The evolution of the Polar Bear has been well documented through the preservation of fossils and their relation to the different layers of sedimentary rock they have been discovered lying in.
It can be seen that about 100,000 to 250,000 years ago a group of brown bears - the same family as the Grizzly, became trapped on the ice and were cut off from the mainland. The reason for this is unclear but it was probably glacier activity. Many of the bears perished because of the harsh environment, but it would appear from studying the fossil evidence that a few must have survived.
Throughout the ages as each litter was born they underwent rapid evolutionary changes that enabled them to deal with the harsh conditions more easily. The ones who adapted more easily were more likely to give birth to offspring with these physiological advantages passed down. For example the brown bear has a fur that can go from dark to light depending on its climate and habitat. Those bears born on the ice with light colored fur, had an advantage in hunting seals and successful hunters passed this lightness of fur down to future generations.
Over the next 20,000 years the body size became reduced and the skull became much more elongated. Fur became thicker( in fact they developed two layers), paws became much larger and the neck became longer useful for holding up the head out of freezing water whilst swimming. Compared to the brown bear of today, the polar bear has a heightened sense of smell, hearing and sight, all useful traits that enable them to live in their arctic environment.
In comparison to the Brown bears vegetarian diet, the Polar bear has evolved to tear and digest meat. The teeth are much sharper and more widely spaced apart with their cheek teeth being useful for shearing lumps of meat from their kill and the large canine teeth are much longer and sharper for holding their prey.
There are still similarities to the Brown bear however. For example the male Polar bear is very unsociable as is the Brown bear and the females will still create a den to give birth to twin cubs and give shelter to the cubs for the first 3 months of their lives.
The Polar bear has evolved to the point that they can survive in temperatures as low as -50 degrees Fahrenheit. In fierce storms He will dig a hole in a snow bank and shelter from the wind, waiting out the cold spell, sometimes covering his muzzle with a huge paw to stop the heat from radiating out.
In fact the Polar bear has more of a problem with overheating than with the bitter cold. Because of the efficient nature of their physiology they overheat easily and cannot run for very long, choosing to walk at a leisurely pace.
The population is estimated to be at between 20,000 25,000 in the world today and exist in Alaska, Canada, Greenland, Norway and Russia, but these numbers are dwindling due to the changing patterns of their ice habitat. Scientists estimate that sadly the species will be extinct in the wild within 100 years.
Conservation projects are underway now with real life environments being created for them in Zoo's and animal sanctuaries. More and more research is being done to make sure that the captive Polar will have as near to natural a life in captivity, alleviating the terrible boredom that this enclosed life gives them.
Webster World Encyclopedia