What Can We Learn From Homo naledi’s Skull?

  • September 17, 2015 | Andrew Howley

After the excitement of Homo naledi’s discovery and extraction from deep in a narrow cave in South Africa, and the implication that these non-humans may have intentionally carried their dead deep into the earth, we are left with the bones themselves, what they tell us about these creatures, and what new questions they inspire.

These sketches and notes come from interviews and conversations during both the 2013 Rising Star Expedition and the 2014 workshop where established experts and early-career scientists came together to analyze the 1,550 fossil pieces.

The Skull

Modern humans have a very large, high-arching, round cranium (or brain case), and the mandible (or lower jaw), is positioned directly below the front half of the skull.

A very early hominin like an australopithecine (“southern ape”) such as Lucy, has a much smaller, almond-shaped cranium (not that there’s much of Lucy’s actual cranium to go by—this comes from other specimens), with the mandible jutting out in front of the face.

Homo naledi is in the interesting position of having a very small skull, but a very round one, and there is only a shallow slope down from the nose to the teeth.

(Illustration by Andrew Howley)

This is similar to what is seen in Australopithecus sediba, also found by Lee Berger nearby, which while not in the genus Homo, shares more skull shape traits with us than with other australopiths. (Quick Guide: Know Your Hominid Skulls)

That roundness of the skull and flatness of the face are both related to having smaller teeth and chewing muscles, relative to our other relatives. So they probably ate more like us than say chimps or gorillas do.

N.B. on Nose Bones

The jaw changes have other impacts on our facial appearance as well. Instead of thinking that human noses jut out while other ape noses lie flat, to a certain extent you can actually picture that as our our jaws shrank and scooted back, they left our noses sticking out all alone in the front. (Further adaptations then gave the noses of some human groups much more prominent bridges.)

Tiny Skull

The small size of the skull is one of naledi’s surprises. For a long time, large brains have been considered a defining characteristic of the genus Homo. No one expected to find a creature with so many physical attributes of our genus, but with a brain the size of an orange (smaller than a modern chimp’s!). Neurologists will tell you though that the volume of a brain is less important to its abilities than the structure. It raises interesting questions about what the mental capacity of naledi might have been.

There is always the chance that a tiny skull is just from a juvenile. Here though, the bones themselves make the answer clear. First off, the sutures that close between the different skull elements as we grow are all clearly in an advanced state. The more exciting piece of evidence though is that there’s not just one skull, there are pieces of five—and they’re all about the same size.

All Together, Boys and Girls

That brings up another point though: each skull falls into one of two groups: the slightly larger and the slightly smaller (by about 15 percent), which the team members see as male and female, respectively.

Having only a small difference between the physical size and appearance of the sexes is another one of the intriguing aspects of Homo naledi. It is too early to apply this reliably to a newly discovered species, but studies of chimps and bonobos, as well as wolves and dogs, wild and tame foxes, and even human facial preferences, show that smaller, rounder skulls, and lesser differences between the sexes are connected to a selection for tameness, whether through outside pressures or the individual choices of mates.

Long Before Porches or Rocking Chairs

There is one other aspect of the naledi skulls that might give an early clue to their social or emotional lives: the teeth and bone of one mandible are so worn down that they would have come from an individual of considerable age. Combined with the implication that these individuals were all intentionally put into this cave by other members of their group, such a jawbone hints at a story of keeping a group together for multiple generations, and supporting members with impaired capabilities.

It’s almost like they’re human.

But then again, chimps have been known to do this too.

And so have elephants.

Use your reduced mandibular structures to chew on that.

 

NEXT

Homo naledi’s Powerful Hand

Homo naledi’s Nike-Ready Foot

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