However, the brain is scarcely half the size of a modern human one. One that is smaller than any other early Homo that has ever been found. As someone who studies teeth, I might argue these are the coolest fossils found at the site.
The assemblage consists of 190 whole or fragmentary teeth that range in age from very old to very young. Like the skeletons, the teeth present a mix of primitive and derived traits. In modern humans, the third molar is typically the smallest, while the first molar is the biggest, but Homo naledi has the primitive condition where the third molar is the biggest and the first molar is the smallest.
The anterior teeth, or the incisors and canines, are small for the genus Homo, and the lower canine has a cuspulid on it — an extra cuspule that gives it a distinct mitten-like shape that it shares with some specimens of the early human, Homo erectus.
The overall shape of the teeth looked odd to me, so I performed crown-shape analysis on the occlusal surfaces of deciduous teeth, or baby teeth — on your left — and the permanent premolars and molars on your right.
The deciduous teeth are especially narrow, and the premolars are unique in their outline shape compared to other hominids. In fact, when I compare the outlines, when I lay them on top of each other, they look very similar. We say they have “low intraspecific variations,” so the variation within the species is low.
When I compare this to groups like the australopithecines, the intraspecific variation is much larger. Postcranially, the team concluded that the position of the shoulders suggesting naledi was a climber; the flared pelvis and curved fingers are all primitive for the genus Homo.
On the other hand, the humanlike wrist, long slender legs and modern feet are all consistent with other members of the genus.
In 2017, we announced more specimens of Homo naledi from the nearby Lesedi Chamber, also in the Rising Star cave system. In addition, our geology team managed to produce an age estimate. The date’s a big deal because, up until now, we had based our analysis solely on the morphology of the specimens, without previous knowledge of how old something is — something which could unconsciously bias our interpretations.
With its small brain and flared pelvis, we would not have been surprised if the fossils turned out to be 2 million years old. Instead, the fossils dated to 235,000 to 336,000 years, an incredibly young date for such a small-brained individual.
So think back to what I said earlier: we thought that our brains were becoming larger relative to the rest of our body. Now we have a small-brained, young individual complicating this idea. What does all this mean? Homo naledi has taught us that we need to reassess what it means to be in the genus Homo .We need to rethink what it means to be human.
In fact, most of the characteristics that we use to define the genus Homo, such as brain size and hip morphology, are no longer valid. No other species exists with this mix of primitive and derived traits.
Why is there so much morphological variation in the genus Homo? And what force is driving that variation?
Another implication for these fossils is that for the first time, we have concrete evidence of a species coexisting in Africa, at 300,000 years, with modern humans. Until this discovery, we only had large-brained modern humans that existed in Africa. Did they interbreed with each other? Did they compete with each other? Another implication that these fossils have is for the archaeologists studying stone tools in South Africa.
Keep in mind that neither the Dinaledi nor the Lesedi Chambers have any artifacts in them. However, they do overlap in time with several stone-tool industries, the makers of which are considered to be either modern humans or direct human ancestors.
This begs the question: Who made the stone tools of South Africa? Brain size has historically played a key role in identifying a species as a tool user. The idea is that you need to have a large brain to have even the capacity to make stone tools. But that notion has been questioned.
Furthermore, Homo naledi, even with its small brain size, has a hand-wrist morphology similar to other species that did make and use stone tools, suggesting it had the capability. With two species coexisting in Africa at 300,000 years, we can no longer assume we know the maker of tools at sites with no associated species.
So where does Homo naledi fit in our human evolutionary lineage? Who is it most closely related to? Who did it evolve from? We’re still trying to figure all that out. It’s ironic, because paleoanthropologists are renowned for having small sample sizes. We now have a large sample size, and more questions than answers.
Homo naledi has taught us, has brought us a little bit closer to better understanding our evolutionary past. So while Mrs Ples will always hold a special place in my heart, she now shares that space with several thousand others.