The dogs living near the disaster site of the Chernobyl Nuclear Power Plant are not like other dogs—at least as far as their genomes go. In recent years, researchers uncovered a unique genetic profile among this population, but what’s driven this distinction remained unclear.

The explanation that’s perhaps most tantalizing to a sci-fi author is that the persistent radiation at the nuclear site has caused mutations to accumulate in the dogs’ genomes. But there are other ways a population’s genetics can change over time, such as random drift or natural selection. A recent study published in the scientific journal Genes revisited these pups, searching for genetic patterns pointing to one of these processes.

The researchers compared the DNA from dogs living in the area right around the nuclear plant with genetic material from dogs living in the city of Chernobyl, about 16 kilometers away. They also compared these canines to other global dog populations. The contrast alone was extreme. The dogs in Chernobyl city “are more genetically similar to dogs in the Galapagos Islands than they are to the dog population” at the nuclear plant, said Megan Dillon, a postdoctoral researcher at North Carolina State University and first author of the study.

But more striking still, the comparison revealed that random chance alone could not explain the differences between the dog populations. Instead, it seems the dogs at the nuclear site have been experiencing natural selection.

“When I was first doing those analyzes, I had to kind of pause and was like, ‘Wow, I have to make sure I'm doing all of this right,’ because this is very interesting, very surprising,” Dillon said.

Not only did the dogs seem to be evolving, but some of the genomic regions under the highest selective pressure were associated with functions such as DNA repair, immunity, and cell survival. In other words, certain genetic differences in the dogs from the plant could be relevant for coping with environmental stress.

Instead of being mutations, these genomic variants likely already existed among local dogs but became more common in the population over the past 40 years. “These dog populations already had this high degree of genetic diversity,” Dillon said. “Maybe this founding population at the power plant had the kind of genetic toolkit to endure and survive in these harsh conditions. It’s not necessarily that they were being bombarded with new mutations.”Taken together, the results support the hypothesis that natural selection contributed to the difference seen in the dogs at Chernobyl compared to other populations.

What remains unclear is which environmental factors favored these variants and whether they provide any specific advantage. “Even though we found this evidence of selection and it is around these biologically relevant genes, we still cannot say that this is tied to the exposures that the dogs have faced,” Dillon noted.

Radiation at the disaster site might seem like the easiest explanation to point to, but there are many other selective pressures this dog population has faced since nuclear meltdown. Chernobyl’s dogs are descended from pets left behind during the evacuations. Since then, they’ve persisted through Ukraine’s harsh winters and weathered other pollutants including heavy metals and pesticides also deposited in the area. “A lot of these contaminants have similar signatures and similar pathology,” Dillon said. “There are just too many different things to really be able to tease apart a single variable.”

The researchers also don’t have access to longitudinal samples that would show exactly how this population has changed over time. “It's almost like looking at a single still image and trying to figure out what came before it,” said Matthew Breen, a professor of genomics at North Carolina State University and the study’s senior author. “Those first few years were probably the most critical period—if they were selected, that's when that selection process would have happened.”It’s not possible to go back and get samples from the dogs who lived there in 1986, a time when no organism—not even a bacterium—had its full genome sequenced.

Breen was a graduate student in England then. “I was involved in a project driving around the Welsh mountain hills, collecting sheep from the pastures because the radioactive cloud had blown over Western Europe,” he recalled. There wasn’t anything to see in the data gathered then, but the technology has evolved, and those sheep had also not been living at the center of a nuclear meltdown for generations.

Nearly four decades after the accident, the Chernobyl exclusion zone has also become a refuge for wildlife. Wolves, deer, wild boars, and lynx now roam the area—a return that researchers largely attribute to the decline in human activity. Among some of these residents, such as the wolf population, researchers have also observed similar genomic distinctions to those seen in the dogs.

“It really demonstrates the resilience of nature to catastrophic events caused by humans,” Breen said. “You push something in one direction, it can respond to that and either push back or deviate its path going forward. I think this is what we're seeing here.”This story originally appeared on WIRED en Español and has been translated from Spanish.