According to an international team of anthropologists led by Binghamton University, tiny ear bones from two species of early human ancestors in South Africa – Paranthropus robustus and Australopithecus africanus – could provide clues about our evolution and the development of modern-day humans.
Reconstruction of Australopithecus africanus and Paranthropus robustus (John Gurche / Myartprints.com)
The team has painstakingly studied a complete ossicular chain (malleus, incus, and stapes) of P. robustus and ear ossicles from A. africanus. These bones are the smallest bones in the human body and are among the rarest of human fossils recovered.
Unlike other bones of the skeleton, the ossicles are already fully formed and adult-sized at birth. This indicates that their size and shape is under very strong genetic control and, despite their small size, they hold a wealth of evolutionary information.
The bones date to around 2 million years ago and come from the well-known South African cave sites of Swartkrans and Sterkfontein, which have yielded abundant fossils of the early human ancestors.
The researchers report their findings in a paper published in the Proceedings of the National Academy of Sciences.
Tiny ear bones of Paranthropus robustus, from left: the incus, stapes, and malleus (Rolf M. Quam et al)
“The malleus is clearly human-like, and its size and shape can be easily distinguished from our closest living relatives, chimpanzees, gorillas and orangutans. Many aspects of the skull, teeth and skeleton in these early human ancestors remain quite primitive and ape-like, but the malleus is one of the very few features of these early hominins that is similar to our own species, Homo sapiens. Since both the early hominin species share this human-like malleus, the anatomical changes in this bone must have occurred very early in our evolutionary history.”
“Bipedalism and a reduction in the size of the canine teeth have long been held up as the ‘hallmark of humanity’ since they seem to be present in the earliest human fossils recovered to date,” explained study lead author Dr Rolf Quam of Binghamton University.
“Our study suggests that the list may need to be updated to include changes in the malleus as well.”
“More fossils from even earlier time periods are needed to corroborate this assertion. In contrast to the malleus, the two other ear ossicles, the incus and stapes, appear more similar to chimpanzees, gorillas and orangutans. The ossicles, then, show an interesting mixture of ape-like and human-like features.”
“The anatomical differences from humans found in the ossicles, along with other differences in the outer, middle and inner ear, are consistent with different hearing capacities in these early hominin taxa compared to modern humans.”
Although the current study does not demonstrate this conclusively, the team plans on studying the functional aspects of the ear in these early hominins relying on 3D virtual reconstructions based on high resolution CT scans.
The scientists have already applied this approach previously to the 500,000 year-old human fossils from the Sierra de Atapuerca in northern Spain. The fossils from this site represent the ancestors of the Neanderthals, but the results suggested their hearing pattern already resembled Homo sapiens.
Extending this type of analysis to Australopithecus and Paranthropus should provide new insight into when our modern human pattern of hearing may have evolved.
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Bibliographic information: Rolf M. Quam et al. Early hominin auditory ossicles from South Africa. PNAS, published online before print May 13, 2013; doi: 10.1073/pnas.1303375110
