<link rel="stylesheet" href="//fonts.googleapis.com/css?family=Roboto%3A300%2C400%2C500%2C700%7CRoboto+Slab%3A400%2C700">Oldest Australopithecus spine discovered, already adapted for bipedalism
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Archaeologists have found the oldest Australopithecus spine to date. 7 vertebrae have been recovered from Ethiopia that clock in at 4.2 million years old. This places the fossils closer to the origins of our lineage than the present day. In fact, these are one of the oldest sets of vertebrae ever discovered1.

However, despite being so ancient it also features many adaptations for walking upright. Including some that we still have. In fact, some of the vertebrae are so similar they can still fit neatly with our spine1.

Australopithecus spine with human bone added
The topmost human vertebra (also called the atlas) with the second ancient Australopithecus vertebra (called the axis) 1.

These ancient vertebrae were amongst more than 30 hominin fossils found in the early 2000s at Asa Issie, Ethiopia. Archaeologists soon realised these ancient fossils belonged to Australopithecus anamensis2. This species is the oldest known member of the Australopiths. In fact, they may be the ancestor of Lucy’s species Australopithecus afarensis3. Which is super cool, since these finds mean Au. anamensis was living 1,000 km further north than we knew2.

Jaws from Australopithecus anamensis (left) from Kenya and A. afarensis from Ethiopia. Courtesy: National Science Foundation

But it turns out that’s not the end these fossils’ story. Now, researchers Meyer and Williams have re-examined this ancient spine, shedding light on the emergence of upright walking. They published their findings in the July 2019 edition of the Journal of Human Origins.

What can we learn from an ancient Australopithecus spine?

The 7 bits of Australopithecus spine represents at least 2 individuals. Yet despite being incomplete, each tells us something about their owners. In particular, Meyer and Williams found key evidence this early species was already walking upright1.

The key fact here is that our spine has to support the weight of our whole upper body. As such, it’s a bit of a chonky boi, to better deal with these extra forces. This becomes especially true further down the spine, where even more weight is concentrated1.

Sure enough, this ancient Australopithecus spine is also on the chunky side. And that size also increases further down the spine. In fact, their vertebrae wind up being larger than the equivalent bones in famous biped Lucy1 (although that may be due to Lucy being on the small side as she’s a dainty lady). This confirms early Australopithecus was also walking upright.

Ancient Australopithecus spine and Lucy
A fragment of Au. anamensis T9 (left) versus the same bone in Lucy (right). Note how much bigger the ancient bone is1.

That discovery refers to something we’ve gained (chonk). However, this ancient Australopithecus spine also tracks something we’ve lost: the excitingly titled “atlantoclavicularis muscle”. This is muscle we no longer have, but the other great apes do. In them, it runs from the atlas to the collarbone; providing an extra muscle to make their shoulder stronger when climbing1,4.

The atlantoclavicularis in chimps and it not in humans4.

Accordingly, we’ve lost the bit of the axis this muscle attaches. And I’ll bet you guess where this is going: it’s also missing in this ancient Australopithecus spine and in other, later, human fossils1,4.

A special spine

Of course, not everything in this ancient Australopithecus spine matched up with ours. There were some differences too.

Notably, the 6th vertebra down from the skull has a massive (for want of a better word) spine. This is associated with a more projecting face as it positions the muscles to better deal with where the head’s centre of gravity is. As such, we now know the face of Australopithecus anamensis stuck out a bit, despite the fact we’ve never actually found a full fossil of their face1.

Au anamensis veretebrae (A-C) and one from Lucy’s species (D) showing how the specieses with the big faces have the big spines on their . . . spine1.

All in all, I’d say that this backbone is definitely supporting its own weight.

References

  1. Meyer, M.R. and Williams, S.A., 2019. Earliest axial fossils from the genus Australopithecus. Journal of human evolution132, pp.189-214.
  2. White, T.D., WoldeGabriel, G., Asfaw, B., Ambrose, S., Beyene, Y., Bernor, R.L., Boisserie, J.R., Currie, B., Gilbert, H., Haile-Selassie, Y. and Hart, W.K., 2006. Asa Issie, Aramis and the origin of Australopithecus. Nature440(7086), p.883.
  3. Kimbel, W.H., Lockwood, C.A., Ward, C.V., Leakey, M.G., Rak, Y. and Johanson, D.C., 2006. Was Australopithecus anamensis ancestral to A. afarensis? A case of anagenesis in the hominin fossil record. Journal of Human Evolution51(2), pp.134-152.
  4. Bramble, D.M. and Lieberman, D.E., 2004. Endurance running and the evolution of Homo. nature432(7015), p.345.

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