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Recent research on the inner ear structures of the fossil ape Lufengpithecus offers new clues about the evolutionary steps towards human bipedalism, revealing the important role of the inner ear and climate change in this evolutionary journey. Reconstruction of locomotor behavior and paleoenvironment of Lufengpithecus. Credit: Illustration by Xiaocong Guo; image courtesy of Xijun Ni, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences
A fossilized ape inner ear, dating back 6 million years, sheds light on the development of human locomotion.
Humans and our closest living relatives, the apes, exhibit an extraordinary variety of ways of moving, ranging from bipedal walking on two legs to climbing trees and quadrupedal walking on all fours.
While scientists have long been intrigued by the question of how humans’ bipedal posture and movement evolved from a quadrupedal ancestor, neither previous studies nor the fossil records have allowed for the reconstruction of a clear, definitive history. of the first evolutionary stages that led to human bipedalism.
However, a new study, which focuses on evidence recently discovered in 6-million-year-old fossil ape skulls, Lufengpithecusoffers important clues to the origins of bipedal locomotion, courtesy of a novel method: analyzing the bony region of the inner ear using three-dimensional computed tomography.
“The semicircular canals, located in the skull between our brain and the outer ear, are essential for providing our sense of balance and position when we move, and provide a fundamental component of our locomotion that most people are probably unaware of,” he explains. . 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 the Innovation. “The size and shape of the semicircular canals correlate with the way mammals, including apes and humans, move in their environment. 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.”
Three different views of the reconstructed inner ear of Lufengpithecus. Credit: Image courtesy of Yinan Zhang, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences
Evolutionary steps towards bipedalism
“Our study points to a three-step evolution of human bipedalism,” adds Terry Harrison, a New York University anthropologist and one of the co-authors of the article. “First, the first apes moved in trees in a style that was very similar to aspects of the way gibbons do in Asia today. Second, the last common ancestor of apes and humans was similar in its locomotor repertoire to Lufengpithecus, using a combination of climbing and scrambling, 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 comparisons of the bones of the limbs, shoulders, pelvis, and spine and how they are associated with the different types of locomotor behaviors observed in apes and humans alive. However, the diversity of locomotor behaviors in extant apes and the incompleteness of the fossil record have hindered the development of a clear picture of the origins of human bipedalism.
Technological advances in the examination of fossils
the skulls of Lufengpithecus—originally discovered in China’s Yunnan province in the early 1980s—have given scientists the opportunity to address, in new ways, unanswered questions about the evolution of locomotion. 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 Archeology and Cultural Relics (YICRA), used three-dimensional scanning technologies to illuminate these portions of the skulls and create a virtual reconstruction. of the bony canals of the inner ear. They then compared these scans 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 IVPP professor Xijun Ni, who led the project. “It appears that the inner ear provides a unique record of the evolutionary history of ape locomotion that offers an invaluable alternative to the study of the postcranial skeleton.”
“Most fossil apes and their inferred ancestors have a locomotor mode intermediate between gibbons and African apes,” adds Ni. “Later, the human lineage diverged from the great apes with the acquisition of bipedalism, as seen in australopithecus, one of the first human relatives of 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 of apes and humans.
“Colder global temperatures, associated with the accumulation of glacial ice sheets in the Northern Hemisphere approximately 3.2 million years ago, correspond with an increase in the rate of bone labyrinth change and this may indicate a rapid increase in the growth rate of apes and human locomotor evolution,” explains Harrison.
Reference: “The inner ear of Lufengpithecus provides evidence for a common locomotor repertoire ancestral to human bipedalism” by Yinan Zhang, Xijun Ni, Qiang Li, Thomas Stidham, Dan Lu, Feng Gao, Chi Zhang and Terry Harrison, February 14, 2024 , The innovation.
DOI: 10.1016/j.xinn.2024.100580