New Insight Into The Auditory Abilities Of Early Humans

We are so used to seeing the world through our eyes that we sometimes forget the sensory layers which work in tandem with it, particularly sound. Take away our sense of hearing and we can still function, sign language allowing us to exchange information. Take away our sense of sight and we are instantly disoriented.

Evolution has blunted our once keen-eared ancestors auditory faculties. We can easily locate the direction from which sound emanates, but without sight determining what makes the sound or how far away the source lies is extremely difficult. It can take years to learn to utilise hearing in the absence of sight.

Of course our sense of hearing has evolved its own set of benefits. Our ability to communicate orally far surpasses that of any other species. Yet our changing relationship with sound has had a great impact on our evolution. A recent study reveals the auditory capacities of our hominid ancestors. Humans have better hearing across a wider range of frequencies, generally between 1.0-6.0 kHz, than most other primates. Within this same frequency range, which encompasses many of the sounds emitted during spoken language, such primates lose sensitivity in comparison to humans.

The skull of Paranthropus used in the study and the reconstructed inner ear

'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 from Binghamton University. 'So we were interested in finding out when this human-like hearing pattern first emerged during our evolutionary history.'

Previous studies conducted by Quam on Homo heidelbergensis fossils from Sima de los Huesos in Spain showed that their auditory capabilities were identical to our own. This new study focused on older 2 million year old australopithecine fossils from South Africa.

A graph showing the range of peak auditory sensitivity for australopithecines

CT scans and computer models were used to reconstruct the anatomy of the inner ears of Australopithecus and Paranthropus so that they could be studied. The results showed that the region of maximum hearing sensitivity was shifted towards slightly higher frequencies, compared to other non-human primates, and that early hominins showed better hearing than either chimpanzees or humans from about 1.0-3.0 kHz.

In more open environments sound waves don't travel as far as in the rainforest canopy, so short range, high frequency communication is more favourable. '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 made it clear that they are not suggesting that the australopithecines had the capacity to speak. They could certainly communicate. Yet what we know of their anatomy suggests that they had neither the vocal nor cranial complexity to utilise language as complex as our own. Nevertheless the shift to higher frequencies was certainly an important step towards 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,' concluded Quam.