Events in the evolution of these two especially important features of gene–culture coevolution have been difficult to reconstruct because the skeletal and artifact data regarding them are so enigmatic. Here genomic data are likely to prove especially useful. The genome-wide scans for genes under selection in the last few tens of thousands of years described in the main text are based on single nucleotide polymorphisms (SNPs) from a relatively limited sample of genomes. These data provide only a relatively low-resolution picture of genetic variation. The 1000 Genomes Project is in the process of fully sequencing at least 1,000 genomes from 11 populations representing the major regions of the world (http://www.1000genomes. org/page.php). The cost of such full sequences will probably continue to fall. Over the next decade, a large representative sample of high-resolution sequences should be available. We can anticipate that the information in these sequences, together with advances in functional genomics, will offer great insights into the deep evolutionary history of our lineage.
To judge from paleoanthropological data, the period from ~250 kya to 50 kya was the time interval over which people became behaviorally modern. African populations had rather modern, but not completely modern, skeletons and large brains early in this period (Rightmire, 2009b), but mostly made comparatively simple stone tools until about 40 kya. About this time, anatomically modern Africans dispersed from Africa to Eurasia. In western Eurasia and northern Africa, anatomically modern populations began making sophisticated Upper Paleolithic stone tools and art objects about 40 kya. Ephemeral episodes of more sophisticated tool making do occur much earlier in Africa (Jacobs et al., 2008). The early, if ephemeral, occurrence of sophisticated stone tools at the same time period as largebrained early modern humans is consistent with behavioral modernization being toward the beginning of this period. If so, the fact that anatomically modern humans were confined for so long to Africa, usually making fairly simple stone tools, is puzzling. If people were capable of modern behavior, why did they so seldom exhibit it? Why was their dispersal out of Africa so late? Klein (2009) suggests that a fortuitous mutation perhaps ~60 kya led to the final modernization of humans and to our movement out of Africa. An uptick in the millennial- and submillenial-scale climate variation after about 70 kya might have advantaged the more cultural hominins and led to a substantial bout of gene–culture coevolution. Or perhaps the explanation is entirely environmental and genes played little or no role. Simply increasing human population densities in some times and places could support the evolution of more complex technology (Henrich, 2004b; Powell et al., 2009).