He talked about the transformations wraught in the post-Columbian era. For example, he discussed the dramatic environmental impact of earthworms as an invasive species that turned many forest ecosystems dependent on masses of leaf litter into bare evergreen forests. He also addressed the important role of the introduction of malaria into the warmer parts of the New World (which West Africans survived better than European indentured servants who died at rates of up to 40% in the first year or so from malaria) on slavery and the social structure of the American South.
The publisher's summary, in part, reads as follows:
Driven by the economic goal of establishing trade with China, he accidentally set off an ecological convulsion as European vessels carried thousands of species to new homes across the oceans.
The Columbian Exchange, as researchers call it, is the reason there are tomatoes in Italy, oranges in Florida, chocolates in Switzerland, and chili peppers in Thailand. More important, creatures the colonists knew nothing about hitched along for the ride. Earthworms, mosquitoes, and cockroaches; honeybees, dandelions, and African grasses; bacteria, fungi, and viruses; rats of every description—all of them rushed like eager tourists into lands that had never seen their like before, changing lives and landscapes across the planet. . . . [T]he Columbian Exchange underlies much of subsequent human history. Presenting the latest research by ecologists, anthropologists, archaeologists, and historians, Mann shows how the creation of this worldwide network of ecological and economic exchange fostered the rise of Europe, devastated imperial China, convulsed Africa, and for two centuries made Mexico City—where Asia, Europe, and the new frontier of the Americas dynamically interacted—the center of the world. In such encounters, he uncovers the germ of today’s fiercest political disputes, from immigration to trade policy to culture wars.
One of the deep questions in history is the issue of causation. How did our world get the way it is today and why? What decisive decisions and events shaped it. The Columbian Exchange analysis Mann conducts addresses those questions, pointing out little heralded ecological and unnoticed causes that may have been as critical as decisions on war and affairs of state by political leaders.
Of course, shedding light on something like the impact of malaria on the slave trade doesn't absolve of responsibility or explain, for example, why the institution European indentured servitude developed in the shadow of contract law, while African slavery became a dehumanized chattle institution.
A look at this episode also recalls the fact that at the time of the slave trade, while Africa lacked many guns and transoceanic ships, the slave trade with food producing agricultural societies well past the iron age and organized into chiefships and small states, rather than with stone age hunter-gatherers. Indeed, the Pygmy and San and Khoisian hunter-gathers of Africa at the time (nor Australian aboriginal hunter-gathers and Native American hunter-gathers later on) were largely not enslaved. Even in the Americas, societies that were already agricultural and food producing, rather than hunter-gatherer societies were incorporated into the new political systems created by the arriving Europeans. Native Americans and New Zealand Maoris didn't have guns at first, but swifted acquired them and integrated them into their cultures and ways of war. Why was Africa so heavily plundered, despite a relatively small gap in technology and political organization when the triangular trade started?