Homo naledi brain was tiny yet modern

Researchers have managed to reconstruct the Homo naledi brain from fossils, revealing it was surprisingly small yet shared many modern features with us.


Homo naledi is the newest and most mysterious member of the human family, with a weird mixture of modern and archaic traits. And despite those ancient bits, it lived quite recently. So it seems fitting that research has revelaed the Homo naledi brain is no less confusing.

Researchers working on the fossils recovered from the enigmatic Dinaledi chamber have managed to recreate the shape of their brain from impressions on the inside of the skull. This reveals it was tiny, like earlier hominin species, yet was organised much like the modern human brain1.

As well as adding more mystery to the species, this unique combination has the potential to shed light on the evolution of our own brain. So let’s not be too judgy about their small size.

Homo naledi brain cast

The team working at Rising Star Cave have been doing great work in intense conditions. After crawling through a passage too narrow for most adults, they’ve pulled out more than 1500 fossils. This treasure trove is thought to represent more than a dozen individuals2.

Sadly, amongst all this data, we have yet to find any brains. Which is what you’d expect, given they’re squishy and tend to rot away.

However, the researchers did find a series of skull fragments. Between them, one relatively complete braincase can be put back together, which is a big deal as the brain leaves impressions on the braincase. Based on these, we can reconstruct the size and shape of the Homo naledi brain1.

3D prints of the Homo naledi brain. Most of the research was actually carried out on virtual models, where precise measurements and statistical analysis can be done. I just thought these looked super cool.

The first thing apparent about these casts is that they’re very small, with a volume of only ~550 ml1. To help you visualise it, that’s about the same size as a pint.

For reference, this is comparable to a chimp or Australopithecus like Lucy. Meanwhile, modern humans – and most other members of Homo – have at least 2 pints of brain in their skull. We go up to a whopping 2.5 pints, whilst Neanderthals show us up with a 3-pint brain.

The brain size of hominin species. Homo naledi is the red star I added in MS paint

The dramatically different size of the Homo naledi brain confirms that this is definitely a new species. Which raises the question: how did this tiny brain evolve?

The shape of their brain may hold the answer.

Homo naledi brain shape

So, the Homo naledi brain looks surprisingly old. It lived less than very recently, yet has a brain the same size as ancient hominin species. However, size is really the only bit of their brain that looks old-fashioned.

First, a quick anatomy lesson. As our brain evolved and expanded, different bits became bigger and covered up other parts, whilst also bringing new brain regions to prominence. For example, our expanding frontal lobe obscured brain groves seen in chimps whilst creating a new border elsewhere1.

The effect of this is that wrinkly pattern of the human brain is a unique “fingerprint” compared to chimps.

A chimp (left) and human (not left) brain with some of the defining groove patterns highlighted

It turns out that other species of human, like Homo erectus, Homo habilis, Neanderthals, etc. also share our brain fingerprint. More or less.

And in a surprising plot twist, the Homo naledi brain also has that same fingerprint. Like us, they have the same expansion of the frontal lobe, obscuring the same parts of the brain and forming new grooves in the same area1.

Homo naledi (right) has a more human pattern of grooves than Australopithecus sediba (the other one)

As well as helping confirm this Homo naledis place in the human group, this discovery has interesting implications for their intelligence. As you might expect any revelation about the brain to have.

Notably, many of the changes to the fingerprint are associated with the enhanced cognitive capabilities seen in our lineage. The changes to the frontal lobe I keep harping on about, for instance, has been linked to tool use1.

Thanks to all this, Homo naledi may have had a greater intellect than their small brain might imply. However, the implications of this research don’t end there.

Shedding light on our own brain

This discovery helps confirm that the human brain fingerprint is essentially universal in our group. How’d it get to be that way? Well, either each individual human species evolved it independently, or we all inherited it from a common ancestor.

Parsimony suggests the latter, and the evidence does seem to support it. The earliest members of our genus we’ve found, such as Homo habilis, also share many of these fingerprint features. And the fact that weird offshoots like Homo naledi have it suggests that it evolved before they split off from the rest of the human family. Which may have been quite a while ago.

Places where Homo naledi could have split off the human family tree. Researchers prefer the earlier options.

Now, evolution has gone on to shape our brain in different ways. It’s now rounder than anyone else’s. But it seems many of the foundational aspects of the human brain are simply inherited from earlier species. A larger frontal lobe, so critical to tool use and social behaviour, might be millions of years old.

We’re just another in a long line of human species.

References

  1. Holloway, R.L., Hurst, S.D., Garvin, H.M., Schoenemann, P.T., Vanti, W.B., Berger, L.R. and Hawks, J., 2018. Endocast morphology of Homo naledi from the Dinaledi Chamber, South Africa. Proceedings of the National Academy of Sciences115(22), pp.5738-5743.
  2. Bolter, D.R., Hawks, J., Bogin, B. and Cameron, N., 2018. Palaeodemographics of individuals in Dinaledi Chamber using dental remains. South African Journal of Science114(1-2), pp.1-6.

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Homo naledi brain was tiny yet modern

by Adam Benton
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