A new analysis of seven-million-year-oldremains from what may be our earliest hominin ancestor has revealed key anatomical characteristics suggesting this ancient primate likely walked upright.
If confirmed, the discovery, made by researchers from the University of Washington, Chaffey College, and the University of Chicago, would help Sahelanthropus tchadensis cement its position as the earliest known hominin candidate, replacing the 6-million-year-old Orrorin tugenensis, discovered in Kenya. The findings also provide researchers with previously unavailable insights into the origin of human bipedalism and the evolution of the species.
“Our analysis of these fossils offers direct evidence that Sahelanthropus tchadensis could walk on two legs, demonstrating that bipedalism evolved early in our lineage and from an ancestor that looked most similar to today’s chimpanzees and bonobos,” explained Scott Williams, an associate professor in New York University’s Department of Anthropology who led the research.
According to a statement detailing the new analysis, the Sahelanthropus remains were discovered in Chad’s Djurab desert by paleontologists from the University of Poitiers in the early 2000s. The initial research focused on the skull fragments, which led to the classification as a new species. Another early analysis of an S. tchadensis cranium revealed clues that this early primate carried its head like later, bipedal hominins, including Homo sapiens.
Now, researchers have expanded their investigation to other parts of the remains, including the forearms (ulnae) and the thigh bone (femur), to search for more clues of possible bipedalism. Such a determination would open the possibility that the seven-million-year-old remains belonged to the earliest known hominin, making S. tchadensis the oldest human ancestor ever found.
The team began their analysis of the ulnae and femur using a multi-fold trait comparison between living and fossil hominin species. This comparison included Australopithecus, an early human ancestor that lived between 4 million and 2 million years ago and was made famous by its 1970s discovery and subsequent nickname “Lucy.”
Next, the team used 3D geometric morphometrics, which enabled them to examine the geometric features of the remains in fine detail. This comparison also included Australopithecus and other hominin species.
Along with the cranium study that found hints of bipedalism, the new analysis found three distinct anatomical features supporting the idea that this species was the earliest human ancestor ever found.
First, dual analysis of the femur found the presence of a femoral tubercle. This feature, which has only been found in hominins, connects the iliofemoral ligament to the pelvis, enabling bipedal motion.
The team’s dual-pronged analysis also found a femoral antetorsion, or natural twist, of the femur that matched the range of known hominins. This twist helps point the legs forward, assisting bipedal motion.
Finally, the team’s 3D analysis indicated the presence of gluteal muscles generally similar to those found in other early hominins. In humans and other bipedal primates, the gluteal muscles stabilize the hips and support bipedal movements such as standing, walking, and running. The team notes that previous studies had identified both femoral antetorsion and the gluteal complex, which the new analysis confirmed.
Another tantalizing hint that this species is the oldest human ancestor ever found is the relatively long femur in relation to the ulna. The researchers said this ratio offers “additional evidence of bipedalism.”
For example, apes have long arms and short legs, whereas hominins have comparatively longer legs. While the subject’s ratio was not as pronounced as in human femora and ulnae, the team said the S. tchadensis limb bones were “distinct from apes and approached Australopithecus in relative femur length,” another indication of bipedal motion.
“Sahelanthropus tchadensis was essentially a bipedal ape that possessed a chimpanzee-sized brain and likely spent a significant portion of its time in trees, foraging and seeking safety,” Williams explained. “Despite its superficial appearance, Sahelanthropus was adapted to using bipedal posture and movement on the ground.”
The study “Earliest evidence of hominin bipedalism in Sahelanthropus tchadensis” was published in Science Advances.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.