A reminder from last lecture: From the point of divergence from the ancestors of chimps through all this times, humans seem to have lived in small bands. Comparisons with both non-agricultural human societies and with non-human hominids suggests these were almost all bands of about 30 or so at most, made up of closely related individuals: everyone would be a first or second cousin. There would be occasional exchange of genes between neighbors, but aggression between them would help maintain cultural and linguistic separateness. At the larger end would be tribes: units of a few hundreds, still closely related by birth and maintaining identity by means of unique customs, languages, and a near constant state of low-level "warfare" (although having far more in common with "drive-bys" than with later wars). The ability for any one band/tribe to conquer or absorb another would be relatively limited. Food acquisition would be various combinations of hunting and gathering, and at the most sophisticated end limited and primitive versions of animal husbandry and gardening.
This was the condition for all of our species form 200 ka until around 10 ka (i.e., about 8000 BCE). By then some populations within the Fertile Crescent had discovered agriculture: the Neolithic Revolution had begun. Truly agricultural societies could:
Many cultures practiced limited agriculture or animal husbandry. The Neolithic Revolution starts one experiment in the development of totalitarian agriculture (deriving nearly all one's food from farming, and consequently the wholesale conversion of most of the landscape to farm and pasture) allowed for those humans who practiced it to capture most of the productivity of an area. Consequently, those cultures who practiced totalitarianism expanded (in population and in area) at the expense of those which practiced limited or no agriculture. Looking at estimates of population:
The spread of humanity across the Earth is the study of great study. Some of the traditional tools for examining this spread (fossils and artifacts) are joined by other lines of evidence, such as: