>> The specifics of Paleo-Indian migration to and throughout the Americas

The specifics of Paleo-Indian migration to and throughout the Americas, including the exact dates and routes traveled, provide the subject of ongoing research and discussion. According to archaeological and genetic evidence, North and South America were the last continents in the world with human habitation. During the Wisconsin glaciation, 50–17,000 years ago, falling sea levels allowed people to move across the land bridge of Beringia that joined Siberia to north west North America (Alaska). Alaska was a glacial refugia because it had low snowfall, allowing a small population to exist. The Laurentide Ice Sheet covered most of North America, blocking nomadic inhabitants and confining them to Alaska (East Beringia) for thousands of years. 

Map of early human migrations

Indigenous genetic studies suggest that the first inhabitants of the Americas share a single ancestral population, one that developed in isolation, conjectured to be Beringia. The isolation of these peoples in Beringia might have lasted 10–20,000 years. Around 16,500 years ago, the glaciers began melting, allowing people to move south and east into Canada and beyond. These people are believed to have followed herds of now-extinct Pleistocene megafauna along ice-free corridors that stretched between the Laurentide and Cordilleran Ice Sheets. 

Another route proposed involves migration - either on foot or using primitive boats - along the Pacific Northwest coast to South America. Evidence of the latter would have been covered by a sea level rise of more than 120 meters since the last ice age. 

Quechua woman and child in the Sacred Valley, Andes, Peru

The time range of 40,000–16,500 years ago is debatable and probably will remain so for years to come. The few agreements achieved to date include: 

• The origin from Central Asia
• Widespread habitation of the Americas during the end of the last glacial period, or more specifically what is known as the Late Glacial Maximum, around 16,000–13,000 years before present

The Clovis culture, the earliest definitively-dated Paleo-Indians in the Americas, appears around 11,500 RCBP (radiocarbon years Before Present , equivalent to 13,500 to 13,000 calendar years ago.
(source)