According to anthropologist Pat Shipman, the reason for humanity's evolutionary success may have have been our kinship with domesticated dogs. The Atlantic reports on research first published in American Scientist:
Shipman analysed the results of excavations of fossilized canid bones — from Europe, during the time when humans and Neanderthals overlapped. Put together, they furnish some compelling evidence that early humans, first of all, engaged in ritualistic dog worship. Canid skeletons found at a 27,000-year-old site in Předmostí, of the Czech Republic, displayed the poses of early ritual burial. Drill marks in canid teeth found at the same site suggest that early humans used those teeth as jewelry — and Paleolithic people, Shipman notes, rarely made adornments out of animals they simply used for food. There's also the more outlying fact that, like humans, dogs are rarely depicted in cave art — a suggestion that cave painters might have regarded dogs not as the game animals they tended to depict, but as fellow-travelers.
Shipman points out two possible reasons why man cozied up to his best friend: On the utilitarian side, dogs could be helpful in the hunt. Dogs could help humans track down animals to hunt and help carry the haul afterwards. Cooperation between humans was mutually beneficial for both species.
But more intriguingly, Shipman speculates that humanity's "expressive" eyes — which allow us to communicate without talking — might have something to do with our friendly relationship with dogs. Research suggests that like humans and unlike other animals dogs can interpretate eye communication. This in turn would have aided our cooperation. Shipman notes that there's a lot more work that needs to be done before we can prove that eye contact had anything to do with it, but here's some highly unscientific evidence for how this works: