Oldest fossil hominin ear bones ever recovered: Discovery could yield important clues on human origins

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 study, led by Binghamton University anthropologist Rolf Quam, was carried out by an international team of researchers from institutions in the US, Italy and Spain. They analyzed several auditory ossicles representing the early hominin species Paranthropus robustus and Australopithecus africanus. The new study includes the oldest complete ossicular chain (i.e. all three ear bones) of a fossil hominin ever recovered. The bones date to around two million years ago and come from the well-known South African cave sites of Swartkrans and Sterkfontein, which have yielded abundant fossils of these early human ancestors.

The researchers report several significant findings from the study. 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. Says Quam, "Bipedalism (walking on two feet) 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.

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, says Quam. 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 team has 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 Neandertals, 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.