Maybe Darwin was wrong after all?
As for the human remains, how could they have remained undiscovered for so long as heavily travelled and explored as the canyon is?
Like many national parks, Grand Canyon has it places where people congregate and other locations that will not see human presence for years. The Bright Angel and Kaibab Trails see heavy traffic almost year round, but venture only a few miles away and you can find true solitude - absolutely no one else around. Many of the trails that serve sections of the park are unmaintained, and access can be quite tricky. I have spent a week in the section of the canyon about five airline miles from the BA-K trail corridor. Aside from my two companions, there was no one around.
I was talking with a member of the NPS staff and she mentioned going out for a stroll, off trail, not at all very far from the South Rim village, and finding a corpse hanging from a tree limb (suicide) She remarked that she had finally found her man.
Grand Canyon usually has a very high number of SAR operations relative to other national parks, a combination of high visitation figures and very rough terrain. They don't always get them all...