So, throughout stops, starts, reversals and recoveries, they spread across the world. They move by land bridge when the cold comes, then by way of bustling houseboat armadas when the seas are high. Their species settles every continent except Antarctica, and everywhere their approach is a pragmatic one. They possess an iron-clad drive to survive and increase their number, eliminating countless species that might threaten their communities and their kits. Their societies are clannish: enormous blood families splintering into subfamilies under the weight of time. Hosts of brides and grooms are exchanged with distant polities to keep their genetics healthy, building a culture with similar values across the globe. These rat cities become ever more elaborate, extending above and below ground, made of earth and wood and stone and concrete. The inhabitants grow larger than their ancestors, but stabilize at around half the size of a grown human. And while their brains are smaller than ours, they are highly organized—complexity making up for volume. If it is true that a human mind can maintain relationships with perhaps three hundred individuals, for the rodents, three thousand is nearer the mark. Most of those are close relatives. You have to get very smart, very fast, to juggle so many in-laws.

However, despite their earlier start, their technological development is only a little ahead of your own. Their close-packed living conditions lead to several cataclysmic, civilization-ending epidemics. They always recover fast, though, because even a handful can breed a society of thousands in a short period of time.

While you had your Renaissance, they had their leap into industry. This included the joys of fossil fuels, mechanized agriculture and factory-line mass production. For them, far more than for you, it was a desperate necessity. There were already seven billion of them, and starvation was whetting its knife in every major city. By the time you had your coal boom, your railroads, your first telecommunications, they were reaping the result of centuries of burned prehistory. Colossal cities had turned grey under clouds of sulphurous pollution—and the deep chambers of their underground metropoli were overflowing with choking bodies and sifting ash.