Here Be Monsters
Werewolves, vampires, elves, fairies, yeti, Bigfoot ... Since the dawn of time, humans have told tales of people that were not people. Nowadays, most of us are pretty damn sure there's not such things as elves or fairies. We're fairly sure there's no such thing as werewolves or vampires (during daytime, anyway!). Yeti and Bigfoot ... well, they could exist, couldn't they?
Indeed, they could. And indeed, only a few months ago, newspapers all around the world were proclaiming the finding of recent fossils (that's recent in archaeological terms) of "hobbits" - a somewhat undersize, humanly speaking, hominid species that appears to have lived on an Indonesian island a mere 12,000 years ago (see Flores Man). As newspapers said: "If another species of humans existed so recently, perhaps stories of other human-like creatures might be founded on grains of truth?"
Actually, I was on to this inescapable truth some time ago, and my own writerly imaginings derived much support from well-respected scientists of human evolution. Here's a couple of quotes I have always regarded as justifying my "discovery" of the Pack:
“If there’s one thing of which we can be pretty certain, it is that during the last million years there was a lot more going on in human evolution than we have yet been able to discern.” 
"the fossils and artefacts are sparse nonetheless relative to the number and variety of peoples from whom they derive, and the intricacies of their lives. It is entirely possible that pivotal events and populations have disappeared without any trace, and that we will never have direct knowledge of them. .. So there is room for speculation …" 
- Ian Tattersall: Becoming human: Evolution and human uniqueness, p145
- Colin Tudge: The day before yesterday. London: Jonathan Cape. 1995. p232