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Hearing pattern of humans 2m years ago resembled that of chimpanzees

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Through study of fossils dated 2 million years old, researchers have established that hearing pattern of humans was quite similar to that of chimpanzees, but there are slight differences in the direction of humans.

A team of international researchers including Rolf Quam, assistant professor of anthropology at Binghamton University analysed internal anatomy of ears of early hominin species – Australopithecus africanus and Paranthropus robustus – using CT scans and computer reconstructions to establish the latest finding. The motivation behind the study was to find out when did human-like hearing pattern first emerge during our evolutionary history.

According to researchers, humans have better hearing across a wider range of frequencies, generally between 1.0-6.0 kHz, while chimpanzees and most other primates lose sensitivity compared to humans in these frequencies.

Previous studies on the hearing abilities in several fossil hominin individuals from the site of the Sima de los Huesos (Pit of the Bones) in northern Spain using fossils dated 430,000 years ago and considered to represent ancestors of the later Neandertals had led researchers to conclude that hearing abilities in the Sima hominins were nearly identical to living humans. However, the much earlier South African specimens had a hearing pattern that was much more similar to a chimpanzee.

In the South African fossils, the region of maximum hearing sensitivity was shifted towards slightly higher frequencies compared with chimpanzees, and the early hominins showed better hearing than either chimpanzees or humans from about 1.0-3.0 kHz. It turns out that this auditory pattern may have been particularly favorable for living on the savanna. In more open environments, sound waves don’t travel as far as in the rainforest canopy, so short range communication is favored on the savanna.

This is a lateral view of the Paranthropus robustus skull SK 46 from the site of Swartkrans, South Africa showing the 3-D virtual reconstruction of the ear and the hearing results for the early hominins. Credit: Rolf Quam

This is a lateral view of the Paranthropus robustus skull SK 46 from the site of Swartkrans, South Africa showing the 3-D virtual reconstruction of the ear and the hearing results for the early hominins. Credit: Rolf Quam

“We know these species regularly occupied the savanna since their diet included up to 50 percent of resources found in open environments” said Quam. The researchers argue that this combination of auditory features may have favored short-range communication in open environments.

That sounds a lot like language. Does this mean these early hominins had language? “No,” said Quam. “We’re not arguing that. They certainly could communicate vocally. All primates do, but we’re not saying they had fully developed human language, which implies a symbolic content.”

The emergence of language is one of the most hotly debated questions in paleoanthropology, the branch of anthropology that studies human origins, since the capacity for spoken language is often held to be a defining human feature. There is a general consensus among anthropologists that the small brain size and ape-like cranial anatomy and vocal tract in these early hominins indicates they likely did not have the capacity for language.

“We feel our research line does have considerable potential to provide new insights into when the human hearing pattern emerged and, by extension, when we developed language,” said Quam.

Ignacio Martinez, a collaborator on the study, said, “We’re pretty confident about our results and our interpretation. In particular, it’s very gratifying when several independent lines of evidence converge on a consistent interpretation.”

How do these results compare with the discovery of a new hominin species, Homo naledi, announced just two weeks ago from a different site in South Africa?

“It would be really interesting to study the hearing pattern in this new species,” said Quam. “Stay tuned.”

The study was published on Sept. 25 in the journal Science Advances.