Merlin Donald does not use the term co-evolution
in this strictly biological sense when he writes:
Our brain has developed in co-evolution with culture (p.15)
In Donald’s work, it is not an issue of two independent creatures or of mutual selection pressure but rather a mutual benefit of culture and brain. If culture, like the brain, were a human organ, we could speak of a symbiosis. Regardless of the terms, it is clear what Donald means.
According to his hypothesis, the human brain has developed together with human culture; brain and culture are dependent on each other. Donald can rely thereby on Charles Darwin, who explained in the Descent of Man: :
A great step in the development of intellect must have taken place as soon as semi-artificial and semi-intuitive language were used; for the constant use of language will have had an effect on the brain and produced a hereditary effect; and this in turn will have benefited the perfection of language.
As Chauncey Wright correctly observed, the size of the human brain compared to its body, compared to the brain of lower animals, may be largely due to the early use of a simple form of language (…) The greater intellectual abilities, such as coming to conclusions, abstraction, self-confidence, etc., probably arose from the constant perfection and practice of other mental abilities. (p. 255)
Elsewhere in the same treatise Darwin wrote:
But however great the difference between the souls of humans and the higher animals may be, it is only a gradual one and not a fundamental one.
We have seen that the feelings and opinions, the various affects and abilities, such as love, memory, attention, curiosity, imitation, reflection, etc., of which man boasts, are present in his predispositions and sometimes in a rather developed state in animals as well. They are also capable of a certain hereditary perfection as the dog proves in comparison to wolf and jackal. (pp. 156s)