The great ice age shaped more than the geological history of North America. It also contributed to the origins of the continent's human history. through recent (and still highly controversial) evidence suggests that some early peoples may have reached the Americas in crude boats, most probably came by land.
Some 35,000 years ago, the ice age congealed much of the world's oceans into massive ice-pack glaciers, lowering the level of the sea.
As a sea level dropped, it exposed a land bridge connecting Eurasia with North America in the area of the present day Bering Sea between Siberia and Alaska.
Across that bridge, probably following migratory herds of game, ventured small bands of nomadic Asian hunters, the "immigrant" ancestors of Native Americans.
They continue to trek across the Bering isthmus for some 250 centuries, slowly peopling the American continent.
As the ice age ended and the glaciers melted, the sea level of rose again, inundating the land bridge about 10,000 years ago. Nature thus barred the door to further immigration for many thousands of years, leaving this part of the human family marooned for millennia on the now isolated American continents.
Time did not stand still for these original Americans. The same climate warming that melted the ice and drowned the bridge to Eurasia gradually opened ice-free valleys though which vanguard bands grouped their way southward and Eastward across the Americas.
Roaming slowly throughout this awesome wilderness, they eventually reached the far tip of South america, some 15,000 miles from siberia.
By the time Europeans arrived in America in 1492, perhaps 54 million people inhabited the two American continents.
Over the centuries they split into countless tribes, evolved more than 2,000 separate languages, and developed many diverse religions, cultures, and ways of life.
Incas in peru, Mayans in Central america, and Aztecs in Mexico shaped stunningly sophisticated civilizations.
Their advanced agricultural practices, based primarily on the cultivation of maize, which is Indian corn, fed large populations, perhaps as many as 20 million in Mexico alone.
Although without large draft animals such as horses and oxen, and lacking even the simple technology of the wheel, these people built elaborate cities and carried on far-flung commerce. Talented mathematicians, they made strikingly accurate astronomical observations.
The Aztecs also routinely sought out the favor of their gods by offering human sacrifices, cutting the hearts out of the chest of living victims, who were merely captives conquered in the battle. By some accounts more than 5,000 people were ritually slaughtered to celebrate the crown in the one Aztec Chieftain.