BY ROUGHLY TWO MILLION years ago, humans had spread out of Africa and into Europe and Asia. Hundreds of thousands of years later, they migrated to Australia and New Guinea, which were then connected as a single continent. Because they did not yet have boats and could not endure the cruel cold of Siberia, tens of thousands of years more passed before they crossed the Bering Land Bridge and made their way into the Americas. When they finally began to populate North America, however, human beings quickly dispersed throughout the continent and, by crossing the Panamanian Land Bridge, soon reached
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