Transitional fossil

Transitional fossils are the fossilized remains of transitional 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 and a "transitional form" is a recognition of a form that vividly represents a particular evolutionary stage.

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Transitional fossils and the theory of evolution

In 1859, when Charles Darwin's The Origin of Species was first published, the fossil record was poorly known, and the lack of transitional fossils was an objection to the theory of evolution; Darwin stressed that this lack was the most formidable obstacle to his theory. However, the discovery of Archaeopteryx two years later was seen as strong support for Darwin's theory of common descent. Many more transitional fossils have been discovered since then, and estimates based on the observed fossil density of various rock formations indicate that billions of such fossils exist for the reptile/mammal transition in particular (see Evidence of common descent). Currently, some gaps remain in the fossil record but most scientists accept that the rarity of fossils means that many extinct animals will always remain unknown. Those who oppose the theory of evolution point to a purported lack of transitional fossils as evidence that the theory of evolution is unproven.

Examples of transitional fossils

Main article: Evolution of the horse

Main article: Evolution of cetaceans

Main article: Evolution of mammalian auditory ossicles

Though the evolution of the horse and its relatives, as Othniel Charles Marsh assembled surviving fossils in his reconstruction of the evolution of horses in the form of a single, consistently developing lineage with many "transitional" types, is often cited as a family tree with a number of clear transitional fossils, 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.

Transitional forms and 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.

Transitional forms vs. '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 stated by critics of evolution that there are no known transitional fossils.^[1] This position is based on a misunderstanding of the nature of what represents a transitional feature. A common creationist argument is that no fossils are found with partially functional features. It is plausible, however, that a complex feature with one function can adapt a wholly different function through evolution. The precursor to, for example, a wing, might originally have only been meant for gliding, trapping flying prey, and/or mating display. Nowadays, wings can still have all of these functions, but they are also used in active flight.

Although transitional fossils elucidate the evolutionary transition of one life-form to another, they only exemplify snapshots of this process. Due to the special circumstances required for preservation of living beings, 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 be "caught in the act" as it were. Critics of evolution often cite this argument as being a convenient way to explain the lack of 'snapshot' fossils that show crucial steps between species. However, progressing research and discovery are managing to fill in gaps.

The theory of punctuated equilibrium developed by Stephen Jay Gould and Niles Eldredge 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 periods of morphological stability. To explain these jumps, Gould and Eldredge envisaged comparatively long periods of genetic stability separated by periods of rapid evolution.

The 'missing link'

A popular term used to designate transitional forms is "the missing link". The term is especially used in the regular media, but is inaccurate and confusing, partly because it implies that there is a single link missing to complete the picture, which must be discovered. In reality, the continuing discovery of more and more transitional fossils is further adding to our knowledge of evolutionary transitions. The term probably arose in the nineteenth century where the awaited discovery of a "missing link" between humans and so-called "lower" animals was considered to be the final proof of evolution. The Australopithecus afarensis fossil (more commonly known as "Lucy"), a key transitional fossil, has often been represented as "the missing link".

The discovery of Australopithecus africanus (Taung Child), Java Man, Homo erectus, Sinanthropus pekinensis (Peking Man), etc. are also vital to the study of links.

See also

• List of transitional fossils
• Gawis cranium