This is a review and synthesis of Michael Tomasello's and others' studies on what differentiates the human species from its closest evolutionary ancestors, the great apes. He describes the development of the human child and its abilities from birth to 6 years of age, and contrasts it to the abilities of mainly chimpanzees. It is during this time that most, if not all, capabilities that makes humans human manifest themselves. It's a story of how the hypersocial nature of humans is made possible by evolutionary changes in various capabilities that in some form or another existed already in the last common ancestor of humans and great apes. These changes eventually produced a species for which accumulative culture, language, cooperation and reason are rooted in biology, but at the same time go far beyond biology. Tomasello calls this the "shared intentionality theory", since it relies heavily on the notion of shared intentionality, in which two or more individuals share a common goal and understanding in a situation, and they understand that the other understands this also. This develops into collective intentionality, which is essentially human culture. The text is dense, so this is not an easy read. But it is rewarding, inspiring and provocative.