Awesome article AA!
Just speculating here, but along the lines of Satanus' info on similarities between people and animals, there's one interesting tidbit, if I'm recalling it accurately.
Narkissos brought up thinking symbolically and expressing it through language as a key human trait, and I think he's right on with that.
Now this is where I may be wrong, but I think I heard it's been found that people and apes have about the same number of phonemes in their vocabulary. Apes apparently are limited in how they relate phonemes to form a "word" or "object". So that really limits their range of expression. People can combine phonemes in many more ways and modify these symbols, and so vastly increase their range of expression. I don't know what change in the neural makeup is needed for that but there's one possible stepping stone for selection to work on and build that gap between the two.
They've also found a direct relationship between the size of a mammal's neocortex and the size of its social groups. (Makes sense that it takes a good deal of grey matter to keep track of individuals, how you should recipricate etc.)
Being more socially skilled, can give you an edge in a group nowadays. Maybe that was also a factor, to some extent, in more archaic bands. If it fetches you more mates, selection can begin to build on that. It may seem circular because the process is iterative in nature and in this particular case I'm postulating the people themselves contributing to shaping their evolution to an extent. It may explain how those traits that aren't usually critical for survival, can be kept and not lost, because they can make life more interesting.