The latest on Homo naledi

Relatively complete skeletons from early human ancestors typically generate a ton of buzz in the popular media. None, however, has generated as much in the last 10 years as the finds of Homo naledi. The remains of at least 15 individuals have been unearthed within the Rising Star Cave System in South Africa since its discovery by cavers in 2013. In 2017, an additional three individuals were announced from a separate chamber in the cave. The finds show an interesting, and perplexing, mixture of traits that recall everything from Homo habilis to H. erectus and even Australopithecus.

Last fall, the President of our Student Anthropological Society here at UNCG, Cory Henderson (who was recently accepted to graduate school at Penn State), asked me if I could get in touch with a friend of mine, Dr. Zach Throckmorton, who is a member of the Rising Star team, to talk about Homo naledi. Zach generously agreed, and on Friday, April 7th the Department of Anthropology and Student Anthropological Society at UNCG hosted Zach and another Rising Star researcher, Dr. Chris Walker. The background to the Rising Star discovery and the rapid fire, open-access approach to research dissemination are well documented and controversial, and I'll leave it to the reader to delve into those issues. Here, I'll just summarize what I found to be particularly intriguing. First up was Zach, who is an expert on feet and discussed the post-cranial anatomy:

• H. naledi has long thumbs, just like modern humans (this is one of the features that gives us the ability to precisely manipulate objects with our hands). However, the fingers are somewhat curved.
• The torsion, or twisting, of the humeral shaft relative to the humeral head is odd in that it falls within a range of modern monkeys. 
• The lower limb, including the foot, appears to be more-or-less modern human-like. However, H. naledi was small--Zach estimates that the complete foot would have fit into a size 3 shoe. 
• So, overall, the upper limbs retain some features that may be adaptations for climbing, while the lower limbs share many similarities to modern humans. 

Chris followed with a discussion of growth and development in H. naledi:

• A majority (eight out of thirteen, to be exact) of the individuals from the cave for which age can be determined are juveniles. Most of the complete long bones are also from juveniles. 
• Based on the dental data, it seems that H. naledi followed a modern-human like growth trajectory. 
• The legs are very long relative to body size (which was pretty small, perhaps 147 cm and around 45 kg), but the intermembral index is human-like. This means that H. naledi had longer legs than would be expected for a hominin of its body size but they are not longer relative to their arm length than would be expected for a modern human. Such long legs may be an adaptation for efficient terrestrial bipedalism. 

The anatomy of H. naledi, and the provocative interpretation put forward by the Rising Star team for the assemblage's origin (the intentional disposal of the cadavers within the cave by other hominins), was even more puzzling because no one knew how old the fossils were. Based on their morphology, some estimated the fossils to be early Pleistocene in age (perhaps one or two million years old). A colleague who has seen the bones first hand told me that they didn't appear to be fossilized, which is difficult to accommodate with such an ancient date. Earlier this year the Rising Star team announced a much younger date of ~300,000 years. While deliberate placement of the bodies in the cave is perhaps more easily reconciled with a younger age, the brain size (less than half the size of a modern human) and the complexities of cave geology make this hypothesis far from demonstrated. Either way, this is extremely interesting, and I am looking forward to seeing where it goes...