Very early in the book, Diamond makes it clear that he is searching for "ultimate causes" for why different societies developed in different ways. These ultimate causes are distinguished from "proximate causes," which, he argues, only provide a superficial explanation. For example, using a very prominent example from Guns, Germs, and Steel, the Spanish conquistadores conquered the Inca because they had better weapons, deadly germs, and far better information about their opponents than the other way around. But these are proximate factors that immediately gave the Spaniards an immediate advantage in the conflict. Diamond argues that these proximate factors can be understood by other, deeper, factors. For instance, because of geographical factors, agriculture had developed much earlier in Eurasia, including the Iberian peninsula than in South America. Europeans also lived in closer proximity to domesticated animals, which caused them to develop many of the deadly diseases that wreaked havoc among Indians. Settled agriculture also led to the development of the technologies that gave Europeans such a marked advantage. One of the proximate causes for conquest, then, was the development of steel weapons. But what allowed Europeans to develop such technologies? The ultimate cause for this, Diamond says, is geographical.
When Jared Diamond talks about ultimate and proximate causes in Guns, Germs, and Steel, he is talking about causes that happened a long time ago and causes that happened closer in time to the events being explained. Put differently, he is talking about causes that led directly to a result (proximate cause) and causes that led to those proximate causes.
Let us think about this in terms of a sports team. Let’s say that the volleyball team that I coach lost a match. I could say that we lost because we kept serving out and because our hitters could not get any kills. Those would be proximate causes because that is what led directly to our loss. But then we would need to ask why those things happened. Maybe our serving was bad because the players do not concentrate while practicing. Maybe our hitting was bad because our school does not have many tall girls and so we are trying to use short girls as hitters. These are causes that happened before the night of the match, but which caused us to have problems on that night.
This is what Diamond means by proximate causes and ultimate causes. Some causes, like the Europeans’ possession of guns, germs, and steel, were the things that directly caused them to have power. Other causes, like having lots of domesticable plants and animals and living on a land mass with a long east-west axis were ultimate causes that led to the Europeans having the guns, germs, and steel.