In 2001, researchers unearthed a partial fossil leg bone and two forearm bones in the Central African nation of Chad. Those fossils come from the oldest known hominid, which lived about 7 million years ago, and reveal the creature walked upright both on the ground and in trees, a new study proposes.
But a lively debate surrounds the fossils over whether they really belong to the hominid species, known as Sahelanthropus tchadensis, or an ancient ape, and to what extent either species might have adopted a two-legged gait. These have become vexing questions as scientists increasingly suspect that ape and hominin species evolved a variety of upright walking ways, some more efficient than others, around 7 million years ago.
Since its discovery, the leg bone has also prompted conflicting accusations of scientific misconduct and an official investigation by the French government-funded research organization CNRS in Paris.
Previously, finds of skulls, jaws, and teeth discovered at the Chad site in 2001 and 2004 were classified as remains of S. tchadensis (Serial number: 4/6/05). The finds are the only other fossils attributed to the species, though some researchers have also suggested those fossils represent an ancient ape.
Analyzes of the three limb bones show that they belong to those previously identified Sahelanthropus species, say paleontologists Guillaume Daver and Franck Guy, both of the University of Poitiers in France, and their colleagues. And the internal and external characteristics of the leg bone indicate that Sahelanthropus walked uprightscientists report on August 24 in Nature. The shapes and structures of the two forearm bones suggest that the hominin moved on two legs through trees while grasping branches with its hands, the team says.
“The Chadian species has a set of anatomical features that clearly indicate that our oldest known [hominid] representative [walked] on the ground and in the trees,” says Guy. It is difficult to say how efficiently or how quickly Sahelanthropus it moved on two legs, he adds.
Guy’s team studied three-dimensional digital models of the fossils derived from CT scans. The leg bone was compared to fossils from ancient apes and other hominins and to modern apes and humans. Features including thickening of the leg bone’s tough outer layer at key points and the presence of an internal bony projection near the hip joint indicate an upright posture, the scientists say.
Fossils from the African site, including bones from all three limbs, suggest that Sahelanthropus it was the first known hominid, agrees paleoanthropologist Kristian Carlson of the University of Southern California, Los Angeles, who was not involved in the new study. But exactly how it moved while upright is unknown, he says. Sahelanthropus it exhibits a combination of upper leg and forearm features that differ from those of modern apes and humans, suggesting that it adopted novel posture and limb movements when walking.
whatever the posture Sahelanthropus assumed, probably resembled that of two other early hominids, more or less 6 million years Orrorin tugenensis Y more than 5 million years Ardipithecus kadabbasays paleoanthropologist Yohannes Haile-Selassie, director of the Institute for Human Origins at Arizona State University in Tempe (D.N.: 09/11/04; Serial number: 3/3/04). The walking abilities of those hominins remain poorly understood due to limited fossils: a partial leg bone for O. tugenensis and a toe bone for the Ardipithecus species.
Haile-Selassie considers all three hominins to be part of a single genus that evolved around 7 to 5 million years ago. On that topic, “the debate is open, even among our team members,” says Guy.
Another debate concerns the internal bony projection of the upper leg that researchers cite as crucial for standing. That trait sometimes appears in modern African apes and is occasionally absent in humans, paleoanthropologist Marine Cazenave of the American Museum of Natural History in New York City and her colleagues report in June. Journal of Human Evolution. The presence of this bone growth does not show definitively that Sahelanthropus walked uprightCazenave says.
Other researchers maintain that the the leg bone probably comes from an ancient ape – not a hominid – which may have occasionally walked upright. Measurements of shape, including the curvature of the fossil’s axis, closely resemble those of modern chimpanzee upper leg bones, University of Poitiers paleoanthropologist Roberto Macchiarelli and colleagues reported in December 2020. in the Journal of Human Evolution.
“There may have been ancient apes that had distinctive types of [upright movement] unlike any living ape, including humans,” says paleoanthropologist Bernard Wood of George Washington University in Washington, DC, who co-authored the 2020 study.
This is where charges of scientific misconduct come into play. The 2020 study was based on measurements of the Sahelanthropus Leg fossil taken in 2004 by a graduate student at the University of Poitiers doing a project on how fossilization affects bones.
That student, Aude Bergeret-Medina, had access to fossils from the Sahelanthropus site that Daver and Guy’s team had not labeled hominid or, more generally, primate. He noted that one specimen, the leg bone, appeared to belong to a primate, possibly an ape. Macchiarelli confirmed his observation. Plans by Bergeret-Medina to open up the bone to study its mineral content were stopped.
Macchiarelli informed his university and the CNRS of the identity of the fossil. He spent the next 16 years, he says, sending repeated complaints to those institutions that the Sahelanthropus The discoverers were violating codes of scientific conduct by not providing information about the leg bone in scientific papers or talks.
The CNRS then launched an investigation of possible misconduct by Macchiarelli himself when the 2020 study appeared before the Sahelanthropus team published findings about the leg bone in their possession. No sentence has yet been handed down.
In companion information released with the new study, Guy and colleagues write that they identified the forearm bones among the fossils in storage after Macchiarelli brought to their attention the identity of the leg bone. Further excavations were carried out in Chad before a detailed study of the three-limbed fossils was released in 2017, the team says.
But Sahelanthropus The team does not cite Bergeret-Medina, now curator of the Muséum d’Histoire Naturelle Jacques de La Comble in Autun, France, by name for her role in identifying the leg bone. The researchers write that “a taphonomy master’s student” received several fossils for a research internship in early 2004 before those finds were carefully examined by experienced scientists. The student, “seeking experience,” gave the leg fossil to Macchiarelli, who identified it as a hominid, Daver and his colleagues say.
That is incorrect, Macchiarelli maintains. Bergeret-Medina initially identified the fossil as the upper leg bone of a primate, followed by her observation confirmation of it. The fossil was not claimed to have come from a hominid, she says. But without the keen observation of fossils from Bergeret-Medina, the new study would never have been done, says Macchiarelli.