Short answers, pulled from the story.
Archaeological evidence suggests these migrations started 25,000 years ago and continued until around 12,000 years ago. Some groups may have arrived even earlier by fishing in kayaks along what is known as the Kelp Highway.
European colonization resulted in a precipitous decline due to newly introduced diseases such as smallpox which caused populations in some regions to fall by 90 percent or more in the first century after contact. By the end of the 18th century numbers had collapsed to around 600,000 due to disease warfare and genocide.
The Indian Appropriations Act of 1871 ended recognition of independent Native nations and started treating them as domestic dependent nations subject to applicable federal laws while preserving rights including a large degree of tribal sovereignty.
There are currently 574 federally recognized tribal governments and 326 Indian reservations in the United States. These tribes possess the right to form their own governments and enforce laws within their lands.
States have limited ability to forbid gambling on reservations as codified by the Indian Gaming Regulatory Act of 1988. As of 2011 there were 460 such operations run by 240 tribes with total annual revenue of $27 billion.