The story of the North American extinction of the horse would have been cut and dried had it not been for one major and complicating factor: the arrival of humans.Humans, too, made use of the land bridge, but went the other way – crossing from Asia into North America some 13,000 to 13,500 years ago.The distribution and nature of much life on earth has been greatly influenced by this crucial land bridge.

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These terrier-sized mammals were adapted to forest life.

Over millions of years they increased in size and diversified.

While climate change dominates headlines in the modern era, it loomed large in the lives of the many species that inhabited the Americas thousands of years before mankind began belching carbon dioxide into the atmosphere.

© Yukon Beringia Interpretive Centre" width="259" height="224" data-recalc-dims="1" / A definitive answer has eluded scientists, but improving techniques and the growing value of DNA analysis has painted a clearer picture of events surrounding their demise.

Evidence suggests North America was hardest hit by extinctions.

This extinction event saw the demise of the horse in North America.

Horses got larger in size and underwent other changes to their feet and teeth to adapt to changing environments.

From five million to 24 million years ago, a number of horses occupied niches to which they had adapted, including grazing the spreading grasslands.

Camels and horses instead went westward from the Americas, where their respective species had developed.