How did the early human beings hear? What were their frequency ranges? Were they able to hear speech? These are questions that anthropologists seek to answer. This week’s Hearing 
International looks at the studies of human hearing by anthropologists using modern methods of research to determine how humans heard two million years ago.
This week’s story begins in south central New York state in the USA where studies have been conducted by a team of anthropologists headed by Dr. Rolf Quam 
of Binghamton University. Among Dr. Quam’s interests is the audition of ancient species of man. As he reviews the temporal bones of ancient man, he states that, “We know that the hearing patterns, or audiograms, in chimpanzees and humans are distinct since their hearing abilities have been measured in the laboratory in living subjects. So we were interested in finding out when this human-like hearing pattern first emerged during our evolutionary history.”
Toward that end the researchers review temporal bones and the fossils of auditory systems from ancient species of Man that are known to have lived in different times.
How Can You Surmise Hearing From A Fossil?
To study these hearing patterns of Man at different times, Dr. Quam and his team 
studied the temporal bones of several fossil hominin individuals from the site of the Sima de los Huesos (Pit of the Bones) in northern Spain. These Homo heidelbergensis fossils are about 430,000 years old and are considered to represent the ancestors of the later Neanderthals [Martinez et al 2004, 2013), were found to have characteristics similar to modern Man.
The team then contrasted those data with those obtained from a much earlier species of Man found in South African caves of Sterfontein and Swartkrans hill. The South African fossils were the Australopithecus africanus that lived 3.3 to 2.1 million years ago and the Paranthropus robustus that lived 1.8 to 1.2 million years ago.
While the hearing abilities in the Sima 
hominins (Homo heidelbergensis) were nearly identical to living humans, the Australopithecus africanus and the Paranthropus robustus had hearing patterns that were more similar to a chimpanzee.
How Can They Tell?
To those of us that assess hearing everyday, it is often difficult enough to assess the hearing of live patients let alone those that have been dead for millions of years. Of course, there are no behavioral responses, no evoked potentials, so how is it done?
According to Dr. Quam, the current study and the previous ones (Martinez et al 2004, 2013) rely on the use of 3 dimensional CT Scans and virtual computer reconstructions to study the internal anatomy of the ear. Once the researchers have scanned the skull, the processing and reconstruction of the skull with models on a computer can be completed without having to touch and possibly damage the delicate fossils.
According to Quam and colleagues,
“of all the special senses, audition is particularly amenable to study in fossils because it is strongly related to the physical properties that can be approached through their skeletal structures.“
Earlier CT Scans and modeling studies (Martinez et al 2004, 2013) demonstrated that the fossil temporal bones of hominins of the Middle Pleistocene period in the Sierra de Atapuerca in Spain (Homo heidelbergensis) were similar to that of living human beings. In their quest to determine when Man began to hear as 
we do today, Dr. Quam’s current study was of temporal bone specimens in earlier Man, the A. africus and P.robustus at the Pilo-Pleistocene period sites in South Africa. He and his colleagues compared the CT Scans and modeling data from these earlier temporal bones to the later Man (H.heidelbergensis) and determined that the earlier South African species probably had hearing closer to that of the chimpanzee.
The figure at left comes from the team’s 2015 study published in Science Advances. Area A is the temporal bone, area B is the tympanic antrum, area C is the ear canal and area D presents the auditory ossicles for the H. heidelbergensis. The data for the species from the two periods were compared as to ear canals, tympanic membrane, auditus ad antrum, auditory ossicles, the level ratios of the ossicular chain, mastoid air cells, size of the basal turn, and other anatomical structures. There were significant differences found in the species from the two periods.
OK….So What?
Their findings demonstrated that the skeletal variables and modeling differences between these two species created differences in the sound power transmission. They felt that the areas of
the ear canal, the tympanic membrane and the auditory ossicles had more influence on the hearing in early man than the tympanic cavity size, auditus ad antrum and mastoid air cells. They also discuss the possibly that the shorter basal turn of the cochlear found in A. africanus and the P. robustus would likely have made some difference in the derived audiograms.
Their data indicates that the A. africanus and the P. robustus probably heard much like a chimpanzee where hearing rolls off at about 3KHz and the later species had better hearing through 3.2 kHz and better hearing at 4 kHz. Compared to current humans whose hearing is relatively flat to 4200 Hz. presented in the blue. Their results demonstrate that “between 1.5 kHz. and 3.0 kHz., the early hominins are more sensitive to than either chimpanzees or modern humans. Above 3.5 kHz, the early hominins are less sensitive than modern humans but similar to chimpanzees.”
Dr. Quam indicates that in the past anthropologists felt that the early hominins (A.africus and P.robustus) were restricted to a vowel type of communication. In this study, however, their 3D scans and computer models demonstrated that there was enough high frequency sensitivity to support more than vowel communication. Dr. Quam’s conclusions after the research are many but he does state that “although the precise nature of early hominin communication remains elusive, it may have involved some form of ‘low fidelity social transmission’.”
References:
Martínez, I., Rosa, M., Arsuaga, P., Quam, R., Lorenzo, C., Gracia, A., Carretero, J.,Berudez de Castro, J., Carbonell, E. (2004). Auditory capabilities of Middle Pleistocene humans from the Sierra de Atapuerca in Spain. Proc. Natl. Acad.Sci USA 101, 9976-9981. Retrieved October 5, 2015.
Martínez, I., Rosa, M., Quam, R., Jarabo, P., Lorenzo, C., Bonmati, A., Gomez-Olivencia, A., Gracia, A., Arsuaga, J. (2013). Communicative capacities in Middle Pleistocene humans from the Sierra de Atapuerca in Spain. Quat. Int. Vol 295, pp 94-101. Retrieved October 5, 2015.
Medical News Today (2015). Two million year old fossils reveal hearing abilities of early humans. Retrieved October 5, 2015.
Quam, R. (2015). Early hominin auditory capacities. Science Advances, Vol 1(8), September 25. Retrieved October 5, 2015.
Wong, S. (2015). Ears of early humans could hear frequencies used in speech. New Scientist. Retrieved October 5, 2015.
Images:
Quam, R. (2015). Early hominin auditory capacities. Science Advances, Vol 1(8), September 25. Retrieved October 5, 2015.
