What we’ve learned from the Rising Star cave system this month
A possible hominin skeleton emerges, and the team finds fossils in unexpected places.
I’m watching as Becca Peixotto and Elen Feuerriegel take hundreds of photos of a mass of bone. They are 30 meters underground and I am watching on a screen, my view for the last few weeks as our team has been excavating in the site. They have gone 170 meters through some of the narrowest rock passages that any human can squeeze through, to reach a point where more hominin remains have been uncovered than anywhere else in Africa.
This mass of bone has emerged just at the base of the Chute, the narrow entry channel into which the team enters the chamber. After weeks of careful excavation, the team can now see part of an articulated hand, ribs and a possible shoulder, even some teeth in what appears to be proper anatomical order.
This may be the partial skeleton of a single hominin individual. We do not know how much additional bone may yet remain just beneath the surface.
A continuing mission
Our team has been in the Rising Star cave system this month searching for answers. In 2013, we uncovered remains from at least fifteen ancient hominin individuals in the Dinaledi Chamber. After more than a year of study, our team of more than 60 scientists were able to identify this sample of bones as a species new to science, which we named Homo naledi.
Just this year, our team was able to publish the first estimate of the date of these fossil remains. These bodies of H. naledi are only 236,000 to 335,000 years old — much younger than most experts had assumed, they lived at around the same time that modern humans were evolving within Africa.
At the same time, we published the first description of a new chamber, the Lesedi Chamber, which contains more of the fossil bones of H. naledi including the partial skeleton that we have named “Neo”. The discovery of this second chamber, with remains so similar to the first, has deepened the mystery of how these ancient skeletal remains came to enter this cave system.
Our excavation this month has tested some hypotheses about how the remains may have entered the Dinaledi Chamber and whether more of the Neo skeleton may remain within the Lesedi Chamber. I reviewed our expedition goals in an article near the beginning of the month (“Renewed excavations in the Rising Star cave”). Now that we are approaching the end of this expedition, I can give an update on what we have achieved, and what we will be doing over the next several months.
A skeleton below the Chute
One of the goals of this excavation has been to collect information from the area of the chamber just below the Chute. The Chute is the only access point we have into the Dinaledi Chamber today, a vertical descent of 12 meters, with jagged edges and a minimum squeeze width of 18 cm. We have not found any other access point into the Dinaledi Chamber, and the geological evidence appears to rule out any easy access during the time that H. naledi accumulated.
Yet it seems remarkable to think that bodies of H. naledi might have entered the chamber by this difficult route. Most of the evidence we collected in 2013 came from more than 15 meters away from the “landing zone” where our team enters the chamber. At the time, we found a few bone fragments in the area just below the Chute, including one embedded within a flowstone high above the floor.
Could it be that the “landing zone” area was once full of sediment and bodies? The team opened a small excavation area here — the main unit a square of 50 centimeters — to see whether the remaining sediment actually holds fossil remains consistent with those in the main chamber.
Only a few centimeters below the surface, a pale area of highly fragmented bone mixed with calcite and flowstone fragments began to emerge. For several days, the team worked slowly to determine the limits of this feature of poorly-preserved bone. Just finding it there was very revealing — it showed that at least some skeletal remains must have been in this area of the chamber in the past. But we were looking at mere shards.
Early last week, everything changed. As Becca worked toward the lower edge of the bone mash — slowly expanding the exposed area of its surface downslope — she uncovered a tooth. Soon it was clear that several teeth of an upper jaw were there, in close to their anatomical configuration. We were looking at parts of a skull, crushed and smeared onto the top of this slope by the passage of time and slow drip of water.
Centimeter by centimeter, the team brushed sediment from around the edges of this feature, leaving these precious fragments in place. The bone nearest the wall of the antechamber was extremely poorly preserved, mixed with flowstone fragments, and very thin. But downslope there appeared to be more bones, with much better preservation.
On Saturday, wrist and hand bones began to emerge, including finger bones in articulation. This is highly significant evidence. Our team found articulated hand, foot, and other remains in the main chamber in 2013. Now, at least part of an articulated hand was here, just near the Chute. Soon Hannah Morris and Becca found other bones: ribs, pieces of long bones, a possible scapula. From what we can see, none of them are repeated.
Could we be looking at one individual’s skeleton? What is still hidden underneath the bones we can see?
In our 2013 excavation, we were able to document the articulations in place and recover the bones safely from the chamber. But to study this new feature, with its large size, poorly preserved sections, and potential spatial relationships among different anatomical areas, we are going to need to work on it in a laboratory.
That means removing it from the cave intact. We are now making plans to enable the safe removal of this block of sediment and fossil material. For now, we are covering it and leaving everything in place. For the next few weeks, the team will be studying and experimenting with water-soluble consolidants that may be able to preserve and solidify the remains within these moist sediments. This is a real challenge, because many of the chemical preservatives that anthropologists sometimes use on fragile bone are dissolved in acetone, making them unsuitable for wet or moist contexts. The team has put out a call for advice from curators — including a direct appeal on Facebook Live, and we will be working to find the best way to remove this material safely to the laboratory.
One thing is for sure. When the team brings this feature up through the Chute, it will be the largest and most delicate operation we have yet attempted within the cave system.
Lower down the sediment slope
Two meters below the “landing zone”, as we began our expedition this month, Marina Elliott pointed to a few fragments of cranial bone and a tooth. They were in an area where rocks from the chamber wall had fallen, just at the mouth of the narrow passage leading down into the main Dinaledi Chamber. The team opened an excavation unit here to stabilize the situation and recover these bone elements. Much of the work here during the last week has been carried out by Elen Feuerriegel, with key assistance from Hannah and Becca.
Throughout this unit, down to a depth of some fifty centimeters, the team has found several dozen specimens of cranial bone, some long bone fragments, and many teeth — so far, all seemingly consistent with a single individual of H. naledi. We do not know whether this area may represent parts of the same individual we may be finding at the top of the slope, or whether this material comes from yet another skeleton.
The finds here place H. naledi into the downslope area of this first antechamber, and suggest that more material may be within the narrow passages connecting to the main Dinaledi Chamber.
Deepening the map
While some team members worked on the excavation in the Chute area, other members of the team were pushing into the extremely tight passageways that lie further behind the main Dinaledi Chamber area.
I cannot exaggerate how difficult, narrow, and craggy these passages are. Sharp ledges of chert and flowstone threaten to rip the cavers’ overalls and skin as they wedge their bodies through the cracks. In some areas, they must hoist themselves more than a meter above the floor and slither snakelike around sharp corners.
Only a few members of the caving team are able to work within many of these passages. Dirk van Rooyen and Steven Tucker pushed some of these passages over the past year, finding and photographing bone fragments on the floor of some areas — in some cases, atop chert shelves on the side walls.
Last week, Steven, Becca, and Marina went into some of the farthest of these to collect fossil material. The hours that they spent pushing meters deeper into these passages inspired their names: “Hades”, “Purgatory”, and “Limbo”. In two places, they found bone material embedded within flowstone, precious evidence of the geological age of the bones and their depositional history. They were able to return with a hand drill to remove the flowstone samples, bone intact.
In another passage, which they named “Pandemonium”, Becca and Marina recovered and documented several teeth that may represent a very young H. naledi individual. It is twice as far from the Chute than our 2013 excavation area, and we have trouble imagining how this infant’s remains could have ended up here. We need to study them in the laboratory to confirm this identification. More bone is in Pandemonium, and over the next few months we will try to figure out how to excavate within this almost-unbelievably challenging situation.
As the team worked in these extremely difficult passages, in some cases twenty meters or more from the main chamber where we excavated in 2013, we started to think very carefully about the nature of the Dinaledi Chamber area.
Until now, we have been thinking mainly about how material first got into the chamber. Finding articulated body parts in the main chamber area, with fragments of bone on the surface of the Chute area, focused our attention on the possible slump or slow movement of sediment downslope from the Chute into the main chamber.
But a look at the detailed map shows some problems with this idea. If a mass of material flowed through two narrow passages from the antechamber near the Chute, it is hard to see how body parts could have remained articulated where we found them in the main chamber. Initially below the Chute may have been a talus slope with material tumbling down — that seems like what we may be seeing in the antechamber area. But could this slope have moved material from the Chute to the opposite end of the main chamber, more than 15 meters away, through the narrow passages between them?
Discovering more fossil material back into the tiny passages behind and to the sides of the chamber tells us something that we didn’t know before. Hominin material now is separated by up to 30 meters within the overall Dinaledi Chamber area, within some of the most improbable places.
As we move forward, we will be looking for more ways to test hypotheses about how this extraordinary distribution of material could have arisen. Obviously one idea is that H. naledi may entered the Dinaledi Chamber alive, remarkable as that seems. But we aren’t ruling anything out. Before coming to any conclusions, we will need to undertake a close examination of the surfaces of bones once we get them into the laboratory, try to incorporate any new information about the ages of the remains, and model the conditions of the sediment and bone assemblage within the cave.
One thing we have learned is the value of naming all these passages and antechambers. The Rising Star system as a whole is very complicated, and the Dinaledi Chamber area houses its own complexity. Describing every part of this space will be essential to understanding how the hominin assemblage may have formed.
The ultimate Lesedi core workout
Our expedition goals in the Lesedi Chamber were to excavate in a tiny area of the North-South Fracture Passage to see if additional parts of the Neo skeleton might be there. This excavation area is just adjacent to the blind tunnel in which much of Neo’s remains were found during 2014 and 2015.
The excavation in the Lesedi Chamber was a very small area, but it turned out to be one of the most brutal parts of the expedition. As the team began three weeks ago, the space at the top of the North-South Fracture passage was only enough for one team member to squeeze head and shoulders into. Two chert shelves cut into the sides of this passage, and had to be cleared of sediment before excavation on the passage “floor” could begin.
Now that we’re approaching the final day of the excavation, the level in the North-South Fracture Passage has reached approximately the same level as the Neo skeleton in the neighboring Blind Tunnel. Reaching that level, just a half-meter of excavation depth from our initial level, has been painstaking. The walls continue vertically downward, punctuated by more chert shelves. The excavator reaches the unit upon a horizontal ladder, and as the level has gone downslope, more and more of the area becomes hard to reach.
Some of our cavers have described working here as the ultimate core body workout.
Over the last two weeks, the team has recovered some hominin material from this unit. A fragment that we think may be part of an atlas vertebra, and a blob that may be part of a calcaneus, will take preparation and close study in the laboratory. As I am writing this, Hannah and Elen have called up to say they’ve found a possible hominin toe bone. Amid the sediment, the team has also found a number of additional faunal specimens, like elsewhere in the chamber, mostly small carnivore pieces.
But there is more here to accomplish. The levels near the Neo skeleton are the most crucial, and it will take more work to test the hypothesis that more of his skeleton, including the crucial pelvic remains, may be here.
We’re just finishing this month’s work, and we’ve learned a lot, but the discoveries have raised new questions that we will be investigating as we go forward. It has been a great expedition, wonderful to work with the entire team of explorers and cavers.
This expedition is just the start of a round of new science at the site. Sediment and flowstone samples we have collected this month have gone for testing in laboratories, both for refinements to the dating of the site, and for chemical investigation. If there is evidence of fire in the chamber, it may come from this kind of testing. As we examine the fossils from this excavation, we will have more to say about the anatomy and variation in H. naledi.
Meanwhile, we have science in press right now that we’re waiting for publication. I’m sure we’ll have more news from the Rising Star cave system coming very soon.