alternatehistory.com

It's less unlikely than you'd think, depending on which theory of migration you subscribe to.

http://en.wikipedia.org/wiki/Beringia
"A study published in 2007 in PLoS Genetics, led by University of Michigan and University College London researchers, suggests that the Bering land bridge migration occurred 12,000 years ago, that every human who migrated across the land bridge came from Eastern Siberia, and that every Native American is directly descended from that same group of Eastern Siberian migrants. The authors note a "unique genetic variant widespread in natives across both continents — suggesting that the first humans in the Americas came in a single migration or multiple waves from a single source, not in waves of migrations from different sources"."

In the Old World, we can expect history to progress pretty much unchanged until Columbus discovers the Americas, or perhaps when Vinland is created. In the New World, we can expect pretty drastic differences in ecology compared to OTL, since a large number of megafauna extinctions are attributed to human settlement and subsequent overhunting. Ground sloths, glyptodons, toxodons, mammoths, American lions and even saber-tooth cats would be a fairly common sight.

Post-Columbian colonization rates might be increased or decreased. In South America, the lack of Aztec/Inca/Toltec/Mayan gold would deter Spanish expeditions of conquest, though a "gold rush" for all the gold deposits that the Native Americans would have mined OTL might happen once they are discovered. In North America, there would be no Native American tribes to resist European expansion.

Much of the American megafauna surviving in this timeline might end up being hunted to extinction by European settlers.

Timeline

1st POD- Somewhere between 40,000 and 10,000 BCE: Bunch of Siberians mauled to death by bears.

2nd POD- 1,000 CE: Viking ship hits a rock, sinks.

1493: Christopher Columbus returns from his voyage to Asia, bearing tales of a mysterious, uninhabited land of fell beasts. Dead specimens of these animals cause a stir in Seville.
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