Recreation of modern humans, the children of Africa, surveying their new continent: America in the documentary First Peoples.
For decades it has been thought that the first Americans arrived from Asia after walking across Siberia.
Looking at a modern map today this journey seems impossible. But turning back the clock to the last Ice Age reveals there was once a land bridge, which scientists call Beringia, connecting North America to Siberia across the Bering Straits.
Despite this connection, two Canadian ice sheets—over 100ft thick—blocked a route through to North America up until 13,000 years ago.
But how do we know where the ice was, how long it lasted, and how deep it was?
The answers are in the rocks, the soil, and the Great Lakes.
Over the course of the last two million years there have been at least four ice ages and each of these has ploughed out an assortment of giant rocks from Canada, moving them hundreds of miles south on waves of melting ice, leaving deposits across the Midwest plain.
The age and chemical composition of these rocks indicates when these ice ages struck, how long they lasted, and how much meltwater was needed to shift these boulders southwards.
Even today, layers upon layers of Canadian gravel, sand, and silt can be found in the soil of Ohio and the Upper Mississippi River valleys, indicating how long ago meltwater carried them to North America.
18,000 years ago the last Ice Age was at its height. At this time the ice sheets reached as far south as the Ohio and Missouri Rivers. Such was their size that they formed huge depressions in the earth’s crust which, as the last Ice Age finally lifted, filled with meltwater, forming the Great Lakes.
But recent fossil evidence, excavated from the Yucatán peninsula in Mexico, proves that modern humans were in Mexico 13,600 years ago. So: How did they get there?