Somewhere during the mid-Pleistocene period (roughly 100,000 to 250,000 years ago), a number of brown (same as grizzly) bears (Ursos arctos) probably became isolated by glaciers. many probably perished on the ice; however, they apparently did not all disappear. Some survived due to the fact that "organisms vary" (Steve Gould's terminology and logic is used here), that is, every litter of grizzly's has a variation in coat thickness, coat color etc. which imparted a slight evolutionary advantage to some indivials of each litter. These successful individuals underwent an apparent rapid (rapid, probably because of the small population, and extreme selection pressure) series of evolutionary changes in order to survive (note they were not necessarily "better" in any absolute sense, or on any absolute "bear" scale of perfection - they were simply more in keeping with their new environment than their siblings). Today, polar bears are adapted to their harsh northern environment.
Hecht (in Chaline, 1983) describes polar bear evolution: the first "polar bear", Ursus maritimus tyrannus, was essentially a brown bear subspecies, with brown bear dimensions and brown bear teeth. Over the next 20,000 years, body size reduced and the skull elongated. As late as 10,000 years ago, polar bears still had a high frequency of brown-bear-type molars. Only recently have they developed polar-bear-type teeth.
Kurten (1976) describes bear transitions: "From the early Ursus minimus of 5 million years ago to the late Pleistocene cave bear, there is a perfectly complete evolutionary sequence without any real gaps. The transition is slow and gradual throughout, and it is quite difficult to say where one species ends and the next begins. Where should we draw the boundary between U. minimus and U. etruscus, or between U. savini and U. spelaeus? The history of the cave bear becomes a demonstration of evolution, not as a hypothesis or theory but as a simple fact of record." He adds, "In this respect the cave bear's history is far from unique."
Ursus minimus (Pliocene) First little bear, with very bearlike molars, but still had the first premolars and slender canines. Shows gradual tooth changes and increase in body size as the ice age approached. Gave rise to the modern black bears (U. americanus & U. thibetanus), which haven't changed much since the Pliocene, and also smoothly evolved to the next species, U. etruscus: Ursus etruscus (late Pliocene) A larger bear, similar to our brown bear but with more primitive dentition. Molars big & square. First premolars small, and got smaller over time. Canines stouter. In Europe, gradually evolved into:
Ursus savini (late Pleistocene, 1 Ma) Very similar to the brown bear. Some individuals didn't have the first premolars at all, while others had little vestigial premolars. Tendency toward domed forehead. Slowly split into a European population and an Asian population.
U. spelaeus (late Pleistocene) The recently extinct giant cave bear, with a highly domed forehead. Clearly derived from the European population of U. savini, in a smooth transition. The species boundary is arbitrarily set at about 300,000 years ago.
U. arctos (late Pleistocene) The brown ("grizzly") bear, clearly derived from the Asian population of U. savini about 800,000 years ago.. Spread into the Europe, & to the New World.
U. maritimus (late Pleistocene) The polar bear. Very similar to a local population of brown bear, U. arctos beringianus that lived in Kamchatka about 500,000 years ago (Kurten 1964).
If you have gotten this far, you are rather hard core! I should say
that I do not work on fossils; if you want to know what I work on, go here
Phil Candela
