Researchers say the findings, the first documented case of rising temperatures driving genetic change in a mammal, offer a "glimmer of hope" for the Arctic species, which is facing possible extinction this century.
The comet is the third object ever confirmed to have entered our cosmic neighborhood from elsewhere in the galaxy. Space telescopes and orbiters have been documenting the rare visit.
New research suggests a volcanic eruption around 1345 cooled the climate, leading to crop failures. On the ships that carried imported grain to fill the gap came plague-carrying fleas.