During the last ice age, approximately 12,000 years ago the Laurentide ice sheet separates in two, providing an access route from Beringia (the land exposed at the bottom of the Bering Sea) to the North American continent for peoples moving in from Asia.
The ice does not separate, and people can not make their way from Asia.
Due to the American continents being unpopulated, the majority of the mega-fauna survives until historical times.
In the year 1001 A.D., Norseman Lief Ericson and some Norse colonists settle in Vinland (North Eastern Canada in reality) due to no local populations to compete with the settlement grows rapidly and settlements are set up along the east coast of Vinland by 1015.
Mastodon and Mammoth ivory makes its way back to Europe, and is found to be far superior to Elephant ivory for carving.
By 1150, the population of Vinland is estimated to be over 10,000 and growing rapidly, and vast areas of fertile land have been discovered to the south (reality Virginia down south to Georgia)