Since our species came into existence, there have been only two sustained periods of warm climate of the kind that might support an agricultural economy for long enough to leave some trace in the archaeological record.11 The first was the Eemian interglacial, which took place around 130,000 years ago. Global temperatures stabilized at slightly above their present-day levels, sustaining the spread of boreal forests as far north as Alaska and Finland. Hippos basked on the banks of the Thames and the Rhine. But the impact on human populations was limited by our then restricted geographical range.
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