The question of whether a 7-million-year-old primate, nicknamed 'Toumai,' walked on two or four legs has whipped up drama amongst palaeontologists - complete with a vanishing femur.
Since the discovery of Sahelanthropus tchadensis's first fossil back in 2001, it has often been cited as our earliest known hominin ancestor. Initial analysis suggested that Sahelanthropus regularly walked upright and had a combination of ape-like and human-like features.
These conclusions, however, were based on a single skull.
The skull has anatomical features that potentially indicate this primate had an erect spine, and therefore spent some of its time walking on two legs only. Its small teeth also appear more human than ape-like. A later reconstruction supported these findings.
But other researchers have since argued that this alone is not enough evidence to class Sahelanthropus as a hominin biped - a primate directly ancestral to humans - rather than a related, but not directly ancestral hominid.
Around the same time and at the same location where the skull was found, in Toros-Menalla in Chad, a partial left femur was also recovered. The femur vanished after another researcher started to examine it in 2004, having come across it supposedly by chance.
Aude Bergeret-Medina and her supervisor, palaeoanthropologist Roberto Macchiarelli from the University of Poitiers in France, eventually continued their analysis based on measurements and photos. They have just published their findings, which cast doubt on Sahelanthropus's place in our family tree.
"Based on our analyses, the partial femur lacks any feature consistent with regular bouts of terrestrial bipedal travel," Macchiarelli and team write in their paper.
"Thus, if there is compelling evidence that S. tchadensis is a stem hominin, then bipedalism can no longer be seen as a requirement for inclusion in the hominin clade."
Meanwhile, another palaeontologist, Martin Pickford from the French National Museum of Natural History, wonders if the femur even belongs to Toumai, or at least another Sahelanthropus.
Still, others agree with Macchiarelli's assessment of the femur.
"I saw the pictures 10 or 12 years ago, and it was clear to me that it's more similar to a chimp than to any other hominin," University of Tübingen palaeontologist Madelaine Böhme, who was not involved in any of the studies, told New Scientist.
Analysis of molecular differences in our DNA suggests that humans parted ways with chimpanzees and bonobos (our closest still living relatives), around 6-8 million years ago. The only other fossil evidence of a possible hominin from that time is from Orrorin tugenensis.
Macchiarelli and team compared the femur with one from O. tugenensis and determined that there's at least species-level difference between them.
After also comparing them with Australopithecus, gorillas, and modern humans, they believe these differences suggest the mode of locomotion of the two oldest species was also different.
They suspect Sahelanthropus may be an ancestral relative with no remaining living descendants - a primate lineage that went extinct.
They also point out others have suggested the small teeth found in the original study could just indicate the primate is female. But the team agrees that fascinating questions nonetheless remain, particularly around the lines that we use to define what exactly makes a primate a human, quoting a 2017 paper in their conclusion:
"Exactly where in Africa, and under what circumstances, the ape-human demarcation began, and when, how and why the ape-human boundary became irrevocably established, are important research challenges that are still unresolved."
We'll need many more fossils before we know the answers.
This research was published in the Journal of Human Evolution.