In the three or four decades after 1490, the human experience on planet Earth arguably changed more than it had since the Year One. Voyages of discovery transformed a world of isolated societies and sent potatoes and tomatoes sailing from the New World to Ireland and Italy, and horses and apples from Europe to the Americas in return. Trade became truly global, and cross-cultural exchange the norm. The invention of movable type and the printing press fostered popular literacy. In the West, the Reformation broke the stranglehold of a single system of religious belief. Gunpowder destroyed feudalism and sustained large standing armies and robust nation-states.

At the time of Columbus’s first voyage, direct trade and communication between Europe and East Asia was mostly blocked by the Islamic regimes that lay between them. There was little contact between Europe and sub-Saharan Africa. The Eastern and Western Hemispheres were mysteries to each other. Two generations later, as Charles Mann writes in his book 1493, “slaves from Africa mined silver in the Americas for sale to China; Spanish merchants waited impatiently for the latest shipments of Asian silk and porcelain from Mexico; and Dutch sailors traded cowrie shells from the Maldive Islands, in the Indian Ocean, for human beings in Angola, on the coast of the Atlantic.”

Now fast-forward. As Barack Obama prepares to take the oath of office for his second term as president, his country and the planet as a whole are experiencing a transformation every bit as revolutionary as the one that shook the world of Renaissance kings and Popes. The scale of that transformation is in some ways deceptive—it’s relentless and yet also quiet, at times almost invisible. But there hasn’t been anything like it in 500 years.

When Ronald Reagan was sworn in for his second term, in 1985, the Human Genome Project was still years away, but the era of genetic engineering would soon be upon us, bringing capabilities we may not want but cannot forestall. Cell phones in the Reagan era were bigger than bananas (if not breadboxes)—it’s impossible to watch the movie Wall Street today without laughing—and the Internet was in an embryonic state, known to few and used by fewer. The rise of the Internet has been the biggest leap forward in communications since Gutenberg; it has changed the nature of information, made privacy obsolete, put vast new power in the hands of corporations and government agencies, and become a weapon of war that anyone can deploy. Money ricochets around the world like so many charged electrons, making a mockery of national borders and undermining the very idea of the nation-state. (China owns two-thirds as much of the U.S. debt as the Federal Reserve itself does.) At home and abroad the availability of sophisticated weaponry has the same destabilizing effect. The migration of peoples from one place to another sparks conflict and violence but also establishes new realities on the ground. When Reagan took office, the United States was 83 percent white; last year, for the first time, more than half of American children under one year of age belonged to a minority group. Meanwhile, the world is run by a new, multi-national global elite that is educated and affluent and owes loyalty mainly to itself, rather than to any cause or country. The Financial Times is its constitution. The “Ambassador” lounges at airports are its embassies.

But here’s a big difference between the last transformative age and our own: our forebears had scant knowledge of what was happening to them, and what little they knew they usually didn’t understand. Oh, people were aware of Columbus’s discoveries (or at least the elites were). And soon enough ordinary people were eating foods that would have been unknown to their grandparents. But information was hard to come by, and causation often went uncomprehended. Chinese peasants in the 16th century understood all too well the effects on their lives of ruinous, runaway inflation, but they could not know that it was caused by the silver-mining policies of imperial Spain in South America, half a world away. Villagers living along the marshy coasts of England left haunting records of their sudden decimation by a “sweating sickness,” unaware that the slave trade, whose profits would turn Bath into a Georgian showpiece, was also carrying a disease called malaria from Africa to every part of the planet.