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). 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