Diamond said the reason for this phenomenon is that Eurasia stretches 10,000 miles from east to west, whereas the Americas stretch 10,000 miles from north to south.

He said that in Eurasia, migration of domesticated animals and crops was easy because the climate and conditions are relatively the same across most of the continent. The Americas, by contrast, have dramatically different climates from north to south, making migration, and thus diversification, difficult.

The contrast is even more striking in Africa, Diamond said. In the fertile crescent, in the area of the Persian Gulf, cultivation was always easy and fruitful. In the warmer climates in the rest of Africa, such agriculture was impossible, thus preventing the spread of culture and people since those who were successful in agriculture stayed put for the most part.

Diamond said many people have tried to explain this diversification phenomenon in racial terms, saying that Africans were simply lazier than Europeans and did not have the energy to migrate. His discoveries have shed new light on this situation, though, showing those ideas to be incorrect.

Still, Diamond does not see his findings as entirely conclusive.

"I see this as an opening of a research program rather than a closing point. There are lots of questions to be answered in this area," he said, "I'm optimistic that we can come up with explanations for these broad patterns of human history."

Diamond's lecture was the second Roger Tory Peterson memorial lecture. The lecture, created in 1996 by the Harvard Museum of Natural History, honors Peterson, author of the Peterson Field Guides and a world famous ornithologist and author.