Evolutionary biogeography addresses the historical relationship between geographic space and the processes of biological differentiation, such as speciation and adaptation. Darwin observed that the evolution of related species in different locations required that they also share a common ancestral location he called the “center of origin.” Darwin thought this requirement was so obvious that it constituted a self-evident truth and to call it into question was to appeal to the agency of a miracle. The occurrence of related species in different locations, especially those considered to be separated by geographic or environmental barriers, was explained as the result of their having migrated away from their original centers of origin according to their individual abilities to disperse (walking, flying, rafting, floating, and so on). Dispersal ability was seen to be the key to geographic distribution, and biogeographic evolution was simply a compendium of unique, individual, and unrelated dispersal events. This perspective justified each group being explored in isolation, whether upon the static geography of Darwin’s time or the currently accepted plate tectonic theory of geological history.
Evolutionists following in Darwin’s footsteps did not question his theory of evolution through centers of origin and dispersal. The science of biogeography was reduced to the practice of creating historical narratives or stories about imagined centers of origin and dispersal routes for each individual group of organisms. These stories were constructed according to prevailing beliefs about evolutionary age, dispersal ability, geological and ecological history, and most important, particular beliefs about how the center of origin could be identified. A variety of contradictory criteria were theorized to identify the center of origin, among the most popular being the location of the oldest fossil or the most primitive (and therefore oldest) member of the evolutionary group. Biogeographic narratives were always a product of prevailing beliefs and knowledge never advanced beyond what was already presumed to be known from geological or ecological history. In this role, biogeography is rendered, at best, a subdiscipline of ecology or systematics, and not a very informative one at that.
Darwin’s theory of geographic evolution faced its first serious challenge from Leon Croizat, who was perhaps the first biogeographer to formally recognize geographic location as an independent source of historical information about the evolution and origin of species. Croizat’s unique approach was first developed in the 1950s and became known as panbiogeography. His research program analyzed the geographic relationships between different taxa at different localities using line graphs or “tracks.” Tracks are generally drawn to connect localities over the shortest geographic distance, since that provides the minimum amount of geographic space and therefore the minimum number of ad hoc geographic hypotheses required to explain the spatial relationships. The line graphs allow direct comparison of spatial geometry for groups of organisms and tectonic features (such as ocean basins and geosynclines) associated with earth history. These biological and geological components comprise the raw data of biogeography. Darwin’s predication that dispersal ability would be the key to understanding the evolution of geographic distribution was not supported by this approach. When Croizat compared tracks, he found that supposedly “poor” dispersers could be as widely distributed as “good” dispersers. He also found that animal and plant distributions could be correlated with geomorphologic features, and this suggested that geological history had more to do with the evolution of a distribution than with dispersal ability.
The overlap between tectonic features and multiple distribution patterns suggested to Croizat that geological and biological patterns share a common history. This shared history may be explained as the result of an ancestor occupying a widespread geographic range across the tectonic feature. Descendant taxa now occur in different localities because each evolved through local differentiation over different parts of the ancestral range, giving the appearance that each had moved to their respective locations. The spatial pattern linking the descendant species is still correlated with the tectonic feature involved with the ancestral dispersal.
Croizat referred to his differentiation model as “vicariant form-making.” For each taxon (whether a species, genus, or family, and so on), the “center of origin” is, in effect, the combined range of the related taxa, rather than a localized part of the range as in Darwin’s theory. With the panbiogeographic method, historical inference is the product of spatial comparisons between distributions, rather than prevailing beliefs about the age of taxa or historical events theorized from other historical disciplines such as geology or ecology. It is this spatial dimension of evolution that is missing from most historical accounts of primate biogeography and evolution.
The biogeography of primates is usually interpreted according to Darwin’s theory of centers of origin, which are usually identified as the location of the oldest fossil. Migrations from the center of origin are then imagined according to the geological age of various fossil members, molecular clock theories, and theories about the sequence of continental connections or their absence. For primates as a whole, the center of origin is assigned to a location in the Old World, apparently because the primitive prosimians are absent in the New World. The presence of monkeys in South America is therefore explained by the theory that their common ancestor either hopped across former islands in the Atlantic or rafted across the sea. Prosimians, even though they represent an older lineage, were somehow unable to make the trip. The possibility that monkeys made the trip from Africa while it was adjacent to South America is usually discounted because it is assumed that the oldest monkey fossil, dated at 35 million years, shows that monkeys did not evolve until after the Atlantic formed earlier in the Cretaceous. Conversely, despite the geographic isolation of Madagascar in the Cretaceous, this was somehow not a barrier to prosimians, while monkeys were evidently unable to make the trip across the Mozambique Channel, even though Darwinian biogeographers frequently invoke island hopping to account for the presence of myriad other animals and plants on the island. It is this contradictory theorizing that exemplifies biogeographic reasoning that appeals to imaginary centers of origin and dispersal.
Anthropoid evolution is similarily portrayed, with the imagined center of origin swinging back and forth between Africa and Asia according to the fortunes of fossil discovery, sometimes with migrations both ways for monkeys, apes, and even hominids. The result is a biogeographic mess, with primates walking or rafting this way and that and making global migrations by crossing continents, land bridges, or enduring dramatic transoceanic voyages. All of these stories require an imaginary interpretation of fossils as migratory markers and presumptions about the location of older fossils or primitive lineages being at or near the imagined center of origin. Each time an older fossil or more primitive lineage turns up or a new molecular clock theory is produced, the current story will be supplanted by another with the caveat that somehow there has been scientific progress that is different from what was “previously thought.”
A panbiogeographic approach to primate evolution requires only a consideration of how the geographic distribution of any one group compares with biogeographic patterns in general and how these patterns are spatially correlated with geomorphological features. Fossils provide information on the minimal age of fossilization localities currently not represented by extant forms. There is considerable debate over many aspects of primate phylogeny, and there is a great deal of uncertainty about the relationships between early primate fossils and living taxa. The evolution of hominids and apes emerges out of the common ancestor with Old World monkeys and, in turn, the common ancestor to both Old World and New World monkeys and primates in general. To provide this historical context, the spatial biogeography of apes begins with general biogeographic patterns for primates overall, with a focus on the extant primate groups.
The dispersal (in the sense of geographic differentiation) of extant primates involves three principal groupings: prosimians, tarsiers, and anthropoids (monkeys and hominoids). Prosimians show a pattern centered on the Indian Ocean (as a tectonic basin), with lemurs confined to Madagascar and lorises located in Africa, southern India, and Southeast Asia. Lorises include two families, the Galagonidae widespread in Africa, but absent from outside that continent, and the Loridae in central western Africa, with an eastern boundary at the Rift Valley, India, and Southeast Asia. The spatial break between the western Rift Valley and Southeast Asia is a standard pattern found in other groups of plants and animals. Tarsiers are often seen as being phylogenetically enigmatic due to their many unique features, but they exhibit the unique toothcomb of prosimians and may therefore be seen as an eastern component of the prosimian distribution.
Prosimian biogeography is classic for its concentration in areas around the Indian Ocean, and in this respect, prosimian evolution is similar to many other plants and animals largely or wholly limited to landmasses in the immediate vicinity of the Indian Ocean basin while being largely absent from the Americas and the Pacific. The direct historical inference of this tectonic correlation is that prosimian ancestors were already widely distributed before the formation of the Indian Ocean in the Late Cretaceous and Early Tertiary time, even though the earliest recognized fossils with prosimian affinities (the omomyids) are only about 40 million years old. The biogeographic correlation suggests that the ancestors of lorises and lemurs each emphasized different geographic areas, so that their descendant now vicariate (occupy different areas) with respect to each other. Even the overlap between lorises and tarsiers indicates slightly different ancestral distributions for each group, with modern tarsiers being present in the Celebes and some Philippine islands where lorises are absent. This current biogeographic pattern for prosimians does not preclude fossil forms of each group occurring in other parts of the prosimian range (lemurs in Africa, for example), but such localities are seen as outside the main centers of evolution for each group as represented by their modern diversity.
Unlike prosimians, anthropoids (monkeys, apes, humans) are found in both the Old and New Worlds, with platyrrhine monkeys in the Americas and catarrhine monkeys and apes in the Old World. This trans-Atlantic range is also standard for plants and animals in general and may predict the origin of anthropoids in the Late Cretaceous even though their oldest fossil representatives are currently known no earlier than about 35 million years. The anthropoids share with prosimians a distribution range that is currently largely to the south of a major tectonic feature, the Tethyan geosyncline, which extends between Europe and Southeast Asia, with a Caribbean extension prior to the formation of the Atlantic. The geosyncline formed through the closure of the Tethys Sea in from Late Cretaceous and Early Tertiary time. Although living and fossil prosimians and anthropoids do occur north of Tethys, the predominant diversity to the south may reflect the geographic distribution of ancestral diversity that lay more to the south of Tethys than to the north. A contrasting pattern is represented by the Plesiadapiformes, a fossil group first appearing about 65 million years ago. These fossils may represent early primates or primate relatives of uncertain mono-phyly. As currently recognized, the Plesiadapiformes are distributed to the north of Tethys, over parts of eastern Asia and North America. They are, therefore, largely vicariant (spatially separate) with respect to prosimians and anthropoids. This separation may represent a more inclusive widespread common ancestor that ranged both north and south of Tethys in Mesozoic time.
Apes represent a sister group to the Old World monkeys, and as such they share the same evolutionary age, though they exhibit some highly derived features that separate them from all other primates. The earliest fossil for apes is represented by Proconsul at 18 million years, though the lineage must extend to at least the time of the earliest fossil representatives of catarrhine primates at 35 million years. Living apes comprise the small-bodied hominoids, or Hylobatidae, in Southeast Asia representing the sister group of the large-bodied hominoids in Africa (Pan, Gorilla) and Southeast Asia (Pongo). The fossil record for the Hylobatidae may be represented by a late Miocene fossil in China. Neither Pan nor Gorilla have a recognized fossil record, although there have been suggestions that the putative hominid Sahelanthropus may actually be more closely related to gorillas. Only Pongo is represented by fossils as well as having recognized fossil relatives, such as Sivapithecus, extending the fossil record for this group at least 14 million years.
The biogeography of living and fossil great apes is problematic because of an unresolved conflict between alternative phylogenetic models developed from different lines of evidence. The almost universally accepted model links chimpanzees as the sister group of humans because they share the greatest similarity of DNA base sequence. This pattern of relationship is contradicted by the virtual absence of any uniquely shared morphological similarities between the two groups and by morphological evidence overwhelmingly supporting the orangutans as the closest living relative of humans. Most primate biologists consider the DNA sequence evidence to invalidate the morphological evidence, so the orangutan relationship is generally ignored. What cannot be ignored, however, is the fossil history of primates, which becomes insolvable and unscientific if morphology cannot provide an independently relative source of phylogenetic evidence because fossils lie beyond the reach of DNA analysis. Without morphology, the evolutionary relationships of living hominoids cannot be integrated with their fossil relatives, let alone know what fossils are hominoids (or even primates) in the first place.
As an independent source of evidence for evolutionary relationships among primates, the characters uniquely shared between orangutans and humans may have a dramatic impact on the interpretation of the fossil ape fauna. Of the 40 features uniquely shared between humans and orangutans, only 14 hard-tissue characters are applicable to fossils, and even less are widely comparable due to incomplete skeletal composition of most fossil apes. Only dental remains are broadly represented, and they also comprise a source of uniquely shared human orangutan characters in the form of thick dental enamel and low molar cusps. When these features are considered for fossil apes, it is possible to separate out a distinct clade allied with humans and orangutans that encompass a range of fossil apes previously seen as mostly peripheral to the evolution of modern apes and humans.
The presence of thick dental enamel and low molar cusps is found in a range of fossil apes that are either generally allied with orangutans or of uncertain status (see Figure 1). The fossil record of orangutans extends into the Pleistocene of Southeast Asia and southern China. Within this geographic range are also the closely related genera Langsonia in northern Vietnam (250-300,000 years), Khoratpithecus in Thailand (7-9 million years), and Lufengpithecus in southern China (9-8 million years). The distributions of these genera are complemented by the mostly vicariant range of Gigantopithecus in India and China (2-0.5 million years) and the related genus Sivapithecus (Ramapithecus) in India (14-10 million years). Other fossil apes with thick dental enamel and low molar cusps include Ankarapithecus in Turkey (11-10 million years), Ouranopithecus in Greece (9 million years), and Hispanopithecus in Spain (10-9 million years). The biogeographic track for these taxa shows a pattern closely conforming to the Tethyan geosyncline (see Figure 1). This correlation suggests an ancestral range along the coastlines of the former Tethys between Europe and Asia that was disrupted during the closure of Tethys by tectonic uplift and subduction. This geological process may have promoted local differentiation of each genus over the ancestral range, so that the “center of origin” of this great ape clade extends across both Europe and Asia, rather then being restricted to any one part of the range.
The Tethyan range of the orangutan clade may also apply to the origin of hominids in Africa. Recognized fossil hominid genera such as Australopithecus also exhibit features otherwise unique to orangutans and their fossil relatives, including broad cheekbones with forward facing anterior roots. These orangutan affinities are congruent with morphological evidence, with over 40 known synapomorphies supporting a sister group relationship between humans and orangutans. Fossil australopithecine records extend back only about 4.5 million years, whereas the orangutan lineage extends back at least 14 million years, as represented by its fossil relative Sivapithecus. Other fossil links may be represented by the proposed 6-million-year African hominid Orrorin, but the orangutan relationship for humans would appear to rule out the fossil genera Ardipithecus and Sahelanthropus being hominids at all.
Even with this very brief biogeographic overview, the evolutionary origins of humans may be seen as the western counterpart to the orangutan in the east, with the intervening geographic space occupied by now extinct members of the human-orangutan clade across the Tethyan geosyncline and its extension along the Rift Valley in Africa. The biogeographic and evolutionary resolution of the large-bodied hominoids extends the fossil history for the human lineage at least into the mid-Miocene, as indicated by the minimal age of fossilization for Sivapithecus. Conversely, the question of what became of large-bodied Miocene apes in Africa may now also be answered by the orangutan affinities of the australopiths. Isolated fossil teeth identified in the literature as Australopithecus conforming to the dental characteristics of orangutans suggest that the Miocene African apes did survive to become what are now recognized as Plio-Pleistocene hominids. They are bipedal orangutan relatives by another place and another name.
References:
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- Craw, R. C., Grehan, J. R., & Heads, M. J. (1999). Panbiogeography: Tracking the history of life. New York: Oxford University Press.
- Croizat, L. (1958). Panbiogeography. Caracas, Venezuela: Author.
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