Bears look like textbook mammals, but hidden in their evolutionary history are two dramatic departures from the standard blueprint of growth and adaptation. For the first time, scientists have unlocked when, and how, ancient bears broke the rules and hacked nature out of need.
Scientists at the Bavarian State Collections of Natural History (Staatliche Naturwissenschaftliche Sammlungen Bayerns, or SNSB) have looked into the mouths of bears through time and found something truly unusual for mammals. Ancient bears didn’t just evolve new teeth – they rewrote one of the basic rules that governs how mammalian molars grow. In fact, they switched up the blueprint twice, in response to huge climate-driven shifts in diet. And those changes are still visible in all living bear species today.
Dentition is one of the most underrated “superpowers” in biology – what's in our mouths can tell us so much about evolution, ecology, behavior, diet and even climate. It's for this reason, and that they're so tough, that teeth have been called nature's black box. In mammals, most mammal species follow a predictable pattern when molars develop. As each tooth forms, it releases chemical signals that partially suppress the growth of the next one, producing a neatly arranged gradient along the jaw. This “inhibitory cascade model” (ICM) is reliable enough that carnivores, herbivores and omnivores can be predictably grouped when you compare their teeth. For example, in carnivorous mammals, the first molar is usually larger than the third, while this is reversed in herbivores.
Bears, however, don’t fit the mold. Their middle (second) molar is disproportionately large, and the entire family has always been at odds with the law of nature. In this new study, researchers have examined bear jaws through time, from ancient fossils – at least 13 million years old, dating back to the Miocene – through to modern species, to figure out why these mammals broke evolutionary rules at two distinctive times in history – which were also two periods of extreme environmental upheaval.
The first major rule-break occurred around 3.6 million years ago, in Ursus minimus, a late Pliocene bear – and common ancestor of most modern species – whose oversized middle molar marked a big developmental shift from its predecessor, Ursus boeckhi. The two species straddle a period of climate upheaval during the Early to Late Pliocene, in what is now Europe, where this part of the bear family tree was developing at the time. The habitat shifted from warm, humid, subtropical forest to cooler, drier landscapes dominated by woodland and emerging grassland. Small vertebrates, climbing mammals birds and reptiles – which formed much of the diet of U. boeckhi – became less abundant, and the large grazing mammals that established themselves on these new open ranges were too big for these relatively small bears to hunt. However, plant foods, roots, bulbs, seeds, nuts and invertebrates like beetle larvae became more available. This saw a big shift in bears to a more omnivorous diet – which then demanded a different set of biological tools.
Evolution delivered it, with the bears experiencing a reduction in those molecular inhibitory signals produced by the first molar (m1). This meant the second molar (m2) was free to grow unusually large – and bigger than the standard molar-development rules should allow. The researchers call this new configuration a “partial inhibitory cascade,” because the overall pattern still tracks the general ICM molar-growth direction of other mammals, but the entire system is shifted upward thanks to that oversized middle tooth. As such, U. minimus emerged with an oversized m2, marking a huge break in the rules of development for mammals.
Then a second break happened around 1.25 to 0.7 million years ago, during the Early–Middle Pleistocene Transition, when grasslands spread across Europe and the climate cooled. In this period, the ancestors of the prehistoric cave bears (Ursus spelaeus), Ursus deningeri, became increasingly herbivorous. And once again, there was a tweak made to the molar-growth program, this time a reduction in inhibitory signals from m2, which saw the third molar (m3) grow larger, adding more grinding capacity, to accommodate the plant-heavy menu.
“Apparently, the balance of chemical compounds that inhibit or activate the growth of the different molars shifted during these periods," said Anneke van Heteren, responsible for the mammal collection at SNSB. "These shifts are probably associated with dietary adaptations of bears in the course of their evolution. On their way from carnivores to omnivores or herbivores, bears adapted to a changed food spectrum, but without following the ICM pattern. Their spectrum still ranges from pure carnivores to pure herbivores, with most bears today being omnivores."
Indeed, a million years later, modern bears still carry their ancient modifications. Brown bears (Ursus arctos) and black bears (Ursus americanus) have oversized second molars linked to Pliocene omnivory. Polar bears (Ursus maritimus), despite being highly carnivorous, also have the same makeup even though they no longer need it for plant grinding. Even giant pandas (Ailuropoda melanoleuca) retain the underlying pattern, even though their path deviated a whole lot – they have reinforced jaws and prominent premolars, rather than enlarged molars, to deal with their bamboo wrangling.
While it may seem like a normal evolutionary response to a changing world, the ICM is a tightly controlled developmental rule that governs molar growth across much of the mammal tree. Yet bears found ways to bypass those constraints twice, hacking the program itself – something that's still evident in their descendants' mouths today.
The researchers also flag the South American spectacled bear (Tremarctos ornatus) as a species that doesn’t quite fit the story. It carries the hallmark large-m2 pattern inherited from ancient rule-breaking bears, but because none of its extinct short-faced relatives were represented in this study's fossil dataset, scientists still don’t know how that odd dental arrangement evolved.
"Further research should be conducted on Arctotheriini to determine whether a similar shift took place in one of the ancestors of T. ornatus," they noted.
The research was published in the journal Boreas.
