I can't remember where I read it. Maybe Carl Sagan's The Dragon's of Eden. That which most clearly set human beings apart from other animals, in an evolutionary sense, was not our cranial capacity; elephants, dolphins and whales, even raccoons, are smarter than proto-humans.
It was the humble evolutionary device of the opposable thumb.
With the squaring of our pelvic bones that started to give proto-humans the possibility of an upright posture, and forelimbs freed from the need to assist in locomotion, our evolutionary ancestors could venture out from the trees. They could expand their capacity for tool-making inherited from other primates. As evolution rewarded the survival of more and better tool-makers, more upright, hand-using critters, our cranial capacity, brain-mass to body-mass ratio, and other things we only think make us uniquely human increased.
It all started with the ability, occurring a couple million years ago in small animals that looked an awful lot like chimpanzees, to touch four of the digits on a forelimb with the tip of the other that was both long enough and flexible enough to reach them.
Simply put - hands make humans.
Lisa muses with poesy and humor on the way hands express love. Randall exposes what happens when poets worry too much over a couple verses.