Humans walk thanks to change in ape ears 6 million years ago

How did the ancestors of modern humans first start to walk on two legs? A new study might offer an answer – and it’s an earful.

New evidence from the fossilised skulls of a 6-million-year-old ape called Lufengpithecus has revealed important clues about how our quadrupedal ancestors made the transition to walking on two legs. The analysis is published in the journal Innovation.

“Our study points to a three-step evolution of human bipedalism,” says co-author and New York University anthropology professor Terry Harrison.

“First, the earliest apes moved in the trees in a style that was most similar to aspects of the way that gibbons in Asia do today.

“Second, the last common ancestor of apes and humans was similar in its locomotor repertoire to Lufengpithecus, using a combination of climbing and clambering, forelimb suspension, arboreal bipedalism, and terrestrial quadrupedalism. It is from this broad ancestral locomotor repertoire that human bipedalism evolved.”

Lufengpithecus was discovered in the early 1980s in China’s Yunnan province. It lived in the Late Miocene epoch (10.4–5 million years ago). At this time, Earth was much warmer and wetter, with vast rainforests covering much of the tropics including in Asia and Africa.

Palaeontologists believe the drying and cooling of the global climate over the following millions of years and the replacement of forest with savannah provided the evolutionary pressure for human ancestors to develop bipedalism.

While evidence suggests the last common ancestor of apes and humans lived in Africa, Lufengpithecus might provide insight into what led pioneering apes to develop bipedalism millions of years ago.

The palaeontologists sought answers by analysing the inner ear region of the Lufengpithecus skull using three-dimensional CT scanning.

You might expect information about the evolution of bipedalism to come from leg, shoulders hip or spinal fossils. And most studies of the evolution of how apes move have focused on these parts of the body. But the inner ear plays a crucial role in animal locomotion.

If you’ve ever had an inner ear infection, you might have an idea of this. A common side effect of inner ear health problems is dizziness – that’s because this tiny apparatus contains the semicircular canals used in balance.

Three different views of the reconstructed inner ear of Lufengpithecus. Credit: Yinan Zhang, IVPP, Chinese Academy of Sciences.

“The size and shape of the semicircular canals correlate with how mammals, including apes and humans, move around their environment,” says lead author Yinan Zhang, a doctoral student at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences.

“Using modern imaging technologies, we were able to visualize the internal structure of fossil skulls and study the anatomical details of the semicircular canals to reveal how extinct mammals moved.”

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