A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages (clades) diverge. It will have characteristics typical of organisms on both sides of the split, but because of the incompleteness of the fossil record, there is usually no way to know exactly how close it is to the actual point of divergence.
Transitional fossils serve as a reminder that taxonomic divisions are human constructs that have been imposed in hindsight on a continuum of variation. Numerous examples exist, including those between humans and other primates, tetrapods and fish, and birds and dinosaurs. The phrase "missing link" has been used extensively in popular writings on human evolution to refer to a perceived gap in the hominid evolutionary record. It is most commonly used to refer to any new transitional fossil finds. Scientists, however, do not use the term as it is misleading and inaccurate.
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| These diagrams plot the set of Hominine 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. | |
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 relating it to 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] 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 classes of vertebrates are related, much of it in the form of transitional fossils.[3]
The reconstruction of the evolution of the horse and its relatives assembled by Othniel Charles Marsh from surviving fossils that form 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[4] and various hominids considered to be proto-humans.
A middle Devonian precursor to seed plants from Belgium has been identified predating the earliest seed plants by about 20 million years. Runcaria, small and radially symmetrical, is an integumented megasporangium surrounded by a cupule. The megasporangium bears an unopened distal extension protruding above the mutlilobed integument. It is suspected that the extension was involved in anemophilous pollination. Runcaria sheds new light on the sequence of character acquisition leading to the seed. Runcaria has all of the qualities of seed plants except for a solid seed coat and a system to guide the pollen to the seed.[5]
Not every transitional form appears in the fossil record because the fossil record is nowhere near complete. Organisms are only rarely preserved as fossils in the best of circumstances and only a fraction of such fossils have ever been discovered. The paleontologist Donald Prothero noted that this is illustrated by the fact that the total number of species of all kinds known through the fossil record was less than 5% of the number of known living species, which suggests that the number of species known through fossils must be less than 1% of all the species that have ever lived.[6]
The fossil record is very uneven, and with few exceptions are heavily slanted toward organisms with hard parts, leaving most groups of soft-bodied organisms with little to no fossil record.[6] The groups considered to have a good fossil record, including a number of transitional fossils between traditional groups, are the vertebrates, the echinoderms, brachiopods and some groups of arthropods.[7]
In evolutionary taxonomy, the prevailing form of taxonomy during much of the 20th century and still used in basal textbooks, taxa based on morphological similarity are often drawn as "bubbles" branching off from each other, forming evolutionary trees.[8] Transitional forms, are seen as falling between the various groups in term of anatomy, and are 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.
A source of confusion is the concept that a transitional form between two different taxonomic group must be directly ancestral to one or both groups. This was exacerbated by the fact that one of the goals of evolutionary taxonomy was the attempt to identify taxa that were ancestral to other taxa. However, it is almost impossible to be sure that any form represented in the record is actually a direct ancestor of any other. In fact because evolution is a branching process that produces a complex bush pattern of related species rather than a linear process that produces a ladder like progression, and because of the incompleteness the fossil record, it is unlikely that any particular form represented in the fossil record is a direct ancestor of any other. Cladistics deemphasized the concept of one taxonmic group being an ancestor of another, and instead emphasizes the concept of identifying sister taxa that share a common ancestor with one another more recently than they do with other groups. There are a few exceptional cases, such as some marine plankton micro-fossils, where the fossil record is complete enough to suggest with confidence that certain fossils represent a population that was actually ancestral to a later population of a different species, but in general transitional fossils are considered to have features that illustrate the transitional anatomical features of actual common ancestors of different taxa rather than to be actual ancestors.[9]
The terms 'transitional' and 'intermediate' are for the most part used as synonyms; however, a distinction between the two can be made:
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 intermediate between two or more groups.
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 relates to the links in the great chain of being, a static pre-evolutionary concept now abandoned. In reality, the discovery of more and more transitional fossils continues to add to knowledge of evolutionary transitions,[3][10] making many of the "missing links" missing no more (see List of transitional fossils).
The term "missing links" was used by Charles Lyell in a somewhat different way in his Elements of Geology of 1851, but was popularized 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.[11]
The idea of a "missing link" between humans and so-called "lower" animals remains lodged in the public imagination.[12] The concept has been fueled by the successive discoveries of Australopithecus africanus (Taung Child), Australopithecus sediba,[13][14] Homo erectus (Peking Man, Java Man, Turkana boy), and other Hominina fossils.[15][16]
Proponents of creationism have frequently made claims about the existence or implications of transitional fossils that paleontologists consider to be false,[17][18] and in some cases deliberately misleading.[19] Some of these claims include:
"Since we proposed punctuated equilibria to explain trends, it is infuriating to be quoted again and again by creationists – whether through design or stupidity, I do not know – as admitting that the fossil record includes no transitional forms. The punctuations occur at the level of species; directional trends (on the staircase model) are rife at the higher level of transitions within major groups."