Another look at the "deliberate body disposal" hypothesis in Rising Star Cave

The recovery of over a thousand human fossils from Rising Star Cave in South Africa has ignited debate among paleoanthropologists over the origins of human mortuary behavior. The fossils were deposited some 300,000 years ago and occur deep within the recesses of the cave system. Today, it is extraordinarily difficult to access the bone chambers--so difficult, in fact, that some believe the bones represent the corpses of people who were deliberately placed in the cave by other humans.  As I wrote in a previous post, my colleagues and I are skeptical of this "deliberate body disposal" hypothesis for a variety of reasons.

Late last year, researchers from the Rising Star team announced in a series of papers in the journal PaleoAnthropology the recovery of still more skeletal material in the cave. They again claim that the most likely explanation for the fossils is intentional disposal by humans. I was invited by the editor-in-chief of the South African Journal of Science to write a short commentary on the issue. Let me first emphasize how difficult it is to work in the cave system: it is dark and damp, the passages are tight, and the stratigraphy is extremely complex. Kudos to the Rising Star team not only for taking on the challenge, but for their open approach to the science. Many of the publications and accompanying data appear in open access journals and the public can follow the research via live streams and other venues. Having said that, I remain suspicious of the "deliberate body disposal" claim, and the commentary lays out why:

• There is good reason to believe that the cave system looked different 300,000 years ago than it does today. Importantly, the fossil-bearing chambers that appear to be largely cut off from the surface may not have been so in the past. It is therefore possible that water could have infiltrated the deepest recesses of the cave. What would have arrived with the water? A bunch of debris like sediments, leaves, branches, and, yes, bones.
• The human bones are poorly preserved, so any marks that might indicate their arrival via non-human means--tooth marks from carnivores, abrasion marks from bouncing around in water--will be difficult to detect.
• Baboon bones have also been found in at least one chamber. This shows us that the bones of a large-bodied primate can arrive within a seemingly inaccessible section of the cave system without deliberate disposal. (Unless, of course, baboons or perhaps humans were intentionally dragging baboons corpses into the cave.) 
• Many of the human fossils show cracks that mimic the weathering damage seen on bones that lay out in the open. This suggests that the fossils may have been exposed to the surface before their final interment in the cave.
• The frequencies of bones in the chamber do not represent complete corpses. This means that complete corpses did not enter the chamber in the first place or, if they did, then something disturbed them afterwards.

Paul Pettitt offers a very thoughtful commentary on the Rising Star data as well. He makes what I think is a critical distinction: namely the intentional deposition of corpses in a specific place (what Pettitt calls "funerary caching") versus the complex, symbolic ideas that accompany such behavior among modern humans. He points out that a variety of organisms, including non-human primates, interact with the corpses of their conspecifics, and many organisms also exhibit behaviors in the presence of the dead that we might interpret as grieving. He argues, then, that the deposition of bodies in deep caves can occur "without any sophisticated cognitive rationale behind the behaviour." In other words, there is no reason to automatically dismiss deliberate disposal just because these hominins were not modern humans. I agree completely. Pettitt also raises the important issue of whether or not alternative entrances to the cave existed in the past--other entrances that, if open to the surface at some point, could have served as conduits for stuff, including bones.

Is deliberate disposal a viable explanation for the fossil remains? Sure. But it still seems to me that other explanations--that the remains were washed in from somewhere else, for example--are sufficient and at least as parsimonious. Apparently an announcement from the Rising Star team is on the horizon that may produce new evidence that confirms, or at least strengthens, the deliberate disposal hypothesis (fire, maybe?).   

References:

Egeland, CP, Pickering, TR, Fadem, CM, Domínguez-Rodrigo, M. (2022). "Back from the Dead": another response to the contextual bases of the Rising Star 'deliberate disposal' hypothesis. South African Journal of Science. doi.org/10.17159/sajs.2022/13873

Pettitt, P. (2022). Did Homo naledi dispose of their dead in the Rising Star Cave system? South African Journal of Science. doi.org/10.17159/sajs.2022/15140

Sabertooth cats and early humans

It's been over two years (!) since I travelled down with my colleagues Manuel Domínguez-Rodrigo, Lucía Cobo-Sánchez, and Enrique Baquedano to the Florida Museum of Natural History to examine the fossils from a site called Haile 21A. What initially drew our interest was the co-occurrence of two partial skeletons of the sabertooth cat Xenosmilus hodsonae with over two thousand peccary bones within the sediments of an ancient sinkhole. I returned home from that trip just days before much of the country shut down because of the COVID-19 pandemic. What a two years it has been...but I am happy to report that we have completed our analysis, and the results appear in the journal Nature Scientific Reports. The study has enjoyed some modest media attention from both Rice University and my own institution.

Sabertooth cats—named for their imposing upper canines—roamed the landscapes of Asia, Europe, and the Americas for several million years before the last species went extinct ~10,000 years ago at the end of the last Ice Age. As apex predators, sabertooth cats played an important role in regulating ancient ecosystems. While they are fascinating in their own right, what draws me to these remarkable predators is the fact that they shared the landscape with, and often came into contact with, our early ancestors. The invention of sharp-edged stone tools ~2.6 million years ago allowed early humans to butcher the meat, organs, and other soft tissues from the carcasses of large animals. This change in diet no doubt would have increased the chances of running into a sabertooth. The question is: what were those interactions like? We know that some unlucky australopithecines fell victim to large cats like leopards, so it is likely that sabertooths too preyed upon our ancestors. Some researchers also argue that the abandoned kills of sabertooths would have provided scavengers, including early humans, with a ready meal. This is not an unreasonable idea. The enormous canines of sabertooths, while no doubt efficient killing weapons, were relatively narrow and thus prone to breakage. So, the idea goes, sabertooths intentionally avoided tooth-on-bone contact in order to protect their delicate teeth but, in so doing, left significant chunks of flesh and all the goodies inside of bones (marrow, grease, and the like) behind for others to eat.

Researchers have devised ingenious methods to reconstruct the killing and feeding behavior of sabertooths by analyzing the shape and chemical make-up of their teeth and bones. This information provides a wonderful window onto the adaptations of these animals. In order to directly infer what they ate and how they ate it, however, we really need to look at the bones of their prey. There are many sites around the world where sabertooth fossils are found in association with the bones of potential prey animals. The problem is that it's very difficult to be sure that sabertooths were the culprit. After all, lots of other animals and even geological processes can accumulate bones. So, it is only in very rare circumstances that we can be reasonably sure that sabertooths were responsible for the bones found at a site. It so happens that Haile 21A is just such a place: we can say with a good deal of confidence that the sabertooth cat Xenosmilus consumed about sixty peccaries there some million-and-a-half years ago.  

That's not to say this whole thing is straightforward. In fact, there are three carnivores in addition to Xenosmilus at Haile 21A, and one or more of them could reasonably be considered as suspects: a coyote-sized canid called Canis edwardii, a wolf-sized canid called Canis armbrusteri, and the sabertooth cat Smilodon gracilis. So why do we think the peccaries were eaten by Xenosmilus? The clues lie in the tooth marks left on the peccary bones. The marks are too big to have been created by Canis edwardii, so that's one suspect eliminated. We know, too, that modern wolves tend to gnaw on and break open bones, which results in lots and lots of individual tooth marks (try this at home with your dog). The peccary fossils, when they do preserve tooth marks, often only preserve a single, or perhaps a few, individual marks—quite unlike a large canid but very similar to what we see among modern big cats like lions and leopards. Another suspect down. That leaves us with the two sabertooth cats. To untangle this, we impressed the teeth from fossils of each species into clay in order to determine if the shape of their tooth marks could distinguish them. It turns out that yes, they can, and in most cases the tooth marks match up very well with Xenosmilus. In fact, the teeth of a Xenosmilus fossil from Haile 21A fit snuggly inside several tooth marks, which is about as close to a smoking gun as you can get.

Impressions of Haile 21A sabertooth teeth in clay (photo: Manuel Domínguez-Rodrigo)

Xenosmilus tooth placed inside a tooth mark
(photo: Manuel Domínguez-Rodrigo)

Now that we've identified our culprit, we can proceed with reconstructing how sabertooths consumed their prey. The peccary bones from Haile 21A show evidence for nibbling on or near muscle attachments, so much so that parts of the bone itself was chewed away completely. This is not what you would expect if Xenosmilus was afraid to graze the bone with its teeth. Nor would you see this if large hunks of meat were being left on the bones. The patterns of damage on the Haile 21A fossils in fact match well what we see on the bones of prey consumed by modern lions, who we know leave very little flesh behind. This means that little, if any, meat would be left on the peccary carcasses for a would-be scavenger. What Xenomsmilus did not do, or did not do very often, is break open the peccary bones. This, too, is consistent with observations of modern lions, who can gnaw off the softer parts of bones but typically do not crush or fragment them. So, a scavenger might have been able to scrounge some marrow and grease from the abandoned peccaries at Haile 21A. Our findings from Haile 21A do not appear to be a one-off, either. The 20,000-year-old fossils of juvenile mammoths at Friesenhahn Cave in Texas show that another species of sabertooth cat, Homotherium serum, also stripped clean the carcasses of their prey.   

That's all well and good, but it's important to point out that the fossils from Haile 21A and Friesenhahn Cave were deposited before humans ever set foot in the Americas. How, then, can we use these findings to test the idea that early humans scavenged from the remains of sabertooth kills? Importantly, the teeth and skeleton of Xenosmilus and Homotherium are very similar to those of other species of sabertooth cats that shared the landscape with humans in Africa, Eurasia, and the Americas between 2.5 million years ago and 10,000 years ago. We therefore think it is plausible that most of these cats consumed their prey in a similar fashion. If that is the case, an abandoned sabertooth kill would probably have been an insignificant and irregular source of meat. A scavenger hoping for a more substantial meal would need to be willing and able to drive these big cats from a kill, a strategy referred to as "confrontational scavenging." Cut marks left by sharp-edged stone knives indicate that early humans were indeed fully capable of gaining quick access to carcasses, either as confrontational scavengers or hunters in their own right. In fact, it has been suggested that early humans not only were major competitors but may in fact have contributed to the extinction of some species of sabertooths. I look forward to seeing more research on these iconic predators...there certainly are many more questions to answer.

References:

Domínguez-Rodrigo, M, Egeland, CP, Cobo-Sánchez, L, Baquedano, E, Hulbert, RC (2022). Sabertooth carcass consumption behavior and the dynamics of Pleistocene large carnivoran guilds. Nature Scientific Reports. doi.org/10.1038/s41598-022-09480-7

Social networks at the end of the Ice Age

Humans are in many ways defined by their social interactions with others. Think about it: much of what we do hinges on peoples’ use of social networks to facilitate solutions to collective problems like resource shortfalls, information acquisition and dissemination, and political or conflict resolution. I don't think it's a reach to say that humans live and die within the context of social networks. Material culture—think clothing, artwork, jewelry—is often used to advertise important information about one’s personal and group identity within a social network. One way that people do this is to adopt or manipulate the style of an object to distinguish themselves from, or more closely identify themselves with, other people. (If you've ever put on the merchandise of your favorite sports team, you know what I'm talking about.) Archaeology is uniquely equipped, through the study of artifact style, to track this most fundamental aspect of human behavior across vast expanses of space and time.

Way back in 2016, the Archaeology Program at UNCG organized an Ashby Dialogue around social networks generally, and social network analysis, or SNA, specifically. SNA is a methodology based in graph theory that allows us to visualize and quantitatively describe relationships between social entities (people, organizations, tweets, etc.) and the features that bind them together (kinship, shared identity, common interests, etc.). This approach has gained significant traction in archaeology, as artifacts can be used as representations of social entities and the stylistic attributes of those artifacts can be used as proxies for social proximity. The idea here is that if two people create or use a pot with the same or similar decorations, they must share, at least at some level, aspects of their social identity with each other, whereas people that create or use pots with very different decorations probably share little or no part of their social identity with each other. As part of our Ashby Dialogue, I was asked to lead a discussion on the use of SNA in my period of interest, the Paleolithic. I quickly realized, however, that while there was a ton of research on Paleolithic social networks, only one study had used SNA. How, then, might we apply SNA to the Paleolithic more broadly? 

Some time soon afterwards, I attended a luncheon organized by UNCG's RISE Network. The purpose was to connect STEM researchers in order to facilitate collaboration across academic fields. At one point, I found myself sitting at a table with Dr. Jing Deng, a faculty member in the Department of Computer Science. The attendee list provided to us listed his area of expertise as—you guessed it—social network analysis. Jing showed a great deal of interest when I described the sort of data we Paleolithic archaeologists work with. A small exploratory analysis supported by UNCG's Undergraduate Research, Scholarship and Creativity Office (and the hard work of then-anthropology and computer science students Amanda Chase and Nathaniel Arnold) convinced us that SNA could be a valuable tool for reconstructing Paleolithic social networks. While any time period is ripe for SNA, we decided for two reasons to focus on the Magdalenian of western and central Europe (ca. 20,000 to 14,000 calibrated years before present). First, Magdalenian cultures are famous for the creation of a staggering array of art, including rock paintings, carved and engraved bones and stones, and perforated bones, minerals, and fossils—all of which encode potentially important information about social identity. Second, Magdalenian peoples were confronted with rapidly changing environments as Europe, and the rest of the world, emerged from the last Ice Age. These volatile circumstances no doubt placed stress on the social networks of folks moving into landscapes only recently vacated by ice sheets. To estimate social "distance" between Magdalenian sites, our initial analysis used frequencies of shared artifact (stone tools, beads, engraved bones) and raw material (stone, bone, fossil, ivory, shell) types. As we worked through this, I began to wonder if it was possible to measure social distance not only by the frequencies of artifact types, but by the stylistic motifs of the engravings on individual artifacts. Could we figure out a way to compare the engravings objectively in order to determine which artifacts were stylistically more similar?  

I spoke with Jing about this, and he introduced me (at yet another RISE event) to Dr. Minjeong Kim, another faculty member in the Department of Computer Science. Minjeong's research focuses on image analysis, particularly medical MRIs and CT scans. She was optimistic that a computer could be trained to extract the engraved patterns from 2D digital images of artifacts and overlay and compare the shapes of the extracted patterns to gauge their stylistic similarity. Among the most common engraved Magdalenian artifacts is called a "perforated disk." These small, flat disks were crafted from stone, bone, or ivory and often include engravings of geometric designs and/or animals. Just like humans today make choices about what to wear in order to (sometimes unconsciously) signal something about themselves, Magdalenian people likely created and used perforated disks to signal aspects of their own identity to others. It just so happened that a good friend of mine, Dr. Rebecca Schwendler, had looked at more perforated disks than just about anyone else in the world. (In fact, it was Rebecca's paper on Magdalenian art and social networks that I discussed way back in 2016 as part of the Ashby Dialogue.) We also needed someone to help reconstruct the ancient environments of post-glacial Europe. Enter Dr. Chris Nicholson, a GIS whiz with tons of experience working with paleoclimatological data. With Chris and Rebecca on board, we had an extremely interesting project (at least to us) and a talented research team that was fully capable of pulling it off. 

In 2019, we put together a research proposal to the Archaeology Program at the National Science Foundation. After two years, three submissions, and tons of valuable input from NSF's hard-working review panels, we were awarded ~$230,000 in 2021 for a three-year project entitled "A network approach to Magdalenian social landscapes." As of this writing, we are a little less than a year into the project, and I am very excited to see where things go from here. If you are interested in more information, including updates on our progress, please visit the project's homepage.