Figure 2.

In The Descent of Man, Darwin [2] presented a model, sketched here, to account for the changes that needed to be invoked to account for human divergence from great apes. Darwin postulated environmental change as the spark and natural selection for attributes enhancing skill in defence and hunting as the engine of change, and he envisioned positive feedback linking adaptive changes in technology, intelligence and anatomical structure as the key to the human species' success. He proposed this, of course, in the absence of a fossil record that could have provided a chronology of events for these changes.