A Femur, a Skull, and a Fight Over Humanity's Oldest Ancestor

Paleoanthropology just got messy again, which honestly is how it always goes. A new study is taking a hard second look at a femur pulled from the same Chadian site that produced “Toumaï” — the roughly 7-million-year-old skull that, since its 2001 discovery, has been held up as possibly the oldest known member of the human family tree.

The skull itself, formally named Sahelanthropus tchadensis, made its case for hominin status mostly through the base of the cranium — the foramen magnum, the hole where the spinal cord exits, sits in a position that some researchers read as consistent with an upright, bipedal posture. That’s a big deal, because upright walking is one of the traditional hallmarks separating our lineage from the rest of the great apes. If Sahelanthropus walked on two legs, it pushes the origin of bipedalism back further than almost anything else in the fossil record.

The femur is where things get contentious. It was recovered from the same general area but wasn’t fully described or connected to the bipedalism argument for nearly two decades. Now that it’s being examined in earnest, the researchers behind this new work argue the bone doesn’t clearly show the anatomical signatures you’d expect from a habitual upright walker. No obvious curvature pattern, no clean match to the load-bearing features seen in later, undisputed bipedal hominins. Their read: the evidence for Sahelanthropus walking upright is a lot shakier than the confident headlines from 2001 suggested.

Why a leg bone matters more than a skull here

Skulls are useful, but locomotion is a whole-body behavior, and legs are frankly the more direct witness. A cranial feature can be suggestive without being definitive — skull shape has other explanations too. A femur that was actually used for upright walking tends to leave a more specific signature in its cross-section and curvature. So if this femur genuinely fails to show that signature, it’s a legitimate reason to pump the brakes on the bipedalism claim, or at least to demand better evidence before running further with it.

None of this necessarily knocks Sahelanthropus out of the hominin family entirely — it just unsettles the specific claim that made it famous. And to be clear, this isn’t a slam dunk in either direction yet. Assigning an isolated limb bone to a species known mainly from a skull, found without a clean, indisputable direct association, is inherently shaky ground, and I’d expect defenders of the original interpretation to push back hard on the femur’s provenance and on how confidently anyone can read gait from bone shape alone.

What I like about this story is that it’s a good reminder of how contingent our “earliest ancestor” timeline really is. These headline fossils get anointed based on partial evidence, and then the field spends years, sometimes decades, quietly re-litigating the details once more material or more scrutiny shows up. Expect more back-and-forth papers on this specific femur before anything like a consensus forms — and don’t be surprised if the “oldest known hominin” title ends up more contested than settled for a while.