Testing ancient human hearing via fossilized ear bones

3D virtual reconstruction of two-million-year-old ear. Rolf Quam, CC BY-ND

How did the world sound to our ancient human relatives two million years ago?

While we obviously don’t have any sound recordings or written records from anywhere near that long ago, we do have one clue: the fossilized bones from inside their ears. The internal anatomy of the ear influences its hearing abilities.

Using CT scans and careful virtual reconstructions, my international colleagues and I think we’ve demonstrated how our very ancient ancestors heard the world. And this isn’t just an academic enterprise; hearing abilities are closely tied with verbal communication. By figuring out when certain hearing capacities emerged during our evolutionary history, we might be able to shed some light on when spoken language started to evolve. That’s one of the most hotly debated questions in paleoanthropology, since many researchers consider the capacity for spoken language a defining human feature.

Many primates vocalize; only people have full-blown language. dsg-photo.com, CC BY-SA

Human hearing is unique among primates

We modern human beings have better hearing across a wider range of frequencies than most other primates, including chimpanzees, our closest living relative. Generally, we’re able to hear sounds very well between 1.0-6.0 kHz, a range that includes many of the sounds emitted during spoken language. Most of the vowels fall below about 2.0 kHz, while the higher frequencies mainly contain consonants.

Thanks to testing of their hearing in the lab, we know that chimpanzees and most other primates aren’t as sensitive in that same range. Chimpanzee hearing – like most other primates who also live in Africa, including baboons – shows a loss in sensitivity between 1.0-4.0 kHz. In contrast, human beings maintain good hearing throughout this frequency range.

We’re interested in finding out when this human hearing pattern first emerged during our evolutionary history. In particular, if we could find a similar pattern of good hearing between 1.0-6.0 kHz in a fossil human species, then we could make an argument that language was present.

Testing the hearing of a long-gone individual

To study hearing using fossils, we measure a large number of dimensions of the ancient ears – including the length of the ear canal, the size of the ear drum and so on – using virtual reconstructions of the fragile skulls on the computer. Then we input all these data into a computer model.

Published previously in the bioengineering literature, the model predicts how a person hears based on his ear anatomy. It studies the capacity of the ear as a receiver of a signal, similar to an antenna. The results tell us how efficiently the ear transmits sound energy from the environment to the brain.

We first tested the model on chimpanzee skulls, and got results similar to those of researchers who tested chimpanzee hearing in the lab. Since we know the model accurately predicts how humans hear and how chimpanzees hear, it should provide reliable results for our fossil human ancestors as well.

Excavations at Sterkfontein. This area contained regions of open savanna when these fossil hominins lived here. John Walker

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.