WATCH: Unit 8 Overview | Big History Project

Oh look! A suggestion box. How helpful. “Does Unit 8 have a threshold of increasing complexity?” Well, no, but not all units do. “When is the next threshold?” Well that’s easy: NOT NOW. “If Unit 8 has no threshold of increasing complexity, does that mean complexity didn’t increase?” You know what, you tell me—I mean we are about to see the connection of world zones, empires, transoceanic voyages, the silk roads, the Columbian Exchange... Hey Bob? Are you sure there’s no threshold for this unit? [Bob] Stick to the script, Hansen. Hi, I’m Rachel Hansen, and this is Unit 8: Expansion & Interconnection. In this unit, the human species expands and the Four World Zones are connected in a complex web. Merchant caravans cross oceans and deserts; huge empires conquer and crumble; and intrepid explorers set off into the unknown. Each event created new linkages, which in turn sped up our collective learning. Sounds exciting right? Well, let me introduce you to another little intrepid explorer. Nope, not the gerbil. Look closer. And no, not the flea that’s on the gerbil. Keep zooming in. Yep, that’s the one. Yersinia pestis. That’s the bacteria that moves from animals to humans. How, you ask? It’s when a flea bites us and vomits into our blood. Fun, right? Wait, this is Unit 8. You probably thought we were done with all the science. Why are we talking about bacteria again? Well, you might recognize this bacterium by the pandemic it caused in the fourteenth century: The Black Death, also known as the bubonic plague. From 1346 to 1351, this plague ravaged Afro- Eurasia, killing up to 200 million people. You see, it’s not just humans that played a role in human history. This little bacterium–by hitching a ride on fleas, who hitched a ride on rats–transformed whole societies. It’s also a great example of the importance of interdisciplinarity to the study of human history. In 2012, geneticists working with human remains from a London cemetery confirmed that Yersinia pestis caused the Black Death. By combining historical evidence from written sources with new genetic information, historians like Monica Green have been able to more accurately trace the disease’s early path out of Central Asia almost a century earlier than historians have traditionally assumed. This interdisciplinary evidence suggests that rodents carried the disease while hiding in wagons of grain transported to Iraq to feed conquering Mongol armies. Later, it spread on merchant caravans and ships as the massive Mongol Empire made long distance trade easier. So all those exciting merchants, empires, and explorers? Yeah, diseases like this little guy often traveled along with them. In most of this unit, you’ll learn how new connections helped grow human populations and increased the rate of collective learning. But the story of Yersinia pestis, and other diseases, like smallpox and COVID-19, is a reminder that history doesn’t move in a straight line. In Unit 7, we investigated the transition from foraging to farming. About 11,000 years ago, some humans started farming thanks to the Goldilocks Conditions of a warming climate . We examined how the conditions in some lush river valleys led some foragers to settle down and eventually start farming. Gradually, farming spread to new areas. But we also encountered evidence in Unit 7 that challenged that narrative of increasing complexity. For example, many societies in Africa and the Americas remained foragers for much longer. In some places, the environment was just much better suited to foraging. Many groups became pastoralists and herders —meaning they raised animals as workers and food— but they avoided settled agriculture, continuing to move from place to place. Finally, back in Unit 7, we asked if farming was an improvement over foraging. Some foragers might have noticed that farming is hard work. They saw the taxes, social hierarchy, and gender roles that developed with agrarian societies and decided, “Nah, I’m good”. The transition from foraging to farming was complicated and evolved over thousands of years. Whatever its drawbacks, in Unit 8, we’ll see that most human communities eventually adopted farming and agriculture. As agrarian cities, states, and empires grew, new connections developed between them. In the last unit, you learned that as populations grow, we get more potential innovators, which means more collective learning. In this unit, you’ll see an acceleration of that trend. As those societies grew and connected with each other, they shared ideas and goods, further speeding up collective learning. Still, as the suggestion box pointed out earlier, we don’t have a new threshold for this unit. That’s because the events of Unit 8 didn’t fundamentally transform our use of energy. In general, people still used the same sources of power in 1500 CE as they had for thousands of years: wind, water, the Sun, and muscle power. But there were still some huge changes in this period. The biggest of all was the linking up of the four separate world zones: Afro-Eurasia, the Americas, Australasia, and the Pacific. Exploration and discovery are a big part of this unit. The travels of Ibn Battuta and Marco Polo were legendary. You’ll also learn about the massive Chinese ships commanded by Zheng He that traveled throughout the Indian Ocean. These stories provide evidence of increasing interconnections within Afro-Eurasia. But Afro-Eurasia wasn’t the only world zone where people exchanged goods and ideas. For example, the Americas also had large empires and thriving exchange networks. After 1492, however, these exchange networks in Afro-Eurasia and the Americas became interconnected. This period marks the beginning of the Columbian Exchange when plants, people, animals, ideas, and diseases were exchanged across thousands of miles. As diverse people and societies became connected, collective learning accelerated. Indigenous societies mixed with those from Afro-Eurasia. These exchanges forever reshaped our world, creating new systems of commerce and blending old cultures to create something new. Big transformations like the Columbian Exchange are pretty darn dramatic. For example, Europeans knew nothing of potatoes or tomatoes before they encountered them in the Americas. Indigenous Americans had never seen horses. Try to imagine Italian food without tomatoes! But there’s a very dark side to the Columbian Exchange. As new connections were made, it wasn’t just plants and animals that moved. Indigenous peoples didn’t have immunities to the diseases that Europeans brought with them. This led to a massive loss of life known as “the Great Dying.” In that period, over 25 million Indigenous Americans died from diseases like smallpox. Europeans then turned to Africa to purchase enslaved peoples who would be transported to the Americas and forced to work on plantations and in mines. European slavers transported 12 million enslaved people across the Atlantic Ocean, killing millions and devastating African communities. In Unit 8, you’ll explore many more of the outcomes of human innovation —both positive and negative—as our connections deepened and our collective learning accelerated. You’ll see this theme appear again and again in Units 9 and 10. Okay, I mean I get it, since the way we use energy didn’t change much there’s no full on “threshold”. But hear me out: “Threshold seven-and-a-half: Flea Vomit Kills.” [Bob] That’s a wrap, Hansen. I have a few more…