Humans Evolved Earlier Than We Thought. But What Does That Mean?

Modern humans belong to the genus ; a group of primates that evolved from the much more ape-like Australopithecus ~2.4 million years ago. Or at least, that's what we all used to think ¹. A new discovery from Ethiopia pushes back the appearance of Homo to 2.8 mya².

Which means what exactly? Although the media have been making a big fuss over this find, a date change of 0.4 million years doesn't sound that significant. Particularly when you remember the human family has been around 7 - 14 million years; and primates maybe as long as 85 million ¹. In the grand scheme of things, is this discovery really that important?

Yes! Allow me to tell you why...

But first...

A bit of information on the new fossil. It's a jawbone from Ethiopia, hailing from a region where countless other fossils of Australopithecus (and some of the oldest stone tools) had previously been discovered. It was dated based on the age of the sediments it was found in ².

Now, you may be wondering how they managed to identify this fossil given it's just a jaw. Well it turns out the evolution is in the details. Whilst this new jaw has a similar size and shape to Australopithecus jaws, the specific features are quite different; much more similar to Homo. Tooth crowns are different shapes, symmetries are different, grooves are missing and so forth. The whole list is rather dry and anatomical ², so I'll spare you the horrors of describing it in depth; but hopefully you get the picture.

Obviously more is needed to be sure about what they've found, but everything published seems pretty solid so far. Just don't be surprised if it all turns out to be hokum.

This is particularly important given that the shift in jaw size is one of the more significant changes in our jaw's evolution. In Australopithecus it became very big, with huge teeth; before becoming a lot smaller and gracile in Homo¹.

Taken together, this has some interesting implications like...

1. Humans evolved without our humanity

Defining Homo is a rather tricky task, given evolution is a gradual thing. Everything sort of blends together. Nevertheless, palaeoanthropologists have significant changes that occurred in our genus: the use of stone tools and (relatively) large brains; starting from 1.5 times as big as a chimp ¹. Yet this new fossil appears to lack both of these attributes.

At 2.8 mya, it lived 200,000 years before the oldest known example of stone tools (2.6 mya); so there's no evidence it was making them. And without more of the skull it's difficult to say how big the brain was, but some have linked our brain growth to the increasing gracility of the jaw³ (although just as many dispute this ⁴). Thus the fact the overall size and shape of the jaw is similar to Australopithecus may imply it has a similarly sized brain.

In other words, if the scientific community agrees that the anatomical details discussed above are enough to classify this fossil as Homo (and no big brain cases or older tools are found to contradict these implications) we may well have to reconsider what makes us "special".

2. Australopithecus was dumb

Perhaps one of the most interesting archaeological developments of recent years has been the growing body of evidence Australopithecus made stone tools. As previously mentioned, this is often linked exclusively to Homo. One key piece of evidence in favour of this is that the oldest stone tools pre-date Homo.

Not any more.

Now, to be fair these dates were not the only reason we thought Australopithecus made stone tools. However, the case was far from definitive and now one piece of evidence has been removed it gets even more uncertain. I suspect in the long run it will still be vindicated, but I'm now a lot less sure of that than I was a fortnight ago.

3. Linking Lucy to us

It's easy to only look at this fossil in terms of what it means for us and the evolution of modern humans. However, it has some pretty important implications for earlier species as well (albeit, only in terms of modern humans).

There are more than half a dozen species of Australopithecus¹. Figuring out how they're all related to us is nigh impossible. This fossil may shed light on this, as it contains some "left over" Australopithecus features. Figuring out which species they're most similar too could reveal which of them we evolved from (if any).

Although more finds are needed to say anything for sure; this fossil appears to be most similar to Australopithecus afarensis, Lucy's species.

Conclusion

This new fossil is significant for more than the date it changes. It implies our genus evolved before stone tools, that earlier groups may never have made such tools. It could also help reveal which of those earlier groups we're descended from and a host of other things I don't have the time to discuss here.

Just keep refreshing the home page of EvoAnth and I'll keep you updated on any more developments.

References

1. Boyd and Silk, 2012. How Humans Evolved.
2. Villmoare, B., Kimbel, W. H., Seyoum, C., Campisano, C. J., DiMaggio, E., Rowan, J., ... & Reed, K. E. (2015). Early Homo at 2.8 Ma from Ledi-Geraru, Afar, Ethiopia. Science, aaa1343.
3. Emes, Y., Aybar, B., & Yalcin, S. (2011). On the evolution of human jaws and teeth: A review. Bulletin of the International Association for Paleodontology,5(1), 37-47.
4. McCollum, M. A., Sherwood, C. C., Vinyard, C. J., Lovejoy, C. O., & Schachat, F. (2006). Of muscle-bound crania and human brain evolution: the story behind the MYH16 headlines. Journal of human evolution, 50(2), 232-236.