Homo naledi may be 1 million years younger than expected

A new analysis of Homo naledi revealed the species may be much younger than anticipated. Whilst the results aren’t conclusive, they are interesting


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In 2015 a new member of the human family was discovered. It was called Homo naledi and might be one of the most significant discoveries ever. Their anatomy indicates it could have been an early member of our genus Homo and may be around 2 million years old. Thus, it would potentially reveal how we evolved. Additionally, the placement of the fossils hints at a deliberate burial. This could suggest ritualistic behaviour is more ancient than we imagined.

Unfortunately, life (and its evolution) is a little more complication than that. The evidence for these bones being “buried” is a lot less conclusive than initially thought. Plus, there are no solid dates for how old the fossil is; so we can’t place it in its evolutionary context. We need to know how old this species is before we can really understand it.

There are still no direct dates for this species. However, a recent paper claims to have found a way around this problem; revealing Homo naledi might have lived much more recently than we thought.

Homo naledi’s age

Homo naledi was discovered in 2015 by the Rising Star Expedition. Led by Lee Berger, they were impressed by how Homo naledi shared many features with earlier, ape-like species. In particular, it was remarkably similar to Australopithecus. Ultimately, Berger and his team concluded it shared more features with the Homo group; so they placed in our genus. Something I kind of already spoiled by giving the species its full name. Sorry about that.

The bones recovered from Homo naledi's burial chamber

The bones recovered from Homo naledi’s burial chamber

Nevertheless, these ape-like features led Berger et al to conclude that Homo naledi was an early member of the human genus. Hence, why it still had so many archaic features. Attempts to directly date the fossil failed, but this inference about its evolution led to a tentative date of around 2 million years. This marks when Homo was beginning to take form, with Homo erectus – the first “proper” Homo – emerging shortly after this date

However, a more rigorous statistical analysis has challenged this interpretation. Another group of researchers used Bayesian analysis to study where Homo naledi fits in the human family tree. Its a complex process I won’t even pretend to understand. The key point is that Bayesian analyses actually try and find the most plausible family tree by comparing them with known data. The more time a tree “wins” comparisons with alternative trees, the more consistent it is with the data and thus the more plausible it is.

When this approach is applied to the skull of Homo naledi it produces a very surprising result. Despite its apparent similarities to Australopithecus, the fossils clock in quite young. 912,000 years ago to be exact. It’s basically a baby compared to the predicted date of 2 million years old.

Homo naledi in the family tree

The initial study of Homo naledi revealed something remarkable. It shared a mixture of features with both Homo and our more ape-like ancestors, Australopithecus. In fact, it shared so many features there was some discussion of calling it Australopithecus naledi; although not for long.

The Bayesian analysis of Homo naledi didn’t only reveal it’s age, but it’s potential place in the human family as well. Like Berger, it didn’t think it belongs to Australopithecus. It was clearly a member of our Homo, like us. But much like the young date of 912,000 years; it put it a lot deeper in  the Homo family than Berger.

Berger placed it at the base of our genus. An early offshoot that represents what some of the earliest members of Homo may have looked like. However, this new analysis groups it in with the same bunch as Homo erectus, the Neanderthals, and us. A sister species deep in the Homo group, rather than an early ancestor “on the outside”.

The old family tree of Homo naledi (left) and their new family tree (right)

The old family tree of Homo naledi (left) and their new family tree (right). It is much deeper in the Homo group

Time to change the changes to textbooks?

When Homo naledi was discovered it could change the textbooks. Although that depended on how old it was. Now this analysis indicates its young, is it time to revert those changes?

Not quite.

Whilst Bayesian analysis is a great tool; it’s only as good as the data you feed into it. And not that much was fed into this analysis. The data for the other species came from a previous study by these researchers; which only considered a couple of different individuals from each species. We know our ancestors could be highly variable, so this definitely raises my eyebrows. Plus, they only examined the skull of Homo naledi. Many of the more archaic features are found in the body, so this would definitely bias their results.

Given these factors, I don’t think it’s time to label Homo naledi a baby face just yet. Nevertheless, it does serve as a reminder that we desperately need dates for this species. It shows that without them there’s a huge range of possibilities for where this species might fall; hampering any attempts to properly study it.

Summary

A new analysis of Homo naledi revealed they may be much younger than originally anticipated. Although the data used in this study is lacking, it serves as a powerful reminder Homo naledi might have some surprises for us yet.

References

Berger, L.R., Hawks, J., de Ruiter, D.J., Churchill, S.E., Schmid, P., Delezene, L.K., Kivell, T.L., Garvin, H.M., Williams, S.A., DeSilva, J.M. and Skinner, M.M., 2015. Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa. Elife, 4, p.e09560.

Dembo, M., Matzke, N.J., Mooers, A.Ø. and Collard, M., 2015, August. Bayesian analysis of a morphological supermatrix sheds light on controversial fossil hominin relationships. In Proc. R. Soc. B (Vol. 282, No. 1812, p. 20150943). The Royal Society.

Dembo et al., 2016. The evolutionary relationships and age of Homo naledi: An assessment using dated Bayesian phylogenetic methods. Journal of Human Evolution

Hawks, J. and Berger, L.R., 2016. The impact of a date for understanding the importance of Homo naledi. Transactions of the Royal Society of South Africa, pp.1-4.

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8 thoughts on “Homo naledi may be 1 million years younger than expected”

  1. Belac says:

    Have you heard of that new stem-ape they found? Apparently it shows a more ape-like semi-bipedal stance as opposed to that of proconsul. I believe its name was Pliobates cataloniae. It resembles a gibbon more than anything, so most likely a splitoff.

    1. Adam Benton says:

      I have heard of it; although it’s a bit too early to be inside my field of expertise so I can’t really comment on it.

  2. Paul Braterman says:

    I found your use of the word “deep” confusing. I have aways taken it to mean close to the origin, but you are using it to mean deeply embedded rather than on the fringes; in effect, the very opposite.

    1. Adam Benton says:

      I prefer the term basal for what you describe; as its less ambiguous. Although now you point out this confusion I suspect I’m also going to have to find another term for deep. Any suggestions?

  3. rich lawler says:

    Gonna have to check out this paper. Bayesian analyses often produce pretty wide 95% credible intervals (akin to but not the same thing as a 95% confidence interval); I recall a study of the Y-chromosome phylogeny in the early 90s that had a credible interval between 0 and 2mya, meaning there was a 95% chance that the true date was within that interval. I wonder what sort of intervals (and priors) they used. Thanks for the write-up.

    1. Adam Benton says:

      As I mentioned, this all goes a bit over my head so I’d be happy to hear your thoughts on the subject.

  4. marc verhaegen (@m_verhaegen) says:

    That Naledi resembles different Homo species as well as A.sediba suggests that Naledi lived not long from the last common ancestor of Homo & the S.African australopiths, probably a few million years. Dembo et al.’s evolutionary tree with Naledi is based on the (IMO wrong) assumption that Pan & Gorilla are outgroups to “hominins” (the supposed group of Homo + australopiths). In fact, all Homo-like features in australopiths (thick enamel, valgus knees, low ilia, flat feet etc.) are not derived-human, but primitive-hominid, e.g. Pan & Gorilla fetuses have more humanlike feet, which later become more hand-like: “Only as it approaches its birth does its foot acquire the appearance of a hand” (C.Coon). This implies that australopiths can equally well be human as chimp or gorilla ancestors or relatives.
    If we use a purely scientific (biological) rather than an anthropocentric (just-so) approach, it becomes clear that Naledi did not deliberately bury their dead, did not make tools, did not run in savannas, and were no direct human ancestors. Naledi’s bones fossilised in mudstone, which forms in stagnant water, the curved hand-bones indicate frequent vertical climbing, the flat feet (as in humans & fetal African apes) suggest parttime wading & swimming (as in other flat-feeted animals), not running (ostriches, kangaroos, zebras etc. have very long & strong central digital rays, the opposite of humans). Lowland gorillas & bonobos sometimes wade bipedally in forest swamps & wetlands for sedges, waterlilies etc., google e.g. bonobo wading. Naledi probably spent a lot more time wading in wetlands, which explains their flatter feet. If they died where they fed, their bones sank in the mud, Naledi fossilised in mudstone, and the limestone underground eroded into caves: it was a natural process, no deliberate burial, and IMO there’s no need for anthropocentric interpretations.

    1. Adam Benton says:

      The bones were found in a cone of detritus from the main entrance to the chamber; suggesting the bodies entered there and not through the erosion method you suggest. Any thoughts on that?

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