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What Can We Learn From Homo naledi’s Skull?

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

 

Comments

  1. David
    Auckland NZ
    October 10, 2015, 7:52 am

    I read that the dolomite rock that forms the ‘dragon’s back’ fell from the roof of its chamber. Before it fell, access to the Dinaledi chamber could have been much easier.
    Likewise, the ‘superman crawl; and ‘postbox’ might have been much wider at one time. If so, the story of homo naledi depositing its dead in this chamber is more believable.

    Alternatively, could a tribe have hidden in the cave, say during an earthquake, and been trapped by the falling dragon’s back rock?

  2. Jean-Paul
    Cerro Gordo IL USA
    October 7, 2015, 1:02 pm

    ” it has something to do with rules about destroying bones in South Africa”.
    If this is true, it is a ridiculous rule that hinders science. Typical of politicians, who’s main goals seems to be to hinder science(at least in this case). The amount of fossil used in Carbon Dating this type of specimen is 2 to 10 grams. Considering the value of the data that could be obtained, I think Carbon Dating is warranted.
    I got the above information from Sonia at:
    http://www.radiocarbon.com/ams-dating-samples.htm
    So the issue here seems to be lousy politics.

  3. Jean-Paul
    Cerro Gordo IL USA
    October 7, 2015, 12:42 pm

    “The summary is that both carbon and DNA analysis require the destruction of material, and the team says they wanted to avoid that until after the fossils were all described.”
    ____________________________________________
    There are enough recovered fossils to use a destructive method of dating. I still think the team, avoiding dating the fossils, is a flaw in their scientific methodology. Before any theories can be developed, it is CRITICAL that a date of age be established.

  4. Moses
    Leeds, UK
    September 19, 2015, 9:29 pm

    “Why has no definitive dating or possibly DNA testing been made.” Good question, Jean-Paul

    • Andrew Howley
      September 21, 2015, 11:37 am

      Hi Moses and Jean-Paul,
      Ed Yong at The Atlantic did a great rundown of the techniques that could be used for dating, and their pros and cons: http://www.theatlantic.com/science/archive/2015/09/why-dont-we-know-the-age-of-the-new-human-ancestor-homo-naledi/405148/

      The summary is that both carbon and DNA analysis require the destruction of material, and the team says they wanted to avoid that until after the fossils were all described. There are other techniques as well, such as dating readioactive decay in the flowstones within the cave, but techniques like that will require a lot more time and excavation. The team is investigating these and other approaches though which they hope will corroborate each other and provide a reliable date for the naledi remains.

  5. ted martz
    conyer,georgia
    September 18, 2015, 5:50 am

    I was just wondering if their was any evidence of lighting in the cave? How did the Naledi see to place the bodies so far in the cave? Keep up the great work and I wish I was there.

  6. Andrew M
    NJ
    September 18, 2015, 12:32 am

    Jean-Paul, you should find interviews with Lee Berger online (try Science Friday from NPR). He explains that it has something to do with rules about destroying bones in South Africa, but also about a near lack of sediment, plant, or animal material (things normally used to help date remains) due to the extremely remote and isolated cave environment. It is my understanding from the SciFri interview that they eventually will be able to date the remains more accurately but that they estimate right now they are 2.5 million years old.

  7. Jean-Paul
    Cerro Gordo IL USA
    September 17, 2015, 10:29 pm

    Just a layman asking; Why has no definitive dating or possibly DNA testing been made. I have noticed this question, and have not seen and answer.