Scientists Shine Light on How Humans First Learned to Walk Upright

A 6-million-year-old fossil ape has shed new light on the evolution of human movement.

For a study published in the journal Innovation, a team of scientists employed a novel method to study the skulls of the prehistoric ape, known as Lufengpithecus. These investigations provided intriguing insights into the origins of the upright, bipedal stance—walking on two legs—seen in modern humans.

The question of how the bipedal stance evolved from that seen in our quadrupedal ancestors who walked on four legs has long fascinated scientists. But previous research has not been able to reconstruct a clear and definitive history of the early evolutionary stages that led to human bipedalism.

To date, most studies investigating the evolution of ape locomotion have focused on comparisons of bones—including those of the limbs, shoulders, pelvis and spine—and how they are associated with the different types of movement behavior seen in living apes and humans.

However, the diversity of locomotor behavior seen in living apes—which ranges from walking upright to climbing in trees and walking on all four limbs—as well as the incompleteness of the fossil record have hampered the development of a clear picture of human bipedalism's origins.

The latest study addresses some of these matters by taking a different approach: analyzing the bony inner ear region in skulls of Lufengpithecus—an extinct ape that lived more than 6 million years ago—using three-dimensional CT-scanning techniques. These skulls were originally discovered in Yunnan province, southwestern China, in the 1970s and 1980s.

"The semicircular canals, located in the skull between our brains and the external ear, are critical to providing our sense of balance and position when we move, and they provide a fundamental component of our locomotion that most people are probably unaware of," Yinan Zhang, the paper's lead author, said in a press release.

"The size and shape of the semicircular canals correlate with how mammals, including apes and humans, move around 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," said Yinan, a doctoral student at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences.

The skulls that the scientists examined in the latest study were all found in a crushed state, obscuring the bony ear region. This had led researchers to believe that the delicate semicircular canals had not been preserved.

But the authors of the latest study were able to overcome this problem with the help of the three-dimensional scanning technologies, which enabled them to create virtual reconstructions of the inner ear's bony canals.

"It wasn't until this study was undertaken, using high-resolution CT scanning technology, that the delicate bony ear region was found to be preserved," Terry Harrison, a New York University anthropologist and one of the paper's co-authors, told Newsweek.

This allowed the tiny semicircular canals in the inner ear to be visualized in three dimensions. The size and shape of the canals were then analyzed and compared with those of living apes and other fossil apes, as well as with an early human ancestor, Australopithecus, from Africa.

The study paints a picture of human bipedalism's evolution characterized by three distinct stages.

"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," Harrison said in the press release.

"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," Harrison said.

The study also investigated the rate of evolutionary change in the semicircular canals in apes. The results showed a dramatic uptick in the rate of evolution about 3.2 million years ago. This coincides with a period of global cooling associated with the onset of glaciation in the Northern Hemisphere.

"The changing climatic conditions at this time may have been an important trigger in the accelerated evolution of bipedalism in the earliest species of [the genus] Homo in Africa," Harrison told Newsweek.

"It appears that the inner ear provides a unique record of the evolutionary history of ape locomotion," IVPP professor Xijun Ni, who led the project, said in the press release.

Do you have an animal or nature story to share with Newsweek? Do you have a question about human evolution? Let us know via science@newsweek.com.