Three bones—among the tiniest in the human body—sit in your middle ear, where they transmit vibrations from the eardrum to the cochlea, significantly sharpening your hearing. Collectively known as the auditory ossicles, they are unique to mammals. Reptiles and birds all have only one middle-ear bone—the bones that would otherwise be their “ossicles” are instead fused with the joints of their jaw. Despite their size, the ossicles are such a potent evolutionary innovation that they emerged independently multiple times in mammals—not only because of the resulting boost to hearing but also because their detachment from the jaw freed those animals to adapt to a wider variety of food sources. Without ossicles, ancient mammals might have been merely dinner for larger predators—or died off from starvation—rather than becoming our distant ancestors. In some sense, the emergence of the ossicle may have increased the odds of our being here. And now, according to a study published Thursday in Science, the long-sought fossil evidence of the first step in this epochal evolutionary pathway has been found.
Excavating a 123-million-year-old igneous rock formation in northeastern China, researchers unearthed fossils of six specimens (an impressive number, in paleontological terms) of a previously unknown species they named Origolestes lii. Origolestes is an ancestor to therian mammals, the group that includes all placentals and marsupials alive today. Its discovery, the researchers say, fills a missing link in the evolution of mammals’ auditory arrangement.
The study’s lead author Fangyuan Mao, a paleontologist at the American Museum of Natural History (AMNH) and the Chinese Academy of Sciences, calls the fossils “absolutely breathtaking.” Mao and her colleagues analyzed the ossicles of several of the specimens and noted an apparent gap between the bones and an ossified bit of cartilage that connects to the jaw. Mao says the discovery shows the evolutionary moment when the ossicles decoupled from the lower jaw, dividing the two previously integrated functional systems of hearing and chewing. “This final step removed the physical constraint of the two systems and provided a greater potential for better and more efficient developments of hearing and chewing,” Mao says. “Perhaps this is one of the factors that contributed to the success of therians.”
With the benefit of paleontological hindsight, that scenario certainly seems to be the case. Decoupling the ossicles from the bones in the lower jaw was so useful that at least three evolutionary pathways led to it in mammals alone, a process known as convergent evolution. The study’s corresponding author Jin Meng, a paleontologist at the AMNH and the Graduate Center, City University of New York, says the decoupling also allowed later adaptations to emerge, such as the ability to hear high-frequency sounds or eat different types of food.
Another study that Meng co-authored, published in Nature in November, provides an interesting contrast in a distantly related, previously unknown mammal named Jeholbaatar kielanae, which lived around the same time as Origolestes. Jeholbaatar belonged to a now extinct lineage of mammals called the multituberculates that endured for well over 100 million years. Like all multituberculates, it chewed its food in a back-and-forth cutting motion. The unique approach to chewing probably contributed to multituberculates’ long success, but it also meant Jeholbaatar had to have a very specific jaw configuration, which limited its options for middle-ear evolution, resulting in a completely unique arrangement.
Anne Weil, a vertebrate paleontologist at Oklahoma State University, wrote an accompanying article concerning Meng’s paper in Nature. “The question is, ‘Does this keep happening throughout basal mammalian evolution because it improves hearing? Or is the driving force that it improves food processing?’” she says. For Jeholbaatar, food processing apparently was the more likely impetus. “But that still leaves the question open for other lineages,” she says.
Once hearing and chewing were decoupled in Origolestes, each system was free to evolve independently. Meng says the hearing and chewing systems are each likely regulated by their own distinct developmental genetic mechanism, a process evolutionary biologists refer to as modularity. Stephanie Smith, a mammalogist at Chicago’s Field Museum of Natural History, who was not involved in either study, says it is common to see modules (the evolutionary biology term for such systems) evolving independently in ways that can be linked to behaviors, such as chewing. “It’s not just about the genetic control of those modules,” she says. “It’s also a story about the function of those modules and how being decoupled allowed them to evolve.”
While ossicles and jawbones worked together as one compound organ in ancestors of Origolestes, the decoupling seen in it, Meng says, allowed modularity that, in turn, permitted a diverse array of specialized ear and jaw morphologies to evolve among its descendants.
Zhe-Xi Luo, an evolutionary biologist at the University of Chicago, who studies early mammalian evolution and was not involved in the Science study, is less certain of its claims. After reviewing the images (but not the actual fossils) provided by the authors, he suspects the gap between the ear bones and cartilage is only a fracture rather than a profound evolutionary jump. “It’s a preservation artifact; that is so common in fossils,” Luo says.
Mao and Meng stand by their interpretation of the fossil, noting that the auditory bones are preserved in four of the specimens and that all share a similar morphology. “It is unlikely all of them are artifacts,” Mao says. Meng adds that while different interpretations are common in paleontology, a close inspection of the fossilized ossicle clearly shows it is not fractured. Luo says he thinks more evidence is necessary.
The separation of the auditory ossicles from the jaw—which Mao and Meng have apparently captured a snapshot of with Origolestes—was so valuable that it served as a nexus for mammalian evolution. It formed convergently, via multiple evolutionary pathways, and, at least in therian mammals, gave rise to myriad new innovations in jaw and ear evolution. “It’s a tiny little animal, and it’s 123 million years old, and it’s marvelously well-preserved.” Weil says. “It’s pretty cool to see that separate [auditory bone] in this lineage.”