So? They took the land from other peoples through conquest and genocide. The Mississauga peoples who we took the land from, took the land from the Iroquois peoples who took the land from the Huron-Wendat peoples.
We know this because it happened while Europeans were around to write about it so this land has been conquered and lost countless times in the 10000+ years since the ancestors of Amerindians first crossed into the Americas.
Ultimately you are trying to assign ownership of this land to the last people to have conquered the land before Europeans conquered it.