Surprising New Information About Homo naledi

The Atlantic (and other outlets) has a story on new information on the date of the Homo naledi fossils from Rising Star Cave and it is...surprising.  As Sarah Zhang writes:

The one thing everyone agrees is that the fossils themselves are spectacular. In 2015, researchers unveiled 1,500 hominin fossils found deep in a South African cave, excavated by six cavers who were all skinny, short, and female. The hominin, a new species the team christened Homo naledi, was an unusual mix of the old and modern. Their heads were small, suggesting an early hominin perhaps more than a million years old. But their feet were stiff for walking upright and their hands adept like modern humans.

So in the media frenzy that followed—a National Geographic cover, a documentary, numerous articles—the question kept coming up: How old are these Homo naledi fossils, really? What do they tell us, if anything, about the origin of Homo sapiens?

To that first question, the researchers now have an answer: 236,000 to 335,000 years old. As for the second question, well, it’s complicated. “You can’t tell simple stories anymore,” says Lee Berger, a paleoanthropologist at the University of the Witwatersrand who led the research. “This is the gigantic message out of Homo naledi.” The age of these fossils puts these strange, small-brained yet human-like hominins in South Africa just before the emergence of the first anatomically modern Homo sapiens.

Holy Liang Bua, Batman!  If the bones really are 236k, then that would put them roughly 40-50 k years before the advent of modernity represented by the Omo 1 cranium.  Admittedly, that skull is fragments “swimming in plaster” but we have a pretty good idea of what it looks like. We certainly have modernity in the Bouri remains from Herto at 160 ky.

How do we know how old the fossil are?  From the eLife article:

Now, Dirks et al. – who include many of the researchers who were involved in the discovery of H. naledi – report that the fossils are most likely between 236,000 and 335,000 years old. These dates are based on measuring the concentration of radioactive elements, and the damage caused by these elements (which accumulates over time), in three fossilized teeth, plus surrounding rock and sediments from the cave chamber. Importantly, the most crucial tests were carried out at independent laboratories around the world, and the scientists conducted the tests without knowing the results of the other laboratories. Dirks et al. took these extra steps to make sure that the results obtained were reproducible and unbiased.

More specifically, they did Electron Spin Resonance (or Electron Paramagnetic Resonance), Uranium-Thorium disequilibrium, Optically Stimulated Resonance and radiocarbon dating.  This is probably not the last word on the dating of the fossils and there are quite a few assumptions that have been made regarding the provenance of the fossils to the cave sediments, but this is the closest thing that we currently have. One of the critical problems in dating much of the South African cave deposits is their relationship to the hominins found there.  Often, the caves opened up at the top and the fossil remains either fell in, were dropped in or they crawled in.  What makes the Rising Star remains so peculiar is that this cave did not form in the traditional way and the bones were so far into the cave. 

Where does this leave us? It leaves us with more questions than answers.  It has been common consensus that, while there was considerable variability in hominin morphology throughout the Pliocene and early to middle Pleistocene, that this "settled down" and that variability diminished as we made the transition to modern humans.  These fossils suggest the opposite: that large amounts of variability were, in fact, present throughout the range of human existence and only recently (200ky to the present) did this variability drop off.

I am quite sure that more studies will be done to elucidate the relationship that these fossils (there are more very similar fossils next door in the Lesedi cave complex) have to other forms in Africa.  There are obviously very peculiar cultural interactions going on, though (or lack, thereof).  With the Liang Bua remains in East Asia, hypotheses of endemic dwarfism and isolation have been put forth to explain why these remains are so diminutive and primitive.  It is more difficult to put forth these same arguments here where there are few to no barriers to migration and interaction between groups.