only possible if a particular evolutionary development is already presupposed. From this is then concluded which deformities can be seen as returns to earlier stages. As the matter to be proved is thus already assumed, atavism cannot be regarded as proof of phylogeny. The fact that a small number of deformities are reminiscent of similarities in assumed ancestors of a specific organism is not particularly remarkable, and due to the similarity of many developmental forms it is also not surprising. Added to this is the fact that when atavisms are scrutinised more closely, they are found to be in no way similar in all aspects. The argumentation with atavistic structures themselves is again also selective. For instance, there are horses that have two-toed feet (Fig 201, see above), yet amongst horse fossils only three or four-toed varieties are known.

Pursued consistently, the atavistic interpretation would lead to ridiculous conclusions, as the following example shows. Four-winged fruit fly mutants (Fig. 57, below right) are regarded as an indication that the normally two-winged insects (diptera) have descended from four-winged types. The development of four wings is interpreted as atavism. There are, however, also fruit fly mutants with four halteres and no wings – a pointless construction which certainly cannot be regarded as an indication of phylogenetic ancestors.

It can generally be shown that atavisms, as all deformities, are caused by particular developmental disorders, and can be understood without any reference to hypothetical phylogenetic ancestors. Mutations of regulatory genes are the basis of the wing/halteres mutations (see Homeobox genes and evolution). Cervical fistulas (see above) result amongst other things from a pathological disorder of the outer layer of skin in the neck region, and not from gill slits that have remained open. The reference to phylogeny can be seen as unnecessary and can therefore be disregarded.

Atavistic hind limb stumps in whales could possibly also be interpreted in a similar way (Fig. 202). They could likewise be regarded as a deformed “copy” of parts of the forelimbs. For this reason it is not necessary to postulate about “sleeping genes” that have been “inadvertently” reactivated and thus caused atavism. Apart from this it could be expected that genes not required over a longer period of time by reason of selection through mutation would suffer such extreme defects that they could no longer be reactivated. Yet this example provides a comparatively good argument for the theory of evolution, for – in contrast to the example of the four-winged fruit flies – the atavistic structure is no substitute for another structure (in the fruit fly: substitute for the halteres), but occurs in addition to these. The understanding gained over the past years about the functions of Hox genes (Homeobox genes and evolution) in the development of body axes and extremities in vertebrates has opened up a clarification of this and other phenomena as pathological homeotic deformation.

In the interpretation of deformities as atavism, the same thing is valid as for vestigial organs: all interpretations are overhasty as long as the basic genetic and physiological developmental situation and the significance of functional growth in normal development are not known.

Summary

Some deformities that occasionally occur can be interpreted as atavism and are therefore regarded as proof of evolution. However, this interpretation can only be applied in exceptional cases, and here only when a certain evolutionary process is already presupposed. For this reason atavism is not an independent proof of evolution. Furthermore the occurrence of atavism can be understood without the necessity of referring to a previous evolution.

Literature

Junker R. (2002) Ähnlichkeiten, Rudimente, Atavismen. Holzgerlingen. Studium Integrale Series, Chapter 9.

Translator: Margaret Kahlberg, 16.10.2008

Author: Reinhard Junker

 
© 2008