Scientists conducing the study compared the genetic code of the hemoglobin gene from a 43,000 year-old woolly mammoth to that of modern elephants. Turns out, mammoth hemoglobin has a unique adaptation that seems to have enabled it to continue it's critical oxygen delivery functions even at cold temperatures.
Woolly mammoths evolved in Africa over one million years ago but migrated to Arctic regions just as the global climate began to cool significantly. The recently discovered hemoglobin adaptation must have been among many evolutionary changes that allowed mammoths to survive the brutal cold of the ice ages.
Source: Nature