Mammals have been diversifying rapidly by this point, sets of teeth flicking through new fashions like a time traveller’s fast-forward shop window. The big battalions begin to make their presence known: dog-forms, cat-forms, bear-forms. Early whales too, who took a look at the extinct mosasaurs and decided they wanted a piece of that murderous action. There are the first elephants, the first tiny dawn-horses, even our own primate ancestors. Which of these majestic contenders will feel evolutionary pressure to develop a greater intellect, more complex interactions with its peers?

None, as it turns out. On northern America’s first prairies, under the newly evolved hooves of the herds, a hidden revolution occurs. It happens to an omnivorous rodent species, whose formidable gnawing teeth are not averse to a little predation, a little scavenging, the cracking of small bones. They live in vast colonies, whole subterranean cities, and breed at a remarkable rate to overcome their own mortality, bringing litters of up to two dozen into a harsh, rodent-devouring world every month.

They live cheek pouch by jowl, hundreds, thousands even, crammed into their constantly expanding polities. This is a narrative we’ve seen before, of course. The demands of so many neighbours can’t be ignored, applying powerful leverage when it comes to the crank handle of intelligence. The rodent’s awkward position between eater and eaten is a significant factor too. These creatures are omnivores, yet also delicious to any passing predator with a yearning for a snack. Early on, snakes and proto-mustelids are a terror for them. Give this species a few million years, and it will fortify its homes to keep such threats outside, where they belong. Later these rodents go out mob-handed, wearing coats of thorns like impromptu hedgehogs. Any incautious predator properly rues the day it tried to chow down on them. They leave home with spiny branches smeared with faeces and the juice of poisonous berries, and later with crude atlatls, which can launch envenomed spears into the faces of their enemies.

Give them a few more million years, and predatory species have learned to stay well away. The predators’ range also retreats and retreats—because the rodents never stop breeding, even though their infant mortality rate is far lower now. They are mammals, after all; a love for their young is rooted deep within them. As they develop their high-pitched language, they begin weaving mythologies about themselves and their significance in the world. The number of an individual’s offspring is a measure of worth, in that prehistoric rodent culture. It is their purpose and their pride, and it drives their every advancement.