In "Ends of the Earth," Neil Shubin travels north and south to explore the frontiers of polar science and history.

Two-year-old twin polar bears Kallik (pronounced Kah-LEEK), left, and Kallu (Kah-LOO) take a swim during their public debut ...

Kallik and Kallu made their public debut at the St. Louis Zoo on March 25. Kallik and Kallu: The twin polar bear brothers are ...