THERE are scientists who believe that one day, rats could inherit the Earth. No, not people born in the years of the rat on the Chinese lunar calendar, but actual rats, some of which may evolve to the size of sheep or elephants.

One such scientist is Jan Zalasiewicz. When his views on the matter surfaced three years ago, media responded with headlines like “Scientists say rats could grow to the size of sheep in future” (BBC). But the stratigrapher—someone who studies rock layers and layering in order to understand the Earth’s history—didn’t mean to alarm. It was merely a thought experiment that he and some colleagues had come up with: if humans ever became extinct, what species would be in the best position to take over? Cockroaches and snakes were in the running, but rats seemed ahead of the hypothetical race.

Why rats? They’re on every major landmass, have survived extermination attempts, and can live in diverse conditions. They’re not fussy eaters. The scientist’s point, however, wasn’t for humans to loathe rats more than we already do. Rather, it’s for more humans to become aware of how we are changing conditions on Earth, even to the point of endangering human lives.

Zalasiewic makes an appearance in Elizabeth Kolbert’s “The Sixth Extinction: An Unnatural History,” which won the Pulitzer Prize for non-fiction in 2015. When Zalasiewic studies rocks, he looks two ways. One, he uses clues embedded in the ground—“fossils, isotopes of carbon, layers of sedimentary rock”—to try to understand what life looked like in the past. But two, he also tries to imagine how the way we live today will look, in the distant future, to the trained scientific eye.

“A hundred million years from now,” Kolbert writes, “all that we consider to be the great works of man—the sculptures and the libraries, the monuments and the museums, the cities and the factories—will be compressed into a layer of sediment not much thicker than a cigarette paper.” The idea reassures, but also humbles us. On one hand, everything that worries us today will, one day, be dust. (Or sedimentary rock.) On the other, most of the things we do today, especially the way we consume, are already altering the world our descendants will inherit.

One thing scientists and science writers are trying to teach us is to think of the consequences of our activities and choices; to see, for instance, the connections between extreme weather events (like longer droughts or fiercer storms) and the fossil fuels we burn. But because the consequences aren’t all immediate, changing the way we act doesn’t always seem urgent. The person who tosses a plastic bottle instead of recycling it can’t see the drain that he or she helps block, or the flash floods that such carelessness can help unleash.

The most recent of the five mass extinction events that preceded the subject of Kolbert’s book was the Cretaceous-Paleogene extinction. An estimated 80 percent of all animal life, including nearly all the dinosaurs, died. The debate continues on whether it was a gradual disaster caused by volcanic eruptions and climate change, or a quicker crisis triggered by Earth’s collision with an asteroid, or a combination of both. But because it occurred some 66 million years ago, it can be difficult for a lot of people to understand why it matters.

Of course, if the K/T extinction hadn’t occurred, life as we know it now would probably be different. We might not even exist. We know we should ponder the probability that we are on the brink of the next mass extinction, but it strains the limits of our memories and understanding. Rats!