Transitional fossil

From Wikipedia, the free encyclopedia

 This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (December 2007)

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. 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.

1850 Described Hominin species, 1850
1900 Described Hominin species, 1900
1950 Described Hominin species, 1950
2002 Selection of described Hominin species, 2002
These diagrams plot the set of Hominin species known to science as of a given year. Each species is plotted as a box showing the range of cranial capacities for specimens of that species, and the range of dates at which specimens appear in the fossil record. The sequence of diagrams shows how an apparent "missing link" or gap between species in the fossil record may become filled as more fossil discoveries are made.

Contents

[edit] Transitional fossils and 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.[1] 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.[2][3] Indeed, Archaeopteryx was discovered just two years later, in 1861, and represents a classic transitional form between dinosaurs and birds.[3]

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.[3]

[edit] Examples of transitional fossils

See also: Evolution of the horse, Evolution of cetaceans, and 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[4] and various hominids considered to be proto-humans.

[edit] 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.

[edit] 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.

[edit] Misconceptions

It is commonly claimed by critics of evolution that there are no transitional fossils.[5][3][6] Such claims may be based on a misunderstanding of the nature of what represents a transitional feature[5] or may be an active tactic by creationists seeking to distort or ignore the evidence that exists.[3] The claim has been called a "favourite lie" of creationists by Donald Prothero which is "manifestly untrue".[3]

A common creationist argument is that no fossils are found with partially functional features, although this is more to do with irreducible complexity than transitional fossils. 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 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 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.[citation needed]

[edit] Missing links

A popular term used to designate transitional forms is "missing links". The term is especially used in the regular media, but is inaccurate and confusing. In reality, the continuing discovery of more and more transitional fossils is further adding to the 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 discovery of Australopithecus africanus (Taung Child), Java Man, Homo erectus, Sinanthropus pekinensis (Peking Man), etc. are also vital to the study of links.

[edit] Footnotes

  1. ^ Darwin 1859, p. 279-280
  2. ^ Darwin 1859, p. 341-343
  3. ^ a b c d e f New Scientist, 27 February 2008, Issue 2645, pages 35-40, "Evolution: What missing link?" by Donald Prothero
  4. ^ Shubin, Neil (2008). Your Inner Fish. Pantheon. ISBN 9780375424472. 
  5. ^ a b Claim CC200 at www.talkorigins.org. Retrieved on 2007-06-19.
  6. ^ [The Scientific Case for Creation http://www.creationscience.com/onlinebook/LifeSciences27.html], Life Sciences, 23. Fossil Gaps

[edit] References

[edit] See also

[edit] External links