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.

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