Difference is the shadow of similarity.

In the first chapter of the book, Ridley begins by illustrating how we perceive differences through similarities. For instance, a man and a woman are obviously different biologically but are perceived as similar when the two of them are compared to a dog. In this case, the dog is perceived as the odd one out. This leads to Ridley quoting an excerpt from the writings of a young Charles Darwin, who had just arrived in Tierra del Fuego. Darwin notices the differences between the locals who live there and the "civilized" Fuegians he had encountered on the ship. These examples, which highlight the precarious nature of interpreting differences, sets up the thesis of the book, which is that the argument of nature versus nurture is outdated. Rather, Ridley proffers the theory that human development is the outcome of genes interacting with the environment—that is, both nature and nurture, hence the title of the book.

So little was known about the behavior of apes that it was easy to go on thinking of them as primitive versions of people, rather than sophisticated animals that were brilliantly good at being apes.

This excerpt emphasizes the misconceptions the general public and scholars alike have had over time regarding differences among living creatures. Humans would rather project their self-image onto other creatures, such as apes, than accept the natural essence of those creatures. This also adds another layer to Ridley's argument that the mainstream conflict of nature versus nurture is anachronistic. Ridley later argues that the debate over nature versus nurture has become deeply partisan and thus has suppressed the exploration of other theories regarding human development.

Either human beings must be more instinctive, or animals must be more conscious than we had previously suspected.

Using observations of primates—this time through the field notes of Jane Goodall—Ridley hints at the primary argument of the book: that humans are just as influenced by instincts and social structures as primates and other animals. The psychological development of primates, like that of humans, is influenced by their environment, allowing them to access complex thoughts and experience deep emotions that were previously thought to be characteristics exclusive to human beings.