A new study shows that bears, which look like typical mammals, have made two "irregular operations" in the development of molars in their long evolutionary history, breaking what is considered a universal tooth growth blueprint for mammals. These ancient modifications are still clearly preserved in the oral structures of living bears.
The study was carried out by scientists from the Bavarian State Natural Science Collection (SNSB). They systematically analyzed bear jaw and molar fossils spanning at least 13 million years, and compared them with living species. They found that ancient bears not only evolved new forms of molars, but also reprogrammed the rules of the "inhibitory cascade model" (ICM) in the production of mammalian molars at the molecular signal level. This "program modification" was also carried out during two major climate and environmental upheavals.
In conventional mammals, molar development follows a highly stable program: the first molar releases a chemical signal when it is formed, partially inhibiting the growth of the following molar, and forming an orderly volume gradient from front to back along the lower jaw. Therefore, just based on the size combination of the molar teeth, one can roughly determine whether one is carnivorous, herbivorous, or omnivorous. However, research has found that ursids are inherently "difficult to classify": their second molars (m2) are extremely large and never match this classic model. This weird feature appeared early in their evolution.
The first key "rewriting" occurred about 3.6 million years ago. The protagonist was Ursus minimus, who lived in the late Pliocene and is regarded as the common ancestor of most living bears. Research shows that compared to its predecessor Ursus boeckhi, the middle molars of bears during this period were significantly enlarged, which coincided with Europe's transition from warm and humid subtropical forests to a cooler and drier environment dominated by woods and emerging grasslands: the number of originally abundant small vertebrates and tree climbers declined, and although large herbivores increased, it was difficult to hunt them. Plant food, underground roots, seeds, nuts and various invertebrates became easier to obtain, forcing these small bears to switch from carnivores to typical omnivores.
In order to adapt to this change in diet structure, bears between U. boeckhi and U. minimus have undergone essential adjustments in molar development signals: the inhibitory signal released by the first molar (m1) is weakened, giving the second molar the opportunity to "break through the specification" and become extremely large. The research team calls this new configuration a "partial inhibition cascade", that is, the entire sequence still generally follows the gradient direction of mammalian molars, but the entire sequence is "raised" by the enlarged m2 as a whole, thereby obtaining stronger grinding ability without completely overturning the original model. U. minimus thus becomes the first bear representative in the history of mammalian evolution to clearly record the "modification of molar tooth program".
The second "exception" occurred during the Early-Middle Pleistocene transition period about 1.25 million to 700,000 years ago, when European grasslands continued to expand and the climate continued to cool, closely coinciding with the emergence of cave bear ancestor Ursus deningeri. Bears during this period further shifted toward herbivory. Research shows that the program of molar development was adjusted again: this time the inhibitory signal of the second molar (m2) was weakened, so the third molar (m3) was enlarged, increasing the chewing surface of the rear segment, and significantly strengthening the "grinding capacity" for plant foods. In early cave bear fossils such as U. deningeri, the volume ratio of m3 to m2 has significantly exceeded the expectations of the classical model.
Anneke van Heteren, head of the SNSB Mammalia Collection, pointed out that these two adjustments indicate that during the evolution of bears, the ratio of inhibitory and activating signals that control the growth of each molar has shifted significantly, and these shifts are highly related to the change in diet from carnivorous to omnivorous and even highly herbivorous. In other words, when bears face the rapidly changing climate and ecological environment, they do not just respond by adjusting the shape of their crowns or bite methods, but directly "touch the source code" and change the balance of chemical signals when teeth are produced.
What’s even more intriguing is that these ancient “program patches” still have a profound impact on living bears today. Brown bears (Ursus arctos) and American black bears (Ursus americanus) still retain large second molars that echo their omnivorous phase in the Pliocene, clearly inheriting that rule-breaking. Even the polar bear (Ursus maritimus), which is almost exclusively carnivorous, still uses this old plan for the configuration of molars in its mouth, but they no longer need to rely on these teeth to grind plants efficiently.
Another "maverick" branch is the giant panda (Ailuropoda melanoleuca). Research points out that giant pandas can still see the shadow of breakthroughs in ancient rules in their basic patterns, but their subsequent evolution path is more radical: instead of relying on enlarged molars, it is better to build a "heavy-duty chewing machine" specifically for a high-fiber bamboo diet by strengthening the jaw structure, thickening the cheek teeth, and allowing some premolars to undertake heavy grinding tasks.
However, the spectacled bear (Tremarctos ornatus) in South America has temporarily become an "unsolved mystery" in this research framework. This species also shows a typical pattern of giant second molars, but because the fossil samples of its closely related short-snouted bears were not included in this analysis, scientists have not yet been able to determine when and through what signal adjustment this system embarked on a unique tooth evolution trajectory. The research team called for further fossil research on the Arctotheriini group to help determine whether spectacled bears also experienced a similar "inhibitory cascade" reprogramming process.
The authors emphasize that the inhibitory cascade model is extremely robust across a wide range of mammalian taxa, so the record of two clear deviations from this rule in bears is particularly striking. Not only did they adapt to changes in diet by changing their tooth morphology, they also "bypassed" the strict internal restrictions at the level of developmental biology and directly "rewritten" the growth program of their molars. This bold evolutionary attempt is still engraved in their dentition.
Relevant research has been published in the journal "Boreas" and was led by the Bavarian State Collection of Natural Sciences. It further shows that teeth, the "black box of nature", are much more "eloquent" than they appear in terms of reconstructing the story of environmental changes and species evolution.