Evolution takes place over long stretches of time: millennia and epochs. But some new research shows that animals may be changing much faster than nearly anyone thought. And those changes seem to be linked to humans.

Tall Trees Park is a little patch of green in Glenview, Illinois. About a hundred years ago, a lot of this area looked a lot like this. It was mostly farms and pasture and forest. And now of course it’s a lot of strip malls and subdivisions and stuff. And the population has grown seventyfold. The climate has changed, too. It’s gotten a little warmer, a little wetter. And all this has made life a lot different for the northern white-footed mice who live here. It’s actually not just made life different for the mice, it’s changed the mice themselves. That gets back to a guy named Oliver Pergams. He’s an ecologist, and in the mid-90s, he was looking at deer mice who live on the Channel Islands off the coast of California. And he was taking these mice and comparing them to museum samples of the same kind of mice from the same place, but decades earlier.

PERGAMS: And I found something kind of strange, that the older specimens were larger than the newer specimens, so they shrunk in size over a period of about 30 or 40 years.

That got him wondering if those relatively quick changes are happening in other places. So years later, now on the faculty at the University of Illinois at Chicago, he gathered up some of those white-footed mice from the Chicago suburbs. And he went to the Field Museum of Natural History, where they have a hundred years of little dead mouse specimens stored in white metal cabinets.

PERGAMS: He we have trays and trays of the northern white-footed mouse. And these are some of the exact specimens that I utilized.

With a tiny caliper, he’d measure their skulls, their tails … the distance from their eyes to their noses.

PERGAMS: So this one here was collected by Aikley in 1903, in Lake County.

And he compared mice from before and after 1950.

PERGAMS: Here are some skulls, this one is in the Lake Bluff area. And this was collected in 1989.

Lo and behold: these mice had grown: by more than 10 percent on some measures. This phenomenon goes way beyond Chicago. Pergams measured more than 1,200 rodents from four different continents – Alaskan lemmings, Mexican gophers, Filipino rats. Some of them got bigger, some smaller. But all over the world, the animals are changing over time spans thought to be mere evolutionary eyeblinks.

PERGAMS: The next step is to figure out why, right? You certainly want to give it a shot. Even though, you know, it’s hard to do. You can’t get in a time machine and go back and look to see what actually happened. So the next best thing is to see if there’s associations or correlations with big factors.

What he found was that the variations correlate with changes in climate and human population density. The exact reasons aren’t clear: more people might mean more yummy trash for the mice to eat, for example. That’s going to take more research to figure out, and these critters may be overdue for some extra attention.

HEANEY: One of the questions is, well, why didn’t anybody notice this before? And I think the answer is, they haven’t looked.

That’s Larry Heaney, who curates the mammal collection at the Field Museum. He says Pergams’s research could complicate some long-held assumptions about the pace of evolution.

HEANEY: The implication is, these animals are changing very, very rapidly in response to changes that humans are largely responsible for. So in a sense, it’s good: they can change. But the other side of the coin is, they’re having to change.

Now, the question is, is this happening in other species, too? Pergams looked at rodents, but might it be true for birds? Bugs? Plants?

PERGAMS: There’s been this default attitude that if you go to one place and you capture or you observe animals or plants, that essentially there’s going to be the same animals or plants 50 years or 100 years later. I don’t think that’s possible to assume at all. We have to include the fact of this change in all of our decisions, from ecology to evolutionary biology to conservation.

Pergams’s findings show that even the most common creatures have more to teach us, when we ask the right kinds of questions.