5.4 Refugial landscapes

Many evolutionary anthropologists and archaeologists have found themselves thinking in terms of refugial landscapes - places where great apes can find relief from predation and/or access to critical resources. It has been argued, for example, that complex, tectonically active and broken landscapes may have been key refugia for early hominin populations and an important bridging environment between forests and open landscapes (Winder et al. 2013). Refugial landscapes are thought to have become important again in the evolution of Homo during the successive glaciations and deglaciations that dominated the Eurasian Pleistocene (Stewart and Stringer 2012). These authors in fact propose that a process of interglacial expansion across Eurasia by H. heidelbergensis, followed by glacial contraction into several refugia, some in Africa and others perhaps in Europe and/or the Levant, might have been responsible for the divergence of Neanderthals, humans and (potentially) the Denisovans. This would suggest that their divergence is not solely the result of constraint-based (heroic) selection, but might also involve founder effects and the random flushing out and accumulation of mutations in small populations.

Of course, these glacial refugia are different geographically and environmentally from those proposed for earlier African species, but their use would have produced similar demographic and genetic signatures in both groups, just as they have done in various other species (Stewart et al. 2010). The evidence for reticulate evolution and disrupted molecular clocks just summarised would be fully consistent with one or more extended periods of low-density presence in refugia, where populations were perturbed over centuries, possibly even millennia, by many waves of immigrants and by expansive radiation when conditions were favourable. If gene flow occurred between successive waves (as the new DNA data on Neanderthals and Denisovans seems to suggest; see Winder and Winder 2014 for a review), then populations would have remained genetically vulnerable and unstable over long periods. Cascades of physical, behavioural, genetic and ecological emergents would be likely and some of the organisms and communities that emerged would have been able to leave the refugia and colonise new habitats.