This question refers to a part of the Prologue (pages 20 and 21 in the paperback edition) in which Diamond is refuting the idea that people in New Guinea are less intelligent than Westerners. He says this is an idea that has been put forward to explain why Westerners dominate the world and he says that idea is false. The connection Diamond makes is that lifestyle and cause of death combine to create “my impression that New Guineans are smarter than Westerners.”
Diamond says that evolution has been working to make New Guineans more intelligent. This is where causes of death comes in. Diamond says that, for centuries, people have lived and died in Europe for fairly random reasons like resistance to disease. Thus, evolution is not selecting Europeans for intelligence. By contrast, people in New Guinea have long died from things that intelligence can help to avoid such as accidents and “problems in procuring food.” Therefore,
Intelligent people are likelier than less intelligent ones to escape those causes of high mortality in traditional New Guinea society.
Diamond is saying, then, that evolution selects more intelligent New Guineans to pass their genes on but has not done the same in Europe.
As for lifestyle, Diamond says New Guineans’ lifestyles force them to think and develop mentally. They don’t just sit around watching TV. Instead, they are constantly interacting and doing things.
Thus, to Diamond, lifestyle and cause of death combine to make New Guineans more intelligent than Europeans.