Without a corpse, hair follicles collected at sightings can't be traced back to a host. Officially being declared as a hair sample belonging to a unknown primate.
Dermal ridges in footprint castings made in the 70's.
FBI analysts specializing in lie detection during questioning/interviews examining video of eye witnesses and confirming they were being truthful. Some of these eye witnesses were fishing guides and park rangers. Folks who know the woods an the indigenous wildlife that inhabit the areas of the sightings.
Jane Goodall and a handful of other respected primatologists (who have examined the scientific evidence) supporting more research into the subject.