Working out how ancient species lived is often difficult, especially when trying to figure out how they sensed things. This is because most sensory organs, like the eyes and tongue, are made of soft tissue that don’t preserve well, forcing scientists to make assumptions from limited data. Yet there is one sense – hearing – that can be studied because it is based, in part, on skeletal structures, albeit very tiny ones. Looking at the ear bones and auditory canal from two species of human ancestor from South Africa, researchers have been able to reconstruct their hearing ability.
What they found was that while in general the early hominins' hearing was similar to that of chimpanzees, they could hear a slightly wider range of frequencies, which is more similar to modern humans. These alterations in how they could hear things, moving away from chimps and closer to humans, could have impacted the way they communicated and ultimately put them one step closer to the development of language. Interestingly, it also seems that the ancient hominins had an increased sensitivity to higher frequencies, perhaps related to the environment they were living in, say the researchers.
“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,” explains Rolf Quam, co-author of the study published in Science Advances, in a statement. “So we were interested in finding out when this human-like hearing pattern first emerged during our evolutionary history.”
A lateral view of the Paranthropus robustus skull SK 46 that was found at the Swartkrans site in South Africa. The above image shows the 3D virtual reconstruction of the ear and the hearing results for the early hominins. Credit: Rolf Quam
The study comes on the back of the discovery a few years ago of the complete set of tiny middle-ear bones from two species of ancient hominins, Australopithecus africanus and Paranthropus robustus, which were living in South Africa between 3.3 and 2.1 million years ago. Using these and CT scans of the fossil skulls, the team was able to reconstruct the internal anatomy of the ear. From this, they were able to recreate the audiogram for each species.
It appears the ancient hominins had a greater sensitivity to higher frequencies, from around 1.0 - 3.0 kHz, when compared to either chimpanzees or modern humans. This, the researchers suggest, could be an adaptation to living on the savanna, where sound waves don’t travel as far as in the forest canopy and where animals have to rely on short-range communication.
This could have implications for the development of language within the human lineage, although Quam in no way thinks that these earlier species were at that stage yet. “They certainly could communicate vocally. All primates do, but we're not saying they had fully developed human language, which implies a symbolic content,” he says.
Others, though, have previously questioned the accuracy of using the middle-ear bones to calculate hearing ability, claiming that it’s actually the shape and size of the outer ear that has a greater influence on hearing ability in primates. The researchers of this new study are, however, confident that their work is accurate, and could provide valuable insights into when language potentially developed.
Main image: Tim Evanson/Flickr CC BY-SA 2.0