This study addresses the question of what makes humans unique, of how sharply we differ from other animals. It begins with evidence on the split of the hominids from the apes, and on the main steps in hominid evolution since then. Its main theme is that the distinctive versatility of human thought and mental representation depends on special properties of the left side of the brain. One manifestation of this lop-sidedness is the predominance of right-handedness in human populations, a tract that does not seem to exist in animals. Another is the special properties of human language, which is represented in the left cerebral hemisphere. The text argues that the human left hemisphere has evolved a specialization for an open-ended, generative form of mental representation. This underlies language and also the way in which we construct, represent and understand objects. It probably colours many other specifically human activities, such as complementary mode of thought that has more holistic, analogue properties, and that goes back much further in evolution.