Evolutionary geography and the Afrotropical model of hominin evolution

Evolutionary geography and the Afrotropical model of hominin evolution

Abstract

During the second half of the twentieth century, the evidence that Africa was central to hominin evolution became overwhelming. The earliest occurrences of most of the fossil hominin taxa and lithic technologies are to be found in Africa, and there is strong evidence that humans are closely related to African apes, and that the genetic origins of modern humans lie in Africa. The aim of this paper is to consider the possible evolutionary and ecological basis for this - why should Africa be so central? After considering biases in the record that might promote an African record, the paper uses evolutionary geography - the spatial and distributional properties of the evolutionary process - to consider the factors that lead to higher rates of speciation, novelty and dispersals, as well as the way in which the African ecological context is structured and changes through time. Critical factors identified are the variable role of the Sahara, the different extent of the Afrotropical realm as climate changes, the impact of basin structure, and the effect of variable topography and surface water distribution. The key factor is biogeographic regionalisation and the shared evolutionary histories that this reflects. It is proposed that hominin evolution is globally part of the Afrotropical realm and its history, and that biogeographical variation within Africa is key to understanding the diverse nature of African hominins and their potential to disperse beyond the continent. More broadly, the paper shows the importance of placing hominin evolution into a comparative and theoretical framework, particularly evolutionary geography, and proposes a more general basis for the Afrotropical Model of Hominin Evolution.

Introduction: the Afrotropical Model of hominin evolution

When Raymond Dart described the first fossil hominin in nineteen twenty-four at Taung, few would have predicted that by the end of the century Africa would have moved from a peripheral to a central role in human evolution. Both theory and data seemed to point to Eurasia as a more probable place of origins. Ironically, Darwin's prediction that Africa would be the place of human origins had largely been forgotten.

Now, no-one would challenge the substantial role of Africa - especially sub-Saharan Africa - in the evolution of the hominin lineage. A simple measure is to look at first appearances. The earliest hominins, the earliest (and only) australopithecines, the earliest genus Homo, the earliest hominin with significantly larger brain size and human body proportions, the earliest anatomically modern humans. Against that, Eurasia can claim one definite palaeontological first appearance - Homo neanderthalensis - plus the genetic ghost lineage, the Denisovans. Homo georgicus, Homo antecessor, Homo floresiensis. Homo erectus and Homo heidelbergensis are hard to place, as it depends upon taxonomic definitions, but could have Eurasian origins. Of the twenty-eight taxa so far identified in the hominin fossil record, twenty-four occur in Africa, and twenty-one exclusively in Africa. All hominin genera have African origins.

A similar story can be found among the lithic traditions - the first stone tool technology (Lomekwian), the first Oldowan, the earliest Mode two technologies, and the earliest Mode three. Looking more broadly at the archaeological record, fire, use of ochre, and microliths are all found earlier in Africa than elsewhere, and there is widespread acceptance of the evidence for what might broadly be called modern human behaviour in Africa by around one hundred thousand years ago.

It is not just the palaeontological and archaeological records that underpin the Afrotropical Model of human evolution. Comparative genetics have shown that the sister clade of the hominins is the chimpanzee/bonobo one, and that in turn is part of an African ape clade, suggesting, in line with Darwin's proposal, that hominin origin is an African event.

Currently genetics cannot inform on australopithecine diversity, or the early evolution of Homo, but both ancient and contemporary DNA has also shown a) that the ancestral population of modern humans was in Africa; b) that Africans have much larger genetic diversity than all Eurasia; c) that Eurasian populations are descended from African ones; and that African ancestral populations did not interbreed with Neanderthals and Denisovans. This has been clear for some time from the uniparental markers, mitochondrial DNA and Y-chromosome, and now firmly confirmed by whole genomes and multiple autosomal markers.

Even when the focus moves beyond Africa, the framework remains African. Eurasian evolution and prehistory is defined in terms of 'out of Africa' models - Out of Africa one (Homo ergaster/erectus), Out of Africa two (modern humans), and more extensively, 'out of Africa again and again' (multiple dispersals).

There is, on current evidence, little chance of this Afrocentric model being overturned, although there may be challenges to some elements of it - for example, that the African apes were in fact 'recent' Eurasian immigrants, or that Homo erectus (defined broadly) evolved in Asia and subsequently dispersed into Africa, or that northern rather than sub-Saharan Africa may have played an important role in modern human origins. Neither is there much to be gained by thinking of this pattern in terms of 'importance'; it is not that Africa is more important than other regions, but that the ecological and demographic conditions in different regions prompt different evolutionary processes. Higher latitudes, for example, are subject to more magnified habitat change, and so responses are likely to be different (often extinction), compared to tropical regions. Cold climates demand particular adaptive responses which may be less likely to lead to novelty. This is certainly the pattern across broader biological systems, leading to such patterns as the declining levels of diversity with latitude.

A more profitable approach is to ask how and why Africa seems to have been, for hominins, such a promotor of change and diversity. The framework adopted here is broadly speaking referred to as evolutionary geography. Evolutionary geography investigates the role of spatial factors in the evolutionary process. It differs from biogeography in that it operates at a variety of scales. While phylogenetic patterns are an important element, its primary focus is the investigation of the environmental (biotic and abiotic) and demographic conditions for both range expansions and contractions in the history of a lineage, and the way this plays out from the micro to the macroevolution levels. Geographical approaches to evolution are significant because spatial distributions and their context underlie most processes. "Evolutionary change occurs first as a result of spatial distribution - population dispersals and contractions, fragmentation and isolation in response to vicariance, shifting habitat boundaries, and so on. Changes in distribution result in altered selective pressures, different demographic parameters, new levels of gene flow or discontinuities, and as such evolutionary change through time is the end product of geographical processes." This paper will examine the primacy of Africa in human evolution in the context of geographical patterns, and in particular that hominins share the evolutionary history and processes of the biogeographical area to which they belonged, and that is the basis for an Afrotropical Model of hominin evolution.