For all the attention devoted to human evolution, we still don’t know who our direct ancestors were, let alone our distant ones. But now Sahelanthropus tchadensis, a primate that lived about 7 million years ago in North Africa, and looked like an ape, is also looking like a prime candidate for our earliest forefather.
Sahelanthropus’ classification as hominin or ape has been difficult to determine partly because of its extreme antiquity. Its fossils date to around the time we think the Homo line split from the Pan line of chimps, also about 7 million years ago, and there are sadly so few of specimens, all found in 2001 in Chad: a distorted skull with a few teeth, based on which the species was classified; some jaw fragments; two arm bones; and a partial femur. Scant evidence on which to consider on how many legs it walked.
But Sahelanthropus was indeed a hominin, argues a new analysis of the same old bones published Friday in Science Advances by Scott Williams of New York University with colleagues.
Fresh analysis of the femur using the most cutting-edge techniques, including Williams running his fingers over it, observed a subtle bump in the femur bulb where it connects to the hip joint that hadn’t been noticed before.
It was a femoral tubercle, the structure where a ligament connects the leg bone to the pelvis, they realized. It enabled Sahelanthropus to do what possibly no animal had ever done before: hold its body upright, in vertical posture. Apes can’t do it.
The new interpretation of old bones supports findings in a paper from 2022 in Nature by Guillaume Daver and colleagues, that this 7-million-year old hominin was bipedal rather than a knuckle-walker, which is how all apes walk – on all fours. Gibbons and such may rear onto their hind legs briefly but are not habitually bipedal.
Williams qualifies by Zoom with Haaretz that they aren’t saying no apes ever had femoral tubercles – who knows? But out of the “tons” of fossil apes known from Africa and Asia between 20 to 7 million years ago, none had it. This one did, and they believe this small bump at the top of the femur is the smoking adaptation to bipedalism.
Hallmark of hominin
The year after the Sahelanthropus was discovered, analysis of the reconstructed skull nicknamed Toumaï suggested that it carried its head vis-à-vis the spine as we do. On that basis, its discoverers postulated that it was a very early bipedal hominin. That position was supported by Daver’s 2022 paper, which identified the leg and arm bones as belonging to Sahelanthropus and suggested that its morphology was best explained by habitual bipedality plus substantial arboreal activity.
Yet some fretted that the femur smacked more of Pan than Adam and suggested the creature had been a proto-gorilla. Now the new paper is Team Hominin.
There still cannot be consensus that Sahelanthropus was hominin as opposed to ape because there isn’t enough evidence, Williams clarifies. More fossils would be nice. But he existed at the very time that Pan and Homo began to diverge. He could have been our earliest ancestor.
Or he could have been an early chimpanzee or gorilla or something else entirely. But Williams and the team argue that Sahelanthropus primarily walked and “vertical bipedalism” is a characteristic distinct to the hominin line, ergo, he was a hominin.
What makes humanity special? Arguably our brains, but we’re starting to wonder what cephalopods like cuttlefish are doing with their eight brains and about the souls of dogs. Humans can be distinguished more cleanly from all other primates and animals by vertical bipedalism.
This applies to two-legged extinct and extant dinosaurs too. None walk or walked in the vertical body position. Their heads were not in a straight line with their feet.
The tubercle shows that Sahelanthropus was already a way away from “chimphood,” and chimps and human diverged at roughly the same time that he existed, Williams adds.
We just add – chimps and humans cannot interbreed today despite alleged efforts to achieve that, but our forefathers apparently did continue to sleep with the chimps for a few million years after the divergence began.
When did the “hominin” suite of characteristics emerge? Williams considers. “When you get animals no longer relying heavily on trees for food and safety,” he suggests. “Our ancestors were arboreal and eating a lot of tree foods and probably sleeping in trees [as the great apes do]. Homo habilis and australopithecines were quite arboreal but were also competent terrestrial bipeds.”
They were all small, with small brains. Perhaps the first we would recognize as “like us” was Homo erectus about 2 million years ago; he had a large body, stood upright, craved meat, and as for his brain size, that depends on which specimens you consider to be erectus, but it wasn’t small.
Chimp resistance
How had the femoral tubercles, which confirm hominin-style hip and knee function, escaped notice in the 25 years since the bone’s discovery? Was Williams looking for it?
No, he wasn’t. “I was just looking at the overall bone to see if it was more similar to chimps, gorillas, or fossil hominins that we have – it took weeks of examining it casually,” he says. “It’s a really important little bump because it is indicative of hominins and not apes. All hominins have it. When sitting the ligament is loose – when we stand, it tightens, and there seems to be direct correlation between its presence and bipedal hominins. We looked at a bunch of mammals including primates and it seems to be associated specifically with hominins.”
Given his chimp-like proportions, the way Sahelanthropus walked wouldn’t have been the same as the way australopithecines walked or the way we do. But maybe our hominin swagger began with him.
Australopithecines, who lived from about 4 to 2 million years ago, were more derived for bipedalism than Sahelanthropus; the shape of their femur looks a lot more ours, though they maintained the tree life too. Research has indicated that A. afarensis babies had “monkey feet”.
“The younger ones probably had grasping ability with their feet but we think australopithecines had an inline big toe like we do,” Williams says. “A bit farther back, Ardipithecus ramidus had a grasping big toe. I’m sure Sahelanthropus also had a grasping big toe.”
Years ago, he adds, most researchers would have argued that the first adaptation to bipedalism would be losing the grasping big toe. “But that’s out the window now because we have early hominins with grasping big toes like ramidus. Sahelanthropus shows the change focused on the pelvis and head of the femur.”
Even before argument over the emergence of walking, there was and is argument over the emergence of knuckle-walking in the great apes. Did it evolve once, or independently in gorillas and chimps?
“It emerged once, I think,” Williams answers. “Thinking about this phylogenetically, chimps and humans split and the next branch is gorillas. It’s contentious whether knuckle-walking evolved in the common ancestor of chimps and gorillas and humans. Half the field thinks it emerged separately.”
Why?
“There has been a resistance to an apelike ancestor and still is. There is an anti-chimp sentiment,” he answers. “But the remains definitely look like they belong to the “African ape human clade” – “it’s in the group with chimps and gorillas and us. I think the evidence is indicating more and more that Sahelanthropus is a hominin – the time is right and the femur evidence and a little bit of the ulna suggests this is a very early hominin with the first adaptations to bipedalism.”