Although there is still much that we do not understand, it is likely that the selective forces working upon the humanization of man lay essentially in the nature of the socio-cultural world itself. Man, in other words, once he had “crossed over” into this new invisible environment, was being as rigorously selected for survival within it as the first fish that waddled up the shore on its fins. I have said that this new world was “invisible.” I do so advisedly. It lay, not so much in his surroundings as in man’s brain, in his way of looking at the world around him and at the social environment he was beginning to create in his tiny human groupings.
He was becoming something the world had never seen before - a dream animal - living at least partially within a secret universe of his own creations and sharing that secret universe in his head with other, similar heads. Symbolic communication had begun. Man had escaped out of the eternal present of the animal world into a knowledge of past and future. The unseen gods, the powers behind the world of phenomenal appearance began to stalk through his dreams.
- Loren Eiseley, The Immense Journey (1957)