After the wars of the 1760s, american colonists started streaming into the western territory ceded by france, which include the present-day states of:

Louisiana, Mississippi, Arkansas, Kentucky, Ohio, Indiana, Illinois, Michigan, Wisconsin, Minnesota, Iowa, Missouri and parts of: PA, West Virginia, Tennessee, Alabama, Texas, Oklahoma, Kansas, N Dakota, S Dakota, Nebraska.

Compare these two maps to see yourself:

Because the discoveries were some of the oldest human remains.