Evolutionary mystery: Why are mammals the only vertebrates to have only one lower jawbone?

When you look at the jawbones of a blue whale, a hyena, or even a human, you’ll notice something interesting. Each one, regardless of its size or purpose, consists of a pair of lower jawbones.

This structure is a hallmark of mammals. It gives us everything from the blue whale’s 20-foot-long filter-feeding jaw, to the bone-crushing prowess of the hyena, and our own delicate human chin bones.

Yet, it might seem like a disadvantage at first. After all, why would having a single bone on each side of the head, forming a stiff lower jaw or mandible, be a good thing?

Is there an advantage to having just one lower jawbone?

Other vertebrates such as crocodiles and snakes have between two and 11 bones in their lower jaws. This shows that a jaw with multiple bones can be advantageous.

For instance, despite having about five bones on each side of their jaws, crocodiles have a bite strength relative to their size that’s more powerful than a hyena’s.

Snakes, on the other hand, can open their mouths wider than baleen whales relative to their size. This is due to their lower jaws’ unique structure with around four bones. This feature even allows snakes to dislocate their jaws to swallow prey larger than their heads.

So what’s the benefit of having two single jawbones. These are fused at the chin into one solid mandible in humans and other primates?

That’s the question paleontologist Jack Tseng, an assistant professor of integrative biology at the University of California, Berkeley, aimed to answer.

How the jawbone study was conducted

He built a database of over 1,000 vertebrate jaws. This represented just a small fraction of the approximately 66,000 jawed vertebrate species on Earth.

His goal? To determine if the mammalian jaw structure was an evolutionary improvement over the multi-boned jaws of fish, lizards, snakes, and other non-mammals. To do this, Tseng even created 3D models of many lower jaws to test their durability.

What the research team learned about lower jawbones

The answer he found was surprising. As it turns out, having a single lower jawbone on each side doesn’t provide a significant advantage over a jaw with multiple bones.

“Should we interpret the innovation of the mammalian jaw as a pure adaptation that enabled, in all ways, mammals to be more successful after the dinosaurs went extinct? I think the answer is no,” Tseng said.

Instead, he found that multiple jawbones actually give animals an advantage when it comes to biting. These bones work together, providing more flexibility and speed. On the other hand, the single bone per side in mammals actually restricts their evolutionary options.

Interestingly, despite this limitation, mammals have managed to adapt and consume a wide variety of foods. They rival the dietary diversity of vertebrates with more than one bone in their jaw.

“There’s this idea of a trade-off between the additional flexibility and maybe additional speed that you could achieve with multiple bones in a jaw — which essentially compounds or enlarges movement — and the increased stiffness or increased bite strength when you have a single bone in the jaw,” Tseng explained.

“That sort of dichotomy between mammals and non-mammals supposedly enabled mammals to essentially become eaters of all things.”

Tseng’s research is into jawbones is groundbreaking

“Nobody’s tried to combine information from all of these groups of jawed vertebrates to ask general questions about how jaw shape and function are related,” he noted.

He concluded that the structure of the jaw isn’t as linked to its function in vertebrates as one might assume. Mammal jaws, despite being more diverse in shape, are more constrained in their biomechanical characteristics compared to non-mammals.

“Mammal jaws can take, on average, more different shapes compared to non-mammal jaws, but those different shapes have narrower range of different mechanical properties than non-mammals,” Tseng observed.

“This is a new observation that could potentially open up different ways of looking at mammal jaw biomechanics.”

“The main finding was that, yes, indeed, mammals, given their single-boned lower jaw, have significantly higher strength or stiffness on average compared to any non-mammal jaw,” Tseng said.

“This is true overall, regardless of what specific mammals are doing — it doesn’t matter if you are a carnivore or an herbivore. Stiffness is not a predator trait or herbivore trait, it’s a mammalian trait, a signature of the mammal jaw.”

So, why do mammals have fewer bones in their lower jaw? They didn’t lose them. Those extra bones evolved into the mammalian inner ear. This potentially gave mammals superior hearing compared to their vertebrate relatives.

“A solid, stiff jaw in mammals is thought to be a side effect of establishing a uniquely mammalian hearing system,” Tseng added.

Examining the jawbone through an engineering lens

This transition from a complex jaw to a simpler one, but with a complex inner ear, has been primarily studied concerning the ear.

Tseng has a background in studying bone-crushing animals like hyenas. He wanted to explore this transition from the jaw’s perspective and through an engineering lens.

He digitized two-dimensional jaw shapes from over 1,000 vertebrate species. Next, he established the key characteristics of these jaws. Finally, he simulated their mechanical performance, including shapes not seen in nature.

This research revealed how mammalian and non-mammalian jaws compare in terms of strength and function. Both groups spanned the range.

This indicated a similar potential for adaptation. However, mammalian jaws tend to be stiffer than non-mammalian jaws.

More research planned for lower jawbone evolution

Tseng plans to expand his research by adding more vertebrate species to his database and incorporating 3D scans of jawbones for a more accurate biomechanical assessment of stiffness and strength.

He hopes that other researchers will further investigate the role of genetics in the evolution of mammals’ complex ear structure and simple jawbone structure.

He’s interested in understanding the consequences of this transition for mammalian evolution and why this trait seems to have become permanently fixed in mammals.

“We hope our findings prompt some people to search for genetic bases for why this is a one-way street,” Tseng concluded.

This study was published last month in the journal Philosophical Transactions of the Royal Society B. It could lead to a greater understanding of how this characteristic separation of structure and function in mammals helped them adapt to new environments during key geological times.

These include the extinction of non-avian dinosaurs and the emergence of land bridges connecting continents. Both enabled the mixing of different ecological communities.

Along with Tseng, the study’s co-authors include Sergio Garcia-Lara and Emily Holmes of UC Berkeley, John Flynn from the American Museum of Natural History in New York, Timothy Rowe of the University of Texas at Austin, and Blake Dickson from Duke University in North Carolina.

More about jawbones

Jawbones, also known as mandibles, play a critical role in a variety of creatures, from mammals and reptiles to birds and fish. They are part of the body’s skeletal structure. Thus, they are instrumental in eating and communication. Here are some key aspects about jawbones:

Structure and Function

In humans and many other animals, the jawbone is a horseshoe-shaped bone. It is the only bone in the human skull that can move and is responsible for the up and down movement of the mouth. The jawbone holds the lower teeth in place and works with the upper jaw to help in chewing food and in the articulation of speech.

In mammals, the mandible is typically made of a single bone on each side, which is a unique characteristic among vertebrates. Other vertebrates, like fish, reptiles, and birds, usually have complex, multipart jaws.

Evolution

The evolution of the jawbone is a fascinating topic in the field of evolutionary biology. Jawbones have evolved significantly over millions of years.

The first jawed animals appeared during the Silurian period (approximately 440 million years ago). These early jawed vertebrates, or gnathostomes, included both fish and amphibians.

The evolution of jawbones in mammals is particularly interesting. Early mammalian ancestors, or synapsids, started with a jaw composed of several bones.

However, over time, these bones simplified and eventually, mammals ended up with a single bone on each side of the jaw. Some of the other bones migrated to the ear, forming the auditory ossicles. This is a significant evolutionary change that possibly enhanced hearing in mammals.

Variation Across Species

Jawbones differ considerably among various animal species, not only in size and shape but also in terms of strength and the number of constituent bones.

For example, the blue whale, the largest creature on Earth, has massive jawbones that enable filter feeding. Crocodiles have incredibly strong and powerful jaws, and snakes have highly flexible jaws that can dislocate to swallow large prey.

Health and Disease

The jawbone can also be affected by various health conditions. Disorders like temporomandibular joint disorder (TMJ) can cause pain in the jawbone and the muscles controlling jaw movement.

Other conditions, like osteonecrosis, can cause damage to the jawbone itself. In dentistry, maintaining the health of the jawbone is crucial.

This is especially true for procedures like dental implants, where the implant must fuse with the jawbone to provide a sturdy foundation for artificial teeth.

Fossil Records and Anthropology

Jawbones are crucial in the field of anthropology and paleontology. They can provide a wealth of information about extinct species, including their diet, age, and size.

Jawbones can even offer hints about social structures and mating habits. In human evolution, changes in jawbone size and structure are linked with dietary shifts. These include the transition from raw to cooked food, and the advent of farming.

In summary, jawbones are fascinating from many perspectives, including biology, evolution, health, and anthropology. They’re critical for various life functions and hold many secrets about our evolutionary past and the diversity of life on Earth.