Many scientists believe that the land bridge theory explains where the earliest Americans found boat-building material. why the Ice Age delayed migration to South America. how people first migrated to North America. why North America experienced an Ice Age.

The first Americans - the earliest people to cross from Siberia to Alaska and begin the colonisation of two vast continents linked by a narrow isthmus - could not have simply followed the deer and the buffalo across dry land during the last ice age 13,500 years ago. They would have been in the right place, but at the wrong time, a new study shows.

What is now the Bering Strait would indeed then have been dry land. There was, as scientists have known for many years, an open 1500km corridor of grassland between two great ice sheets that would have made migration deep into North America possible.

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But, according to a new study in Nature, this route wasn’t fully open for traffic until 12,600 years ago.

This means the very first pre-Columbian settlement of America, perhaps by people known to archaeologists as the Clovis culture, must have been either by sea, or by hugging the Pacific shoreline, long before the ice sheets retreated and the ocean closed in to flood the Bering Strait and separate the Old World from the New.

Studies based on radiocarbon dating, pollen, fossils and ancient plant and mammal DNA from lake sediments, found that before 12,600 years ago, there were no grasses, trees, bison, woolly mammoth or rabbits to serve as food and shelter along the corridor.