Friday, 25 January 2013

The case for earlier Out-of-Africa (Boivin et al. 2013)

An informative review critical of the ~60kya coastal-Out-of-Africa hypothesis. On the left, the authors' estimate of the distribution of hominin groups during MIS5.

From the paper:
Another under-appreciated issue is the anomalous nature of the genetic evidence for a rapid spread of modern humans from Africa to Asia. Echoing the fossil date anomaly, the mtDNA branch lengths for sampled populations are longest for those which are farthest east, in Near Oceania, and shortest in the Asian areas that would have been encountered first (Merriwether et al., 2005; Oppenheimer, 2009). The real problem, however, is that the variation in branch lengths suggests that a single genotype engaged in the expansion actually existed for 30 ka, which does not support a rapid expansion. The anomaly can be explained by what we call an an ‘M buffer’ effect (see Supplementary material A) which implies that the branch ages we observe are considerable underestimates of the time of arrival of the genotype to these areas. Such anomalously long-lived genotypes have been directly observed through ancient DNA in species such as the Iberian lynx (Dalen et al., 2011).
We have focused here on the possibility that the modern human exit recorded by fossil evidence in the Levant in MIS 5 does not represent a failed dispersal, and that in fact our species was not only in the Levant but also the Arabian peninsula during this marine isotope stage, and spread to India before the Toba eruption at 74 ka (Petraglia et al., 2007). Another valid hypothesis we do not explore here is that H. sapiens was able to leave Africa in MIS 6 via a grassland corridor (Frumkin et al., 2011; see also Scally and Durbin, 2012). Yet another is that our species dispersed out of Africa shortly after its first appearance c. 195 ka, in MIS 7 (Dennell and Roebroeks, 2005: 1102). One other possibility is that there were several, separate dispersals of our species out of Africa (Dennell and Petraglia, 2012). At the same time, we acknowledge that major demographic changes occurred in MIS 4 and MIS 3, perhaps explaining the relatively young mtDNA coalescence age in living populations. The increasing evidence for complexity as well as the clear patterns of bias for all records, whether archaeological, fossil or genetic, suggests the need for an open mind to multiple scenarios for Out of Africa, as well as for more rather than less complex models of H. sapiens dispersal across Eurasia.
Quaternary International doi:10.1016/j.quaint.2013.01.008

Human Dispersal Across Diverse Environments of Asia during the Upper Pleistocene

Nicole Boivin et al.

The initial out of Africa dispersal of H. sapiens, which saw anatomically modern humans reach the Levant in Marine Isotope Stage 5, is generally regarded as a ‘failed dispersal’. Fossil, archaeological and genetic findings are seen to converge around a consensus view that a single population of H. sapiens exited Africa sometime around 60 thousand years ago (ka), and rapidly reached Australia by following a coastal dispersal corridor. We challenge the notion that current evidence supports this straightforward model. We argue that the fossil and archaeological records are too incomplete, the coastal route too problematic, and recent genomic evidence too incompatible for researchers not to remain fully open to other hypotheses. We specifically explore the possibility of a sustained exit by anatomically modern humans, drawing in particular upon palaeoenvironmental data across southern Asia to demonstrate its feasibility. Current archaeological, genetic and fossil data are not incompatible with the model presented, and appear to increasingly favour a more complex out of Africa scenario involving multiple exits, varying terrestrial routes, a sub-divided African source population, slower progress to Australia, and a degree of interbreeding with archaic varieties of Homo.


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