Out of Africa1Homo sapiens ancestors almost certainly lived in eastern and/or southern Africa, along with his bipedal cousins. There is some archeological evidence that humans with skulls shaped like ours go back over 100,000 years. Evidently, the climate in southern and eastern Africa was relatively harsh during the first 50,000 years of that period. Our ancestral population remained small, and isolated in pockets. There are suggestions of a rudimentary intentionality in a few ancient hearths, tools, and bones, plus rare traces of red ochre (possibly used symbolically for body decoration). The absence, so far, of a continuous chronology during this early period suggests that sapience may have flickered on briefly here and there and died out again, until finally, like a forest fire, it took over. Around 50,000 years ago, climatic changes made the southern parts of Africa more hospitable. It seems reasonable that as the food supply improved and population increased, more people had more opportunity to leave their isolated pockets and mix with other groups, expanding cultural interchange. This may have produced surroundings enabling a few sparks to ignite a more widespread awakening. If we can extrapolate back in time, we can infer that this awakening led to more food and, hence, to expanding population. This need not necessarily have taken a long time, as long as there was plenty of food. Whatever the proximate causes, our sapient ancestors emigrated out of Africa in a couple of waves, or more, beginning about 50,000 years ago. The first, or one of the first, was a group that followed the coastline from Africa east all the way around to Australia. (Remember that this period was during the last ice age, which tied up so much water in glaciers that sea levels were lower and coastlines were quite different). Other waves of these emigrants went east, northeast, and northwest to Europe. The Neanderthals disappeared, except for some intermixture with our ancestors. (European DNA tests now show a small percentage contribution, less than 5%.)2 The erectus cousins of the Neanderthals farther east suffered a similar fate, also leaving behind a small contribution to the new bloodlines there. Some of the easternmost ancestral humans crossed the land bridge to the Americas by about 12,000 BCE and spread from there to populate the New World. How did these migrations differ from those of the more primitive peoples who preceded them? Intentionality based on language and altruism made all the difference. It enabled cooperation between larger social groups, with all the accompanying benefits size confers, in terms of division of labor, ability to survive in bad times, and to prevail in competition with other groups. Our ancestors developed new and more efficient kits of stone tools during this period, as shown by the archeological record. DNA evidence (of body lice, of all things!) shows that they learned early on how to fabricate warm clothing, permitting bodies that evolved in warm regions to capitalize on abundant food supplies in the colder northern climes. And we also have signs of early culture, including ornaments, flutes, and of course the famous cave paintings. All this newly minted adaptability allowed the newly minted people to eat better and to move much more rapidly, both across space and through time, than their erectus predecessors Natural selection, as we've already established, involves changes punctuated by generations. The length of a generation didn't change much when people became sapient, though much else did. When erectus burst out of Africa about 200,000 years ago, they settled in places where the weather and land forms were very different from the home turf where they had evolved. However, the tempo at which they spread out was slow, spaced over many generations, so adaptive pressures on each generation were not as strong. When Homo sapiens emigrated much later, the tempo was faster. The new intentionality factor started a positive feedback loop; increased adaptability enabled more rapid dispersion into new environments, and the challenges of adapting to the new environments tilted the selection process toward favoring the most adaptable individuals. This feedback loop applies in principle to many other behavioral and physical features that distinguish Homo sapiens from Neanderthal and other cousins, so it is reasonable to regard this period, from 50,000 till 10,000 BCE, as a rich period for rapid change. It is illustrative to turn this tempo change around and apply it to the environment instead of the invaders. Various species of large herbivores that Homo relied on for food during both invasions survived the first invasion but during the second one, in an eerie preview of coming attractions, they went extinct (the wooly mammoth, for example). Our sapient ancestors had become too efficient as hunters and too disruptive of the natural balance for the existing equilibrium to survive its introduction. Great herds of herbivores survive to this day in the African heartland because they co-evolved with these strange new bipedal carnivores; there were enough generations while Homo was growing up to permit them to grow up too, through natural selection.
Social OrganizationBecause the climate was harsh and fluctuated rapidly, humans were scattered thinly across Eurasia. The food supply for our hunter-gatherer ancestors wasn't predictable enough to support denser populations. People probably arranged themselves through space in ways somewhat analogous to the ways they do now in remote mountainous areas, where inhabited patches are separated by mountainous terrain. If that analogy has merit, then populations might be centered around nuclei of up to about 150 persons, which is the usual maximum for communities where everybody knows everybody else. Where food supplies were abundant and regular enough, such groups might coalesce into somewhat larger units.3 The more isolated these groups were, the sooner they might develop dialects and other distinguishing features. Small neighboring groups might get together from time to time for the leaders to talk out matters of general concern while everybody else did business or celebrated. Most people would live and die within a few miles of where they were born. When a group's total population would grow beyond the level the land could comfortably sustain it would split, with a part of it moving on to new turf. This raises the issue of genetic drift. In biology generally, if an emigrant population finds itself in an environment that differs enough from that in the homeland it will evolve differently, and if it remains isolated from the parent group it may develop into a new species. Something along those lines happened to our own ancestors, accounting for present racial differences, but no speciation developed. Presumably the stresses of environmental change came too rapidly to be fully met by anatomical evolution, while our new sapient adaptability stepped in, as with warm clothing in northern climes, to help cope with the new stresses. Another factor was increased mobility, which apparently allowed enough gene flow across clinal borders to offset trends towards speciation. Cultural evolution during this period opened new opportunities for a more complex division of labor. If the economic unit is no bigger than perhaps fifteen people, there's not much chance you can spare one of them to specialize exclusively in tool making, or basketry, or learning about the therapeutic qualities of naturally occurring herbs. With 150 in the unit, it's easier to spare someone who does nothing much but knap flint, since others who hunt and gather can feed him in exchange for his product. With intentionality this kind of specialization becomes not only possible but, eventually, necessary for survival. With a minimum of coercion, this kind of group can maintain a kind of equilibrium which, at the time, was something new on the planet. Why? Not because it united many individuals into a cooperative whole–ants and termites can do that, and have been for a very long time. The sociality of our sapient forebears was unique because its unity depended on the capacity of the individuals composing it for altruism based on intentionality and language. And unlike the bees and the ants, our new capacity for organizing contained the potential for further evolution. A massive climate change came about 10,000 years ago and sparked the introduction of agriculture, which in turn kicked off further radical changes in the ways most people organized themselves in communities.
 Stringer, Lone Survivors, Ibid., good general reference for sapiens as well as erectus during this period.
 Very recent research suggests that Neanderthal genetic material affecting hair and skin survives in modern humans in higher proportions than genes affecting other parts of the anatomy. More at Nature here.
 There is a library of books dealing fictionally with that remote period. Jean Auel’s ‘Clan of the Cave Bear’ series, and William Golding’s The Inheritors are classics in this genre. Auel is better on life with the Cro-Magnon, Golding is probably more accurate for the Neanderthal.