Rare Fossil Discovery Reveals New Insight Into The Evolution Of Human Hearing

Early hominins’ middle ear structure may have looked a lot like that found in humans today, according to a new study led by Binghamton University anthropologist Rolf Quam.

The report, published by the National Academy of Sciences, includes an analysis of the world’s oldest discovery of middle ear bones taken from two species of early humans, including one complete set from a 1.8-million-year-old specimen and an incomplete set from a specimen estimated between 2.1 and 3.3 million years old.

As a first of its kind, the discovery, which occurred in the well-excavated South African cave sites of Swartkrans and Sterkfontein, offers a window into human evolution that hitherto remained closed.

Furthermore, the ossicles are especially interesting for researchers because, unlike other bones, they are already fully formed and adult-size at birth, thus indicating strong genetic control over their size and shape.

In all, the researchers report several key findings based on their analyses thus far.

First of all, despite their early origins, the malleus bone in both cases is clearly human-like with a size and shape easily distinguishable from humans’ closest living relatives - chimpanzees, gorillas and orangutans.

Knowing this, Quam said he believes early evolutionary milestones should be expanded to fit his team’s discovery.

“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,” he said in a press release. “Our study suggests that the list may need to be updated to include changes in the malleus as well.”

Ultimately, though, Quam said more fossils from even earlier time periods are needed in order to prove this to be true.

The team further noted in their study that the incus and the stapes, unlike the malleus, appear similar to chimpanzees, gorillas and orangutans, thus revealing a mixture of human-like and ape-like features.

Such differences are, along with differences in the outer and inner ear, consistent with the different hearing capacities scientists believe early human ancestors had compared to modern humans, though the team has yet to prove this theory conclusively, which they plan on doing via 3D virtual reconstruction based on high resolution CT scans.