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Atransitional fossil is anyfossilized 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 bygross anatomy and mode of living from the ancestral group. These fossils serve as a reminder that taxonomic divisions arehuman 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, whenCharles Darwin'sOn the Origin of Species was first published, the fossil record was poorly known. Darwin described the perceived lack of transitional fossils as "the most obvious and gravest objection which can be urged against my theory," but he explained it by relating it to the extreme imperfection of the geological record.[3] He noted the limited collections available at the time but described the available information as showing patterns that followed from his theory ofdescent with modification throughnatural selection.[4] Indeed,Archaeopteryx was discovered just two years later, in 1861, and represents a classic transitional form between earlier, non-aviandinosaurs andbirds.Many more transitional fossils have been discovered since then, and there is now abundant evidence of how allclasses ofvertebrates are related, including many transitional fossils.[5] Specific examples of class-level transitions are:tetrapods andfish,birds and dinosaurs, and theevolution of mammals from"mammal-like reptiles".
The term "missing link" has been used extensively in popular writings onhuman evolution to refer to a perceived gap in thehominid 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.
In evolutionary taxonomy, the prevailing form oftaxonomy 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 branchedclade.[7]
With the establishment ofcladistics in the 1990s, relationships commonly came to be expressed incladograms 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 givenphylogenetic 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 amisnomer. 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]
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 relatedspecies 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 marineplanktonmicrofossils, 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 tobe actual ancestors.[2]
Archaeopteryx is agenus oftheropod 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 theLate Jurassicperiod around 150 million years ago, when Europe was anarchipelago in a shallow warm tropical sea, much closer to the equator than it is now. Similar in shape to aEuropean magpie, with the largest individuals possibly attaining the size of araven,[13]Archaeopteryx could grow to about 0.5 metres (1.6 ft) in length. Despite its small size, broad wings, and inferred ability to fly or glide,Archaeopteryx has more in common with other smallMesozoic dinosaurs than it does with modern birds. In particular, it shares the following features with thedeinonychosaurs (dromaeosaurs andtroodontids): jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes ("killing claw"),feathers (which suggesthomeothermy), and various skeletal features.[14] These features makeArchaeopteryx a clear candidate for a transitional fossil between dinosaurs and birds,[15] making it important in the study both of dinosaurs and of the origin of birds.
The first complete specimen was announced in 1861, and ten moreArchaeopteryx 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 offlight feathers,Archaeopteryx fossils are evidence that feathers began to evolve before the Late Jurassic.[16]
The hominidAustralopithecus afarensis represents an evolutionary transition between modern bipedal humans and their quadrupedalape ancestors. A number of traits of theA. afarensis skeleton strongly reflect bipedalism, to the extent that some researchers have suggested that bipedality evolved long beforeA. afarensis.[17] In overall anatomy, the pelvis is far more human-like than ape-like. Theiliac 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 theknee extensors, implying an upright posture.[17]: 122
While thepelvis is not entirely like that of a human (being markedly wide, or flared, with laterally orientated iliac blades), these features point to a structure radically remodelled to accommodate a significant degree ofbipedalism. Thefemur angles in toward the knee from thehip. This trait allows the foot to fall closer to the midline of the body, and strongly indicates habitual bipedal locomotion. Present-day humans,orangutans andspider monkeys possess this same feature. The feet featureadducted big toes, making it difficult if not impossible to grasp branches with thehindlimbs. Besides locomotion,A. afarensis also had a slightly larger brain than a modernchimpanzee[18] (the closest living relative of humans) and had teeth that were more human than ape-like.[19]
Thecetaceans (whales, dolphins and porpoises) aremarine mammal descendants of landmammals. Thepakicetids are anextinctfamily of hoofed mammals that are the earliest whales, whose closest sister group isIndohyus from the familyRaoellidae.[20][21] They lived in the EarlyEocene, around 53 million years ago. Their fossils were first discovered in North Pakistan in 1979, at a river not far from the shores of the formerTethys Sea.[22][page needed] Pakicetids could hear under water, using enhanced bone conduction, rather than depending ontympanic membranes like most land mammals. This arrangement does not give directional hearing under water.[23]
Ambulocetus natans, which lived about 49 million years ago, was discovered in Pakistan in 1994. It was probably amphibious, and looked like acrocodile.[24] In the Eocene,ambulocetids inhabited the bays and estuaries of the Tethys Ocean in northern Pakistan.[25] The fossils of ambulocetids are always found in near-shore shallow marine deposits associated with abundant marine plant fossils andlittoralmolluscs.[25] Although they are found only in marine deposits, their oxygen isotope values indicate that they consumed water with a range of degrees of salinity, some specimens showing no evidence of sea water consumption and others none of fresh water consumption at the time when their teeth were fossilized. It is clear that ambulocetids tolerated a wide range of salt concentrations.[26] Their diet probably included land animals that approached water for drinking, or freshwater aquatic organisms that lived in the river.[25] Hence, ambulocetids represent the transition phase of cetacean ancestors between freshwater and marine habitat.
Tiktaalik is a genus of extinctsarcopterygian (lobe-finned fish) from the LateDevonian 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 onEllesmere Island inNunavut, Canada.[29]
Tiktaalik lived approximately 375 million years ago.Paleontologists suggest that it is representative of the transition between non-tetrapod vertebrates such asPanderichthys, known from fossils 380 million years old, and early tetrapods such asAcanthostega andIchthyostega, 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 characterizeTiktaalik as a "fishapod."[30][31] Unlike many previous, more fish-like transitional fossils, the "fins" ofTiktaalik have basic wrist bones and simple rays reminiscent of fingers. They may have beenweight-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 et al. it is noted thatPanderichthys, due to its more derived distal portion, might be closer to tetrapods thanTiktaalik, which might have independently developed similarities to tetrapods by convergent evolution.[32]
Tetrapod footprints found in Poland and reported inNature in January 2010 were "securely dated" at 10 million years older than the oldest knownelpistostegids[33] (of whichTiktaalik is an example), implying that animals likeTiktaalik, 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]
Pleuronectiformes (flatfish) are anorder ofray-finned fish. The most obvious characteristic of the modern flatfish is their asymmetry, with both eyes on the same side of the head in the adult fish. In some families the eyes are always on the right side of the body (dextral or right-eyed flatfish) and in others they are always on the left (sinistral or left-eyed flatfish). The primitivespiny turbots include equal numbers of right- and left-eyed individuals, and are generally less asymmetrical than the other families. Other distinguishing features of the order are the presence of protrusible eyes, another adaptation to living on theseabed (benthos), and the extension of the dorsal fin onto the head.[35]
Amphistium is a 50-million-year-old fossil fish identified as an early relative of the flatfish, and as a transitional fossil.[36] InAmphistium, 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 vianatural selection—not suddenly, as researchers once had little choice but to believe."[36]
Amphistium is among the many fossil fish species known from theMonte BolcaLagerstätte ofLutetian Italy.Heteronectes is a related, and very similar fossil from slightly earlier strata of France.[37]
A Middle Devonian precursor toseed plants has been identified from Belgium, predating the earliest seed plants by about 20 million years.Runcaria, small and radially symmetrical, is an integumentedmegasporangium surrounded by acupule. The megasporangium bears an unopeneddistal extension protruding above the multilobedintegument. It is suspected that the extension was involved inanemophilous pollination.Runcaria sheds new light on the sequence of character acquisition leading to the seed, having all the qualities of seed plants except for a solidseed coat and a system to guide the pollen to the seed.[38]
Not every transitional form appears in thefossil record, because the fossil record is not complete. Organisms are only rarely preserved as fossils in the best of circumstances, and only a fraction of such fossils have been discovered. PaleontologistDonald Prothero noted that this is illustrated by the fact that the number of species known through the fossil record was less than 5% of the number of known living species, suggesting that the number of species known through fossils must be far less than 1% of all the species that have ever lived.[39]
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 ofsoft-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, theechinoderms, thebrachiopods and some groups ofarthropods.[41]
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'sOn 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 imaginenatural selection producing the variation seen withingenera and families, the transmutation between the higher categories was harder to imagine. The dramatic find of theLondon specimen ofArchaeopteryx in 1861, only two years after the publication of Darwin's work, offered for the first time a link between theclass of the highly derived birds, and that of the more basalreptiles.[43] In a letter to Darwin, the palaeontologistHugh 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 theArchaeopteryx.[44]
Thus, transitional fossils likeArchaeopteryx came to be seen as not only corroborating Darwin's theory, but as icons of evolution in their own right.[45] For example, the Swedishencyclopedic dictionaryNordisk familjebok of 1904 showed an inaccurateArchaeopteryx 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]
Transitional fossils are not only those of animals. With the increasing mapping of thedivisions of plants at the beginning of the 20th century, the search began for the ancestor of thevascular plants. In 1917,Robert Kidston andWilliam Henry Lang found the remains of an extremely primitive plant in theRhynie chert inAberdeenshire, Scotland, and named itRhynia.[47]
TheRhynia plant was small and stick-like, with simpledichotomously branching stems without leaves, each tipped by asporangium. The simple form echoes that of thesporophyte ofmosses, and it has been shown thatRhynia had analternation of generations, with a correspondinggametophyte in the form of crowded tufts of diminutive stems only a few millimetres in height.[48]Rhynia thus falls midway between mosses and early vascular plants likeferns andclubmosses. From a carpet of moss-like gametophytes, the largerRhynia sporophytes grew much like simple clubmosses, spreading by means of horizontal growing stems growingrhizoids that anchored the plant to the substrate. The unusual mix of moss-like and vascular traits and the extreme structural simplicity of the plant had huge implications for botanical understanding.[49]
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 strovetowards 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]
AfterOn the Origin of Species, the idea of "lower animals" representing earlier stages in evolution lingered, as demonstrated inErnst Haeckel's figure of the human pedigree.[52] While the vertebrates were then seen as forming a sort of evolutionary sequence, the variousclasses were distinct, the undiscovered intermediate forms being called "missing links."
The term was first used in a scientific context byCharles Lyell in the third edition (1851) of his bookElements of Geology in relation to missing parts of thegeological column, but it was popularized in its present meaning by its appearance on page xi of his bookGeological Evidences of the Antiquity of Man of 1863. By that time, it was generally thought that the end of thelast glacial period marked the first appearance of humanity; Lyell drew on new findings in hisAntiquity of Man to put the origin of human beings much further back. Lyell wrote that it remained a profound mystery how the huge gulf between man and beast could be bridged.[53] Lyell's vivid writing fired the public imagination, inspiringJules Verne'sJourney to the Center of the Earth (1864) andLouis Figuier's 1867 second edition ofLa Terre avant le déluge ("Earth before the Flood"), which included dramatic illustrations of savage men and women wearing animal skins and wielding stone axes, in place of theGarden of Eden shown in the 1863 edition.[54]
The search for a fossil showing transitional traits between apes and humans, however, was fruitless until the young Dutch geologistEugène Dubois found a skullcap, a molar and afemur on the banks ofSolo River,Java in 1891. The find combined a low, ape-likeskull 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 namePithecanthropus erectus ("erect ape-man"), it became the first in what is now a longlist 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 intermediaryArchaeopteryx.[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]
The theory of punctuated equilibrium developed byStephen Jay Gould andNiles 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 bycreationists—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.
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.
