Transitional fossil
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A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group.[1] This is especially important where the descendant group is sharply differentiated by gross anatomy and mode of living from the ancestral group. These fossils serve as a reminder that taxonomic divisions are human constructs that have been imposed in hindsight on a continuum of variation. Because of the incompleteness of the fossil record, there is usually no way to know exactly how close a transitional fossil is to the point of divergence. Therefore, it cannot be assumed that transitional fossils are direct ancestors of more recent groups, though they are frequently used as models for such ancestors.[2]
In 1859, when
The term "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 refers to a pre-evolutionary view of nature.
Evolutionary and phylogenetic taxonomy
Transitions in phylogenetic nomenclature
In evolutionary taxonomy, the prevailing form of taxonomy during much of the 20th century and still used in non-specialist textbooks, taxa based on morphological similarity are often drawn as "bubbles" or "spindles" branching off from each other, forming evolutionary trees.[6] Transitional forms are seen as falling between the various groups in terms of anatomy, having a mixture of characteristics from inside and outside the newly branched clade.[7]
With the establishment of cladistics in the 1990s, relationships commonly came to be expressed in cladograms that illustrate the branching of the evolutionary lineages in stick-like figures. The different so-called "natural" or "monophyletic" groups form nested units, and only these are given phylogenetic names. While in traditional classification tetrapods and fish are seen as two different groups, phylogenetically tetrapods are considered a branch of fish. Thus, with cladistics there is no longer a transition between established groups, and the term "transitional fossils" is a misnomer. Differentiation occurs within groups, represented as branches in the cladogram.[8]
In a cladistic context, transitional organisms can be seen as representing early examples of a branch, where not all of the traits typical of the previously known descendants on that branch have yet evolved.[9] Such early representatives of a group are usually termed "basal taxa" or "sister taxa,"[10] depending on whether the fossil organism belongs to the daughter clade or not.[8]
Transitional versus ancestral
A source of confusion is the notion that a transitional form between two different taxonomic groups must be a direct ancestor of one or both groups. The difficulty is exacerbated by the fact that one of the goals of evolutionary taxonomy is to identify taxa that were ancestors of other taxa. However, because evolution is a branching process that produces a complex bush pattern of related species rather than a linear process producing a ladder-like progression, and because of the incompleteness of the fossil record, it is unlikely that any particular form represented in the fossil record is a direct ancestor of any other. Cladistics deemphasizes the concept of one taxonomic group being an ancestor of another, and instead emphasizes the identification of sister taxa that share a more recent common ancestor with one another than they do with other groups. There are a few exceptional cases, such as some marine plankton microfossils, 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.[11] 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.[2]
Prominent examples
Archaeopteryx
Archaeopteryx is a genus of theropod dinosaur closely related to the birds. Since the late 19th century, it has been accepted by palaeontologists, and celebrated in lay reference works, as being the oldest known bird, though a study in 2011 has cast doubt on this assessment, suggesting instead that it is a non-avialan dinosaur closely related to the origin of birds.[12]
It lived in what is now southern Germany in the
The first complete specimen was announced in 1861, and ten more Archaeopteryx fossils have been found since then. Most of the eleven known fossils include impressions of feathers—among the oldest direct evidence of such structures. Moreover, because these feathers take the advanced form of flight feathers, Archaeopteryx fossils are evidence that feathers began to evolve before the Late Jurassic.[16]
Australopithecus afarensis
The hominid Australopithecus afarensis represents an evolutionary transition between modern bipedal humans and their quadrupedal ape ancestors. A number of traits of the A. afarensis skeleton strongly reflect bipedalism, to the extent that some researchers have suggested that bipedality evolved long before A. afarensis.[17] In overall anatomy, the pelvis is far more human-like than ape-like. The iliac blades are short and wide, the sacrum is wide and positioned directly behind the hip joint, and there is clear evidence of a strong attachment for the knee extensors, implying an upright posture.[17]: 122
While the
Pakicetids, Ambulocetus
The
Tiktaalik
Tiktaalik is a genus of extinct sarcopterygian (lobe-finned fish) from the Late Devonian period, with many features akin to those of tetrapods (four-legged animals).[27] It is one of several lines of ancient sarcopterygians to develop adaptations to the oxygen-poor shallow water habitats of its time—adaptations that led to the evolution of tetrapods.[28] Well-preserved fossils were found in 2004 on Ellesmere Island in Nunavut, Canada.[29]
Tiktaalik lived approximately 375 million years ago. Paleontologists suggest that it is representative of the transition between non-tetrapod vertebrates such as Panderichthys, known from fossils 380 million years old, and early tetrapods such as Acanthostega and Ichthyostega, known from fossils about 365 million years old. Its mixture of primitive fish and derived tetrapod characteristics led one of its discoverers, Neil Shubin, to characterize Tiktaalik as a "fishapod."[30][31] Unlike many previous, more fish-like transitional fossils, the "fins" of Tiktaalik have basic wrist bones and simple rays reminiscent of fingers. They may have been weight-bearing. Like all modern tetrapods, it had rib bones, a mobile neck with a separate pectoral girdle, and lungs, though it had the gills, scales, and fins of a fish.[27] However in a 2008 paper by Boisvert at al. it is noted that Panderichthys, due to its more derived distal portion, might be closer to tetrapods than Tiktaalik, which might have independently developed similarities to tetrapods by convergent evolution.[32]
Tetrapod footprints found in Poland and reported in Nature in January 2010 were "securely dated" at 10 million years older than the oldest known elpistostegids[33] (of which Tiktaalik is an example), implying that animals like Tiktaalik, possessing features that evolved around 400 million years ago, were "late-surviving relics rather than direct transitional forms, and they highlight just how little we know of the earliest history of land vertebrates."[34]
Amphistium
Amphistium is a 50-million-year-old fossil fish identified as an early relative of the flatfish, and as a transitional fossil.[36] In Amphistium, the transition from the typical symmetric head of a vertebrate is incomplete, with one eye placed near the top-center of the head.[37] Paleontologists concluded that "the change happened gradually, in a way consistent with evolution via natural selection—not suddenly, as researchers once had little choice but to believe."[36]
Amphistium is among the many fossil fish species known from the Monte Bolca Lagerstätte of Lutetian Italy. Heteronectes is a related, and very similar fossil from slightly earlier strata of France.[37]
Runcaria
A Middle Devonian precursor to
Fossil record
Not every transitional form appears in the
Because of the specialized and rare circumstances required for a biological structure to fossilize, logic dictates that known fossils represent only a small percentage of all life-forms that ever existed—and that each discovery represents only a snapshot of evolution. The transition itself can only be illustrated and corroborated by transitional fossils, which never demonstrate an exact half-way point between clearly divergent forms.[40]
The fossil record is very uneven and, with few exceptions, is heavily slanted toward organisms with hard parts, leaving most groups of soft-bodied organisms with little to no fossil record.[39] The groups considered to have a good fossil record, including a number of transitional fossils between traditional groups, are the vertebrates, the echinoderms, the brachiopods and some groups of arthropods.[41]
History
Post-Darwin
The idea that animal and plant species were not constant, but changed over time, was suggested as far back as the 18th century.[42] Darwin's On the Origin of Species, published in 1859, gave it a firm scientific basis. A weakness of Darwin's work, however, was the lack of palaeontological evidence, as pointed out by Darwin himself. While it is easy to imagine natural selection producing the variation seen within genera and families, the transmutation between the higher categories was harder to imagine. The dramatic find of the London specimen of Archaeopteryx in 1861, only two years after the publication of Darwin's work, offered for the first time a link between the class of the highly derived birds, and that of the more basal reptiles.[43] In a letter to Darwin, the palaeontologist Hugh Falconer wrote:
Had the Solnhofen quarries been commissioned—by august command—to turn out a strange being à la Darwin—it could not have executed the behest more handsomely—than in the Archaeopteryx.[44]
Thus, transitional fossils like Archaeopteryx came to be seen as not only corroborating Darwin's theory, but as icons of evolution in their own right.[45] For example, the Swedish encyclopedic dictionary Nordisk familjebok of 1904 showed an inaccurate Archaeopteryx reconstruction (see illustration) of the fossil, "ett af de betydelsefullaste paleontologiska fynd, som någonsin gjorts" ("one of the most significant paleontological discoveries ever made").[46]
The rise of plants
Transitional fossils are not only those of animals. With the increasing mapping of the divisions of plants at the beginning of the 20th century, the search began for the ancestor of the vascular plants. In 1917, Robert Kidston and William Henry Lang found the remains of an extremely primitive plant in the Rhynie chert in Aberdeenshire, Scotland, and named it Rhynia.[47]
The Rhynia plant was small and stick-like, with simple
Missing links
The idea of all living things being linked through some sort of transmutation process predates Darwin's theory of evolution. Jean-Baptiste Lamarck envisioned that life was generated constantly in the form of the simplest creatures, and strove towards complexity and perfection (i.e. humans) through a progressive series of lower forms.[50] In his view, lower animals were simply newcomers on the evolutionary scene.[51]
After On the Origin of Species, the idea of "lower animals" representing earlier stages in evolution lingered, as demonstrated in Ernst Haeckel's figure of the human pedigree.[52] While the vertebrates were then seen as forming a sort of evolutionary sequence, the various classes were distinct, the undiscovered intermediate forms being called "missing links."
The term was first used in a scientific context by
The search for a fossil showing transitional traits between apes and humans, however, was fruitless until the young Dutch geologist Eugène Dubois found a skullcap, a molar and a femur on the banks of Solo River, Java in 1891. The find combined a low, ape-like skull roof with a brain estimated at around 1000 cc, midway between that of a chimpanzee and an adult human. The single molar was larger than any modern human tooth, but the femur was long and straight, with a knee angle showing that "Java Man" had walked upright.[55] Given the name Pithecanthropus erectus ("erect ape-man"), it became the first in what is now a long list of human evolution fossils. At the time it was hailed by many as the "missing link," helping set the term as primarily used for human fossils, though it is sometimes used for other intermediates, like the dinosaur-bird intermediary Archaeopteryx.[56][57]
While "missing link" is still a popular term, well-recognized by the public and often used in the popular media,[58] the term is avoided in scientific publications.[5] Some bloggers have called it "inappropriate";[59] both because the links are no longer "missing", and because human evolution is no longer believed to have occurred in terms of a single linear progression.[5][60]
Punctuated equilibrium
The theory of punctuated equilibrium developed by Stephen Jay Gould and Niles Eldredge and first presented in 1972[61] is often mistakenly drawn into the discussion of transitional fossils.[62] 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. Gould made the following observation concerning creationist misuse of his work to deny the existence of transitional fossils:
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.
See also
References
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- ^ "Australopithecus afarensis". Human Evolution. Washington, D.C.: Smithsonian Institution's Human Origins Program. Retrieved 15 May 2015.
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- ^ Wilford, John Noble (5 April 2006). "Scientists Call Fish Fossil the 'Missing Link'". The New York Times. Retrieved 17 May 2015.
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- ^ Donovan & Paul 1998
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- Kalmbach Publishing. Retrieved 10 September 2011.
- ^ Sambrani, Nagraj (10 June 2009). "Why the term 'missing links' is inappropriate". Biology Times (Blog). Retrieved 19 May 2015.
- ^ "Newly found fossils could link to human ancestor". CBC News. Ottawa, Ontario, Canada: Canadian Broadcasting Corporation. 8 April 2010. Retrieved 19 May 2015.
It's tempting to call the new species a 'missing link' between earlier species and modern humans, but scientists say the concept no longer applies, given new knowledge of human evolution. [...] Researchers now say the evolution of humans consisted of a number of diverse species in many branches, not a single smooth line from ape-like species to humans.
- ^ Eldredge & Gould 1972, pp. 82–115
- ISSN 0819-1530. Retrieved 6 July 2014.
- Theunissen, Lionel (24 June 1997). "Patterson Misquoted: A Tale of Two 'Cites'". TalkOrigins Archive. Houston, TX: The TalkOrigins Foundation, Inc. Retrieved 19 May 2015.
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External links
- Lloyd, Robin (11 February 2009). "Fossils Reveal Truth About Darwin's Theory". Purch. Retrieved 19 May 2015.
- Hunt, Kathleen (17 March 1997). "Transitional Vertebrate Fossils FAQ". TalkOrigins Archive. Houston, TX: The TalkOrigins Foundation, Inc. Retrieved 19 May 2015.
- "Tiktaalik roseae". Chicago, IL: University of Chicago. Retrieved 19 May 2015.
- "Whales Tohorā". Wellington, New Zealand: Museum of New Zealand Te Papa Tongarewa. Retrieved 19 May 2015.
- Hutchinson, John R. (22 January 1998). "Are Birds Really Dinosaurs?". DinoBuzz. Berkeley, CA: University of California Museum of Paleontology. Retrieved 19 May 2015.