Chapter 3 Introduction

Young man: Mike Powell/Photodisc/Getty Images; background: gui jun peng/Shutterstock.
Licensed Material is being used for illustrative purposes only; person depicted in the licensed Material is a model.

Have you ever stood before a chimpanzee enclosure at a zoo and watched for a while? If you haven’t, we urge you to seize the next opportunity to do so. It is impossible, we think, to look for long without sensing strongly the animal’s kinship to us. Its facial expressions, its curiosity, even its sense of humor, are so like ours that we intuitively see it as a hairy, long-armed cousin. Indeed, the chimpanzee is our cousin. It is—along with the bonobo, a chimp-like ape discussed later in this chapter—one of our two closest animal relatives. Geneticists have lined up the DNA molecules of chimpanzees against those of humans and found that they match at 98.8 percent of their individual base units (Cyranoski, 2002). In genetic material, we are just 1.2 percent different from a chimpanzee. Language and culture, and the knowledge these have given us, have in some ways separated us markedly from our nonhuman cousins. But in our genes—and in our basic drives, emotions, perceptual processes, and ways of learning—we are kin not just to chimpanzees, but in varying degrees to all of the mammals, and in lesser degrees to other animals as well.

More than 150 years ago, in The Origin of Species, Charles Darwin (1859/1963) presented a theory of evolution that explains both the similarities and the differences among the animal species. According to Darwin, all species are to varying degrees similar to one another because of common ancestry, and all species are to some degree unique because natural selection has adapted each species to the unique aspects of the environment in which it lives and reproduces. Darwin presented massive amounts of evidence for his theory, and essentially everything that scientists have learned since, about our own and other species, is consistent with it.

This chapter is primarily about the application of evolutionary theory to the behavior of humans and other animals. It is also the first of a two-chapter sequence on the adaptiveness of behavior. Adaptation refers to modification as a result of changed life circumstances. Evolution is the long-term adaptive process, spanning generations, that equips each species for life in its ever-changing natural habitat.

Darwin developed his theory of evolution before genes were discovered, but the theory is best understood today in the light of our knowledge of genes. This chapter begins with a discussion of basic genetic mechanisms and their implications for the inheritance of behavioral characteristics. With that as background, the rest of the chapter is concerned with the evolution of behavior and with ways in which we can learn about our own behavior by comparing it to that of our animal relatives. Among other things, we examine patterns of mating, aggression, and helping, in our species and in others, from an evolutionary perspective.