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Report on SAGA2/COE Symposium



Evolution of the Apes and the Origin of Human Beings

Jo Thompson, Lukuru Wildlife Research Project


     Pan paniscus holds a unique place in the range of modern day great apes. Pan paniscus is the only Pongidae living south of the Congo River, a semi-transcontinental barrier that reproductively separated members of this ape lineage from other members of the large-bodied non-swimming primate family. The central African equatorial tropical zone constitutes the principal theater of evolution for the African apes and root of human origins. Thus examination of the biogeographical journey of Pan paniscus from the Pan root is an important component to consider when discussing the establishment of hominids and evolution of the African apes. This paper considers a reconstruction of the paleogeographic events between 10 to 2 MYA which help explain how present species distributions may have come about and permits us to hypothesize about challenges and opportunities which fostered the emergence and extinction of other life forms. The most critical events which influenced the occurrence of Pan paniscus included the presence of a wider belt of tropical forest vegetation, the north/south shift in the position of the caloric equator, tectonic activity creating uplifting of the southeast continental shelf and down-warping of the central basin, the resulting change in surface relief and watershed causing Lake Tanganyika to overflow via the ancient Lukuga River gorge feeder, creation of an inland lake and it's subsequent release, the north/south sorting of fauna and floral, the capture and draining of the inland lake exposing area for forest recolonization, wetter and dryer climatic phases resulting in forest contraction and expansion, the redistribution of Kalahari sands, and human and Australopithecine settlements.
     Within the study of genetics we know that analysis of different loci tell different stories of divergence. However, we can conclude that in geological time, as the Proto-Pan distribution was divided across the Congo Basin, residual gene flow may have occurred in some degree for a limited time across the marsh and broken land mass to the west of the basin between diverging Pan species. There are a number of congenital anomalies observed in wild Pan paniscus at an unusually high incidence of occurrence which suggest effects of a small population undergoing accelerated change through founder effects and random genetic drift.
     The African continental northern plateau shelf is markedly lower in elevation than the southern upland plateau shelf. With this relatively dramatic change in elevation to the south, the southern periphery had a more abrupt shift to savanna habitat and little or no open woodlands with intermediary aborescent growth as found in the northern periphery. Because of the shape of the continent and the presence of the Eastern Rift, the forest vegetation belt perimeter zone to the north was obviously much more extensive than the corresponding southern forest vegetation perimeter zone thus permitting a greater portion of the northern Proto-Pan population to adapt to transitional habitat and contribute to the gene pool. At the southern periphery the transitional vegetation is geographically a smaller surface area. The presence of forested habitat along the western coast south of the equator is altered by the location of the Intertropical Convergence Zone, further cutting into available forested surface area to the south.
     There have been important changes in the distribution of tropical forest and grassland in central Africa. White (1977) stressed the marked difference between floral biodiversity north compared to south of the central basin forest zone. The narrower transitional vegetation belt south of the central basin forest block supports greater floral species diversity than the comparably positioned broader transitional vegetation belt associated with Pan troglodytes distribution to the north of the central forest block. Exemplifying the unequaled opportunities for evolution of floral types, geoxylic suffrutice families are found endemic to the southern transitional vegetation region and evolved as a response to the unfavorable soil conditions characterized at the southern periphery of the central basin permeated by wind-blown sands from the Kalahari Desert. Through geological time these ancestral climax trees adapted to the oscillating environmental conditions south of the equator by evolving from massive, woody, mature-growth arboreal structures into a subterranean form. The persistence of geoxylic suffrutices in the southern transition is remarkable and may play an important role in the distribution and availability of food resources and equally the distribution and persistence of forest dependent organisms through geologic time in the southern region.
     Findings from the ecological study of a wild population of Pan paniscus living at the most southern limit of modern day distribution highlight adaptive strategies which diverging species may have utilized in the archaic environment of mosaic forest and grassland habitat where Australopithecine evolved. To date three grassland fruit species have been observed to be consumed by the study subjects within the study area. These are: Annona senegalensis, Anisophyllea quangensis, and Landolphia lanceolata. These fruits are not reported at any other Pan paniscus study site. Of special note, Anisophyllea quangensis is a geoxylic suffrutice. This fruit is an example of a food resource utilized by Proto-Pan evolving in the fluctuating habitat south of the major paleogeographical water barrier. Biochemical analysis was performed on the grassland fruit types. Remarkably, ripe Anisophyllea quangensis are the most sugary grassland fruit and have the lowest level of tannins. Sweetness, through increased levels of sugar, is an important attractant. We can infer from this evidence that Proto-Pan was able to utilize a different adaptive feeding strategy to forage on fleshy fruits in a drier ancestral environment of mosaic forest and grassland habitat similar to where Australopithecine evolved.
     Pan paniscus is the contemporary result of a unique set of evolutionary forces. Some southern Proto-Pan individuals survived during the paleogeographic upheavals and climatic instability of the Pliocene. This progenitor survived when topographic events affected their dispersal capabilities and periods of aridity (forest contraction and fragmentation) and higher rainfall (optimum habitat expansion) changed the distribution of their habitat. The fluctuating environmental pressures influenced divergence, distribution, demography, and socioecolgy and moderated the emergence of Australopithecine from the Proto-Pan ancestor and independent of the northern forest habitat.



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