These questions have been debated for decades. But Shapiro and her colleagues Gemma Murray and André Soares have found some new twists to the old answers by collecting bits of skin from around 200 passenger pigeons, whose century-old, taxidermied bodies sit in museums around the world. Using these samples, they sequenced the full genomes of four individuals, and compared them to the genome of the band-tailed pigeon—a close relative that still exists but lives in considerably smaller flocks.

At first, nothing jumped out. On average, the passenger pigeon’s genome looked to be extremely diverse—two to three times more so than that of any other bird that had been sequenced thus far. That made sense, given how many of them there were.

But averages are deceptive. DNA is packaged in chromosomes, and the team found that the genetic diversity at the ends of these chromosomes was exceptionally high, while the diversity in the middle was exceptionally low. The band-tailed pigeon doesn’t share the same pattern; its genome has roughly the same level of diversity throughout. Indeed, Shapiro had never seen anything like this before. This pattern—and the evolutionary forces that produced it—have important implications for understanding both why the passenger pigeon died out, and whether it'll be possible to bring it back.

Here’s why the pattern exists. When animals reproduce, their chromosomes mix and mingle, shuffling their genes into new groups. This process, known as recombination, breaks up blocks of genes, allowing natural selection to weed out the worst mutations and keep only the best ones. But in birds, recombination happens more often at the ends of chromosomes than in the middle.

Imagine that you’re going through your wardrobe trying to chuck out any clothes you hate, while keeping only the ones you love. Unfortunately, you find that some miscreant has stitched all the shirts, skirts, and pants together. If you want to keep a particular shirt, you’re forced to keep everything else that goes with it. That’s what happens in the middle of the pigeon’s chromosomes. Recombination is low, so genes stick together in large blocks, making it hard to select for one without getting all the hangers-on. But in your wardrobe, the hats, scarves, socks, and shoes are still free and loose, allowing you to consider each item individually and choose the best ones for your ensemble. Same goes for the ends of the passenger pigeon’s chromosomes: That’s why they’re more diverse than the middle.

It’s commonly assumed that animals with massive populations should be genetically diverse. But “the passenger pigeon’s genome is that of both a low-diversity species and a high-diversity one,” says Ben Novak, who worked on the study. “In either case, it was well-adapted for its preferred lifestyle.”