A blackened, broken leg bone from Earth’s prehistoric past may hold the answer to when early humans split from the apes and began their own evolutionary path.
The fossilized find, first discovered two decades ago, suggests that early humans regularly walked on two feet around seven million years ago. This new analysis published today in Naturemakes a strong case that Sahelanthropus tchadensis, a species that lived during the critical time when our human lineage split from chimpanzees, usually walked on two legs. Since bipedalism is considered by many to be the major milestone that put our own lineage on a different evolutionary path than the apes, Sahelanthropus it could be the oldest known hominid: the group made up 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. But while the fossil femur appears to have withstood the demands of usual upright walking, Sahelanthropus The chimpanzee-like forearms show that it still spent a lot of time in the trees. Two surviving arm bones reveal that the species used a grasping climbing technique to maintain a type of hybrid lifestyle that could have persisted among early hominins for some three million years.
Because he lived during the era when humans branched off to evolve separately from apes, Sahelanthropus tchadensis it must 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 the Djurab desert in Chad. The reasonably complete skull, jaw and teeth became known as Toumaï, meaning “hope of life” in the local Goran language, and was described as a new species in 2002.
“In most respects it looks like an ape,” says Daniel Lieberman, a paleoanthropologist at Harvard University who was not involved in the new study. The species sported a brain smaller than that of a chimpanzee and an elongated skull with a prominent forehead. “But it has some really key features that make it look like it belongs to 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. The evidence for bipedalism began with previous studies of the skull. The passageway through which the spinal cord connects to the brain points down into the skull, as it does in humans and other upright walkers, while in quadrupeds it points backward toward a more horizontal neck.
But not all experts agree that the Sahelanthropus the skull definitely suggested bipedalism. And it has taken scientists nearly 20 years to describe in detail other bones that could shed light on the debate, most notably the femur.
The femur and two forearm bones were not initially recognized as part of the Sahelanthropus fossil, although they were found near the skull. While scientists don’t know for sure if the limb fossils belonged to the same individual as the skull, there were no other large primates at the site, so the bones have been attributed to Sahelanthropus.
Analysis of the femur was complicated because the bone is missing joints at each end, and with them key diagnostic features that might have prevented debate over whether the species was bipedal. The neck of the femur, which connects to the hip socket, would reveal whether the femur was adapted to support the full weight of the body on one leg at a time. Similarly, the distal end of the knee would show alignment if body weight was kept below the body’s center of gravity, another sign of habitual bipedalism.
“It was also very challenging, because the bone has been gnawed, most likely, by a porcupine,” says co-author Jean-Renaud Boisserie of the Université de Poitiers. “And yet, a great deal of information about both external morphology and internal structures was preserved that we accessed via micro-computed tomography.”
Boisserie and colleagues, some of whom originally described Sahelanthropus in 2002, compared more than 20 features of the femur and forearm bones with a large sample of living chimpanzees, gorillas and orangutans, Miocene fossil apes, early hominin bipeds such as Orrorin tugenensis other Ardipithecus Ramidusand remains of prehistory homo other Homo sapiens. They compared external shapes, curves, internal structures, and thicknesses to see if the bone had the same characteristics as those known to be well suited to the demands of strength, balance, and other requirements of upright walking. the femur of S. tchadensis it showed many similarities to other hominin species, while features of the femur were not found exclusively in apes either. “Therefore, it seemed clear to us that the most parsimonious interpretation of these results is that the morphology shared by Sahelanthropus and other hominids reflect their common evolutionary history, but also similar locomotor adaptations,” says Boisserie.
“I think the authors did everything humanly possible to try to analyze whether it is a biped or not. They make a compelling case, with these hard-to-describe bits of anatomy,” says Daniel Lieberman. “Is it smoking gun evidence by itself? Absolutely not. But in my opinion, in combination with the skull, it should put an end to the debates about whether it is a biped.”
But some debate is likely to continue. Just two years ago a study in the Journal of Human Evolution he suggested that the same femur belonged to an individual that was not usually 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 any of the femur studies, has questioned whether SahelanthropusThe skull and teeth mark it as an upright hominid. He finds the disconnect between analyzes of the femur puzzling and more than a little frustrating, particularly since the fossil in question was discovered two decades ago. “The two teams that have collected data from the femur seem to be in complete disagreement about what the femur shows,” he says. They are looking at the same piece of bone. I don’t understand how they disagree with this. If either group could just post data (surface 3D and internal CT scan) for all of us to examine, there would be no reason for this disagreement.”
Yes Sahelanthropus it was usually bipedal, further analysis of the study of the ulnae, the larger of the two forearm bones extending from the elbow to the little finger, shows that its arms were extremely ape-like, resembling chimpanzees. So the species was also very proficient in trees.
Boisserie suggests that this could have fitted into 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 mosaic of forest cover, palm groves, and less forested and richer grass areas in what is now desert,” he notes. “Sahelanthropus tchadensistherefore, it had at its disposal resources from arboreal, terrestrial and aquatic environments.”
“I love the analysis of the cubits and the demonstration that Sahelanthropus also lived or moved habitually in the trees”, adds Rick Pottsdirector of the smithsonian Human Origins Program. This evidence suggests, Potts says, that hominins may have adapted to move both habitually bipedally and in trees for nearly four million years, from the time of Sahelanthropus a afarensiswhich showed such adaptations until about three million years ago.
Naturally, any debate about bipedalism is only part of a larger and even more intriguing question; it is Sahelanthropus Really the oldest known member of our human lineage?
Some scientists do not believe that the species is a hominid at all. 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 it could be an ancestor of chimpanzees or humans, or some common ancestor of both lineages, or even a close relative who is not actually an ancestor of either.
Rick Potts points out that walking on two legs could have evolved somewhat differently, multiple times, and still led to similar-looking evidence among the leg bones of ancient skeletons. So it’s possible that there were ancient bipedal walkers that aren’t ancestors of any of the later hominins who continued to refine that ability.
but Sahelanthropus it appears to display two of the fundamental adaptations shared by all later hominins, and not found among chimpanzee relatives; a restructured femur for normal upright walking and, as shown in earlier studies of the skull, reduced canine teeth that restructured the mouth. Other known early hominin fossils also share these traits, notably Orrorin tugenensiswho lived about six million years ago, and Ardipithecus Ramiduswho lived about 4.4 million years ago.
“So what are the chances that fossil apes from 7 to 4.4 million years old in Africa that have both of these traits are not hominins?” Potts asks. “The authors argue that the simplest answer is that all three are hominins. And because Sahelanthropus is the oldest known, it could have been the closest we are going to get to the evolutionary branching event that led to us.”