Most of the thinking people I know openly disdain racism while harboring some degree of it. Of course, this wasn't always the case. For most of our history racism was as American (or French or Romanian or Australian or Japanese or Vietnamese or….) as (insert ethnic or nationalistic dessert here). Other people were inferior based solely on their color or place of birth. It's obvious. God is on our side.
Guns, Germs and Steel shows how when racism is combined with resources, the result is imperialism.
When–not so long ago–the entire human population was made up of hunter/gatherers, some of the people found the hunting and gathering a little easier, due to the fact that they had wandered into geographical areas, such as the Middle East's "fertile crescent," where there happened to exist native plants and animals that were particularly well-suited to domestication. This led to surpluses, the resulting development of cities and–to be very important later on–resistance to diseases that had "jumped" from domesticated animals to humans. (See bird- and swine flu for contemporary examples.) In the Old World, these advantages spread and came to full flower in Europe.
On this level, it's simple: Equss ferus caballus (the horse) can be tamed, ridden and made to haul wagons. Its close cousins, the African equids (zebras) cannot. Horses can be made into fearsome cavalries. Wheat, rye, rice and barley can be domesticated toward farming, providing enough food for populations to grow. And as the populations grew and political and economic systems were developed to put some order to them, people got greedy.
When greedy (and ruthless) men such as Cortez and Pizarro came to the New World, they had hardly to lift a sword to subdue the native people, small pox being such a steadfast and untiring warrior in the cause. Then add horses, gunpowder, advanced metallurgy. Today it seems almost inconceivable that such small bands of invaders could so easily conquer so large a land. Troubling however, is how easy it remains to understand that they believed their triumphs were ordained from on high.
It's from such a meticulously constructed and supported argument about the distribution of resources that Diamond then goes on to completely undermine the notion that any one group of human beings is inherently more intelligent or more resourceful than another. Homo sapien is perhaps the most diverse species on earth, enabling it to populate every continent except Antarctica. Diamond samples communities from across the globe, showing that the skills we evolved as a species are consistent in their refinement and ability to adapt to the local environment.