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This week marked the 4th anniversary of the discovery of Homo naledi. Years later, research on this weird human species is still ongoing, revealing more and more unusual things about them. Like recent work on Homo naledi legs, which has revealed they walked weird, for a human1.

Legs 11

Homo naledi is just a weird human species overall. It shares many traits with us and other recent humans, as well as many with the earlier Australopithecus. Plus a few unique features are thrown in for good luck.

Yet despite looking like a weird halfway point between humans and Australopiths, it lived nowhere near the halfway point between those two groups. Instead, it lived less than half a million years ago; hundreds of thousands of years after the last Australopithecus went extinct.

A handy summary of Homo naledi from NatGeo

A mystery is like a magnet to scientists, so palaeoanthropologists have spent the past few years going “what’s the deal with Homo naledi“?

The latest effort to answer this question has focused on three Homo naledi legs. Specifically, their thighs. More specifically, the upper part of their thighs in the hip joint. EVEN MORE SPECIFICALLY, the cross-section of those bits of thigh1.

Homo naledi legs with the locations of CT scans taken
The three Homo naledi legs studied, with the red squares where cross-sections were taken via CT scans1.

This part of the body is kind of a big deal on account of us walking upright. As such, the entire weight of your body passes through this joint. Accordingly, the bone here has become increasingly chonky to deal with these forces1,2.

However, evolution is nothing if not frugal. The entire joint is not exposed to that weight, so not all of it has become chonky. In fact, parts of it have weakened compared to chimps as our gait exerts less force on it. No sense growing bone where it isn’t needed2.

Radiograph of human and chimp thigh. Ours has adapted to the forces of bipedalism by becoming stronger on the bottom. However, density has been lost elsewhere as it wasn’t needed2.

The Homo naledi legs

Long story short, the density of bone in the top of the thigh has adapted to the way different species walk. As such, researchers can use it to spot species that walk upright. Specifically, we’re looking for increasing thickness on the bottom of the joint.

Sure enough, this recent research has found the Homo naledi legs fit this pattern, confirming they were well adapted for bipedalism1.

Homo naledi legs cross section
The cross-section of the Homo naledi leg joint, complete with bipedal features1.

Meanwhile, other parts of our hip joint have gotten thinner as our way of walking exposes them to reduced forces. Specifically, the glutes take some of the strain of the top of the joint, hence why only the bottom part gets thicker2.

However, this is where things start to get weird in these Homo naledi legs.

Sure enough, these legs have lost thickness in some places. However, it’s different from our pattern of thickness. Taken together with some of the other weird features in this joint, this means they likely had a different gait to us. One not seen in any other human species1.

Instead, these features look like those seen in the earlier Australopithecus. Their (and thus Homo naledi‘s) gait involved a lot more side to side movement than ours1.

A reconstruction of Australopithecus sediba which features some of the side to side movement Homo naledi like had3.

Yet another thing that makes this weird species of human weirder.

References

  1. Friedl, L., Claxton, A.G., Walker, C.S., Churchill, S.E., Holliday, T.W., Hawks, J., Berger, L.R., DeSilva, J.M. and Marchi, D., 2019. Femoral neck and shaft structure in Homo naledi from the Dinaledi Chamber (Rising Star System, South Africa). Journal of Human Evolution133, pp.61-77.
  2. Ruff, C.B. and Higgins, R., 2013. Femoral neck structure and function in early hominins. American journal of physical anthropology150(4), pp.512-525.
  3. Zhang, A.Y. and DeSilva, J.M., 2018. Australopithecus sediba—Computer Animation of the Walking Mechanics of Australopithecus sediba. PaleoAnthropology2018, pp.423-432.

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