Phylogeny

The closest living relatives to bighorn sheep are the five other sheep species in the genus Ovis. They include argali, mouflon, Dall sheep, Siberian bighorn, and urial. The closest relative to sheep are true goats. The difference between sheep and goats is the glands or lack of glands in each animal. Sheep have preorbital and inguinal glands, absent in all goats. Whereas goats have an odoriferous tail gland, which is lacking in sheep.

Mountain sheep evolved from the Rupicaprini, a large group of animals, including sheep and goats. Two million years ago, there existed a bridge, made from the surface of the Bering Sea when the ice sheets separated. This bridge, known as Beringia, connected northern  Asia to Alaska. However, this bridge was constantly hammered with vicious winds and crashing seas, making it almost impossible to cross. It was these conditions that the ancestors of our modern day bighorns endured for migration. Perhaps one million years ago, the first group, recognized as bighorns, traveled across Beringia, slowly but surely made their way into northern Parts of Alaska. They then began migrating south into Canada. However, these same ice sheets that opened Beringia, melted and separated these sheep from future herds that crossed Beringa. Thus, leading to the isolation of mountain sheep into the two groupings we recognize today, bighorns and thinhorns.