What, according to john o'sullivan, is america's mission?

Here is the quote which clearly shows John O'Sullivan vision of  America's mission :"And that claim is by the right of our manifest destiny to overspread and to possess the whole of the continent which Providence has given us for the development of the great experiment of liberty and federated self-government entrusted to us." So, the general idea is to expand territory of America thoughout its north part.

Because the discoveries were some of the oldest human remains.