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Ape-like ear bones found in hominin skull

TEXAS A&M (US) — Ear bone fossils discovered in the skull of a hominin believed to be about 1.9 million years old have features found in both humans and apes.

The tiniest bones in the human body—the bones of the middle ear—could provide huge clues about our evolution and the development of modern-day humans, according to a new study published in the Proceedings of the National Academy of Sciences.

The team found the skull in a cave called Swartkrans in South Africa. Of particular interest to the team were bones found in the middle ear, especially one called the malleus. It and the other ear bones—the incus and the stapes—together show a mixture of ape-like and human-like features, and represent the first time all three bones have been found together in one skull.

The malleus appears to be very human-like, the findings show, while the incus and stapes resemble those of a more chimpanzee-like, or ape-like creature. Since both modern humans and our early ancestors share this human-like malleus, the changes in this bone must have occurred very early in our evolutionary history.

“The discovery is important for two reasons,” explains Darryl de Ruiter, a professor in the anthropology department at Texas A&M University and co-author of the study.

“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.”

de Ruiter recently authored a series of papers in Science magazine that demonstrate the intermediate nature of the closely related species, Australopithecus sediba, and provide strong support that this species lies rather close to the ancestry of Homo sapiens. The current study could yield additional new clues to human development and answer key questions of the evolution of the human lineage.

The National Research Foundation of South Africa and the Ray A. Rothrock Fellowship at Texas A&M funded the research.

Source: Texas A&M University

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