Martha, the last known passenger pigeon before dying in 1914, can be seen at the Smithsonian's Natural History Museum. Photo: Susan Walsh / AP

A new study suggests passenger pigeons, which once covered North America with massive flocks before their extinction in the early 20th century, may have maintained stable populations for thousands of years until human hunters came along, per The Washington Post. That counters previous research that found the species had already taken a downturn by that time.

The double whammy: Besides the sudden influx of human predators, the birds' genome had been tuned to the size of its population. It had surprisingly low genetic variation in some parts of its genome, which "provided few avenues for the bird to respond to human pressures, which ultimately drove it to extinction," according to the study, published Thursday in Science.

Role of genetics: One way genomes evolve is via random mutation (also called neutral evolution). Those mutations don't necessarily have an immediate benefit but sometimes can in the long run. Another process is selection in which one version of a gene is preferred — or not — over another because it influences survival. Researchers found the passenger pigeon's genome was diverse overall compared to other birds but that diversity wasn't uniform across their chromosomes. The researchers think that suggests their large population size allowed them to adapt quickly to their environment (via selection) but the cost was that there wasn't much neutral evolution happening, which left them with little genetic variation.

"Large population size appears to have allowed for faster adaptive evolution and removal of harmful mutations, driving a huge loss in their neutral genetic diversity," the researchers wrote. "These results demonstrate the effect that selection can have on a vertebrate genome and contradict results that suggested that population instability contributed to this species's surprisingly rapid extinction."

The bottom line: The study says having a huge population was initially a key survival mechanism for the passenger pigeon. However, the birds' surprisingly low genetic variation caused it to be unable to recover from humanity's overhunting practices. As one of the study authors told WashPost, "It's impossible to adapt to gunfire."

Editor's note: This story has been updated to provide further information.

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