12 questions · Science
The further back you go, the thinner the evidence and the hotter the argument. Three candidates stand for the period just after the split from the chimpanzee line: Sahelanthropus tchadensis, found in Chad in 2001 and about seven million years old, nicknamed Toumaï, which means "hope of life" in the Daza language; Orrorin tugenensis from Kenya, some six million years old and dubbed "Millennium Man" for the thigh bone that made it famous; and Ardipithecus ramidus, whose partial skeleton "Ardi" was presented in 2009 after fifteen years of preparation: 4.4 million years old and so crushed that reconstructing it resembled a jigsaw of thousands of splinters. What makes these three so important is the question they are meant to answer: when and why did our line stand up on two legs? Sahelanthropus's foramen magnum sits far forward, as in upright walkers. Ardi, meanwhile, had a grasping big toe for climbing and still walked upright on the ground and lived in woodland, not savannah. That toppled the popular story that humans stood up because the forests turned to steppe. Little of this is settled; each of these species has been hailed as a human ancestor and dismissed as an ape. That is exactly what makes this stretch of time so instructive: here you see science not as a finished result but as a struggle to interpret a few bones.
Practise for freePurFact brings every card back right before you would forget it. That's how knowledge sticks.
Foto: Didier Descouens · CC BY-SA 4.0 · Wikimedia Commons
Toumaï
Sahelanthropus tchadensis, found in 2001 in the Djurab Desert of Chad. Toumaï means "hope of life" in the Daza language: a name given to children born before the dry season.
About seven million years
That places it roughly at the split from the chimpanzee line: whether before, after or exactly between is the heart of the dispute.
In Chad
Some 2,500 kilometres west of the East African Rift. The find showed early hominins ranged far wider than anyone assumed.
By the position of the foramen magnum
In quadrupeds the opening sits at the back, in upright walkers centrally underneath: the head must balance on the spine. In Toumaï it sits far forward.
Foto: Tiia Monto · CC BY-SA 3.0 · Wikimedia Commons
Ardi
Preparation took fifteen years: the bones were so crushed they crumbled at a touch. The analysis appeared in 2009 across eleven papers at once.
A grasping big toe
A climbing foot and yet Ardi could walk upright on the ground. Both at once, a combination that no longer exists.
That the savannah made humans stand up
According to her discoverers, Ardi lived in woodland. The savannah hypothesis (forests vanish, ancestors rise to see over grass) was badly shaken. The reading is disputed, though: since 2010 other researchers have seen a more open, wooded savannah in the same finds.
Orrorin tugenensis
Found in the year 2000, about six million years old. Its femoral neck shows the loading pattern that comes with upright walking.
About six to seven million years ago
Molecular clocks and fossils both point there. The split was probably no clean cut but drew out over hundreds of thousands of years.
Because there are too few bones
Of Sahelanthropus we essentially have one distorted skull and a few fragments. Every interpretation hangs on details that can be read more than one way.
Ardipithecus
About 5.8 to 4.4 million years old, the bridge to Lucy's genus. The name comes from the Afar language: ardi means "ground".
The shrinking canine
Male apes carry long canines for display and threat. In Sahelanthropus and Ardi they are already small: a hint of fewer contests between rivals.
Home · All topics · Legal notice · Privacy · Terms · Contact
