Paleontology is built up out of the most tenuous of evidence. Whole pages of children's dinosaur books are extrapolated out from one or two fragmentary skeletons of unfortunate creatures that just happened to die in places where their corpses were covered up by preserving sediments, instead of being picked apart by scavengers.
Archaeology is a little better, just because it's more recent, and there's more stuff that has managed to survive, but in the absence of ceramics, metals, or large-scale use of stone, almost everything that primitive humans would use is extremely perishable, with the exceptions being where we just happen to have found bodies and graves in extremely preservative conditions, like bog people, or the Peruvian desert cave mummies, or frozen in glaciers. I've an uncle who is an archaeologist specializing in post-contact Native American sites, and the vast majority of the time, the only identifying aspects of a site will be that he's stumbled onto an ancient trash heap, full of discarded and broken mollusk shells.