A blackened, broken leg bone from Earth’s prehistoric past may hold the answer to when early humans diverged from apes and started their own evolutionary path.
The fossilized find, first uncovered two decades ago, suggests that early humans regularly walked on two feet some seven million years ago. This new analysis, published today in Nature, makes a strong case that Sahelanthropus tchadensis, a species that lived during the critical time when our human lineage diverged from the chimps, habitually walked on two legs. Since many consider bipedalism the major milestone that put our own lineage on a different evolutionary path than the apes, Sahelanthropus could be the very oldest known hominin—the group consisting of modern humans, extinct human species and all of our immediate ancestors.
The species could even be our oldest non-ape ancestor, if its lineage led to Homo sapiens instead of dying out. But while the fossil femur appears to have supported the demands of habitual upright walking, Sahelanthropus’s chimp-like forearms show that it still spent plenty of time in the trees. Two surviving arm bones reveal that the species used a grasping climbing technique to support a type of hybrid lifestyle that could have persisted among early hominins for some three million years.
Because it lived during the era when humans branched off to evolve separately from the apes, Sahelanthropus tchadensis should grab. our attention no matter where scientists place it on our extended family tree. The only known example consists of fossils found two decades ago at the Toros-Menalla site, in Chad’s Djurab desert. The reasonably complete skull, jaw and teeth became known as Toumaï, meaning “hope of life” in the local Goran language, and it was described as a new species in 2002.
“In most respects it looks like an ape,” says Daniel Lieberman, a Harvard University paleoanthropologist who wasn’t involved in the new study. The species sported a brain smaller than a chimp’s and an elongated skull with prominent brow. “But it’s got some really key features that make it look like it’s on the human lineage. The most important of those features is that it looks like a biped,” adds Lieberman, who specializes in the evolution of human physical activity. Evidence of bipedalism began with previous studies of the skull. The passage through which the spinal cord connects with the brain points downwards in the skull, as it does in humans and other upright walkers, while in quadrupeds it points backwards towards a more horizontal neck.
But not all experts have agreed that the Sahelanthropus skull definitely suggested bipedalism. And scientists have taken nearly 20 years to describe in detail other bones that might shed light on the debate, most notably the femur.
The femur and two forearm bones weren’t initially recognized as part of the Sahelanthropus fossil, though they were found near the skull. While scientists don’t know for certain if the limb fossils belonged to the same individual as the skull, no other large primates were at the site so the bones have been attributed to Sahelanthropus.
Analysis of the femur proved tricky because the bone is missing the joints on each end, and with them key diagnostic features that might have preempted debate about whether the species was bipedal. The femur’s neck, which connects toward the hip socket, would reveal if the femur was adapted for bearing all the body weight on one leg at a time. Similarly, the distal knee end would show if alignment kept body weight beneath body’s center of gravity, another sign of habitual bipedalism.
“It was very challenging also, because the bone has been gnawed by, most probably, a porcupine,” says co-author Jean-Renaud Boisserie of the Université de Poitiers. “And yet, a great deal of information was preserved both on external morphology and in internal structures that we accessed through micro CT-scanning.”
Boisserie and colleagues, some of whom originally described Sahelanthropus in 2002, compared more than 20 traits of the femur and forearm bones with a large sample of living chimpanzees, gorillas and orangutans, fossil Miocene apes, early hominin bipeds like Orrorin tugenensis and Ardipithecus Ramidus, and remains of prehistoric Homo and Homo sapiens. They compared external shapes, curves, internal structures and thicknesses to learn if the bone had the same characteristics as those known to be well-suited to the demands of force, balance and other requirements of upright walking. The femur of S. tchadensis showed many similarities with other hominin species, while no traits of the femur were also found exclusively in apes. “Hence, it seemed clear to us that the most parsimonious interpretation of these results is that the morphology shared by Sahelanthropus and other hominins reflect their common evolutionary history but also similar locomotor adaptions,” Boisserie says.
“I think the authors did everything humanly possible to try to analyze whether it’s a biped or not. They make a compelling case, with these difficult to describe bits of anatomy,” says Daniel Lieberman. “Is it smoking gun evidence on its own? Absolutely not. But in my opinion, in combination with the skull, it should lay to rest the debates about whether this is a biped.”
But some debate will likely continue. Just two years ago a study in the Journal of Human Evolution suggested that the same femur belonged to an individual that was not habitually bipedal. The new analysis stands in direct contrast to that. John Hawks, who studies human evolution at the University of Wisconsin–Madison and was not involved in either femur study, has questioned whether Sahelanthropus's skull and teeth mark it as an upright hominin. He finds the disconnect between femur analyses puzzling and more than a little frustrating—particularly since the fossil in question was discovered two decades ago. “The two teams who have collected data from the femur seem to disagree entirely about what the femur shows,” he says. “They’re looking at the same piece of bone. I don’t understand how they disagree about this. If either group could just release (surface 3-D and internal CT scan) data so that we can all examine it, there would be no reason for this disagreement.”
If Sahelanthropus was habitually bipedal, the study’s further analysis of the ulnae, the larger of the two forearm bones which stretches from the elbow to the little finger, shows that its arms were extremely apelike, akin to chimps. So the species was also very competent in the trees.
Boisserie suggests this might have fit an opportunistic lifestyle that was probably quite useful in the diverse environment of Toros-Ménalla some seven million years ago. “The extension of wetlands in the northern Chad Basin maintained a patchwork of forest cover, palm groves and less forested, more grass-rich areas in what is now a desert,” he notes. “Sahelanthropus tchadensis, therefore, had resources from arboreal, terrestrial and aquatic environments at its disposal.”
“I delight in the analysis of the ulnas and the demonstration that Sahelanthropus also lived or moved habitually in the trees,” adds Rick Potts, director of Smithsonian’s Human Origins Program. This evidence suggests, Potts notes, that hominins might have been adapted to move both habitually bipedally and in the trees for almost four million years, from the time of Sahelanthropus to A. afarensis, which showed such adaptations up until about three million years ago.
Naturally, any debate over bipedalism is just one part of a larger and still more intriguing question; is Sahelanthropus really the oldest known member of our human lineage?
Some scientists don’t believe that the species is a hominin at all. Because Sahelanthropus lived so close to the divergence between hominins and apes, scientists debate whether the fossils are from an individual who lived after that divergence, or perhaps one who lived just before the divergence. If the latter, Sahelanthropus could be an ancestor of either chimps or humans, or some common ancestor to both lineages, or even a close relative which is actually ancestral to neither.
Rick Potts notes that two-legged walking might have evolved somewhat differently, several times, and still led to similar looking evidence among the leg bones of ancient skeletons. So ancient bipedal walkers may well have existed that aren’t ancestors of any of the later hominins who kept refining that ability.
But Sahelanthropus seems to show two of the foundational adaptations that all later hominins share, and which aren’t found among the chimpanzee kin; a femur restructured for habitual upright walking and, as shown in earlier studies of the skull, reduced canine teeth that restructured the mouth. Other fossils of the earliest known hominins also share these traits, notably Orrorin tugenensis, which lived around six million years ago, and Ardipithecus Ramidus, which lived about 4.4 million years ago.
“So, what are the chances that 7- to 4.4-million-year-old fossil apes in Africa having both these traits are not hominins?” Potts asks. “The authors argue that the simplest answer is that all three of them are hominins. And because Sahelanthropus is the oldest one known, it might have been the closest we're going to get to the evolutionary branching event that led to us.”