Bipedalism: How Did Humans Achieve the Unique Trait of Walking Upright?

Walking on two legs is a rarity in the animal kingdom, with humans, birds, some lizards, and fast-moving cockroaches being the exceptions. Humans uniquely showcase mandatory bipedalism among living primates, and transitioning from tree climbing to upright walking is a significant but mysterious evolutionary milestone. Despite advancements in understanding, scientists are still uncertain about the precise details of this pivotal shift.

Decoding the Mystery of Bipedalism: Limb Bones and Inner Ear Canals' Role in Human Ancestry

Six million years ago, the earliest humans displayed the first signs of walking on two legs, a trait potentially beneficial for survival in diverse habitats. To unlock the secrets of upright walking's evolution involves examining the limb, shoulder, pelvis, and spine bones, which are directly linked to key evolutionary transitions.

However, another fundamental aspect influencing walking is found in the bones of the inner ear, particularly the semicircular canals. These canals, serving as an internal gyroscope, contribute significantly to maintaining balance and spatial awareness during movement, a feature often taken for granted.

Nestled deep within the skull between the brain and external ears, the semicircular canals play a critical role in navigating the world on two feet. While limb bones are typically associated with upright movement, the canals, despite their unrelated location, are equally pivotal.

The University of York and the Chinese Academy of Sciences collaborated on an advanced study using 3D CT scanning to analyze the bony inner ear of the fossil ape Lufengpithecus, dating back six million years. The precise measurement of the canals' shapes allows researchers to infer the body posture and movement patterns of ancient ancestors.

Yinan Zhang, a doctoral student at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, emphasized the correlation between the characteristics of these canals and mammalian movement. This understanding aids researchers in unraveling how extinct mammals, including our ancestors, navigated their environments.

Three-Step Evolution of Bipedalism

The researchers unveiled a "three-step evolution" of human bipedalism by examining scans of living and fossil apes, as well as humans from different regions.

The process began with early apes exhibiting tree-based movements similar to today's gibbons. Subsequently, the last common ancestor with apes, resembling Lufengpithecus, engaged in a mix of climbing, clambering, walking on two legs in trees, and moving on all fours on the ground. The final step, bipedalism, emerged from this diverse ancestral heritage.

Conventional studies focused on limb, shoulder, pelvis, and spine bones for understanding locomotion, yet challenges arose due to the diverse living apes and an incomplete fossil record. Researchers turned to the inner ear, unveiling a new perspective on comprehending ancestral movement.

Professor Xijun Ni, leading the project at IVPP, emphasized the inner ear's unique role in recording the evolutionary history of ape locomotion, offering a valuable alternative to studying the postcranial skeleton.

The study also proposed a substantial influence of climate change on ape movement evolution. Approximately 3.2 million years ago, during a period of cooler global temperatures associated with glacial ice sheet formation, there was an increased rate of change in the bony labyrinth.

This indicates a swift advancement in ape and human locomotion evolution, per Terry Harrison, an anthropologist from New York University. It prompts contemplation on our ancient ancestors' diverse movements shaping our current ability to stand on two feet.

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