How grizzlies evolved into polar bears: The first book to tell the whole story is now available

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From polarbearscience

Polar Bear Evolution: A Model for How New Species Arise is the fascinating story of the origin of polar bears. It reveals not just when and where the species came to be, but how it happened and why the bears were able to survive repeated cycles of sea ice change, some of unimaginable severity.

No other book like this exists. Despite decades of serving as an icon for the catastrophic climate change narrative, the polar bear has never had its evolutionary history explained so completely, never mind in a fully-referenced, plain-language style. And I couldn’t have done it without the financial help of my many supporters, so I thank you all again for your assistance in getting this important work completed.

One Amazon reviewer said this about Polar Bear Evolution:

The author of Polar Bear Evolution, Susan Crockford, is a good, credentialed scientist. Her writing is clear; her thinking is also. She has a broad understanding of biology and an informed paleo perspective. Crockford condenses a very large literature on polar bear biology and evolution in this book which will help readers understand the science related to the evolution of an Arctic species. Perhaps the most important aspect of this book is its synthesis of information from the fields of wildlife biology, molecular evolution, paleontology, and climate. Her original ideas and hypotheses on thyroid hormone’s role in evolution are very important and add a credible mechanism of phenotypic change which complements the literature on molecular genetic evolution. Polar Bear Evolution is an important contribution to science and its application in evolutionary biology and wildlife biologyMatthew A. Cronin, Ph.D.

You can quote me: “Modern polar bears are essentially pre-adapted to survive extended periods with ice-free summers because they lived through at least two of these events since they became a unique species. One of these, the Eemian, lasted at least 10 thousand years.” [Susan Crockford, 11 June 2023, author of Polar Bear Evolution: A Model for How New Species Arise]

The book includes a detailed account of brown bear and polar bear 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.

Taken together, the evidence indicates the most plausible time and place for the rise of the polar bear as a new species was just south of Ireland, about 140,000 years ago, at the height of a profoundly cold ice age.

And for the first time, a biological mechanism reveals 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 the extinct dwarf proto-humans from Indonesia known as “Hobbits.”  

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.

This comprehensive look at polar bear evolution provides critical insight into why we should expect Ursus maritimus to survive in a warmer world. Polar bear physiology and behavior have been fine-tuned by natural selection to adapt the species to the highly dynamic Arctic environment of the late Pleistocene, including interglacial periods when sea ice was much scarcer than it is today and lasted thousands of years at a time.


  • We have known for a long time that polar bears evolved from a brown bear ancestor because of the close similarities between fossil bones of ancient bears and biological features of living animals.
  • Despite more than a dozen genetic studies and several new fossil finds, before now no one has been able to reconcile all the evidence regarding when and where this speciation event happened.
  • This book argues that polar bears almost certainly evolved off Ireland about 140 thousand years ago, during a very cold ice age.
  • Uniquely, this account also provides an explanation for how this happened, using a testable theory that explains exactly how a brown bear could have transformed into a polar bear within a few generations.
  • The same theory also explains how virtually all other species came to be, including domestic dogs, flightless birds, and dwarf island species.
  • More than the complete story of polar bear origins, this book is a big-picture account of how evolution works for most organisms.
  • Given their evolutionary history, polar bears must be pre-adapted to extended periods of ice-free summers, since we know they have survived at least two of these events, the first of which lasted at least 10 thousand years.

Polar Bear Evolution: A Model for How New Species Arise is available now via Amazon marketplaces worldwide, in print and ebook formats. Read a sample by clicking on the Kindle or paperback icons.  






Biography notes: Susan Crockford is a zoologist (BSc., Ph.D.) who has studied evolution and Arctic ecology for more than 30 years. She is the author of more than a dozen peer-reviewed scientific papers as well as science books for the general public including Sir David Attenborough and the Walrus Deception (2021), and The Polar Bear Catastrophe That Never Happened (2019).