Rethinking the dispersal of Homo sapiens out of Africa

Abstract

Current fossil, genetic, and archeological data indicate that Homo sapiens originated in Africa in the late Middle Pleistocene. By the end of the Late Pleistocene, our species was distributed across every continent except Antarctica, setting the foundations for the subsequent demographic and cultural changes of the Holocene. The intervening processes remain intensely debated and a key theme in hominin evolutionary studies. We review archeological, fossil, environmental, and genetic data to evaluate the current state of knowledge on the dispersal of Homo sapiens out of Africa. The emerging picture of the dispersal process suggests dynamic behavioral variability, complex interactions between populations, and an intricate genetic and cultural legacy. This evolutionary and historical complexity challenges simple narratives and suggests that hybrid models and the testing of explicit hypotheses are required to understand the expansion of Homo sapiens into Eurasia.

Punctuated bursts in human male demography inferred from 1,244 worldwide Y-chromosome sequences

“…This model of geographic segregation within the CT clade requires just one continental haplogroup exchange (E to Africa), rather than three (D, C, and F out of Africa). Furthermore, the timing of this putative return to Africa—between the emergence of E and its differentiation within Africa by 58 kya—is consistent with proposals, based on non-Y data, of abundant gene flow between Africa and nearby regions of Asia 50–80 kya 15 .…”

Genomic analyses inform on migration events during the peopling of Eurasia

“…This favours a small presence of xOoA lineages rather than Denisova admixture alone as the likely cause of the observed deep African-Papuan split. We also show (Methods) that such a scenario is compatible with the observed mtDNA and Y chromosome lineages in Oceania, as also previously argued13,28.…”

“…The inferred date of the xOoA split time (~120 kya) is consistent with fossil and archaeological evidence for an early expansion of Homo sapiens from Africa13,14. Furthermore, the recently identified modern human admixture into the Altai Neanderthal before 100 kya12 is consistent with a modern human presence outside Africa well before the main OoA split time ( ~75 kya).…”

“…Fossil and archaeological evidence13,14, and craniometric studies15 of African and Asian populations, demonstrate that Homo sapiens was present outside of Africa ca. 120-70 kya11.…”

A climatic context for the out-of-Africa migration

“…200 ka) and then dispersed out of Africa between 120 ka and 50 ka, based on fossil, archaeological, and genetic evidence (Groucutt et al, 2015). The exact timing of the dispersal is uncertain, with competing models arguing for an early migration during Marine Isotope Stage 5 (MIS 5; 130-80 ka) (Petraglia et al, 2010;Armitage et al, 2011), a later migration spanning MIS 4 and MIS 3 (75-50 ka) (Soares et al, 2012;Mellars et al, 2013), or a combination of these two events (Groucutt et al, 2015). The role of climate change in driving out-of-Africa migration also remains a topic of debate.…”