This fascinating story finally brings together everything that scientists have learned about the origin of polar bears, in clear language that non-scientists will understand: not just when and where the species arose, but exactly how it happened and why the bears were able to survive repeated cycles of sea ice change, some of unimaginable magnitude. The book includes a detailed account of the fossil evidence, recent hybridization events between brown bears and polar bears, and summaries of more than a dozen genetic studies that have been done on these bears to determine the most plausible time and place for the origin of polar bears.

For the first time, a biological mechanism explains how this rapid transformation from a brown bear ancestor could have happened. Thyroid hormone, essential for countless coordinated body functions including stress responses, the growth of embryos, and the activation of critical genes, seems to have played a vital role in the vast majority of all speciation events. A testable theory based on thyroid hormone not only explains how polar bears came to be but does the same for domestic dogs, flightless birds like the dodo, and extinct dwarf species like Homo floresiensis aka “The Hobbit” from Flores Island, Indonesia.

If you’ve ever wondered whether polar bears could have arisen more than once or if hybridization with brown bears really did play a significant part in polar bear evolution, this book is for you. It also explains how polar bear biology and behavior have been fine-tuned by natural selection, adapting the species to the highly dynamic Arctic environment of the late Pleistocene, which included interglacial times when sea ice was scarcer than it is today. This book is more than the complete story of polar bear origins: it’s a big-picture account of how evolution works for most organisms. It’s a fascinating story that deserves to be a staple in your home library.