Homo naledi is the newest member of the human family, being discovered in 2014. Since then information about the species has trickled out. We learnt about its weird mixture of modern and archaic features. We discovered it may have been burying its dead. But the significance of all this was a bit uncertain since we didn't know how old the fossils were. Without that key piece of context, we were left speculating about where this new species fit in our family tree.
But speculate no more as the Homo naledi age was finally revealed. Multiple techniques were used on the fossils from the Dinaledi chamber, revealing they lived sometime 236 - 335 thousand years ago. For those who don't have the timeline of human evolution memorised, this is very recent. Initial estimates suggested the species might have lived closer to 2 million years ago.
Homo naledi age finally revealed
At this point, you may well be asking why it took so long for the Homo naledi age to be revealed. It was discovered all the way back in 2014 after all. Well, there were actually efforts to date the fossils when they were first discovered. Bones were examined to be carbon dated and rocks were tested for uranium-lead dating. However, it turns out the specimens were too old for the former and too contaminated for the latter.
But the researchers didn't give up. Archaeologists have many dating techniques open to them, it's part of the strength of the field. We can always cross reference results with independent analyses to ensure we're on the right path. Ultimately, the researchers found two more techniques which would work in these circumstances: uranium-thorium dating and ESR.
Uranium-thorium dating was applied to flowstones throughout the chamber where the fossils were found. These are rocks formed by water depositing material on top of the floors or wall at the time. As such, they provide a minimum age for the deposits they overlie. In the case of the deposits featuring Homo naledi bones, these must be at least older than 240,000 years and younger than 500,000 years.
ESR dating was carried out on three teeth of Homo naledi. Because science is awesome and we can figure out how old something is based on the atomic composition of its teeth. This produced consistent results with the flowstone data, placing the age of the teeth 230 - 250 thousand years ago.
A few other dating techniques were also applied, yielding consistent results. This allowed the researchers to say with confidence that Homo naledi lived some time between 236 - 335 thousand years ago.
It's a spooky ghost
Homo naledi is a strange species. It shares many similarities with our genus, Homo. But also with our ancestors, Australopithecus. As such, initial estimates placed the species close to the transition between the two; sometime between 2 - 3 million years ago. Follow-up research suggested the species actually lived closer to 1 million years ago. Whichever estimate you prefer, its still a far cry from the 300,000 year old actual date these researchers discovered.
What's up with the Homo naedli age? The discoverers suggest this could all be explained if the species was a ghost. Not the spooky kind, but the evolutionary kind.
Humans did not move and evolve as one complete species. We lived in many separate populations, each evolving independently to suit our environments. As we spread out of Africa this gave rise to variation in Homo erectus. Later on it also produced the differences between modern humans, Neanderthals, and Denisovans. But what if there were one species that remained isolated and relatively well-adapted to its environment. It might persist for thousands of years relatively unchanged, like a living fossil.
We know living fossils exist in many other lineages. Perhaps it happened in ours as well. A late-surviving member of those first Homo. It could explain the weird Homo naledi age. Of course, this isn't the only option. It could be a later branch that reverted to older characteristics, perhaps through some convergent evolution. And even if it is a late surviving example, who knows how much has changed since then? We've got no fossils either side of it to see how closely it might resemble our earlier ancestors.
In solving the mystery of the Homo naledi age we've opened up another can of worms.
How could it survive?
One key step in understanding if Homo naledi might be some sort of living (although now extinct) fossil is understanding if its possible. Could a relic population survive in South Africa for millions of years?
It might be surprisingly plausible. We've only found a handful of other fossils from this time period down there. Many of them are undated, so we don't know if they'd be contemporary with Homo naledi. As such, it may be the case that the species faced relatively little competition from other human species, allowing it to survive for so long.
Interestingly, dated fossils of more derived humans arriving in the region coincides with the lower estimates for the Homo naledi age. Could they have caused their extinction? We'll only know the answer to all these mysteries with more fossils.
Fortunatley, researchers have just found a bunch more.
References
Berger, L.R., Hawks, J., Dirks, P.H., Elliott, M. and Roberts, E.M., 2017. Homo naledi and Pleistocene hominin evolution in subequatorial Africa. eLife, 6, p.e24234.
Dembo, M., Radovčić, D., Garvin, H.M., Laird, M.F., Schroeder, L., Scott, J.E., Brophy, J., Ackermann, R.R., Musiba, C.M., de Ruiter, D.J. and Mooers, A.Ø., 2016. The evolutionary relationships and age of Homo naledi: An assessment using dated Bayesian phylogenetic methods. Journal of human evolution, 97, pp.17-26.
Dirks, P.H., Berger, L.R., Roberts, E.M., Kramers, J.D., Hawks, J., Randolph-Quinney, P.S., Elliott, M., Musiba, C.M., Churchill, S.E., de Ruiter, D.J. and Schmid, P., 2015. Geological and taphonomic context for the new hominin species Homo naledi from the Dinaledi Chamber, South Africa. Elife, 4, p.e09561.
Dirks PH, Roberts EM, Hilbert-Wolf H, Kramers JD, Hawks J, Dosseto A, Duval M, Elliott M, Evans M, Grün R, Hellstrom J. The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa. eLife. 2017 May 9;6:e24231.
