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The new finds include the most complete skull from the species yet.

In 2015, researchers announced a surprising discovery: a completely new species in our own genus, Homo. Homo naledi was discovered not as a few bone fragments or even a single skeleton, but as a huge collection of bones from 14 individuals, all found in the same chamber of a South African cave.

The new species was small in stature and had a brain smaller than any other member of Homo. Some of its features were shared with Australopiths, while a few were all its own. But many other characteristics were shared with other members of Homo, including ourselves. The features on their own suggested Homo naledi might fit in early in our species' evolutionary history. But, without any dates attached to the bones, any ideas as to where they were largely speculative.

Now the same team that discovered Homo naledi is back with more skeletons from a different chamber in the same cave. With those new skeletons comes some hard dates: only about 250,000 years ago, possibly making H. naledi a contemporary of the earliest modern humans. Those dates upset just about everything we thought we knew about recent human evolution, our genus' use of tools, and how all those skeletons ended up buried in the cave in the first place.

Context is everything

There are a number of ways to get accurate dates on fossil finds. Many rely on the presence of specific minerals in surrounding layers of dirt or the presence of other fossilized species that have well-dated histories. Homo naledi had neither of these. It was pretty much the only species found in this area of the Rising Star cave, and its bones were buried in a mix of solidified mud and a crust of once-dissolved minerals.

So Homo naledi arrived without a date. But its mixture of features from our genus and its ancestors led most people to expect that it dated from close to the origin of Homo, and it might help tell us how we came about.

Since then, however, the team behind Homo naledi's discovery have been looking into alternative ways of getting at the fossil's age. These include uranium-thorium dating. This is based on the relative solubility of uranium, which allows it to seep into wet material. Once in place, however, uranium can decay to thorium, which is insoluble. While the approach has considerable uncertainties, uranium-thorium dating can work with a variety of materials, including the bones and minerals in the cave.

Two other approaches rely on the fluorescence of electrons that end up placed in a high-energy state by radiation. These should provide an indication of how long a material has been in place and exposed to radiation. Getting a date out of that, however, requires a good understanding of the radioactive environment of the cave and, again, has uncertainties. The team also looked at the magnetic orientation of the mud, which will track well-dated reversals of the Earth's magnetic field.

To minimize problems, the team sent samples to two different labs, who performed the dating on them blind. In this instance, "blind" means neither lab learned the other lab's results until both were done. Despite this caution, some samples produced dates that didn't make any sense in light of other samples. So the authors had to do a very detailed analysis of when the different layers of mud and mineral were put in place—which was not simple, because in some areas, the mud appears to have been eroded by later flooding. They also identified a bone sample that seems to have picked up an added dose of uranium when the chamber was submerged.

So, while most of the uranium-thorium dating just tracked the most recent time the cave was submerged, one Homo naledi tooth hadn't picked up new uranium for over 200,000 years, suggesting it (and the rest of the remains) had been buried for at least that long. Both fluorescence techniques put two of the three H. naledi teeth at 250,000 years old (but also produced one much earlier and one much older reading). Dating the material the bones were found in, as well as the mineral deposits above them, also put upper and lower limits on the age.

Putting it all together and throwing out the dates that didn't make any sense, the results seem to suggest the bones were buried between 235,000 and 355,000 years ago. That's remarkably similar to when modern humans arrived on the scene 200,000 years ago. Rather than being some sort of primitive ancestor, H. naledi may have been our contemporary.
What the... ?

As the authors understate matters, "The new age estimates for Homo naledi show that an approximate age for the hominin fossil fragments cannot be simply deduced from their morphology." In many cases, parts of skeletons have been available that also didn't have a date, but they were placed in an approximate chronology based on their mix of early and modern features. But H. naledi has features not seen in any species since Australopiths, yet it lived after Neanderthals and modern humans split. Anything to do with Homo without a date now has to be viewed with suspicion.

There's an interesting corollary to this. Homo naledi surprises us because it seems to have early features long after they've been gone from the fossil record. But there's a chance that some of the fossils we assumed were older (because of how they looked) are also more recent and represent intermediaries on the path to H. naledi. There are also a handful of fossils that have been assigned to Homo erectus (our species' immediate ancestor) but also have a few early-looking features or small brains.

Any discussion also has to consider the fact that modern humans seemed to have mated with their close relatives. Hybridization could potentially explain Homo naledi's odd mix of features: "H. naledi shows that many human-like anatomical aspects of the hand, foot, lower limb, dentition, and cranium, including some aspects that are not present in H. erectus, occurred in a species with a brain size equal to that of Australopiths."

Another option is that Homo naledi split off during the earliest diversification of our genus. Right as Homo started, there seems to have been a collection of overlapping species before Homo erectus dominated the fossil record. H. naledi could have been part of that rapid branching and ended up with features that either re-evolved separately or ended up back in the human linage through hybridization.

But all we know for sure now is that Homo naledi branched off somewhere before modern humans, Neanderthals, and Denisovans split. Everything else is likely to be an ongoing argument for years.

Tools and burials

The date also means that Homo naledi or its ancestors were around as Homo experienced its first revolution in tool technology in nearly two million years. Just this week, a paper inferred that the Oldowan-to-Acheulian tool transition occurred through the formation of a modern brain network. But now we can no longer be sure that the species using the tools was our ancestor, especially since Homo floresiensis apparently represents a different lineage that retained early features.

If some degree of abstract thinking is needed for sophisticated tool manufacture, then a whole lot more is needed to ritualize death. And that's where the new skeletons come in. They're not really exceptionally exciting as skeletons, as they mostly confirm what the first batch told us. The exception is the most complete cranium yet, which happens to come from the largest individual found. It's big enough to finally get a Homo naledi brain to overlap in size with a different species of Homo.

What's most interesting is that the bones of several individuals were found in a completely different chamber in the Rising Star cave, nearly 150 meters away from the first finds. And again, they were the only bones present; there was none of the mixture of other species typically found in randomly buried fossil deposits.

This, the team argues, is further indication that the bones were buried intentionally. They suggested this after the first finds in 2015, but the idea was a bit controversial. Given a second, similar find, it's more likely to be accepted.

But, of course, now that we have a date for Homo naledi, the idea that these individuals were buried is no longer closely associated with burial by other members of the same species. It could have been our own species that placed them in the cave.



[Ars Technica]

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