Map of life expectancy at birth from Global Education Project.

Tuesday, January 21, 2020

Stepping back and looking at the big picture

I'm going to step away from current events for a bit -- okay, I might intersperse a post on something exigent -- and contemplate the nature of the historic epoch in which we find ourselves. I'll begin with the most basic history of Homo sapiens. Paleontologists use the abbreviation Kya for "thousand years ago." So, our species emerged in southern Africa maybe 200 Kya or slightly more. We can't be sure because we might not have found fossils of the oldest of our kind and we also can't be sure if the earliest anatomically modern humans were also behaviorally modern, the most important question obviously being language. Actually around 200Kya there were multiple populations in Africa that appeared very similar to modern humans. Were they really like us in capacity for language and cultural development? We just don't know.

In any case, the early human population remained confined to southern Africa until about 100 Kya, when it expanded into central and then northern Africa and then, around 50 Kya, crossed the Red Sea. Homo sapiens arrived in Europe about 45 Kya, and made it to  southeast Asia and Australia at about the same time. They made it to central and east Asia - al the way to the Pacific coast -- just 5,000 years later, and crossed into North America during the glacial age 15Kya, when the Bering Strait was a land bridge, and made it to the tip of South America in just a couple of thousand years.

It's very important to understand, however, that these migrations were undertaken by very small bands of people. Genetic diversity in Africa is far greater than it is throughout the rest of the world. Something like 1,000 to 2,500 individuals initially moved from Africa into the Near East. Dispersion thereafter occurred in a similar way. The band would settle in a new place and once it grew too populous for the local resources to sustain, a small group would move on and found a new colony. The way of life might change gradually in response to available resources and climate in new places, but people continued to subsist by hunting and gathering, using tools of stone, bone and wood until around 10,000 years ago. They lived in small groups, knowing nothing of different lands. Although some goods traveled surprising distances through trading networks, the people on the ultimate receiving end would have known nothing of their origin.

Recently extant hunter-gatherer bands averaged 28 individuals, organized into larger groups we usually call tribes, of about 500. Population density is on the order of fewer than 1 person per square kilometer. At these densities communicable disease is limited, but presumably as density increased in a new environment it became more of a problem. So we can reconstruct a pattern of peopling of a new environment, a period of relative health and prosperity, followed by a period of scant resources and increasing disease, followed by out-migration and repetition of the pattern. So the total global population increased as its geographic extent expanded, but its density did not, for some 100,000 years.

See Henn, BM, Cavalli-Sforza LL, Feldman MW. The Great Human Expansion. Proceedings of the National Academy of Sciences, 2012.

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