Inner ear of extinct ape species is overlooked aspect of human bipedal evolution, study finds

The inner ear may not seem like a particularly bony place, but human ears in fact have three small bones (also known as ossicles): the malleus, the incus and the stapes. While most people would assume that these bones are necessary for hearing, one would not imagine that they relate much to how we walk.

Yet according to Chinese and American scientists working together for a study in the journal The Innovation, the ear bones of ancient apes can teach us a lot not only about our primate ancestors, but also about ourselves.

It all comes down to bipedalism, or the fact that humans walk on two legs. Because our various primate ancestors were often quadrupedal (walking on four legs), evolutionary scientists have often wondered how we made the shift from being a four-legged species to one that relies on two legs. The experts turned to the seemingly obvious places for answers: They studied the bones of ancient monkeys when they came from their limbs, pelvis, shoulders and spine. Yet in The Innovation study, the team of scientists looked instead to the inner ear. They specifically chose the remains of a Lufengpithecus, an ape from China that has been extinct for 7 to 8 million years.

"The semicircular canals, located in the skull between our brain and the external ear, are critical for our sense of balance and position when we move, and they provide a fundamental component of our locomotion that most people are probably unaware of," Zhang Yinan, first author of the study and a Ph.D. student at the Institute of Vertebrate Paleontology and Paleoanthropology, said in a press statement. "The size and shape of the semicircular canals have mathematical correlation with how mammals, including apes and humans, move around their environment. Using modern imaging techniques, we are able to visualize the internal structure of fossil skulls and study the anatomical details of the semicircular canals to reveal how extinct mammals moved."

After doing this, the scientists observed that the Lufengpithecus inner ear revealed an animal that moved in ways unlike anything previously known about ancient or modern primates. Instead, it is believed that the primate moved around by combining various motion types — clambering, climbing, bipedalism, quadrupedalism and forelimb suspension. The ancestors of the Lufengpithecus did not move anything like this — their locomotion was more analogous to what we see today among gibbons in Asia — and humans developed their bipedalism afterward. In a sense, the inner ear bones of the Lufengpithecus is a overlooked connection in the evolutionary history of human locomotion.

As the authors of the study explain, it is because the inner ear bones yield information about the locomotion of primates that cannot be found through traditional methods.

"The bony labyrinth of the inner ear of vertebrates houses the peripheral vestibular system comprised of three fluid-filled semicircular canals that are functionally tied to sense of balance, spatial orientation, posture, and body movements," the authors explain. "This, in turn, is linked to modes of locomotion among living and extinct taxa."

Want more health and science stories in your inbox? Subscribe to Salon's weekly newsletter Lab Notes.

Interestingly, natural climate change may have also played a role in this ear evolution. After observing that evolutionary rates tend to slow down as global temperatures rise, the authors point out that "the dramatic increase in the average evolution rate of semicircular canals" within these apes may illuminate that "the rapid evolution of bipedalism in the human lineage in response to gradual global cooling." The Lufengpithecus lived during a warm period in the Pliocene era, one that "marks the beginning of Plio-Pleistocene continuous cooling and the onset of Northern Hemisphere alaciation. Against the backdrop of global cooling, the increase of grassy vegetation driven by regional-scale environmental factors may be the trigger for the accelerated evolution of" primates and humans walking on two legs in Africa.

Studying our primate relatives has shed enormous light on our own evolution. A paper last year in the journal iScience studied chimpanzees and bonobos to determine if they possess a trait known as "vocal functional flexibility." Vocal functional flexibility refers to an animal's ability to produce complex sounds that form speech, as opposed to the more simple sounds that come forward from screaming, crying, laughing or making other basic sounds.

Humans are not born with vocal functional flexibility but rather develop it over stages, and it is considered one of the prerequisites to creating actual speech. In their research, the authors of the iScience study discovered that grunting chimpanzees from newborns through to ten-year-old youths display vocal functional flexibility.

"The logic is, if we find good evidence for something in humans and good evidence for something in chimpanzees, then we’re kind of justified in making the inference that this was also a trait that was held by the last common ancestor," Dr. Derry Taylor, the paper's corresponding author and a professor at the University of Portsmouth, told Salon at the time. "So we can say with a reasonable degree of confidence that whatever those vocal communication systems were like, they probably at least had this type of flexibility.”

Another study — this one published last year in the journal Current Biology — involved scientists performing a magic trick known as the "French drop effect" in front of three types of monkeys: common marmosets, Humboldt's squirrel monkeys and yellow-breasted capuchin monkeys. The trick involves a scientist putting food in one hand, presenting it to the money and then putting their other hand over the treat while appearing to grab it.

In fact, the scientists did not grab the treat with their second hand, therefore leading those monkeys that had opposable thumbs to be surprised when discovering it was still in the first hand. Yet the one species of monkey in the group that lacks opposable thumbs, the marmosets, were less likely to be fooled by the trick because they lacked the same digital frame of reference.

"It tells us something about how we think without work," Dr. Nicky Clayton, a professor at the University of Cambridge and corresponding author on the paper — explained to Salon at the time. "We know this because we know . . . there's this massive power in this non-verbal communication. And then I think of seeing non-human animals respond in that way to these non-verbal stimuli. It creates all kinds of questions in your mind, doesn't it? Why is it so soothing to us? What does it mean for the animals that watch it?"

Whether it is in shedding light on the origins of human locomotion and speech or helping us understand our very sense of selves, scientists who study primates both living and extinct continue to learn a lot more about human beings.