Fossil evidence from the geologic record indicates anatomically modern humans evolved from their immediate pre-human predecessors and gradually began replacing them between two hundred thousand and three hundred thousand years ago. At that time there were several different hominid species roaming around Africa and Asia. One after another of those species became extinct until it was just early humans and our cousins, the Neanderthals and the Denisovans, that were left. Physical evidence indicates the last of the Neanderthals died out around 25,000 years ago. But modern genetic testing indicates that modern human non-African DNA contains a few percent of Neanderthal DNA, which indicates that at some point the two different groups were interbreeding.
There are various theories as to why of the several different early species of human-like primates only Homo sapiens survived. A couple of the more convincing reasons for why the others didn’t make it include climate change and being out-competed by early humans. Starting about two and a half million years ago the Earth entered into an Ice Age. Geologists refer to that time to the present period as the Pleistocene Epoch. It is a time period that transitions between periodic temperature extremes on a fairly uniform timetable. On average the Earth is plunged into a cold period with glaciers covering a large percent of the earth spanning from eighty to a hundred thousand years and then a ‘warm period’ where glaciers recede and temperatures warm lasting from ten to twenty thousand years. At present we are living in a ‘warm period’ that began about 11,000 years ago.
During the Pleistocene the fluctuating climate at times created ‘Garden of Eden’ type conditions but also brutal climates of longer duration that would have taxed even the hardiest organisms. It was during this time period that primates would also have been severely stressed encouraging the survival of the best adapted and, in the case of our ancestors, the most intelligent and resourceful. So changing climate is at least a plausible explanation as to why only the human species survived and why we became ‘smart’.
That hypothesis ties into the other theory; that early human ancestors and then early humans simply outcompeted other hominids. Certain characteristics evolved in early humans that enabled them to better secure food and shelter during scarce times. It may be that humans were better at cooperative hunting (socialistic characteristics) and also banding together to defend their ‘group’ from both wild animals and other hominids. That theory has been put forth for the demise of the Neanderthals in particular. Given their strength (they were probably twice as strong as humans), they were likely ‘brute force’ hunters that may not have adapted to different prey animals that required different hunting techniques.
The extremely harsh climatic conditions that were typical of the last glacial advance would have reduced available resources, particularly food resources, and so competition for them was likely ‘cut throat’ between primate groups, other predators, and other hominid species. To survive, early humans developed strong socialistic survival skills including cooperating within the group for hunting and defense. During times of slim pickings, the idea of protecting a resource area for a group from other groups probably developed, leading to the creation of claimed territories much like some other animal species. And when a group ran out of resources in its territory, there was the option of raiding the neighboring territory even if it involved killing those hominids already established there.
The harsh conditions of the Ice Age drove human evolution and created the necessity of adopting harsh measures that were nonetheless instrumental in human survival. Those primitive survival traits, often barbaric from our point of view, are likely encoded in human DNA and are still very much a part of human nature. The irony is that it is those traits, essential at one time, that now may threaten our existence. As evidenced by the Russian invasion of Ukraine, the ‘war gene’ is still alive and well in the human species.
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