Ancient ear bones show how our earliest ancestors evolved, say researchers from Binghamton University and Texas A&M University, USA. The incus, stapes, and malleus are tiny and extremely fragile bones found in the inner ear and are notoriously difficult both to find and to analyse. However, fossilised examples dated to about 2 million years ago from the South African cave sites at Swartkrans and Sterkfontein are being used to identify earliest changes in anatomy between primates and early hominins.

Evidence of bipedalism and of smaller canine teeth size has long distinguished early humans from our ape ancestors; now, ear fossils may represent the earliest modern traits to appear in human evolution – and the oldest fossils of their kind yet discovered. Those under investigation belong to Paranthropus robustus and Australopithecus africanus, set about a million years apart. Comparing these to living apes and modern humans they can show some of the first changes in anatomy of apes to becoming human. It can also offer a unique insight into the sensory experiences of the earliest people.

Having compared the ear bones with those of chimps, orangutans and gorillas, the malleus ear bone appears very modern, while the other two are still ape-like in both species. These early fossils therefore have both modern and ape-like features. Considering that the rest of the skulls, the teeth and post-cranial skeletons appear very ape-like in Australopithecus and Paranthropus, the malleus of the middle ear appears to be one of the very few early features similar to our own species Homo sapiens.

The team’s anthropologist, Darryl de Ruiter, emphasises that the human middle ear bones are already fully formed at birth, which means that despite their small size they are subject to strong genetic control and hold a wealth of evolutionary information. However, the finds imply a distinct early hominin hearing capacity for although the malleus appears modern, the overall hearing at this stage was almost certainly still different to that of later humans. Now, researchers are hoping more ear fossils will be uncovered, which may help show the origins of modern hearing capacities. Meanwhile, a virtual 3D reconstruction is being designed, based on the high resolution CT scans.