There are only two known animals, humans and chimpanzees, that walk on two legs instead of four. This upright posture is known as bipedalism, and it serves not only as our primary mode of transportation but also as our way of standing and sitting. Bipedalism allows us to conveniently carry items in our arms or hands, leaving our other limbs available for secondary activities such as throwing, batting, or climbing.
Interesting animal behavior, with a male chimpanzee walking upright, like a human, across a dirt road. The other four chimps are moving in the usual way, with knuckles to the groundBipedalism has not only set us apart from other apes but has also made us proficient runners and jumpers, according to recent studies. These findings suggest that the urge to run might have been the primary driving force behind the evolution of bipedalism, rather than the other way around. But why did our ancestors bother to make the switch, and why haven’t we reverted to walking on all fours?
The femur’s shape is both intriguing and essential because it dates back to a time when our earliest ancestors closely resembled African apes but diverged from the evolutionary path that led to chimpanzees. “One of the most fascinating and significant questions about our lineage is when and how hominins adopted bipedalism,” says Carol Ward, a paleoanthropologist at the University of Missouri, Columbia. “Fossils that shed light on this topic are vital.” Unfortunately, she notes that “these fossils do not provide conclusive evidence as we had hoped.”
Left: 3D models of the postcranial material of Sahelanthropus tchadensis. From left to right: the femur, in posterior and medial view; the right and left ulnae, in anterior and lateral view. Right: Example of analysis performed to interpret the locomotor mode of Sahelanthropus tchadensis. 3D cortical thickness variation map for the femurs of (from left to right) Sahelanthropus, an extant human, a chimpanzee and a gorilla (in posterior view). This analysis enables us to understand the variations of mechanical constraints on the femur and to interpret these constraints in terms of locomotor mode
According to a recent study, the discovery of a partial thighbone and two lower arm bones in Chad’s Djurab Desert offers new evidence that our ancestors may have been walking upright as far back as 7 million years ago. Sahelanthropus tchadensis, a species whose remains were found in Chad and Niger, is believed to have walked on two legs while also spending some time in trees.
Dan Lieberman, a paleoanthropologist at Harvard University who was not involved in the study, believes that if this creature did walk upright, it would be a significant discovery. He states that it would indicate that walking upright was one of the first traits that set apart the human lineage from chimpanzees, with whom we diverged around this time. However, Lieberman notes that it is not a “slam dunk” due to the incomplete fossil thighbone.
In 2001, paleontologist Michel Brunet of the University of Poitiers introduced the remarkably well-preserved Sahelanthropus skull to the world. Brunet named the creature Toumaï, which means “hope of life” in the local Daza language.
Cast of the Sahelanthropus tchadensis holotype cranium TM 266-01-060-1, dubbed Toumaï. Source: Wikimedia Commons
Sahelanthropus, or Toumaï, was an early hominin with a chimp-sized skull but with a foramen magnum that suggested he balanced his head on a vertical neck like humans do. However, a skull alone was insufficient to prove that the species walked on two legs. In 2004, a thighbone and a lower arm bone were discovered in a drawer of fossils labelled “indeterminate” by the group that found the skull. The bones were identified as belonging to a primate, but they were not investigated in detail until 2017.
In a recent study, French and Chadian researchers analyzed the internal and external structure of 20 bones, including the thighbone and two lower arm bones. They compared the orientation and flattening of the femur’s neck and upper part of the bone with living and fossil apes and hominins and found that they resembled those of hominins or humans rather than African apes. This discovery, along with the femur’s inner density falling within the range of hominins, suggests that Sahelanthropus walked upright. However, some experts believe more study is required to conclusively establish bipedalism in Sahelanthropus.
Although Sahelanthropus was bipedal, the two lower arm bones suggest that the species spent a lot of time in trees to avoid predators. Some experts believe that the chimplike nature of the bones makes it unlikely that the species walked upright like later hominins. Nonetheless, the shaft of the femur is significant as it dates back to a time when our earliest ancestors resembled African apes but diverged from the lineage leading to chimpanzees. Paleontologists hope to find more fossils that provide conclusive information about when and how hominins became bipedal.
