Have you ever heard the expression, "There's no such thing as a free lunch?" Well, if you were a conquering nation that just took over huge tracts of foreign land, there was definitely such a thing as a very, very cheap lunch.

It went something like this:

"Okay Americas, you give me all your money, food, and resources, and I'll give you a religion you don't want and an incurable illness. Oh, and I'll even throw in some new crops you can grow for me."

There you pretty much have the essence of the Columbian Exchange. A phrase coined by historian Alfred Crosby, the "Columbian Exchange" describes the interchange of plants, animals, and diseases between the Old World and the Americas following Columbus' arrival in the Caribbean in 1492. 

New World meets Old World for the first time since Pangaea split a gazillion years ago. What could go wrong? 

For a lot of science-y reasons, it was generally great for Afro-Eurasia and terrible for the Americas.

In a nutshell, these science-y reasons are:

• The population of Africa, Asia, and Europe combined was much higher than that of North and South America. More people means more diseases.
• The people of Africa, Asia, and Europe kept a lot of domestic animals around like cows, pigs, sheep, dogs, etc. By contrast, only a few societies in the Americas kept any animals at all. This meant that more diseases had made the jump from animals to humans in Afro-Eurasia than in the Americas.
• The first time a group of people is exposed to a disease is always the worst. Humans who survive a disease pass resistance on to their kids. Because Afro-Eurasia had so much more disease, they also had more disease resistances. When all the diseases mixed together, the Americas suffered a lot worse than the Afro-Europeans.
• While they weren't so good at avoiding diseases, the large settled states of the Americas were excellent at farming. Both the Aztec and the Inca Empires had developed crops that produced super-healthy, balanced vegetarian diets. These crops were brought to Afro-Eurasia and made the already disease-resistant people there even healthier and more disease resistant.

Even though the Americas didn't have silk and the other cool stuff, the Europeans originally wanted from Asia, they did have goods that the Europeans wanted.

The Native Americans totally got the short end of the stick, though. Even some of the "good" things they got out of the exchange, like coffee, sugar cane, and bananas, required a lot of hard labor to grow, leading to their enslavement and forced labor. Sadly, the fact that Africans had immunities that Native Americans didn't have is why the African slave trade grew as big as it did—Europeans needed people to do the work that Native Americans were dying too much to do.

TL;DR: For reasons beyond human control, rooted deep in the divergent evolutionary histories of the continents, the Columbian Exchange massively benefited the people of Europe and its colonies while bringing catastrophic crumminess to Native Americans.

The Columbian Exchange: It's a relatively obscure concept, developed by a relatively obscure historian. Most people have never even heard of it. Its definition—the transmission of non-native plants, animals, and diseases from Europe to the Americas, and vice versa, after 1492—doesn't sound very sexy. 

And yet the Columbian Exchange just may be the single most important event in the modern history of the world.

The Columbian Exchange explains why Indian nations collapsed and European colonies thrived after Columbus' arrival in the New World in 1492. It explains why European nations quickly became the wealthiest and most powerful in the world. It explains why Africans were sold into slavery on the far side of the ocean to toil in fields of tobacco, sugar, and cotton.

The Columbian Exchange even explains why pasta marinara has tomato sauce.

If you don't understand the Columbian Exchange, you can't truly understand the forces that shape the world we live in today. You can't understand why you speak the language you speak, why you live in the nation you live in, or even why you eat the food you eat.

If you don't understand the Columbian Exchange, much of what you think you know about the history of the Americas may be wrong. Spanish soldiers did less to defeat the Incas and Aztecs than smallpox did. Divine Providence did less to bless the Puritan settlers of the Mayflower with good health and fortune than the Pilgrims' own immune systems did.

In the Columbian Exchange, ecology became destiny. Powerful environmental forces, understood by no one alive at the time and by very few people even today, determined who'd thrive and who'd die. And that may be the most shocking truth revealed to those who take the time to understand the Columbian Exchange: we, as humans, cannot always control our own destinies.

The most important historical actors in this story aren't Christopher Columbus or Moctezuma or Hernán Cortés. They're the smallpox virus, the pig, the potato, and the kernel of corn.