A Short History of the Human Race
Part 1, The Late Pleistocene, A Story of Survival
By Ed Caryl
The story of the human race, Homo Sapiens, is really a story driven by climate, particularly temperatures, rainfall, and sea level. Most of that history has taken place in the last 20,000 years, since the Last Glacial Maximum (LGM). But there was also some pre-history. Before we could advance to civilization, we needed to survive the the last glacial period. This was not easily done. 100,000 years ago, there were several species of Homo. By 10,000 years ago there was just us, and that was just by the skin of our teeth. There is genetic evidence that in the period around 70,000 years ago, there may have been as few as 10,000 Homo Sapiens in the world.
Figure 1 is a plot of Deuterium in a Greenland ice core, GISP2, a proxy for temperature.
Several points are indicated in figure 1. Reading from oldest at the right to the present day at the left, the Toba super-volcano in Indonesia caused an abrupt severe cooling that dropped the global temperature by about four degrees in a very short time. Prior to that, the climate had already cycled by similar amounts several times, but this cooling was much more severe. At that time, our ancestors were mostly confined to tropical Africa, but the cooling was accompanied by severe drying, putting pressure on the savannas in Africa that were our preferred habitat. Fortunately for us, after about a thousand years of starving out, the temperature and rainfall swung the other way, the Sahara Desert became green for a time, and we were able to migrate out of Africa through the Middle East, filling the vacuum left by Homo Erectus and putting pressure on the Neanderthals. Before the Eemian interglacial, Homo Erectus had gone extinct in Asia, except for locally adapted populations like Homo floresiensis in Indonesia and the Denisovans in central Asia. The last non-Modern Human population to die out was the Red Deer Cave people in China. They disappeared about the time Jericho was first settled in the Jordan Valley, 11,500 years ago.
Figure 2 is a detail from figure 1 of the period from 60,000 to 75,000 years ago.
This period is a perfect example of what happens in a cold period (we die) versus what happens in a warm period (we thrive). In a span of 4000 years, mankind went from a severe population bottle-neck, to spreading across three continents.
15,000 years later, about 50,000 years ago, another warm spell triggered a further migration to what is now New Guinea and Australia. All through this period, and for much of the last glaciated phase, sea levels were much lower than today, as much as 120 meters lower, joining islands and continents with dry land. Except for the migration to Australia, this meant that ships and rafts were not necessary for these migrations. Walking sufficed.
About 40,000 years ago, another super volcano erupted, Archiflegreo on the Italian coast. This triggered another 1000-year cold spell, putting more pressure on our neighbors in Europe and Western Asia, Homo Neanderthalensis. After many cycles of warm and cold, even though they were cold-adapted, their population finally collapsed 30,000 years ago.
15,000 years ago, the last great migration, that of the ancestors of the Amerindians to the New World, took place during a period nearly as warm as at present, but before the great ice sheets had melted sufficiently for Beringia to be flooded. Beringia is the continental shelf in the Bering Sea joining Asia and North America, now under 50 to 100 meters of cold sea water.
Figure 3 is a map of Beringia 21,000 years ago. Source here.
The great migrations, out of Africa, the crossing into New Guinea/Australia, the migration into the New World, were all made possible by warm, wet, periods during an ice age. When it is cold and arid, we huddle in our caves, starve and freeze. When it is warm, we multiply, innovate, and go on the move. When possible, we move to warmer and more hospitable climes, or at least empty areas. From Beringia, we populated North America and expanded to South America in about a thousand years. Just like today, when we move to Arizona or Florida, southern France or Spain.