The chimpanzee's closest relatives (part one)

Evolutionary tree of the Hominoidea (emphasis on family Hominidae): after an initial separation from the main line by the Hylobatidae (gibbons) some 18 million years ago, the line of Ponginae broke away, leading to the orangutan; later, the Hominidae split into the tribes Gorillini and Hominini. The Hominini species include humans, chimpanzees and bonobos.

The title of the book refers to how similar taxonomically chimps and humans are, as their genes differ by just 1.6%, whereas chimps and gorillas differ by 2.3% (p. 19). Thus the chimp's closest relatives are not the other apes with which it is classed, but the human (see Homininae). In fact, the chimpanzee-human difference is smaller than some within-species distances: e.g. even closely related birds such as the red-eyed and white-eyed vireos differ by 2.9%. Going by genetic differences, humans should be treated as a third species of chimpanzee (after the common chimpanzee and the bonobo).^[2] Or possibly the chimpanzee's scientific name should be Homo troglodytes instead of Pan troglodytes. Diamond observes in his book that this would provide food for thought to people passing this side of the bars of a cage with the label Homo.

Sexual selection (parts two and three)

Part two considers sexual dimorphism in mammals, and particularly humans, and the mechanics of sexual selection. It considers how across species, females are more careful in selecting their mates than males (they invest far more energy into each offspring). This determines much of human behaviour: how we pick our mates, and how we organize society and child nurturing systems, leading to differing social structures in cultures such as Papua New Guinea, Kerala, and the Christian West. It also considers questions of longevity – the previous generation dies because its biological clock shuts down metabolism and repair as to divert investment from the parent individual to that of the offspring.

Part three extends the effects of sexual selection into language, art, hunting and agriculture, through the idea of honest signaling – sexual signals that also cost the signer. This is extrapolated to explain the appeal of drugs. Finally, the possibility of contact with extraterrestrial intelligence is discussed (Diamond thinks that would be a disaster).

World conquest (part four)

Part four considers conquest. Why is it that the Eurasians came to dominate other cultures? Diamond's answer is that, in part, this was due to the East–West layout of the Eurasian continent, due to which successful agricultural and animal domestication packages (combinations of certain domesticated plants and animals) could easily be adopted in regions farther east or west. On the other hand, extension of domestication packages along the North–South axis – as required in the case of the American and African continents – was much more difficult owing to severe imbalances of climate. Also, long-lasting contact with domesticated animals of agricultural populations permits greater resistance to disease, which is another reason why contact among geographically separated cultures – mostly agricultural versus hunter-gatherer societies – often leads to extinction of the latter through devastating infections.

The process of first contact between differing civilizations is examined through the descriptions of Papua New Guinea highlanders, who were first visited half a century back. Historically, Diamond argues such contacts between widely differing populations have very frequently culminated in the extinction of the disadvantaged groups like many Native American tribes, the Tasmanians, etc. There is a long list of genocides in history.^[3]

The question of why some civilisations conquered others is the main theme of Diamond's later book Guns, Germs and Steel: A short history of everybody for the last 13,000 years (1997).

Environmental impact and extinction (part five)

Here the argument is that civilizations sometimes get caught up in internal superiority contests, and deplete the environment to such an extent that they may never recover. Examples include Easter Island and the ruins of Petra, both of which were the result of deforestation resulting in desertification, according to Diamond.

The question of why some civilizations collapse and others survive is the main theme of Diamond's Collapse: How Societies Choose to Fail or Succeed (2005).