Testing Ancient Human Hearing Via Fossilized Ear Bones

What do the fossils tell us?

Previously, we 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 anthropologists consider them to represent ancestors of the later Neanderthals. Based on ear bone measurements we took, the computer model calculated that hearing abilities in the Sima hominins were nearly identical to living humans in showing a broad region of good hearing.

In our current study published in Science Advances, we worked with much earlier hominin individuals, representing the species Australopithecus africanus and Paranthropus robustus. These fossils were excavated at the sites of Sterkfontein and Swartkrans in South Africa, and likely date to around two million years ago.

Auditory sensitivity between 0.5-5.0 kHz for chimpanzees, humans and the early hominins. Points higher on the curve indicate greater auditory sensitivity. (A) Region of maximum sensitivity. The early hominins are shifted toward slightly higher frequencies compared with chimpanzees. (B) Hearing results. The early hominins are more sensitive than either chimpanzees or humans up to around 3 kHz. Above around 3.5 kHz, the early hominins resemble chimpanzees more closely in showing a drop-off in sensitivity.
Rolf Quam, CC BY-ND

When we measured their ear structures and modeled their hearing, we found they had a hearing pattern that was more similar to a chimpanzee – but slightly modified in the human direction. In fact, these early hominins showed better hearing than either chimpanzees or modern humans from about 1.0-3.0 kHz, and the region of best hearing was shifted toward slightly higher frequencies compared with chimpanzees.

It turns out this auditory pattern may have been a particular advantage for living on the savanna. We know A. africanus and and P. robustus regularly occupied the savanna, since as much as half of their diet was made up of resources found in open environments, based on measurements of isotopes in their teeth.

In more open environments, sound waves don’t travel as far as they do in the rain forest canopy. Sound signals tends to fade out sooner, and short-range communication is favored on the savanna. The hearing pattern of these early hominins – greater sensitivity than humans or chimpanzees to frequencies between 1.0-3.0 kHz and maximum sensitivity at slightly higher frequencies than in chimps – that would work well in these conditions.

The author describes his research.

From hearing to talking

Model of A. africanus individual. flowcomm, CC BY

A. africanus and P. robustus had hearing abilities similar to a chimpanzee, but with some slight differences in the direction of humans.

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.

My colleagues and I aren’t arguing that these early hominins had language, with its implications of symbolic content. They certainly could communicate vocally, though. All primates do, and many species regularly emit a variety of vocalizations including grunts, screams, howls and so on.

But these South African fossils have given us another hearing data point as we try to puzzle out the emergence of language. Two million years ago, it looks like they didn’t have language. But 430,000 years ago, it looks like the Sima de los Huesos hominins did. We suspect that sometime between these early South African forms and the later more human-like forms from the Sima, language emerged. Now we just need to narrow that window.

We hope to continue this kind of work on hearing patterns in different groups of ancient hominins from various places and time periods. The discovery of a new hominin species, Homo naledi, announced just a couple of weeks ago from a different site in South Africa, underscores how much there is left to uncover.

The Conversation

Rolf Quam, Assistant Professor of Anthropology, Binghamton University, State University of New York

This article was originally published on The Conversation. Read the original article.