Tiny, very rare ear bones discovered
By: Wits, 19 May 2013

The recently discovered ossicles (ear bones) consist of a malleus, incus and stapes of a 1.9 million year old Paranthropus robustus hominin and ossicles from Australopithecus africanus. The malleus in both hominids is human-like, whereas the incus and stapes resemble those of great apes.

A group of international scientists, including a Wits researcher, have made the first discovery where all three of the middle ear ossicles have been found in one fossil hominin skull. The discovery of these tiny bones shows a mixture of ape-like and human-like features and represents some of the rarest of fossils that can be discovered.

Dr Darryl de Ruiter, from the Department of Anthropology Texas A&M University in the US as well as from the Evolutionary Studies Institute at Wits University, and his colleagues published their findings this week in an article in the scientific journal, Proceedings of the National Academy of Science (PNAS).

According to their paper, entitled Early hominin auditory ossicles from South Africa, the complete ossicular chain (malleus, incus, and stapes) were discovered in a skull of a hominin believed to be about 1.9 million years old and found in the Swartkrans cave in the Cradle of Humankind World Heritage Site near Johannesburg in South Africa. The skull belongs to a Paranthropus robustus hominin and they also report on additional ear ossicles from Australopithecus africanus.

The paper states the researchers found the malleus in both early hominin taxa to be “clearly human-like in the proportions of the manubrium and corpus, whereas the incus and stapes resemble African and Asian great apes more closely. A deep phylogenetic origin is proposed for the derived malleus morphology, and this may represent one of the earliest human-like features to appear in the fossil record. The anatomical differences found in the early hominin incus and stapes, along with other aspects of the outer, middle, and inner ear, are consistent with the suggestion of different auditory capacities in these early hominin taxa compared with modern humans.”

De Ruiter says the discovery is important for two reasons. “First, ear ossicles are fully formed and adult-sized at birth, and they do not undergo any type of anatomical change in an individual lifetime. Thus, they are a very close representation of genetic expression. Second, these bones show that their hearing ability was different from that of humans – not necessarily better or worse, but certainly different.”

 “They are among the rarest of fossils that can be recovered,” de Ruiter adds.

“Bipedalism (walking on two feet) and a reduction in the size of the canine teeth have long been held to be ‘hallmarks of humanity’ since they seem to be present in the earliest human fossils recovered to date. Our study suggests that the list may need to be updated to include changes in the malleus as well.”