When did human ancestors start walking on two legs?

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The period between 7 million and 4 million years ago is a bit of a nebulous phase in the story of human evolution. There are basically four data points: Sahelanthropus tchadensis from 7 million years ago, Orrorin tugenensis from 6 million years ago and the two species of Ardipithecus from 5.6 million and 4.4 million years ago. Each is known from a handful of incomplete fossils. For a period of 3 million years, that’s not much. For comparison, there are dozens of Neanderthal sites from the past 500,000 years.

This means that the few fossils we have from the nebulous phase of hominin evolution have taken on an outsized importance. Every detail of their interpretation is scrutinised and discussed endlessly.

Despite being the oldest known hominin, Sahelanthropus is a relatively recent discovery, first described in 2002 by a group of researchers largely based at the University of Poitiers in France. The remains were found in the deserts of Chad, which is in north-central Africa, a long way from eastern countries of the continent like Ethiopia and Tanzania that had yielded many famous fossils.

The main find was a skull, which was named Toumaï. In New Scientist’s report at the time, writer Jeff Hecht said it didn’t resemble any modern great ape: “Although its body and brain were the size of a modern chimp’s, its face was quite different, with large brow ridges and much smaller canine teeth.” The researchers also found some teeth and bits of jawbone.

What they didn’t find were leg bones. This was unfortunate, because leg bones ought to tell us about how Sahelanthropus walked: did it walk on two legs like us and many later hominins, did it knuckle-walk like a chimpanzee or did it do something else entirely?

The French researchers who described Sahelanthropus argued it was bipedal. This was based mainly on the base of the skull and how it apparently fitted onto the spine: it looked like the skull sat directly atop the spinal column, as opposed to being at an angle like in an ape skeleton. It was an intriguing argument – but far from conclusive.

For the better part of two decades, this was largely how things rested. Which was weird, because, in fact, limb bones had been found: the original team had found a femur (thigh bone) and two ulnas (forearm bones). A student at the University of Poitiers, Aude Bergeret-Medina, identified the femur as belonging to Sahelanthropus in 2004. Her tutor, Roberto Macchiarelli, supported her. However, they were denied further access to the bone.

In 2018, the pair tried to present their own analyses of the femur at a conference at the University of Poitiers, but they were blocked by the organisers. Other palaeoanthropologists criticised this decision. John Hawks at the University of Wisconsin-Madison wrote a blog post that, while couched in polite academic language, was basically furious. “All of the critics could be silenced within hours by data and evidence,” Hawks wrote. “Instead, silence about these key fossils has reigned for fifteen years.”

The following year, Macchiarelli and Bergeret-Medina submitted a paper about the femur to the Journal of Human Evolution, which was finally published in November 2020. The key point was that the Sahelanthropus femur was curved. This is typical of a great ape like a chimpanzee, and not what you’d expect of an upright-walking hominin. Our leg bones are straight because they need to act like pillars supporting the entire weight of our bodies. I consulted two independent palaeoanthropologists, including the discoverer of Orrorin, and they both agreed: Sahelanthropus didn’t look like a biped.

However, the original Poitiers research team, after years of silence, decided to start talking. In September 2020, two months before the publication of Macchiarelli and Bergeret-Medina’s new paper, researchers led by Franck Guy (a co-author of the original Sahelanthropus paper from 2002) and Guillaume Daver at the University of Poitiers released a preprint, which was eventually published in Nature in 2022.

Guy and his colleagues highlighted a number of features of the femur that they say indicate bipedality. For instance, thicker regions along the shaft of the bone correspond to those seen in modern humans and are different from those in great apes. There was also “a rough surface at the top of the femur where the buttock muscles attach”.

So, case closed? Not quite.

No to bipedalism

In June, a team that included Macchiarelli and Bergeret-Medina published a response in the Journal of Human Evolution. The paper is a point-by-point rebuttal of Guy and Daver’s paper.

“Most of the analysis they published in this paper [does] not show any signal of bipedalism, in our opinion,” says Clément Zanolli at the University of Bordeaux in France, one of the authors of the new study. Features that Daver and Guy claimed were solely found in later bipedal hominins can actually be found in some older ape species, which weren’t bipedal.

For example, the top end of a femur has a ball-shaped bit sticking out, which fits into the pelvis. This ball is missing from the Sahelanthropus femur, but we do have the section immediately below. This includes a thickened section called the calcar femorale, which helps support the weight of the body.

“It was supposed to be a hallmark of bipedalism,” says Zanolli. Daver and Guy interpreted the presence and shape of the calcar femorale as evidence that Sahelanthropus habitually walked on two legs. However, “this feature is actually found also in African apes and in many primates,” says Zanolli. So, he argues, it isn’t useful for deciding whether a species was bipedal.

Zanolli’s team also reanalysed the two arm bones, in order to estimate the relative lengths of Sahelanthropus’s arms and legs. Bipedal hominins have proportionally longer legs and shorter arms than knuckle-walking apes, so this is another clue to how Sahelanthropus got around. On this measure, the team found that Sahelanthropus was most similar to modern gorillas and orangutans, and nowhere near bipedal hominins like Australopithecus or Homo.

The paper has other analyses, but you have probably got the gist at this point. The overall message is that the few pieces we have of Sahelanthropus’s limbs don’t show strong evidence of habitual bipedal walking. “It’s generally indistinguishable from the African apes,” says Zanolli.

At this point, you will be wondering what the authors of the 2022 paper have to say about all this. I’m afraid you will have to keep wondering, because Guy declined to comment. He told me that he and his colleagues are writing a reply, which will be published in the same journal, and which they expect to have finished “before August”. He didn’t want to discuss the results until that was done.

Meanwhile, some of Zanolli’s colleagues are pressing ahead with analyses of computer tomography (CT) scans of the bones. Not all of them, though, because some members, including Zanolli, were denied access by the government of Chad. Zanolli says this is highly unusual: “In most countries I’ve been, it’s quite easy, or at least it’s possible, to access fossil material.” He says this is the only time he has been denied access.

Other parts of the skeleton could also shed more light. “I think the teeth should be investigated further,” says Zanolli. “There is not really a paper published on the teeth.”

The future of the past

Finally, let’s step back from the details of the Sahelanthropus fossils and put them into their full context.

As a first step, suppose Zanolli and his colleagues are right. That means Sahelanthropus isn’t bipedal and possibly isn’t even a hominin. That doesn’t mean it’s suddenly a boring fossil. If anything, it becomes even more important.

That’s because Sahelanthropus would become a key fossil for the evolution of African great apes. That’s something we know almost nothing about. “If it’s an ancestor of gorillas, for example, it would be also fantastic, because we have not a single fossil for the ancestors of gorillas,” says Zanolli.

More broadly, it would help to find more hominin and ape fossils from that nebulous time between 7 million and 4 million years ago. Zanolli says part of the problem is that much of central Africa was tropical forest at the time, and some of it still is: fossils tend not to be preserved in such places. Still, there are places like Chad that are promising but under-investigated.

At the moment, this gap in the fossil record is a target for fascinating but unproven speculations. When and where did bipedality evolve? Did it evolve only once in one lineage of apes, or multiple times independently? What evolutionary pressures led to the emergence of bipedal hominins?

For now, we can’t even be sure that bipedality evolved in Africa. It’s tempting to think so, because the oldest bipedal hominins we know of are African, even if you discount Sahelanthropus. A study published in May combined the locations of known hominin and ape fossils and their suspected relationships, and concluded that the group that includes both chimpanzees and hominins probably originated in north-central Africa.

But animals do move around, especially over millions of years. While apes originated in Africa 26 million years ago, climatic shifts meant that for millions of years, they were more widespread and diverse in Eurasia, before largely moving back to Africa later on. A minority of researchers claim that European apes had signs of bipedal walking as early as 11.6 million years ago. There are also hominin-like footprints on the Greek island of Trachilos, which are 6.05 million years old – about on a par with Orrorin and a million years younger than Sahelanthropus. For added variety, a study from February claimed a key role in the story for Lufengpithecus, apes that lived in what is now China around 6 million years ago.

The evidence for bipedality evolving in Eurasia rather than Africa is strongly contested, because it’s based on incomplete and even fragmentary skeletons. But so is the idea that Sahelanthropus was a biped. Again, this is why I describe this whole period as nebulous.

As for how and why bipedalism evolved, we don’t know. Not that this has stopped people coming up with ideas. A paper from March suggested that the last common ancestor of hominins and other African apes lived in Eurasia, but that a dramatic event separated the population into two, which then evolved independently.

What dramatic event? Why, the Zanclean Megaflood of course. If you don’t know, there was a period between about 6 million and 5.3 million years ago when the Mediterranean almost entirely dried out. The Strait of Gibraltar, which connects the Mediterranean to the wider Atlantic, closed – and the sea gradually evaporated, leaving hypersaline lakes. This was the Messinian Salinity Crisis.

Then, around 5.3 million years ago, the Strait of Gibraltar reopened – and the waters of the Atlantic came rushing in. One reconstruction estimated that it took “from a few months to two years” to refill the Mediterranean basin, which, if not an apocalyptic mega-tsunami, is still pretty fast.

Supposedly, this Zanclean Megaflood cut off one population of apes/hominins on the Arabian peninsula, while others were able to reach Africa – creating an evolutionary split. In May, a separate group went further and linked the closure and reopening of the Mediterranean to changes in the behaviour of the Pacific tectonic plate. In this view, the evolution of bipedality and/or hominins was driven by geological processes on the largest scale.

You may be able to tell that I’m unconvinced by all of this. It seems to me there are far too many intermediate steps from the Pacific tectonic plate and the Zanclean Megaflood to hominins habitually walking upright, and we can’t be sure about any of them. We don’t even know the timing of the origin of bipedality. If either Sahelanthropus or Orrorin was bipedal, the behaviour evolved well before the megaflood.

So, much as I want to link the origin of hominins to the biggest flood of the past 10 million years, I think we probably ought to find some more fossils first.

Source: Humans - newscientist.com