Homo naledi more human than expected

Homo naledi has a mixture of archaic and more derived traits. New research has found that there are even more modern-like traits than we thought.


The bones recovered from Homo naledi’s burial chamber

Homo naledi is the newest member of the human family we’ve discovered. And it really is quite new, living around 300,000 years ago. That might not seem particularly “new”, being several orders of magnitude older than the pyramids. However, in terms of our evolution that would make Homo naledi basically a baby. It’s only a few thousand years older than us!

This gets especially interesting when you take a look at their anatomy, which seems surprising old. Their brain, for example, is the same size as fossils millions of years older like Lucy. In fact, Homo naledi looked so archaic that we used to think they must be 1 – 2 million years older than they actually were. That’s a fairly significant margin of error.

However, researchers are still making discoveries about the species. We’re even still finding brand new fossils in their cave! And it turns out that Homo naledi shares a few more similarities with later species than we first thought. Finally, the new fossils seem to be actings their age.

Setting the stage

The skull of Homo naledi.

When Homo naledi was first discovered researchers dove in head first. Quite literally. A lot of the initial research focused on the head, as that’s where hominin species can really look different. This revealed the species had the unique combination of archaic and derived traits that makes it so confusing for us.

However, ongoing work reveals that the rest of their body seemed to be more derived. Researchers found a few exceptions to this, but for the most part, Homo naledi looked quite modern below the neck. And this trend looks set to continue as researchers have found two new modern traits in the species.

The first isn’t technically a specific trait. Rather, it’s how traits vary across the species. Sexual dimorphism refers to how different the two sexes are. Modern humans – along with other members of later Homo – have relatively low sexual dimorphism. Homo naledi appears to follow this pattern too.

The other modern trait Homo naledi has is a fairly simple one: long legs. We’ve all got them, relatively speaking. In fact, 1/3 of your body weight is below the waist. These long, powerful legs are a great adaptation for the bipedalism we – and other later Homo – are so fond of.

Stupid sexy naledi

It’s easy to think of ways in which males and female anatomy differ. However, we’re actually quite similar relatively speaking. Compared to the other apes we’re practically indistinguishable. Just check out our teeth for proof. Male apes have much larger canines than their female counterparts. Outside of vampires, our tooth dimorphism just can’t compete.

Early members of our family were quite similar to apes. Both in general anatomy, and the degree of sexual dimorphism. Early in our history, there were much more significant differences between males and females. But this dropped off over time. By the time of Australopithecus we were significantly similar. With very interesting social implications.

However, the story isn’t quite that simple. Some Australopiths seem to have retained higher dimorphism, as did later species like Homo erectus. In other words, sexual dimorphism had a fair bit of species dimorphism! Later species began to settle down at human-like levels, but the journey they took was varied.

The relative size of canines of different species, a key indicator of sexual dimorphism Homo naledi = dotted line, modern Homo sapiens = white, chimpanzees = gray, gorillas = black.

All of this raises the obvious question: where did Homo naledi fit into it all? Perhaps they have human-like dimorphism, as their young age would indicate.  Or do they fit into one of the many other patterns of dimorphism seen in earlier species? Well, you can stop holding your breath. Homo naledi falls square within the range of modern human dimorphism. This is particularly interesting given their body size. They were much smaller than us, like earlier species, yet had this modern pattern of dimorphism. What’s up with that?

This is particularly interesting given their body size. They were much smaller than us, like earlier species, yet had this modern pattern of dimorphism. What’s up with that? For each question being answered, more take its place. That keeps the researchers in a job I guess.

Homo naledi had legs for days

Human legs are ruddy massive. They contain the biggest muscles and bones in our body. This isn’t particularly surprising, given they have to support our entire weight. This extra size also helps with stride length. Bigger steps are just more efficient.

Crucially, this isn’t the pattern seen in our earlier relatives. Whilst they also often walked upright, they also climbed trees. In the canopy long legs can be a hindrance, moving your center of gravity away from the tree or branch and making you vulnerable to falling. This resulted in a fairly typical blueprint for early hominins. They had long arms for grabbing, short legs for balancing, and small bodies so they wouldn’t break the branches they were climbing.

As you may have noticed by looking down, we shifted away from this blueprint a fair bit. However, Homo naledi still has some of it. Notably, their body is quite small. Might they have some of the other climbing adaptations too?

Relative leg/arm length of different species. Homo naledi = dotted line, modern Homo sapiens = white, chimpanzees = gray, gorillas = black.

Well if they do, the rest of the blueprint isn’t one of them. This latest set of analyses found that the rest of their body definitely shifted towards the modern human way of doing things. Notably, their legs were pretty big when compared to their arms. They were clearly shifting away from the short leg/lanky arm pattern of earlier species.

But what does it mean?

So, our picture of Homo naledi is getting clearer and clearer. And they’re looking more and more modern. However, the biggest implications from this aren’t about Homo naledi.

See, the thing is that not every bit of them follows this trend. Yes, we’ve found a few more modern-like features, but let’s not forget all the more archaic ones we found earlier. In evolution speak, this is a mosaic of characteristics. They’re independently evolving parts of the body that come together, in a unique combination, to make this unique species.

Importantly, this indicates that these features are mosaic in us. All later species might have long legs, big bodies, and reduced sexual dimorphism. But Homo naledi shows these traits aren’t all necessarily linked. They can evolve and shift independetly, so likely did so in us. We’re as much of a hodgepodge of traits as these earlier species.

We’re just a hodgepodge we’re more familiar with.

References

Garvin, H.M., Elliott, M.C., Delezene, L.K., Hawks, J., Churchill, S.E., Berger, L.R. and Holliday, T.W., 2017. Body size, brain size, and sexual dimorphism in Homo naledi from the Dinaledi Chamber. Journal of Human Evolution111, pp.119-138.

Gordon, A.D., Green, D.J. and Richmond, B.G., 2008. Strong postcranial size dimorphism in Australopithecus afarensis: results from two new resampling methods for multivariate data sets with missing data. American journal of physical anthropology135(3), pp.311-328.

Reno, P.L., Meindl, R.S., McCollum, M.A. and Lovejoy, C.O., 2003. Sexual dimorphism in Australopithecus afarensis was similar to that of modern humans. Proceedings of the National Academy of Sciences100(16), pp.9404-9409.

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