Skull from Broken Hill (Kabwe), Zambia. This kind of human was still around when the Neanderthals were going extinct in Europe. (Wikicommons)
East Africa, 60,000 to 80,000 years ago. The relative stasis of early humans was being shaken by a series of population expansions. The last one went global, spreading out of Africa, into Eurasia and, eventually, throughout the whole world (Watson et al., 1997). Those humans became us.
This expansion took place at the expense of more archaic humans: Neanderthals in Europe, the Middle East, and Central Asia; Denisovans in East Asia; and mysterious hobbit-like creatures in parts of Southeast Asia.
And in Africa itself? We know less about those archaic humans, partly because the archeological record is so patchy and partly because ancient DNA does not survive as long in the tropics. Over time, the double helix breaks down, and this decomposition occurs faster at higher ambient temperatures. We'll probably never be able to reconstruct the genome of archaic Africans.
Yet they did exist. Surprisingly, they held out longer in parts of Africa than their counterparts did much farther away. A Nigerian site has yielded a skull that is only about 16,300 years old and yet looks intermediate in shape between modern humans on the one hand and Neanderthals and Homo erectus on the other. It resembles the skull of a very early modern human, like the ones who once lived at Skhul and Qafzeh in Israel some 80,000 to 100,000 years ago (Harvati et al., 2011; Stojanowski, 2014).
Archaic humans also held out in southern Africa. The Broken Hill or Kabwe skull, from Zambia has been dated to 110,000 years ago and looks very much like a Homo erectus (Bada et al., 1974; Stringer, 2011). This pre-sapiens human seems to have lasted into much later times. Hammer et al. (2011) found that about 2% of the current African gene pool comes from a population that split from ancestral modern humans some 700,000 years ago. They dated the absorption of this archaic DNA to about 35,000 years ago and placed it in Central Africa, since the level of intermixture is highest in pygmy groups from that region.
Cognitive modernity: less awesome on its home turf
Why did archaic humans survive longer in Africa than elsewhere? Some of them were more advanced than the Neanderthals or Denisovans, and perhaps better able to fend off invasive groups. This was the case with archaic West Africans, who seem to have been transitional between pre-sapiens and sapiens. They may have met modern humans on a more level playing field while enjoying the home team advantage.
On the other hand, archaic southern Africans look clearly pre-sapiens. What was levelling their playing field? Perhaps modern humans had advantages that were more useful outside Africa. Klein (1995) has argued that this advantage was cognitive, specifically a superior ability not only to create ideas but also to share them with other individuals via language—in a word, culture. This cognitive edge may have been more useful outside the tropics, where the yearly cycle forced humans to plan ahead collectively and keep warm collectively by building shelters and making garments. The result was a much wider range of human technology: deep storage pits for meat refrigeration; hand-powered rotary tools; kilns for ceramic manufacture; woven textiles; eyed sewing needles; traps and snares; and so on (Frost, 2014).
Modern humans were thus pre-adapted in Africa for later success elsewhere. We see this in their rapid penetration of cold environments unlike anything in their place of origin. By 43,500 years ago, they were already present in Central Europe at a time when it was barren steppe with some boreal forest in sheltered valleys (Nigst et al., 2014).
Pre-adaptation is a recurring oddity of evolution. A new ability may initially be a bit helpful and only later truly awesome. Does this mean that evolution anticipates future success? Well, no. It's just that the difference between failure and success—or between so-so success and the howling kind—often hinges on a few things that may or may not exist in your current environment. By moving to other environments, you increase your chances of finding one that will put your talents to better use. Success is fragile, but so is failure.
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