While the tiny bones of the middle ear have much to teach about evolution and sensory perception, their small size and fragility have made them tough to find in fossils. Recently, however, a complete ossicular chain from a human ancestor was discovered, offering clues as to just how far back in time the human malleus goes and what differences in the incus and stapes may mean for early auditory capacities.
“These are among the rarest bones in the entire human fossil record,” said Darryl J. de Ruiter, PhD, who coauthored the study on these findings in Proceedings of the National Academy of Sciences (2013;110:8847-8851). Dr. de Ruiter is professor in the department of anthropology at Texas A& MUniversity in College Station, TX, and honorary reader in the Evolutionary Studies Institute at the University of the Witwatersrand in Johannesburg, South Africa.
“The interesting thing about ear ossicles, and why we would hope to find more, is that they're fully formed by the time you're born, and they don't change over your lifetime,” he said. “Most other bones in your body respond to stresses and to growth. Your ear bones don't. What that means is they're a very good representation of genetic expression. That makes these bones very useful for evolutionary studies.”
The malleus, incus, and stapes came from a Paranthropus robustus specimen recovered from the Swartkans site in South Africa. Also studied were a left malleus and partial right stapes from an Australopithecus africanus specimen recovered from the Sterkfontein site in South Africa.
“In both of these species, the malleus is essentially humanlike, whereas the other two bones, the incus and the stapes, were both more similar to African apes—to gorillas and chimpanzees,” said lead author Rolf Quam, PhD, assistant professor in the department of anthropology at Binghamton University.
“The fact that this humanlike condition is present in both of these species suggests that they inherited it from their last common ancestor and that this change in the malleus occurred very early in our evolutionary history—maybe even as early as bipedalism, which is one of the hallmarks of humanity. We need to have some fossils from earlier time periods to confirm this, but this is our hypothesis.”
Also needing confirmation is what the bones may indicate about auditory patterns.
“The combination of this humanlike malleus and apelike incus and stapes is consistent with slightly different hearing capacities in these early human ancestors,” Dr. Quam said. “It's very difficult to say more precisely what this difference would have been like because we can't say very much just based on the ear bones. Hearing is a very complicated process involving many different structures, and perception involves the brain.
“There are some other aspects of the temporal bone, like the size and dimensions of the ear canal, that are also consistent with a slightly different hearing pattern, so we think there are several indications that perhaps there was a difference in auditory capacities in these early human ancestors compared with ourselves.”
While the researchers haven't yet demonstrated a distinction, they plan to look at hearing in these ancient species next.
“We want to understand whether they could hear in higher ranges or in lower ranges of sound, and that's going to take a lot of background research that we're just doing right now,” Dr. de Ruiter said. “We know what auditory range humans have, so we need to figure it out in chimps, gorillas, and different apes, and try to figure out what their capacity was more similar to.”
This work represents a novel direction for paleontology, Dr. Quam noted.
“Whatever interpretations are put on this change in hearing, at the very least we can definitely say we are reconstructing an aspect of sensory perception in these human ancestors,” he said. “That's something that has traditionally been believed to be beyond the scope of paleontological research.”
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