Why Mammals Are So Good at Hearing (And Chewing)
Before an evolutionary breakthrough, the tiny bones of ears were part of the jaw.
One hundred and twenty million years ago, when northeastern China was a series of lakes and erupting volcanoes, there lived a tiny mammal just a few inches long. When it died, it was fossilized down to its most minuscule ear bones. And it is these ear bones that have so intrigued scientists: They are evidence of how evolution created the unique ear of mammals, giving modern mammals—including us—a finely tuned sense of hearing.
Today, mammals have three small bones in the ear that transmit sound from the eardrum: the malleus, incus, and stapes. A wealth of evidence from fossils and developing embryos suggests that two of these ear bones were once jawbones. Over millions of years of evolution, they shrank in relative size and detached completely from the jaw. Reptiles—like our nonmammalian ancestors, probably—hear by placing their jaw on the ground to pick up low-frequency vibrations. But mammals, with their three ear bones, can hear high-pitched sounds in the air: insects buzzing, wind rustling, birds squawking, music, speech.
The fossilized mammal found in northeastern China, named Origolestes lii, has an ear that looks close to modern. While parts of its body still look quite ancient, its ear bones, according to the study’s authors, have moved away and detached from the jaw. “That separation is critical because it allows the separation of hearing and chewing,” says Jin Meng, the curator of fossil mammals at the American Museum of Natural History and an author of the paper. And thus, the ear and the jaw could evolve separately in mammals, each specializing in what it does.
In fact, it may have been chewing that initially drove the detachment of the would-be ear bones and the jaw, says Vera Weisbecker, who studies mammalian evolution at the University of Queensland and was not involved in the study. Chewing is actually unique to mammals. It requires a jaw that can move side to side in addition to up and down, teeth that can grind, and lips to keep food inside the mouth while masticating. (Consider a crocodile, says Thomas Martin, a paleontologist at the University of Bonn. It doesn’t have lips. “If it would chew, all the food would fall out of the mouth.”) The ability to chew may have unlocked additional nutrients and food sources for mammals. Ruminants today, such as cows, for example, can eat tough plants because they chew and chew. And the detachment of extra bones in the jaw helped create a more flexible joint for chewing.
Those extra bones, it happens, ended up being co-opted for hearing in the middle ear. Scientists who study mammalian embryos have also noticed that two of the tiny bones of the middle ear start off attached to the jaw. “The ear develops by first developing as part of the mandible,” says Neal Anthwal, a developmental biologist at King’s College London who studies the mammalian jaw and ear. It is only later in embryonic development that the ear bones detach and the piece of cartilage that connects them, called the Meckel’s cartilage, dissolves. Scientists have in fact found ways to mutate mice so that their ear bones remain connected to their jaw—essentially reversing a step in mammalian evolution.
This separation of the middle ear appears to have, interestingly, happened at least three independent times in the evolution of mammals—in the ancestors of monotremes (a group of egg-laying mammals that includes the platypus and echidna), of therians (a group including marsupials and placental mammals), and in another group of mammals that has gone entirely extinct. Somehow, all three groups converged on the same adaptation of separating their chewing and their hearing. Origolestes lii, Meng says, belonged to a group of animals that eventually evolved into placental mammals and into us humans.
The first of the Origolestes lii fossils in this study was actually found in 2003. But Meng says the team waited a long time to analyze the fossils, because they were so tiny and difficult to extract from the rock around them. “It’s not until recently we have high-resolution CT scan,” he says, which can be used to look inside the fossils without breaking them apart.
Zhe-Xi Luo, a paleobiologist at the University of Chicago who specializes in the evolution of the middle ear, says it is possible that the ear bones were actually connected to the jaw and fractured apart only after the animal died—as he has found in a fossil of a closely related mammal from the same time period and region in northeastern China. He agrees that ear bones must have completely detached from the jaw at some point in the evolution of mammals, though. “The only way this can be resolved is, we get another fossil that shows it,” he says.
This area of China, close to the North Korean border, is in fact a hotbed of fossil-hunting activity. The lake beds and volcanic eruptions that rained down ash 120 million years ago created fantastic conditions for fossil formation. Since the 1990s, amateurs and professionals alike have been unearthing exquisite fossils from this region. (Several of the six Origolestes lii fossils presented in the paper were originally found by local villagers.)
Fossil hunters in northeastern China have most famously found dinosaurs with delicately preserved feathers—as well as dozens of new species of birds, plants, mammals, and fish. Together, they are clarifying and complicating the story of life on Earth in the Mesozoic era, some 252 million to 66 million years ago. “We have these complete Mesozoic skeletons in the last 20 years or so, which really dramatically changed our picture of mammalian evolution,” Martin says. “We live in exciting times.” If the story of the mammalian ear is to get any clearer, it will likely be because of fossils found there.