The earliest migrations into Siberia and northern Europe were probably short-lived exploratory probes during brief warm periods. But sites such as Mezhirich show that by twenty thousand years ago, our ancestors could cope with extremely cold environments. Some may have settled permanently in Siberia as early as forty thousand years ago. Twenty thousand years later, at the coldest phase of the last ice age, some Siberians trekked east across the land bridge of Beringia, which was crossable because so much water was locked up in polar glaciers that ocean levels were lower than today. From
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