Toros-Menalla, Djurab Desert, Chad
Toumaï
Between about 7.2 and 6.8 million years (a crushed skull, and a femur argued both ways)
In the Djurab Desert, at a locality called TM 266, a nearly complete but badly crushed cranium came out of sediments dated between about 7.2 and 6.8 million years ago. The endocranial volume is about 360 to 370 cubic centimetres, in the range of a chimpanzee. The teeth and the brow have been read as an early hominin. A femur from the same locality has been read as a habitual biped and, in answer, as a bone that does not show that.
Why this is extraordinary
The Djurab is a desert of wind and old lake beds. At Toros-Menalla, locality TM 266, the sediments hold a crushed skull catalogued as TM 266-01-060-1 and nicknamed Toumaï, “hope of life” in Goran. A cosmogenic clock on beryllium in those sediments brackets the layer between about 7.2 and 6.8 million years, younger than 6.83 plus or minus 0.45 million and older than 7.04 plus or minus 0.18 million, if the fossils were lying in the sand that was dated. The animals in the same unit had already been read as late Miocene, near 7 million, because they look more primitive than a Kenyan fauna of about 6 million. The skull is the object the name attaches to. It is cracked, matrix in the cracks, the face pushed. A cast is what the photograph shows. The original is not on the black cloth.
The brain is small. A virtual reconstruction gives about 360 to 370 cubic centimetres, chimpanzee-sized, the smallest published for an adult placed on the human side of the family. That does not settle the side. The oldest bipeds are not supposed to be big-brained. What is used as the bipedal sign on the skull is the hole for the spinal cord, brought forward under the brain by the reconstruction, and a neck plane flatter than an ape’s. Small canines, worn at the tips, are the other hominin-leaning trait. The catch is the crush. Move a fragment across a crack and the hole moves with it. An ape with a heavy brow and a particular diet is the other reading of the same face.
Then the leg. A femur from the locality, TM 266-01-063, keeps a shaft and the base of the neck. The head is gone and so is the knee. A 2022 study, with two ulnae, says the thigh is most economically a habitual biped’s and the arms are a climber’s. A 2024 reply says that does not hold, point by point, on a bone missing both joints. A geometric study since then finds the limbs closest to chimpanzees in shape and still picks out a tubercle and proportions it reads as early bipedalism. Climbing is the shared ground. Regular walking is the fight.
There is no sky in this room. A desert bone of the late Miocene has no star lore attached. What this changes: if the bipedal reading holds, central Africa had an upright hominin near the age the molecular split from chimpanzees is given, west of the Rift. If the ape reading holds, Toumaï is a remarkable Miocene primate with a famous nickname, and the start of habitual walking is still waiting for a knee.
Why it matters
If Toumaï walked upright, the human lineage was already bipedal in central Africa near the time the molecular clock puts the split from chimpanzees. If the skull is an ape, that split-picture loses its oldest skull, and the femur is an ape’s thigh.
The holotype is TM 266-01-060-1. The nickname Toumaï is a Goran phrase, “hope of life.” Cosmogenic dating, using beryllium in the sediments, brackets the anthracotheriid unit that held the fossils between 6.8 and 7.2 million years, with published figures of younger than 6.83 plus or minus 0.45 million and older than 7.04 plus or minus 0.18 million. That clock assumes the bones were in place in those sediments. The skull is cracked and full of matrix. A virtual reconstruction put the foramen magnum, the hole where the spine meets the head, in a position like later bipeds, and the nuchal plane more horizontal than an ape’s. The braincase and much of the face remain ape-like in size and in many details. Canines are relatively small and worn at the tip, not the long pointed teeth of a male ape. In 2022 a femur and two ulnae from the locality were described as habitual bipedalism plus a lot of climbing. A 2024 reply says the femur lacks the hallmarks that would make bipedalism the regular habit, and that the head and the knee joint are missing, so the shaft is being asked to do too much. A geometric study after that finds the bones closest in shape to chimpanzees and still points to a few features it reads as bipedal, including a femoral tubercle. No sky tradition is attached to a desert fossil of this age. The question is the body.
Photo tour
1 photograph, unedited. On a phone, swipe the row. Choose one to look closely.
- The original cranium is not the photograph. The cast shows the cracks a reconstruction has to cross.
What is still unexplained
These sit beside the consensus account, with the same visual weight. None of them is marked solved.
Location and dating
- Location
- Locality TM 266, Toros-Menalla, in the Djurab Desert of northern Chad. The dating paper prints 16°15′ N, 17°29′ E, which is the pin, 16.25 north, 17.483 east. The cranium is not in the desert now. The photograph is a cast.
- Period label
- Current consensus — open to revision Late Miocene, between about 7.2 and 6.8 million years, on cosmogenic beryllium in the sediments. Biochronology of the animals had already put the unit near 7 million.
- How this dating was done
- Authigenic beryllium isotope ratios in the sediment, checked against the evolutionary stage of the fossil mammals. Not radiocarbon. The walking argument is anatomy, on a crushed skull and an incomplete femur.
- What could change it
- A date that does not depend on the bones being in place, if the burial of the skull was ever shown to be secondary, would move the year. A femur with its head and knee, from the same layer, would replace the argument from a shaft.
What is actually there
- The holotype is a crushed cranium, TM 266-01-060-1.
- Endocranial volume is about 360 to 370 cubic centimetres.
- The sediment age is between about 7.2 and 6.8 million years if the fossils are in place.
- A femur and two ulnae from the locality are published, incomplete.
- Habitual bipedalism is affirmed in a 2022 study, rejected in a 2024 reply, and partly affirmed again on geometric grounds.
- The photograph is a cast.
Open questions
| Question | Status |
|---|---|
| Did Toumaï walk upright as a habit? | Open. The skull reconstruction and the femur are both used, and both are damaged. |
| Is this an early hominin or an ape near the split? | Open. The brain is ape-sized. The teeth and the spinal hole are the hominin side of the argument. |
The current narrative
Where publication stands today. It is the best-supported reading, and it is open to revision. It is not a verdict.
The current narrative — open to revision
The earliest known bipedal hominin
Toumaï is a hominin close to 7 million years old, already carrying the head as a biped, and the femur supports walking as a regular habit alongside climbing.
What supports this
- The reconstructed foramen magnum and the nuchal plane are the cranial signs used for upright posture.
- Small, apically worn canines differ from the projecting canines of living apes.
- A 2022 description of the femur, and a later geometric study, each find features they tie to bipedalism, even while the ulnae show climbing.
What pushes back
- The skull is crushed. A reconstruction can move the spinal hole.
- The femur has no head and no knee. A 2024 reply says the shaft does not show habitual bipedalism.
- The cosmogenic age assumes the bones were found in the sediment they date. That assumption has been disputed for the skull’s discovery.
What would settle it
It is the centre because the skull was published as a hominin and more than one study of the limb bones finds bipedal features. It moves if the femoral tubercle and the shaft angles do not survive the reply’s measurements.
Emerging narratives
Other published readings. Each may be right. None is adopted here unless the evidence carries it.
An emerging narrative
An ape near the split, not a habitual biped
The cranium is an ape-like Miocene primate, and the femur does not demonstrate that upright walking was the regular way it moved.
What supports this
- Brain size, 360 to 370 cubic centimetres, and much of the vault sit with African apes.
- The 2024 point-by-point reply rejects habitual bipedalism on the preserved shaft.
- Geometric work that still argues for bipedalism also says the bones are closest in shape to chimpanzees. Closeness to chimpanzees is the ape case, even inside a bipedal paper.
What pushes back
- A spinal hole tucked under the skull is not what the reconstruction’s critics have fully removed. They dispute the reliability of the crush, more than a measurement everyone shares.
- Small canines still need an explanation if this is a standard male ape.
- “Not habitual” is not the same as “never upright.” Climbing and some bipedalism can share a body, which is close to what the 2022 paper claimed.
What would settle it
It stays because the femur really is incomplete and the 2024 reply is a direct answer, not a sidelong doubt. It is not the centre while several independent descriptions still find bipedal features on the same bones.
Should the current view be reevaluated?
Open to revision
The age of the sediment is a published bracket near 7 million years. The identity of the animal is a crushed skull plus a shaft without joints, with published readings on both sides. Open is required by the bone, not by the nickname.
What drives this
- A cranium of 360 to 370 cubic centimetres with a disputed spinal hole.
- A femur and two ulnae, read as bipedal in 2022, not habitually so in 2024, and partly bipedal again in a geometric study.
- A sediment date between about 7.2 and 6.8 million years, if the fossils are in place.
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If the contested reading is right
What would follow if a reading that is not the consensus turned out to be true, and what could settle it. This explains the stakes. It does not adopt the reading.
Habitual bipedalism at about 7 million years in Chad
Upright walking would be older than the eastern African fossils usually set at the start of the hominin record, and it would already be present far west of the Rift.
Why it is still worth a test
The femoral head and knee, or a published agreement on the tubercle and the shaft, and a clear account of whether the skull was in place in the dated sediment.
Sources
- Cosmogenic nuclide dating of Sahelanthropus tchadensis PNAS, the 7.2 to 6.8 million year bracket and the coordinates
- Postcranial evidence of late Miocene hominin bipedalism in Chad Nature, 2022, the femur and the ulnae
- Postcranial evidence does not support habitual bipedalism in Sahelanthropus Journal of Human Evolution, 2024, the reply
Image credits
- A cast of the holotype cranium TM 266-01-060-1, called Toumaï. The original is not this object. Cracks and distortion are part of what the argument is about. . Didier Descouens, CC BY-SA 4.0. Shown unedited.