y studying the standard phylogenetic tree, it can be seen that every species has a unique genealogical history. Each species has a unique series of common ancestors linking it to the original common ancestor. We should expect that organisms carry evidence of this history and ancestry with them. The standard phylogenetic tree predicts what historical evidence is possible and what is impossible for each given species.
Some of the more renowned evidences for evolution are the explanations it provides for nonfunctional or rudimentary vestigial characters, both anatomical and molecular. Throughout macroevolutionary history, functions necessarily have been gained and lost. Thus, from common descent and the constraint of gradualism, we predict that many organisms should display vestigial structures, which are structural remnants of lost functions. Since there is no apparent reason for their existence, nonfunctional characters can be especially puzzling features of organisms. So are rudimentary structures, which have different and relatively minor functions compared to the same more developed structures in other organisms.
There are many examples of nearly useless or nonfunctional characters of organisms, and these can very often be explained in terms of evolutionary histories. Some examples are the vestigial pelvises of pythons, the rudimentary, nonfunctional legs found in some species of lizards, the eyes displayed by cave dwelling fishes in every stage of degeneration, the flowers of dandelions, human wisdom teeth, and the wings of flightless beetles retained underneath fused wing covers. All of these examples can be explained in terms of the functions and structures of the organism's predicted ancestors (Futuyma 1998, pp. 122-123).
No organism can have a vestigial structure that was not previously functional in one of its ancestors. Thus, for each species, the standard phylogenetic tree makes a huge number of predictions about vestigial characters that are allowed and those that are impossible for any given species.
Shared derived characters and molecular sequence data, not vestigial characters, determine the phylogeny and the characteristics of predicted common ancestors. Thus, if common descent is false, vestigial characters very possibly could lack an evolutionary explanation. For example, whales are classified as mammals according to many criteria, such as having mammary glands, a placenta, one bone in the lower jaw, etc. Snakes likewise are classified as reptiles by several other derived features. However, it is theoretically possible that snakes or whales could have been classified as fish (as Linnaeus originally did). If this were the case, the vestigial legs of whales or the vestigial pelvises of snakes would make no sense evolutionarily and would be inconsistent with common descent.
It follows, then, that we should never find a vestigial placenta or vestigial nipples in any amphibians, birds, or reptiles. No mammals should be found with vestigial feathers (but they can have vestigial tails, as humans do). We should never find any arthropods with vestigial backbones.
Note that this prediction is not invalidated by finding a function for the presumed vestigial structure. Should this happen, the structure merely becomes an example of paralogy, which is considered in prediction 11.
Anatomical atavisms are closely related conceptually to vestigial structures. An atavism is the reappearance of a lost character specific to a remote evolutionary ancestor and not observed in the parents or recent ancestors of the organism displaying the atavistic character. Atavisms have several essential features: (1) presence in adult stages of life, (2) absence in parents or recent ancestors, and (3) extreme rarity in a population (Hall 1984). Of course, without an evolutionary perspective we could not state that an atavism is a structure that was once found in a remote ancestor but has been lost in a recent lineage. Therefore, here we are primarily concerned with potential atavistic structures that are characteristic of taxa to which the organism displaying the structure does not belong. As a hypothetical example, if mutant horses occasionally displayed gills, this would be considered a potential atavism, since gills are diagnostic of taxa (e.g. fish) to which horses do not belong. As with vestigial structures, no organism can have an atavistic structure that was not previously found in one of its ancestors. Thus, for each species, the standard phylogenetic tree makes a huge number of predictions about atavisms that are allowed and those that are impossible for any given species. Atavisms were once considered an "embarrassment" by evolutionary biologists, but now that developmental processes are much better understood their occasional occurrence is expected under common descent if structures or functions are lost between ancestor and descendant lineages (Hall 1984; Hall 1995).
Probably the most well known case of atavism is found in the whales. According to the standard phylogenetic tree, whales are known to be the descendants of terrestrial mammals that had hindlimbs. Thus, we expect the possibility that rare mutant whales might occasionally develop atavistic hindlimbs. In fact, there are many cases where whales have been found with rudimentary atavistic hindlimbs in the wild (for reviews see Burzin 1972, pp. 65-67 and Hall 1984, pp. 90-93). Hindlimbs have been found in baleen whales (Sleptsov 1939), humpback whales (Andrews 1921) and in many specimens of sperm whales (Abel 1907; Berzin 1972, p. 66; Nemoto 1963; Ogawa and Kimiya 1957; Zemskii and Berzin 1961). Most of these examples are of whales with femurs, tibia, and fibulae; however, some even include feet with complete digits.
Many other famous examples of atavisms exist, including (1) rare formation of extra toes (2nd and 4th digits) in horses, similar to what is seen in the archaic horses Mesohippus and Merychippus, (2) atavistic thigh muscles in Passeriform birds and sparrows, (3) hyoid muscles in dogs, (4) wings in earwigs (normally wingless), (5) atavistic fibulae in birds (the fibulae are normally extremely reduced), (6) extra toes in guinea pigs and salamanders, (6) the atavistic dew claw in many dog breeds, (7) dental enamel in the beaks of birds, and (8) various atavisms in humans (one described in detail below) (Hall 1984).
Primarily due to intense medical interest, humans are one of the best characterized species and many developmental anomalies are known. There are several human atavisms that reflect our common genetic heritage with other mammals. One of the most striking is the existence of the rare "true human tail" (also variously known as "coccygeal process," "coccygeal projection," "caudal appendage," and "vestigial tail"). More than 100 cases of human tails have been reported in the medical literature (Matsuo et al. 1993). Less than one third of the well-documented cases are what are medically known as "pseudo-tails" (Dubrow et al. 1988). Pseudo-tails are not true tails; they are simply lesions of various types coincidentally found in the caudal region of newborns, often associated with the spinal column, coccyx, and various malformations. In contrast, the true atavistic tail of humans develops from the most distal end of the embryonic tail found in the developing human fetus (Belzberg et al. 1991; Dao and Netsky 1984), and it is usually benign in nature (Dubrow et al. 1988; Spiegelmann et al. 1985). The true human tail is characterized by a complex arrangement of adipose and connective tissue, central bundles of longitudinally arranged striated muscle in the core, blood vessels, nerve fibres, nerve ganglion cells, and specialized pressure sensing nerve organs (Vater-Pacini corpuscles). It is covered by normal skin, replete with hair follicles, sweat glands, and sebaceous glands (Dao and Netsky 1984; Dubrow et al. 1988; Spiegelmann et al. 1985). True human tails range in length from about one inch to over 5 inches long (on a newborn baby), and they can move and contract (Dao and Netsky 1984; Lundberg et al. 1962). Although they usually lack skeletal structures, they have also been found with cartilage and multiple articulating vertebrae (Fara 1977; Matsuo et al. 1993). Caudal vertebrae are not a necessary component of mammalian tails; contrary to what is frequently reported in the medical literature, there is at least one known example of a primate tail which lacks vertebrae, as found in the rudimentary two-inch-long tail of Macaca sylvanus (the "Barbary ape") (Hill 1974, p. 616; Hooten 1947, p. 23). True human tails are rarely inherited, though familial cases are known (Dao and Netsky 1984; Standfast 1992). As with other atavistic structures, human tails are most likely the result of either a somatic or germline mutation that reactivates an underlying developmental pathway which has been retained in the human genome (Dao and Netsky 1984; Hall 1984; Hall 1995).
It should be noted here that the existence of true human tails is quite shocking for many religiously motivated anti-evolutionists, such as Duane Gish, who has written an often-quoted article entitled "Evolution and the human tail" (Gish 1983; see also Menton 1994). Solely based on the particulars of a single case study (Ledley 1982), Gish has erroneously concluded that atavistic human tails are "nothing more than anomalous malformations not traceable to any imaginary ancestral state." However, Gish's arguments are clearly directed against pseudo-tails, not true tails, since true human tails are complex structures which have muscle, blood vessels, occasional vertebrae and cartilage (Fara 1977; Matsuo et al. 1993), they can move and contract, and they are occasionally inherited (Dao and Netsky 1984; Standfast 1992). Furthermore, Gish argues that human vestigial tails are not true tails if they lack vertebrae - an erroneous claim since M. sylvanus is a primate whose fleshy tail also lacks vertebrae (Hill 1974, p. 616; Hooten 1947, p. 23).
These are essentially the same as for vestigial structures above.
Vestigial characters should also be found at the molecular level. Humans do not have the capability to synthesize ascorbic acid (otherwise known as Vitamin C), and the unfortunate consequence can be the nutritional deficiency called scurvy. However, the predicted ancestors of humans had this function (as do most other animals except primates and guinea pigs). Therefore, we predict that humans, other primates, and guinea pigs should carry evidence of this lost function as a molecular vestigial character.
Recently, the L-gulano-g-lactone oxidase gene, the gene required for Vitamin C synthesis, was found in humans and guinea pigs (Nishikimi, Kawai et al. 1992; Nishikimi, Fukuyama et al. 1994). It exists as a pseudogene, present but incapable of functioning (see prediction 20 for more about pseudogenes).
There are several other examples of vestigial human genes, including multiple odorant receptor genes (Rouquier, Blancher et al. 2000), the RT6 protein gene (Haag, Koch-Nolte et al. 1994), the galactosyl transferase gene (Galili and Swanson 1991), and the tyrosinase-related gene (TYRL) (Oetting, Stine et al. 1993).
Our odorant receptor (OR) genes once coded for proteins involved in now lost olfactory functions. Our predicted ancestors, like other mammals, had a more acute sense of smell than we do now; humans have >99 odorant receptor genes, of which ~70% are pseudogenes. Many other mammals, such as mice and marmosets, have many of the same OR genes as us, but all of theirs actually work. An extreme case is the dolphin, which is the descendant of land mammals. It no longer has any need to smell volatile odorants, yet it contains many OR genes, of which none are functional – they are all pseudogenes (Freitag, Ludwig et al. 1998).
The RT6 protein is expressed on the surface of T lymphocytes in other mammals, but not on ours. The galactosyl transferase gene is involved in making a certain carbohydrate found on the cell membranes of other mammals. Tyrosinase is the major enzyme responsible for melanin pigment in all animals. TYRL is a pseudogene of tyrosinase.
It is satisfying to note that we share these vestigial genes with other primates, and that the mutations that made these genes nonfunctional are also shared with several other primates (see predictions 19-21 for more about shared nonfunctional characters).
It would be very puzzling if we had not found the L-gulano-g-lactone oxidase
pseudogene or the other vestigial genes mentioned. In addition, we can predict
that we will never find vestigial chloroplast genes in any metazoans (i.e.
animals) (Li 1997, pp. 284-286, 348-354).
Figure 2.8.1. Cat and human embryos in the tailbud stage. A cat embryo is shown on top, a human embryo below. Note the post-anal tail in both, positioned at the lower left below the head of each. The human embryo is about 32 days old.
Embryology and developmental biology have provided some fascinating insights into evolutionary pathways. Since the cladistic morphological classification of species is generally based on derived characters of adult organisms, embryology and developmental studies provide a nearly independent body of evidence.
The ideas of Ernst Haeckel greatly influenced the early history of embryology; however, his ideas have been superseded by those of Karl Ernst von Baer, his predecessor. Von Baer suggested that the embryonic stages of an individual should resemble the embryonic stages of its ancestors (rather than resembling its adult ancestors, a la Haeckel). The final adult structure of an organism is the product of numerous cumulative developmental processes; for species to evolve, there necessarily must have been change in these developmental processes. The macroevolutionary conclusion is that the development of an organism is a modification of its ancestors' ontogenies (Futuyma 1998, pp. 652-653). The modern developmental maxim is the inverse of Haeckel's biogenetic law. "Phylogeny recapitulates Ontogeny," not the opposite. Walter Garstang stated even more correctly that ontogeny creates phylogeny. What this means is that once given knowledge about an organism's ontogeny, we can confidently predict certain aspects of the historical pathway that was involved in this organism's evolution (Gilbert 1997, pp. 912-914). Thus, embryology provides confirmations and predictions about evolution.
From embryological studies it is known that two bones of a developing reptile eventually form the quadrate and the articular bones in the hinge of the adult reptilian jaw. However, in the marsupial mammalian embryo, the same two structures develop, not into parts of the jaw, but into the anvil and hammer of the mammalian ear. This indicates that during their evolution, the mammalian middle ear bones were derived and modified from the reptilian jaw bones (Gilbert 1997, pp. 894-896).
Accordingly, there is a very complete series of fossil intermediates in which these structures are clearly modified from the reptilian jaw to the mammalian ear (see Figures 2.9.1 and 2.9.2; also compare the intermediates discussed in prediction 4) (Carroll 1988, pp. 392-396; Futuyma 1998, pp. 146-151; Gould 1990; Kardong 2002, pp. 255-275).
There are numerous other examples where an organism's evolutionary history is represented temporarily in its development, such as mammalian pharyngeal pouches (which are indistinguishable from aquatic vertebrate's gill pouches) and avian teeth (Gilbert 1997, pp. 380, 382).
Based on our standard phylogenetic tree, we may expect to find gill pouches
or egg shells at some point in mammalian embryonic development (and we do), and
we may expect to find human embryos with tails (and we do; see Figure 2.8.1).
However, we never expect to find nipples, hair, or a placenta at any point in
fish, amphibian, or reptilian embryos. Likewise, we might expect to find teeth
in the mouths of some avian embryos (as we do), but we never expect to find
beaks in eutherian mammal embryos (Gilbert 1997, esp. Ch. 23).
Because species divergence happens not only in the time dimension, but also in spatial dimensions, common ancestors originate in a particular geographical location. Thus, the spatial and geographical distribution of species should be consistent with their predicted genealogical relationships. The standard phylogenetic tree predicts that new species must originate close to the older species from which they are derived. Closely related contemporary species should be close geographically, regardless of their habitat or specific adaptations. If they are not, there had better be a good explanation, such as extreme mobility (cases like sea animals, birds, human mediated distribution, etc.), continental drift, or extensive time since their divergence. In this sense, the present biogeographical distribution of species should reflect the history of their origination.
A reasonable nonevolutionary prediction is that species should occur wherever their habitat is. However, macroevolution predicts just the opposite – there should be many locations where a given species would thrive yet is not found there, due to geographical barriers (Futuyma 1998, pp. 201-203).
With few exceptions, marsupials only inhabit Australia. The exceptions (some South American species and the opossum) are explained by continental drift (South America, Australia, and Antarctica were once the continent of Gondwanaland). Conversely, placental mammals are virtually absent on Australia, despite the fact that many would flourish there. Humans introduced most of the few placentals found on Australia, and they have spread rapidly.
Similarly, the southern reaches of South America and Africa and all of Australia share lungfishes, ostrich-like birds (ratite birds), and leptodactylid frogs – all of which occur nowhere else. Alligators, some related species of giant salamander, and magnolias only occur in Eastern North America and East Asia (these two locations were once spatially close in the Laurasian continent).
In addition, American, Saharan and Australian deserts have very similar habitats, and plants from one grow well in the other. However, indigenous Cacti only inhabit the Americas, while Saharan and Australian vegetation is very distantly related (mostly Euphorbiaceae). Humans introduced the only Cacti found in the Australian outback, and they grow quite well in their new geographical location.
The west and east coast of South America is very similar in habitat, but the marine fauna is very different. In addition, members of the closely related pineapple family inhabit many diverse habitats (such as rainforest, alpine, and desert areas), but only in the American tropics, not African or Asian tropics (Futuyma 1998, ch. 8).
From a limited knowledge of species distributions, we predict that we should
never find elephants on any Pacific islands, even though they would survive well
there. Similarly, we predict that we should not find amphibians on remote
islands, or indigenous Cacti on Australia. Closely related species could be
distributed evenly worldwide, according to whichever habitat best suits them. If
this were the general biogeographical pattern, it would be a strong blow to
macroevolution (Brown 1998).
Past biogeography, as recorded by the fossils that are found, must also conform to the standard phylogenetic tree.
As one example, we conclude that fossils of the hypothetical common ancestors of South American marsupials and Australian marsupials should be found dating from before these two landmasses separated.
Consequently, we find the earliest marsupial fossils (e.g., Alphadon) from the Late Cretaceous, when South America, Antarctica, and Australia were still connected. Additionally, the earliest ancestors of modern marsupials are actually found on North America. The obvious paleontological deduction is that extinct marsupials fossil organisms should be found on South America and Antarctica, since marsupials must have traversed these continents to reach their present day location in Australia. Interestingly, we have found marsupial fossils on both South America and on Antarctica. This is an astounding macroevolutionary confirmation, given that no marsupials live on Antarctica now (Woodburne and Case 1996).
We confidently predict that fossils of recently evolved animals like apes and elephants should never be found on South America, Antarctica, or Australia (excepting, of course, the apes that travel by boat).
As a second example, very complete fossil records should be smoothly connected geographically. Intermediates should be found close to their fossil ancestors.
The Equidae (i.e. horse) fossil record is very complete (though extremely complex) and makes very good geographical sense, without any large spatial jumps between intermediates. For instance, at least ten intermediate fossil horse genera span the past 58 million years. Each fossil genus spans approximately 5 million years, and each of these genera includes several intermediate paleospecies (usually 5 or 6 in each genus) that link the preceding and following fossil intermediates. They range from the earliest genus, Hyracotherium, which somewhat resembled a dog, through Orohippus, Epihippus, Mesohippus, Miohippus, Parahippus, Merychippus, Dinohippus, Equus, to Modern Equus. Every single one of the fossil ancestors of the modern horse are found on the North American continent (MacFadden 1992, pp. 99, 156-162). For more detail about the known evolution of the Equidae, consult Kathleen Hunt's thorough FAQ on Horse Evolution.
It would be macroevolutionarily devastating if we found in South America an irrefutable Epihippus or Merychippus (or any of the intermediates in-between) from the Paleocene, Eocene, Oligocene, the Miocene, or anytime before the Isthmus of Panama arose to connect North and South America (about 12 million years ago). Moreover, we should never find fossil horse ancestors on Australia or Antarctica from any geological era (MacFadden 1992; Brown 1998).
As our third example, consider the African apes. Humans are most closely related to the great apes that are indigenous to Africa (as determined by cladistic morphological analysis and confirmed by DNA sequence analysis). Why did the Leakeys, Raymond Dart, and Robert Broom go to Africa in search of early hominid fossils? Why not dig in Australia, North America, South America, Siberia, or Mesopotamia? Charles Darwin gave an answer for this question over 130 years ago - long before any early hominid fossils had been found.
"We are naturally led to enquire, where was the birthplace of man at that stage of descent when our progenitors diverged from the Catarrhine stock? The fact that they belonged to this stock clearly shews that they inhabited the Old World; but not Australia nor any oceanic island, as we may infer from the laws of geographical distribution. In each great region of the world the living mammals are closely related to the extinct species of the same region. It is therefore probable that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; and as these two species are now man's nearest allies, it is somewhat more probable that our early progenitors lived on the African continent than elsewhere." (Darwin 1871, p. 161)
Thus, the theory of common descent predicts that we may find early hominid fossils on the African continent.
Numerous transitional fossils between humans and the great apes have been found in southern and eastern Africa. For examples, discussion, pictures, detail, and extensive references refer to Jim Foley's comprehensive Fossil Hominid's FAQ. These examples include such fossil species as Ardipithecus ramidus, Australopithecus anamensis, Australopithecus afarensis, Australopithecus garhi, Kenyanthropus platyops, Kenyanthropus rudolfensis, Homo habilis, and a host of other transitionals thought to be less related to Homo sapiens, such as the robust australopithicenes. At this point in time, the difficulty in reconstructing exact genealogical relationships among all of these fossils species is that there are too many links, not that there are missing links. Like most family trees, the family tree of the hominids is best described as a wildly branching bush.
We do not expect to ever find any Australopithicus, Ardipithecus, or Kenyanthropus fossils in Australia, North America, South America, Antarctica, Siberia, or on any oceanic islands removed from Africa. Any such findings would be catastrophically problematic for the theory of common descent.
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