What human ancestors’ hearing was like decoded

By: | Published: September 27, 2015 6:33 PM

Early humans who lived around two million years ago had hearing abilities similar to chimpanzees but with some slight differences in the direction of modern humans, a new study has found.

Early humans who lived around two million years ago had hearing abilities similar to chimpanzees but with some slight differences in the direction of modern humans, a new study has found.

An international research team reconstructed an aspect of sensory perception in several fossil hominin individuals from the sites of Sterkfontein and Swartkrans in South Africa.

The study relied on the use of CT scans and virtual computer reconstructions to study the internal anatomy of the ear.

The results suggest that the early hominin species Australopithecus africanus and Paranthropus robustus, both of which lived around 2 million years ago, had hearing abilities similar to a chimpanzee, but with some slight differences in the direction of humans.

Humans are distinct from most other primates, including chimpanzees, in having better hearing across a wider range of frequencies, generally between 1.0-6.0 kilohertz (kHz).

Within this same frequency range, which encompasses many of the sounds emitted during spoken language, chimpanzees and most other primates lose sensitivity compared to humans.

“We know that the hearing patterns, or audiograms, in chimpanzees and humans are distinct because their hearing abilities have been measured in the laboratory in living subjects,” said Rolf Quam, assistant professor of anthropology at Binghamton University in New York, who led the team.

“So we were interested in finding out when this human-like hearing pattern first emerged during our evolutionary history,” said Quam.

Previously, Quam and colleagues studied the hearing abilities in several fossil hominin individuals from the site of the Sima de los Huesos (Pit of the Bones) in northern Spain. These fossils are about 430,000 years old and are considered to represent ancestors of the later Neandertals.

The hearing abilities in the Sima hominins were nearly identical to living humans. In contrast, 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 favourable 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 favoured on the savanna.

“We know these species regularly occupied the savanna since their diet included up to 50 per cent of resources found in open environments,” said Quam.

The researchers argue that this combination of auditory features may have favoured short-range communication in open environments.

The study was published in the journal Science Advances.

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