Transitional fossil

Transitional fossils are the fossilized remains of intermediary forms of life that illustrate an evolutionary transition. They can be identified by their retention of certain primitive (plesiomorphic) traits in comparison with their more derived relatives, as they are defined in the study of cladistics. "Missing link" is a popular term for transitional forms. Numerous examples exist, including those of primates and early humans.

According to modern evolutionary theory, all populations of organisms are in transition. Therefore, a "transitional form" is a human construct of a selected form that vividly represents a particular evolutionary stage, as recognized in hindsight. Contemporary "transitional" forms may be called "living fossils", but on a cladogram representing the historical divergences of life-forms, a "transitional fossil" will represent an organism at the point where individual lineages (clades) diverge.

Relation to the theory of evolution
In 1859, when Charles Darwin's On the Origin of Species was first published, the fossil record was poorly known, and Darwin described the lack of transitional fossils as "the most obvious and gravest objection which can be urged against my theory", but explained it by the extreme imperfection of the geological record. He noted the limited collections available at that time, but described the available information as showing patterns which followed from his theory of descent with modification through natural selection. Indeed, Archaeopteryx was discovered just two years later, in 1861, and represents a classic transitional form between dinosaurs and birds. Many more transitional fossils have been discovered since then and it is now considered that there is abundant evidence of how all the major groups of animals are related, much of it in the form of transitional fossils.

Examples
The reconstruction of the evolution of the horse and its relatives assembled by Othniel Charles Marsh from surviving fossils form of a single, consistently developing lineage with many "transitional" types, is often cited as a family tree. However, modern cladistics gives a different, multi-stemmed shrublike picture, with multiple innovations and many dead ends. Other specimens cited as transitional forms include the "walking whale" Ambulocetus, the recently-discovered lobe-finned fish Tiktaalik and various hominids considered to be proto-humans.

Cladistics
Before the general acceptance of cladistics in paleontology, evolutionary trees were often drawn as the emerging of one group from another. The transitional forms were placed at the borders of these. With the establishment of cladistic methods, relationships are now strictly expressed in so-called cladograms, illustrating the branching of the evolutionary lineages.

The different so-called 'natural' or 'monophyletic' groups form nested units that do not overlap. Within cladistics there is thus no longer a transition between established groups, but a differentiation that occurs within groups, represented as a branching in the cladogram. In this context, transitional organisms can be conceptualized as representing early examples on the different branches of a cladogram, lying between a particular branching point and the "crown-group", i.e. the most-derived group, which is placed at the end of a lineage.

Comparison to 'intermediate' forms
The terms 'transitional' and 'intermediate' are for the most part used as synonyms; however, a distinction between the two can be made:


 * "Transitional" can be used for those forms that do not have a significant number of unique derived traits that the derived relative does not possess as well. In other words, a transitional organism is morphologically close to the actual common ancestor it shares with its more derived relative.
 * "Intermediate" can be used for those forms that do have a large number of uniquely derived traits not connected to its derived relative.

According to this definition, Archaeopteryx, which does not show any derived traits that more derived birds do not possess as well, is transitional. In contrast, the platypus is intermediate because it retains certain reptilian traits no longer found in modern mammals and also possesses derived traits of a highly specialized aquatic animal.

Following this definition, all living organisms are in fact to be regarded as intermediate forms when they are compared to some other related life-form. Indeed there are many species alive today that can be considered to be transitional between two or more groups.

Misconceptions
It is commonly claimed by creationists that there are no transitional fossils. Such claims may be based on a misunderstanding of the nature of what represents a transitional feature but are also explained as a tactic actively employed by creationists seeking to distort or discredit evolutionary theory and has been called the "favourite lie" of creationists.

A common, though fallacious, creationist argument is that no fossils are found with partially functional features. Vestigial organs are common in whales for example. Also, there is evidence that a complex feature with one function can adapt to a wholly different function through evolution in a process known as exaptation. The precursor to, for example, a wing, might originally have only been used for gliding, trapping flying prey, and/or mating display. Nowadays, wings may still have all of these functions, while also being used for active flight.

Although transitional fossils demonstrate the evolutionary transition of one species to another, they only exemplify snapshots of this process. Due to the specialized and rare circumstances required for a biological structure to fossilize, only a very small percentage of all life-forms that ever have existed can be expected to be represented in discoveries. Thus, the transition itself can only be illustrated and corroborated by transitional fossils, but it will never demonstrate an exact half-way point between clearly divergent forms. Creationists have often claimed that this analysis of the fossil record is merely a convenient way to explain the lack of 'snapshot' fossils that show crucial steps between species. Progress in research and new discoveries continue to fill in such gaps, however, and in modern thinking, evolutionary lines of development are understood as being bush-like in appearance, not as the simplistic ladder of progress that was common before Darwin published his theory and still influences popular opinion.

The theory of punctuated equilibrium developed by Stephen Jay Gould and Niles Eldredge and first presented in 1972 is often mistakenly drawn into the discussion of transitional fossils. This theory, however, pertains only to well-documented transitions within taxa or between closely related taxa over a geologically short period of time. These transitions, usually traceable in the same geological outcrop, often show small jumps in morphology between extended periods of morphological stability. To explain these jumps, Gould and Eldredge envisaged comparatively long periods of genetic stability separated by periods of rapid evolution.

Missing links
A popular term used to designate transitional forms is "missing links". The term tends to be used in the popular media, but is avoided in the scientific press as it is inaccurate and confusing. In reality, the discovery of more and more transitional fossils continues to add to knowledge of evolutionary transitions.

The term was used by Charles Lyell in a somewhat different way in his Elements of Geology of 1851, but was popularised in its present meaning by its appearance in Lyell's Geological Evidences of the Antiquity of Man of 1863, p. xi. By that time geologists had abandoned a literal biblical account and it was generally thought that the end of the last glacial period marked the first appearance of humanity, a view Lyell's Elements presented. His Antiquity of Man drew on new findings to put the origin of human beings much further back in the deep geological past. Lyell's vivid writing fired the public imagination, inspiring Jules Verne's Journey to the Center of the Earth, and Louis Figuier's 1867 second edition of La Terre avant le déluge which included dramatic illustrations of savage men and women wearing animal skins and wielding stone axes, in place of the Garden of Eden shown in the 1863 edition.

The idea of a "missing link" between humans and so-called "lower" animals remains lodged in the public imagination. The concept was fuelled by the discovery of Australopithecus africanus (Taung Child), Java Man, Homo erectus, Sinanthropus pekinensis (Peking Man) and other Hominina fossils.