You can answer this question by looking in the Prologue. There, Diamond tells us that people often try to answer "Yali's question" by saying that cold weather made Europeans superior to other people. According to this theory, people who live in cold climates have harder lives than those who live in warm climates. Their climate challenges them more than a warm climate. This requires them to be "more technologically inventive to survive (p. 22 in the paperback edition of the book)." They have to invent more kinds of clothing. Basically, they just have to be more innovative in order to live well. In these ways, the challenges of living in a cold climate made (this theory claims) Europeans smarter and more developed than other people.