30

By February 2014 it was time to explore the chamber we had named 102, the adjacent one that Rick and Steven had told me about. They were so excited about its potential that I wanted to see for myself. The first squeeze was a tight one for me. Its two vertical walls pressed hard against my chest as I shimmied along the downward slope. Alia and Rick had gone through ahead of me, their smaller frames unbothered. John was behind me. Seeing me squeeze between the rock faces, he shrugged. “I’ve got some inches on you, Lee,” he said. “I guess I’ll sit this one out…again.”

Now, after a few hours underground, I was coming back up the same passage, but it seemed even narrower than before. The walls wedged my body in more tightly than they had before. I couldn’t get a foothold. I couldn’t figure out how to move. I kept my focus, though—it reminded me of my scuba training, when you learned to respond to danger with calm analysis—and I systematically tried new ways to twist upward. It took 45 minutes of groaning and sweating until I finally popped through to freedom.

“I think from now on we’ll call that one the ‘Berger Box,’ ” said Rick. All of us stood exhausted in the shady entryway of the Rising Star cave, laughing. My face, caving gear, and clothing were all covered in grime.

That was the last time I went down to site 102, but that one trip had been well worth the struggle. The passage leading down to this new underground expedition site went almost at right angles from the passage that led to the Dinaledi Chamber. On a map, the two chambers were nearly a hundred meters apart. But they were in totally different sections of the cave system. Going from one chamber to the other underground would be much farther. There was no way that any natural processes could have moved fossils from one site to the other, and the layout of the cave made it impossible for gravity alone to have filled both sites with fossils from a single cave entryway.

And 102 was full of fossils! That day we plotted and recovered pieces of hominin skull, jawbone, and a few other fragile bits, all found exposed on the sediment surface. Our work began again, doing the same meticulous scanning, documenting, collecting, and cataloging of fossil finds from this new cave site. Marina Elliott became the primary underground astronaut in this new phase of excavation, and over the next two years she would dig one or two days at a time in this chamber, working with the exploration team and joined at one time or another by Becca, Hannah, or Elen. By 2016, they had uncovered large parts of an adult hominin skeleton, together with at least one bone from another adult as well as teeth and bones from what looked like three young children.

So what were we finding in Chamber 102? At first we did not assume that the fossils necessarily were Homo naledi, like those found in Dinaledi. Swartkrans cave, just a few hundred meters away, contained fossils of both Australopithecus robustus and some form of Homo, probably erectus. Maybe the Rising Star cave complex would also prove to be home to more than one species.

Over months, as we examined the fossils and compared them to other hominins, it became clear that 102 held remains very similar to those from the Dinaledi Chamber. The femur had the same long neck with the oval cross section. The vertebrae had the same small size, with large canals for the spinal cord. The clavicle was short and curved, just like those we had found for naledi. Over time the team found all 32 teeth belonging to the adult skull, and they looked just like the Dinaledi teeth, primitive in proportion, with the interesting shape of the premolars and canines. Every piece of evidence brought up from Chamber 102 suggested that these bones were Homo naledi, almost identical in measurements to those from the Dinaledi Chamber. This did not seem like a coincidence. These bones looked like they all came from the same biological population.

WE HAD ALREADY FACED the question of how the fossils had gotten down into the Dinaledi Chamber. Their extraordinary context—the twisting pathway underground that led to the Chamber—ruled out any easy explanations. There were no teeth marks on the fossils, so we know they had not been chewed or left there by carnivores. The mineral composition of the sediment told as they could not have been washed into the Chamber. The fossils represented at the very least dozens of individuals, ranging in age from youth to elderly, so we had to rule out the possibility that the Dinaledi Chamber contained the remains of a few unlucky naledi cave explorers. When we had announced the fossils to the world, a few people suggested that a naledi group had become lost and trapped there. Knowing the Chamber well, though, we knew that it was highly unlikely that a group including infants and young children would have ever gone there.

So what was left? Intentional deposition: a deliberate decision by this group of hominins to place their dead in this underground chamber. By a process of elimination, that was looking to be the likely answer to the question of how the bones got there. But that hypothesis, we all admitted, was extremely hard to test.

In Chamber 102, though, the context was somewhat different. Soft, fine sediment filled a small alcove, spilling out and slumping downslope. Marina and the team found hominin bones here and in another area of the chamber, but because of the erosion it was difficult to say exactly how or when they may have arrived there. No stone tools or other artifacts accompanied them. The team recovered a few bones of other animal species from the chamber, but not clearly within the sediment where they found the hominin fossils. These animals may have reached the chamber long after the hominins. We could tell that cavers had come through, and maybe other creatures had as well. The passage to 102 was tight, but it was nothing like the extremely challenging Chute above the Dinaledi Chamber.

There were a lot of unanswered questions, but one thing was certain. No accident, no cave collapse, and no death trap like the one we envisioned at Malapa could account for these two chambers, far from one another within one cave system, both full of remains of the same ancient hominins. Granted, it is hard to be definitive as you make the leap between the scientific evidence and your best guess about ancient behavior. But we had formulated a hypothesis about the Dinaledi Chamber, and the 102 Chamber added important corroborating evidence. The best hypothesis to account for these fossils is that Homo naledi used these chambers intentionally as places to deposit their dead.

Our team is still working to uncover how naledi may have used the cave as a whole, and this work will undoubtedly go on for years. Even though the configuration of the two chambers suggests that they were always relatively inaccessible, there may be other areas of the cave that were once open and are now filled with sediments or breccias—areas that naledi might have been able to enter. We do not know yet whether they needed artificial light, as we do today, to use these caves. Maybe we will find evidence that naledi lived somewhere within the cave system, or used it regularly for shelter. Our exploration continues: We have found some other promising sites within the Rising Star cave system, and we will continue to investigate them to find more evidence of naledi or other hominins.

Our excavations in 102 helped us know Homo naledi even better. We recovered nearly every part of an adult skull, even the nose and the tiny parts of the eye socket that contain the tear ducts. With his typical patience and anatomical expertise, Peter Schmid reconstructed the skull, working with exquisite care upon these thin pieces. As it took shape, we began at last to see the face of naledi. This skull was slightly bigger, with more noticeable muscle markings, than the other naledi skulls we had excavated. It had well-worn teeth, suggesting that this adult male had had a long life. His face was not so broad as the face of Homo erectus; the bridge of his nose was flat, turning out just slightly at the bottom. His jaw must have been powerful for its small size. His face made him seem almost human, but he had that same tiny brain and primitive teeth we knew so well from the individuals found in the Dinaledi Chamber.

WE STILL HAD TO answer the question of how old the Homo naledi fossils were. For that, we needed to turn to the geologists. Over the same months that we were exploring Chamber 102, our team of geologists was busy analyzing the flowstone—the thin mineral coating found throughout the Dinaledi Chamber. Earlier they had attempted to date the flowstone samples by the technique that had been so successful at Malapa, but the mineral content of the Dinaledi flowstone demanded a different process. Analysis of the thin flowstone residues on the Chamber walls above the fossil-bearing sediment indicated that they were less than 250,000 years old. But that just told us that the fossils had to be older than that—no surprise. We needed a way to find an upper limit—a maximum age—without destroying any more of the precious fossil material than we had to.

For that the team turned to electron spin resonance (ESR), the technique that helps date fossils by analyzing how long-term exposure to radiation changes the electron energy within crystals, including those in tooth enamel. By this time we were willing to sacrifice a few naledi fossils for the sake of dating our finds. We sent three teeth that had been brought up from the Dinaledi Chamber to be zapped with lasers, drilled, and sampled. The ESR assessment of all three teeth indicated that these hominin remains are less than 450,000 years old.

So now we had our age range for Homo naledi: These fossils must come from a time between 450,000 and 250,000 years ago. The time range may seem broad, but to a paleoanthropologist, it makes very little difference whether naledi was in the Rising Star cave system 450,000 or 250,000 years ago. This hominin’s anatomy had our colleagues—and ourselves!—guessing that the fossils might be nearly two million years old. Now we had discovered that they are vastly younger than anyone assumed.

Now we understood why the naledi fossils were so well preserved, and why in both chambers we had found them in soft, unconsolidated sediments instead of hard breccia. We were lucky to have found them before erosion destroyed the deposits altogether. Explorers didn’t look for fossils in places like these. I had worked in caves for years, and whenever I saw bone fragments in soft sediments on the cave floors, I tended to dismiss them. Those dusty bones had to be too recent—surely not interesting, I used to think. Who knows how many other troves of important fossils might have once existed in caves like this—or might still exist, waiting to be discovered?

FROM SEDIBA TO NALEDI: All along the way, behind all the research and analysis of the findings from Malapa and Rising Star, lay the question of the origin of Homo, the line of hominin species that ultimately led to Homo sapiens, humans of today. How did these two new species relate to us? Were these our direct ancestors or offshoots of the human family tree that died out while our ancestors thrived? The ESR dating of naledi made the answers to these questions more challenging—and more interesting.

The hominin fossil record tells us that the species that most closely resemble naledi, like Homo habilis or early members of Homo erectus, lived more than 1.5 million years ago. Long after that, more humanlike species appeared—species that we assumed were the immediate ancestors of modern humans. Now the Rising Star discoveries show that within Africa, at a later time than anyone had ever guessed, a remarkably primitive hominin species still survived. Maybe naledi evolved from an early form of Homo erectus. Or maybe an early form of naledi really did exist much earlier, before erectus, and gave rise to both our much later naledi fossils and other forms of Homo, even modern humans. We cannot rule out anything at this point: The anatomical mosaic of naledi’s skeleton makes it hard to be sure exactly where it fits on our family tree. If there’s one thing that the Rising Star discoveries tell us, it is that we haven’t found everything that’s out there. That’s what an explorer wants to hear.

The species we call Homo sapiens—modern humans—includes everyone living in the world today. Archaic forms of humans, like Homo erectus or Neanderthals, no longer exist. How did that happen? Sometime before 200,000 years ago, an African population of humans began to grow in numbers. This population gave rise to more than 90 percent of the genetic ancestry of people living now throughout the world—but it’s not clear exactly who that population of ancestral people were.

Four known fossil specimens from Africa share a basic skull shape with living people—three from Ethiopia that are more than 150,000 years old, and one from Tanzania that is a bit younger. That much we can observe, but we really don’t know if those fossils represent the direct ancestors of the special population from which we all arose—or their cousins. Each of those fossils shares some similarities with modern humans, and yet they are much more different from one another than anyone alive in the planet is today. We have evolved a lot since that time, making it hard to tell which hominin fossils, if any, might be our direct ancestors. And when we look back even further in time, into the time of naledi, no fossil remains look very similar to modern humans.

In the last few years, DNA evidence has added tremendous complexity to our understanding of this story of modern humans. Genetic testing shows us that when one tiny branch of the modern human species first migrated out of Africa, they encountered some of their archaic cousins—the Neanderthals and Denisovans. Those species interbred, scattering a small percentage of Neanderthal and Denisovan genetic ancestry through much of the human population. Meanwhile, Africa remained the center of our origins, and—as we continue to discover—within the huge diversity of African environments lived other distant archaic cousins. The fossil record of Africa is so sparse that we have no idea who these distant cousins may have been. But inside the genomes of living African populations, we see traces of DNA interlaced from unknown populations. The origin of our human species now looks like a braided stream. Branches formed and flowed separately for some distance before they merged again with the growing river as it flowed on to today.

Which brings us back to the mystery of Homo naledi. We have now discovered the largest sample of fossils on the continent, representing a species never known before. We have determined that it lived only a few hundred thousand years ago, and we suspect that it may have engaged in deliberate body disposal. These two facts are astounding in themselves, but they raise much larger questions. In the midst of such a diverse continent, what was the place of this unexpected species? Could it have interbred with other populations? Might its DNA have merged into the stream of our own origins?

One comparison is Flores, where a tiny, small-brained species may have been living when modern humans arrived there no earlier than 50,000 years ago. In one way, the Flores discoveries showed that surprises might be in store for us in unexpected places—a revelation we have echoed now in South Africa. But in another way the Flores story confirmed a stereotype about Homo sapiens, and that is that humans are fierce competitors. Even small groups of human hunter-gatherers can kill dangerous predators, dominate landscapes, and consume nearly every edible food source. According to conventional thinking, a different hominin population like the Flores hobbits might evolve in isolation, but once large-brained humans arrived on the scene, that isolated species was doomed. So much of the bitter fight about H. floresiensis arose from this assumption: Many scientists could not imagine how a small-brained species could survive when the larger-brained human species was in the picture.

Looking at naledi we have to throw away this stereotype. This species was not isolated on an island. It lived smack in the heartland of human origins. Naledi was not small and dwarfed: It was the size of modern hunter-gatherers, with hips, legs, and feet apparently as well made for walking as those of modern humans. Its teeth show that it likely relied upon meat and other high-energy foods; its hands and fingers show the capability to make almost any kind of stone tool; and despite its small brain, it appears to have developed some very interesting behaviors. Naledi did not survive by being safely isolated, and its anatomy gives us no reason to think it avoided competition for the same resources as larger-brained human species. Naledi did not survive merely by being different. In some ways, at least, naledi could only have survived by being better.

What does all this mean? We need to look at the dawn of modern human behavior in a new way. Conventional theories tell us that African technology in the last 400,000 years followed a slow, gradual progression. First some groups of ancient hominin toolmakers moved from hand axes and chopping tools toward more deadly spear points and complicated flaking methods. Somebody started to dabble in pigment, and to travel longer distances to seek out useful stones to bring home. By 70,000 years ago, some African peoples were making objects that communicated information and took on value as symbols, like inscribed ostrich eggshells and shell beads.

Archaeologists have assumed that all the people who took these great steps in human development were direct ancestors of modern Homo sapiens—that human evolution happened in a single straight line. But how do we know?

Naledi shows us how incomplete this idea must be. Imagine the possibility: Several hundreds of thousands of years ago, another species was out there with its own society and social behaviors. It was very different from modern humans, but it was clever and capable of making things. Granted, no tools have yet been found with the bones of naledi—but no tools have been found in close contact with any other hominin bones from around the same time period. We simply cannot assume who was making the artifacts we have found.

We are only at the beginning stages of learning about this remarkable species. These discoveries force us to ask new questions and to question old assumptions. Right now we can only speculate how the larger story will fit together as we continue to find more evidence and test new hypotheses. So much of Africa remains almost entirely unexplored.

Very likely there are more early hominin species to be discovered. We would be foolish to assume that naledi was the only one that has been hidden. If we stop connecting the dots, assuming evolutionary change happens in a straight line, and open up to larger possibilities, we can see that at the time of the dawn of modern humankind, Africa was full of different traditions, each one diverse and reflecting an accumulation of learning over thousands of generations. One of these traditions may have belonged to the ancestors of the first modern humans, but we don’t know which, or where they may have lived. Another tradition, maybe several, must have been made by species like naledi. Maybe our ancestors could draw upon many traditions, learning not only from parents and grandparents but also from distant cousins—like naledi.