not want to discount the possibility of convergent evolution in the two species. Neandertals had brains as large as anatomically modern humans (Klein, 2009). By some accounts, Neandertals proved as capable of sophisticated culture as anatomical moderns. Just before we came into contact with them, and after the uptick in millennial- and submillennial-scale variation ~60 kya, Neandertals may have independently evolved the modern behaviors ascribed to the makers of the Upper Paleolithic industries of western Eurasian anatomical moderns (d’Errico, 2003; Zilhão et al., 2010). Introgression between anatomically modern humans and Neandertals is a possibility (Cochran and Harpending, 2009), and what genes did introgress would be informative if they can be reliably detected, particularly if they generated parallel selective sweeps in the two species.
For this period, we have nothing like the unmistakable signature of cultural changes driving genetic changes that we see in the Holocene. If anything, genetically determined traits such as brain size seem to appear in the paleoanthropological record preceding, rather than following, the most conspicuous cultural changes. Perhaps the most interesting single question here is whether genes underlying modern behavior evolved early or late in this period. The durable artifacts tend to support a late interpretation, because a great number of traits that are most diagnostic of modern behavior, such as symbolic behaviors (art), develop rather late. If Neandertals did independently evolve modern behavior, then perhaps parallel or convergent genetic or cultural responses to increased climate variation can explain the pattern. The capacity for modern behavior need not necessarily have been present in the last common ancestor. The skeletons of early anatomically modern humans are still very robust and nonmodern in other ways (Rightmire, 2009b). The fossils and stone tools do not necessarily contradict the hypothesis that large-brained but archaic anatomical modern genes were coevolving in response to Middle Paleolithic cultural innovations. The combination of large brains and comparatively simple technology is a major puzzle nonetheless. How were our ancestors supporting such an energetically expensive organ unless by modern or near-modern behavior? Even the anatomically modern humans that left Africa and moved eastward to eastern Eurasia and Australia did so using relatively simple Middle Paleolithic toolkits (Foley and Lahr, 1997). The most dramatically modern Upper Paleolithic industries rich in symbolic artifacts were seemingly confined to western Eurasia and northern Africa for tens of thousands of years after 40 kya.
The especially intense pattern of millennial- and submillennial-scale variation after 70 kya suggests that environmental conditions potentially played some role. We might imagine that the adaptive advantages of Middle Paleolithic stone tool traditions were sufficient to induce the evolution of very large brains in both anatomical moderns and Neandertals.