Towards a theory of modern human origins: Geography, demography, and diversity in recent human evolution

Abstract

The origins of modern humans have been the central debate in palaeoanthropology during the last decade. We examine the problem in the context of the history of anthropology, the accumulating evidence for a recent African origin, and evolutionary mechanisms. Using a historical perspective, we show that the current controversy is a continuation of older conflicts and as such relates to questions of both origins and diversity. However, a better fossil sample, improved dates, and genetic data have introduced new perspectives, and we argue that evolutionary geography, which uses spatial distributions of populations as the basis for integrating contingent, adaptive, and demographic aspects of microevolutionary change, provides an appropriate theoretical framework. Evolutionary geography is used to explore two events: the evolution of the Neanderthal lineage and the relationship between an ancestral bottleneck with the evolution of anatomically modern humans and their diversity. We argue that the Neanderthal and modern lineages share a common ancestor in an African population between 350,000 and 250,000 years ago rather than in the earlier Middle Pleistocene; this ancestral population, which developed mode 3 technology (Levallois/Middle Stone Age), dispersed across Africa and western Eurasia in a warmer period prior to independent evolution towards Neanderthals and modern humans in stage 6. Both lineages would thus share a common large-brained ancestry, a technology, and a history of dispersal. They differ in the conditions under which they subsequently evolved and their ultimate evolutionary fate. Both lineages illustrate the repeated interactions of the glacial cycles, the role of cold-arid periods in producing fragmentation of populations, bottlenecks, and isolation, and the role of warmer periods in producing trans-African dispersals.

Inconsistencies in Neanderthal Genomic DNA Sequences

“…Under this scenario, either the modern European–Neanderthal split time is very recent (i.e., ≤60 Kya) or the Neanderthal admixture proportion is extremely high (>>50%). Paleoanthropological evidence suggests that Neanderthals formed a distinct group of fossils at least 250 Kya [6,7], so the more recent modern European–Neanderthal split time is highly unlikely. In addition, most of the available evidence from paleoanthropology suggests that the Neanderthal contribution to the modern gene pool is limited [7], while previous Neanderthal mtDNA studies concluded that the Neanderthal contribution could be no more than 25% [8,9].…”

“…Paleoanthropological evidence suggests that Neanderthals formed a distinct group of fossils at least 250 Kya [6,7], so the more recent modern European–Neanderthal split time is highly unlikely. In addition, most of the available evidence from paleoanthropology suggests that the Neanderthal contribution to the modern gene pool is limited [7], while previous Neanderthal mtDNA studies concluded that the Neanderthal contribution could be no more than 25% [8,9]. Furthermore, preliminary analyses of additional nuclear Neanderthal sequences suggest a much older human–Neanderthal sequence divergence time than was found by Green et al [10].…”

Long-Distance Dispersal Shaped Patterns of Human Genetic Diversity in Eurasia

“…Even though AMH genetic diversity could have been impacted by cold periods ( Hewitt 2000 ; Arenas et al 2013 ), the specific genetic effect of the last glacial age that occurred between 13 and 29 Ka ( Straus 1991 ) has been little investigated. Similar to what happened with many other species during the last glacial maximum (LGM) ( Lahr and Foley 1998 ), modern humans might have retreated to refuge areas such as southern Europe as well as southern and south-eastern Asia ( Banks et al 2008 ; Stewart and Stringer 2012 ), and the extent of genetic differentiation between populations from different refugia might have increased during this period.…”

Tracing the peopling of the world through genomics

“…However, the number, the geographic origin and migratory routes and the timing of major dispersals remains elusive. For instance, there is evidence to support the origins of modern humans in eastern, central and southern Africa 18,31–33 , single and multiple dispersals out of Africa 7,34–36 , a north or south dispersal route 37,38 and estimates for the timing of dispersals occurring about 50 kyr–100 kyr ago 20,39–43 . Three studies 44–46 that leverage fresh, high-quality whole-genome sequencing data from geographically diverse individuals from more than 270 locations worldwide help to resolve some of these questions.…”