Kenyanthropus platyops vs Australopithecus afarensis: The Flat-Faced Mystery

In 1999, a team led by Meave Leakey pulled a cracked, sediment-filled cranium from the Lomekwi badlands on the western side of Lake Turkana. Catalogued as KNM-WT 40000, it was roughly 3.5 million years old — and it had a face unlike anything from that era. Where Lucy and her relatives carried muzzle-like, forward-jutting faces, this skull was startlingly flat. In 2001 the discoverers named a new genus and species for it: Kenyanthropus platyops, the "flat-faced man of Kenya."

The claim was bold. If the skull was what its finders said, then the central question of Kenyanthropus platyops vs Australopithecus afarensis is not really which came first — it is whether more than one hominin lineage walked East Africa at 3.5 million years ago. That would turn the orderly, single-file story of human origins into a crowded, branching bush. But a powerful counterargument soon followed: the cranium is so badly fractured and warped that its famous flat face might be an illusion of the fossilisation process. Here is how the two stack up.

Who was Kenyanthropus platyops?

Kenyanthropus platyops is known mainly from one fossil: the type cranium KNM-WT 40000, found in 1999 at Lomekwi, west of Lake Turkana in northern Kenya, in sediments dated to about 3.5 million years ago. A second specimen, a partial upper jaw catalogued as KNM-WT 38350, comes from the same region. Meave Leakey and colleagues formally named the genus and species in Nature in 2001.

The defining feature is right there in the name: platyops means "flat face." Compared with other hominins of its age, KNM-WT 40000 has a tall, relatively vertical mid-face with high cheekbones positioned well forward, giving it a strikingly modern, un-muzzled profile. It also pairs this with surprisingly small molar teeth and small external ear openings (auditory meatuses). That combination — flat face plus small chewing teeth — was unusual enough that the discoverers argued it could not be squeezed into Australopithecus.

What we do not have is a confident brain size. The cranium is fragmentary and distorted, so any endocranial volume is an estimate at best. The honest description is that the brain was small and australopith-grade — broadly in the same league as Lucy's species, on the order of a few hundred cubic centimetres — rather than a precise published figure. Treat any single number you see for Kenyanthropus brain size with caution.

Who was Australopithecus afarensis?

Australopithecus afarensis is, by contrast, one of the best-sampled early hominins in the entire fossil record. It lived in East Africa from roughly 3.9 to 2.9 million years ago, with hundreds of specimens recovered across Ethiopia, Tanzania, and Kenya. Its fame rests on Lucy (catalogue number A.L. 288-1), the ~40% complete skeleton found at Hadar, Ethiopia, in 1974, which showed the world that upright walking came long before big brains.

The species' anatomy is well understood. The face is prognathic — it projects forward into a muzzle — and the jaws carry relatively large molars built for a tough plant diet. Brain volume runs roughly 380 to 430 cc, only modestly above a chimpanzee's. Yet from the neck down, A. afarensis was committed to bipedalism: the Laetoli footprints in Tanzania (~3.66 million years old) preserve a trail of upright walkers, even as skeletal traits like curved fingers and mobile shoulders hint at time still spent in the trees. Other landmark fossils include the Dikika child ("Selam") and the large male partial skeleton nicknamed "Kadanuumuu." You can read the fuller account on the Australopithecus afarensis species page.

Key differences

The face

This is the heart of the matter. A. afarensis has a projecting, prognathic face — jaws and teeth thrust forward beneath a small braincase. Kenyanthropus, as reconstructed by its discoverers, has the opposite: a flat, orthognathic face with the cheekbones set high and forward, more reminiscent of much later hominins. If real, that flatness is the single most important reason to keep Kenyanthropus separate from Lucy's species.

Teeth

The two also differ in the chewing apparatus. A. afarensis carries comparatively large molars suited to a coarse diet. Kenyanthropus, despite its robust-looking face, has notably small molars with thick enamel. That mismatch — a big, flat face above small teeth — is part of what made the discoverers reach for a new genus.

Brain

Here there is less daylight between them. Both were small-brained by later standards. A. afarensis sits in the well-measured ~380–430 cc range, while Kenyanthropus brain size is too uncertain to pin down. Brain size, in other words, is not where these two part ways — the face and teeth are.

Is Kenyanthropus even a valid species?

This is the question that hangs over everything, and it deserves an honest answer. The type cranium, KNM-WT 40000, is not a clean skull. It is heavily fractured and packed with hardened sediment (matrix), and many of the cracks are filled and displaced — a condition called taphonomic distortion, where the processes of burial and fossilisation deform a specimen after death.

In 2003, paleoanthropologist Tim White argued in Science that this distortion could manufacture the very flatness the new species was built on. In a paper pointedly titled "Early hominids — diversity or distortion?", he suggested that a warped, expanded cranium can look flat-faced even if the living animal was not, and that KNM-WT 40000 might be better read as a distorted Australopithecus afarensis rather than a distinct genus. On this view, "Kenyanthropus" could be a fossilisation artefact dressed up as a new branch of the family tree.

The discovery team and many other researchers disagree, holding that the skull preserves real, diagnostic features that distortion alone cannot explain. The current state of play is genuinely unsettled: Kenyanthropus platyops appears in plenty of textbooks and family trees, but always with a caveat. It is one of the clearest cases in paleoanthropology where the validity of a named species turns on how you read a single broken fossil.

Did they overlap?

Yes — and that is the whole point. Kenyanthropus at ~3.5 million years ago and A. afarensis across ~3.9–2.9 million years ago share the same broad time window, and they share the same broad neighbourhood: the Turkana Basin and the wider East African Rift. A. afarensis fossils are known from Kenya as well as Ethiopia and Tanzania, so the two were not separated by some vast distance.

If Kenyanthropus is valid, then at least two hominin lineages were living side by side in East Africa in the mid-Pliocene. That is no longer a fringe idea: another contemporary, Australopithecus deyiremeda, has since been named from ~3.4-million-year-old deposits in Ethiopia, reinforcing the picture of a crowded Pliocene. The mystery is not whether overlapping species are possible — it is whether this particular fossil is one of them.

Why it matters

The argument over Kenyanthropus platyops vs Australopithecus afarensis is really an argument about the shape of our family tree. One view sees a tidy single lineage marching from Australopithecus toward Homo. The other sees a bushy tree — multiple hominin experiments running at once, most of them dead ends. Kenyanthropus is a test case for which picture is right.

There is a tantalising extra wrinkle. In 2015, Sonia Harmand and colleagues reported the Lomekwi 3 stone tools — deliberately struck flakes and cores dated to about 3.3 million years ago, the oldest known stone tools, and roughly 700,000 years older than the previous record. They come from the same Lomekwi region that yielded Kenyanthropus. Because Kenyanthropus lived in that landscape, it has been floated as a candidate toolmaker — but the truth is the maker is unknown, and A. afarensis or others could equally be responsible. What is clear is that toolmaking now predates the genus Homo, and the flat-faced mystery of Lake Turkana sits right at the centre of that revolution.