Chapter 5 focuses on human reasoning and starts by introducing Darwin and Wallace. These two scientists independently arrived at natural selection as how species evolve, but where Darwin saw no problems with the human brain (and mind) evolving through natural selection, Wallace found the issue to be a sticking point. He argued that foragers and those in non-industrial civilizations would not need a very large or complex brain. Therefore, once we were foragers, human intelligence should have stagnated and not reached the level it has reached in modern humans.

Like many arguments against the human brain evolving, Wallace’s contention doesn’t consider the basic tenet of natural selection: It does not have a goal. Scientific thinking is slow and methodical, which could be a problem when needing to make quick decisions. Heuristics and knowledge of the current situation are more highly valued in some environments, and that doesn’t mean that the brain required to handle that thinking is any less complex or advanced than a brain molded for scientific thinking. Each person’s brain is shaped by the environment of the brains that came before it and its own environment to maximize skills needed for survival in that environment.

One common thread in human thinking is categorizing.