Inner ear of 6-million-year-old ape fossil reveals clues to the evolution of human movement

Humans and our closest living ape relatives exhibit a remarkable diversity of types of locomotion, from walking upright on two legs to climbing trees and walking using all four limbs.

While scientists have long been intrigued by the question of how human bipedal stance and movement evolved from a quadrupedal ancestor, neither previous studies nor the fossil record have been able to reconstruct a clear, definitive history. of the first stages of evolution that led to human bipedalism.

However, a new study, focusing on recently discovered evidence in the skulls of a 6-million-year-old fossil ape, Lufengpithecus, offers important clues to the origins of bipedal locomotion through a new method: scanning of the bony region of the inner ear using three-dimensional scanning.

“The semicircular canals, located in the skull between our brain and the outer ear, are essential to our sense of balance and position when we move, and they are a fundamental part of our locomotion that most people “People probably don’t know,” says Yinan Zhang, a doctoral student at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences (IVPP) and lead author of the paper, which appears in the journal Innovation.

“The size and shape of the semicircular canals correlate with the way mammals, including monkeys and humans, move through their environments. Using modern imaging technologies, we have been able to visualize the internal structure fossil skulls and study the anatomical details of the semicircular canals to reveal how extinct mammals moved.

“Our study highlights a three-stage evolution of human bipedalism,” adds Terry Harrison, an anthropologist at New York University and one of the paper’s co-authors.

“First, early apes moved in trees in a style that most closely resembled that of Asian gibbons today. Second, the last common ancestor of apes and humans was similar in its locomotor repertoire to that of Lufengpithecus , using a combination of climbing and climbing, forelimb suspension, arboreal bipedalism, and terrestrial quadrupedalism. It is from this broad ancestral locomotor repertoire that human bipedalism evolved.

Most studies on the evolution of ape locomotion have focused on comparing the bones of the limbs, shoulders, pelvis, and spine and how they are associated with the different types of locomotor behaviors observed in living apes and humans. However, the diversity of locomotor behaviors among living apes and the incompleteness of the fossil record have hampered the development of a clear picture of the origins of human bipedalism.

The Lufengpithecus skulls, originally discovered in China’s Yunnan province in the early 1980s, have given scientists an opportunity to approach unanswered questions about the evolution of locomotion in new ways. However, the severe compression and distortion of the skulls obscured the bony region of the ear and led previous researchers to believe that the delicate semicircular canals were not preserved.

To better explore this region, Zhang, Ni and Harrison, along with other researchers from IVPP and the Yunnan Institute of Cultural Relics and Archeology (YICRA), used three-dimensional scanning technologies to illuminate these parts of the skulls to create a virtual reconstruction of the bony canals of the inner ear. They then compared these analyzes to those collected from other living and fossil apes and humans from Asia, Europe and Africa.

“Our analyzes show that the first apes shared a locomotor repertoire ancestral to human bipedalism,” explains Professor Xijun Ni of the IVPP, who led the project. “It appears that the inner ear provides a unique record of the evolutionary history of ape locomotion, offering an invaluable alternative to the study of the postcranial skeleton.”

“Most fossil apes and their presumed ancestors are intermediate in locomotor mode between gibbons and African apes,” adds Ni. “Later, the human lineage moved away from the great apes with the acquisition of bipedalism, as seen in Australopithecus, an early human relative in Africa.”

By studying the rate of evolutionary change in the bony labyrinth, the international team proposed that climate change may have been an important environmental catalyst in promoting locomotor diversification in apes and humans.

“Cooler global temperatures, coupled with the buildup of ice sheets in the Northern Hemisphere around 3.2 million years ago, correspond to a slight acceleration in the rate of change of the bony labyrinth, which could signal a rapid increase in the pace of life of apes and human locomotor evolution,” explains Harrison.