Sahelanthropus may be the earliest human ancestor that we know about, but it is mired in controversy.
This is a diary series in which we take a closer look at hominins, fossil ancestors of the human family.
3d print of Sahelanthropus skull, from front
From side
In the 1980s, French paleontologist Michel Brunet and his partner David Pilbeam were searching for hominin fossils in Cameroon, in deposits formed along an ancient shoreline when Lake Chad had been much larger than at present and was surrounded by a mix of tropical forest and open grassland. Then in the 1990s, they received permission from the government of Chad to take an expedition into some remote northern areas of that country, and settled on a place called Toros-Menalla. In 1995, another expedition had already found fossils in Chad, about 100 miles away, of a hominid species that they named Australopithecus bahrelghazali. It was the first early hominin to be found outside of eastern or southern Africa.
In July 2001, Brunet’s team found the badly-broken pieces of a nearly-complete skull, which they concluded was an ancient hominin. Chad had been a French colony and had undergone years of revolution and civil war, and the new find quickly became a symbol of Chadian nationalism. When the skull was shown to Idriss Deby, the President of Chad at the time, he called it “Toumai”, from a local Daza word meaning “hope of life”—but it was also the name of a comrade of his who had been killed while fighting. Brunet himself announced the discovery on TV, declaring, “The ancestor of humanity is Chadian ... The cradle of humanity is in Chad. Toumai is your ancestor.”
Over the next year, eight more fragments, consisting of jawbones and teeth, were also found, representing somewhere between six and eight individuals.
The skull had been found exposed at the surface, and there were no volcanic deposits to allow radiodating, but the fossils of other known mammals found along with the skull indicated an age of 6-7 million years, making it potentially the oldest hominin discovered so far. Brunet gave it the Latin name Sahelanthropus tchadensis (“southern Sahara man from Chad”), suggested to him by President Deby. Later, a new method called “cosmogenic nuclide dating” was used on some nearby deposits. This process measures how long ago a rock sample has been exposed to cosmic rays by measuring the amount of specific isotopes of beryllium that were produced by these interactions. This gave an age of 6.8-7.2 million years.
Pieces of a femur (thighbone) and two ulnas (one from each forearm) were also found nearby, but it was not clear that these belonged to the skull. Brunet had them shipped separately to the University of Poitiers in France in 2003 along with some fossil animal bones.
When the Sahelanthropus skull was reconstructed (taking almost a year), it showed some interesting traits. The brain was small like an ape’s, with a volume of around 360 cubic centimeters—about the size of a chimp, and one-third that of a modern human. There was a heavy brow ridge above the eyes (known technically as a “supra-orbital torus”), and a prominent “nuchal ridge” running around the back of the skull where the neck muscles were attached, like an ape. In addition, the lower part of the face was slanted forward, making a projecting muzzle as in apes, rather than the flat vertical face found in humans.
Nuchal ridge at the back of the skull
The canine teeth, though larger than those in humans, was smaller than an ape’s, and “small canines” is a human characteristic. Further, the canine showed wear only at its tip, and had no indications of the “honing” mechanism found in apes, in which the upper canines are continually sharpened by rubbing against one of the lower teeth. On the other hand, the Sahelanthropus teeth formed a rectangular shape in which the back teeth were about as far apart as the front—an apelike pattern that differs from the flared parabolic tooth pattern seen in humans.
Most interesting, though, was the foramen magnum, the hole in the skull where the spinal cord enters to connect with the brain. This gives important information about whether an animal walked upright. In apes, which are quadrupedal and move on the ground using all four limbs, the spine is held in a more or less horizontal position, and the spinal cord enters at the very back of the skull. In two-legged bipeds like humans, though, the spine is vertical and the skull sits on the top, where the spinal cord enters at the very bottom. When the Sahelanthropus skull was examined, it showed a foramen magnum that was located near the bottom of the skull as in a human, indicating that the skull belonged to a bipedal animal. Brunet therefore concluded that Sahelanthropus had walked upright and, based upon this, was an early hominin.
Foramen magnum (rounded area) at the bottom of the skull
Comparing the skulls of a chimp, Sahelanthropus and modern human, we can see that Toumai’s foramen magnum lies closer to the center of the skull as in humans, rather than at the back as in apes.
Brunet’s conclusion that the skull was a hominin, however, came under intense fire from some quarters. Critics pointed out that the skull had been badly crushed during preservation and had to be reassembled, and the position of the foramen magnum may have been inadvertently shifted too far forward while the pieces were being put back together. If that were so, then the skull would belong on the “ape” side of the split and not the hominin side. However, in 2005 another team did a virtual reconstruction using computer tomography, and concluded that the reconstructed skull fell into the hominin range instead of the ape.
Others questioned the dating. Photos of the site showed that the skull had been surrounded closely by ancient mammal bones that appear to lie in two parallel lines. This, it was argued, meant that the skull, along with the surrounding bones including the femur and the mammal bones, may have been reburied together at some time in deposits in which it had not really lived, perhaps by Muslim tribesmen who had found it on the surface as it eroded out of younger deposits and had mistaken it for a modern human skeleton. This would mean that the fossil was not really as old as it appeared to be and the bones did not really belong together.
That interpretation was supported by genetic studies, which at that time indicated that the genetic split between humans and apes had occurred between 5 and 6 million years ago—at least a million years after Sahelanthropus. However, new genetic studies in 2012 pushed the date of that split back to 6-7 million years, once again placing Toumai back into the correct possible age bracket.
The suggestion was also put forward that the skull had some affinities with some of the australopithecines—which would possibly remove the Ardipithecus genus from the direct line to humans. That hypothesis has not been widely accepted. Meanwhile, the French team that had discovered the Orrorin fossils, Brigitte Senut and Martin Pickford, judged that the Sahelanthropus skull seemed to be consistent with a female “proto-gorilla” ancestor. (Most studies have concluded from the heavy brow ridge that the skull is a male.) Perhaps not coincidentally, that would leave Orrorin (which did not have a skull for comparison) as the celebrated “oldest known hominin”.
Then there was the femur. Brunet had not cited this bone in his papers announcing the new species, since he was not certain that the leg bone actually belonged to the skull. The femur was also fragmented and hard to interpret because it did not contain either end at the hip or knee joints, which are the most informative parts. The bone lay unstudied in France until it was stumbled upon by a grad student in 2004. This in turn led to conspiracy theories that Brunet had intentionally “buried” the femur because it presumably did not support his argument that Toumai was a biped.
For the most part, researchers have assumed that the skull and femur belong together—largely because Sahelanthropus is the only hominin known from the area. A French team in 2020 studied the femur and concluded that it did not exhibit any signs of habitual bipedalism. Another French team in 2022, however, concluded that it did come from a bipedal walker and that it shared some important traits with later Orrorin and the australopithecines, including apparent attachment points for muscle groups that are essential in upright walking.
The two teams have not released any discussion with each other over their contradictory conclusions, but it is possible that the conflict comes from the fact that they studied different specific traits on the bone, and they may have different definitions of “habitual bipedalism”. We won’t know until things get clarified further. Sahelanthropus may not have been fully bipedal to the extent that later hominins were, but may have been capable of moving at least some of the time on two legs while on the ground. And it certainly seems to have been more bipedal than any of the earlier apes that we know about.
The arm bones were also examined by the second French team, who concluded that they were curved in shape and thickened from front to back, indicating that they belonged to a climber. So even if Sahelanthropus was bipedal, it apparently still spent a lot of time climbing in trees.
Today, it remains very confusing, but the majority view seems to be that Brunet is correct and that Sahelanthropus is indeed a very early bipedal hominin and the earliest known human ancestor, though this is still controversial. It will take the discovery of some more fossils to definitively settle the matter.
Before the discovery of Toumai, nearly all of the earliest hominin fossils had been found in eastern and southern Africa, and it had been assumed that this was where the human line had first evolved. But Toumai changed that view, and raised the possibility that the earliest hominins were more widespread than had been thought, and may have branched off from apes somewhere else on the continent, perhaps in the north while it was wetter than today and the Sahara Desert had not yet formed. Even if Sahelanthropus turns out to be an ancient ape and not a hominin, it still demonstrates that northern Africa was the home of some very interesting apes at around the time of the hominid/hominin split, and that southern and eastern Africa were not necessarily where all of the evolutionary action was happening.
NOTE: As some of you already know, all of my diaries here are draft chapters for a number of books I am working on. So I welcome any corrections you may have, whether it's typos or places that are unclear or factual errors. I think of y'all as my pre-publication editors and proofreaders. ;)
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