In some places on this planet, nature is a stern and discriminating gatekeeper.
Lee Berger had a taste of this at a cave system in South Africa. While attempting to explore it, he became trapped within a narrow underground squeeze for nearly an hour. His own efforts to extract himself proved unsuccessful. He eventually had to be pulled out using ropes tied to his wrists.
For Berger, Research Professor in Human Evolution and the Public Understanding of Science at the University of the Witwatersrand, getting stuck in a narrow squeeze would not be considered unique in the world of cave spelunkers. But this effort was part of a broader project to explore the very-difficult-to-navigate Rising Star cave system, the cave system in the fossil-rich Gauteng province of South Africa that recently yielded the largest trove of early human bones found in Africa. In this instance, he and his colleagues were investigating a new chamber found more than 100 meters away from the now famous Dinaledi Chamber. The Dinaledi Chamber was discovered in 2013 and eventually yielded thousands of bones representing a new extinct human species, Homo naledi (1). The new chamber, called Lesedi (or ‘light” in the Setswana language), was in some ways a bit less difficult for explorers than the challenge presented previously by the Dinaledi. But only slightly. “After passing through a squeeze of about 25cm, you have to descend along vertical shafts before reaching the chamber,” said Dr. Marina Elliott about Lesedi. She is Exploration Scientist at Wits and one of the original “underground astronauts” on the expedition to the Dinaledi Chamber in 2013. “While slightly easier to get to, the Lesedi Chamber is, if anything, more difficult to work in due to the tight spaces involved.”* So tight, in fact, that only one person can work in the chamber at a time, as opposed to the Dinaledi, where at least several could move about within the space at once. The Lesedi Chamber is approximately 30 meters underground and, though 145 meters away from the Dinaledi Chamber, it is part of the same interconnected subterranean system. So when spelunkers Rick Hunter and Steven Tucker first spied the hominin fossil remains in the Dinaledi Chamber in September 2013, they had also entered the Lesedi Chamber. But it wasn’t until November when the scientific team, while carrying out fieldwork in the Dinaledi Chamber, actually discovered that there were also fossil hominin remains within the Lesedi Chamber; and like the Dinaledi project, the Lesedi effort marshaled the participation of a large team of researchers, including the University of the Witwatersrand (Wits), James Cook University, Australia, the University of Wisconsin, Madison, United States, and more than 30 additional international institutions.
View of the Rising Star property in 2017, with Base Camp in the distance. Photo: Wits University/Marina Elliott
The Lesedi Finds
What they initially discovered within Lesedi is less prolific than the Dinaledi haul, but without doubt equally revelatory. Here, excavated from three locations within the chamber, scientists recovered 131 hominin fossil specimens representing at least three individuals — two adults and one juvenile. The majority of the fossil specimens belonged to a single adult male. They designated him LES1, nicknaming him “Neo” (meaning “gift” in the Sesotho language). There were enough fossil parts, including a nearly complete skull, to catapult Neo into a rare position among hominin fossil finds.
“The skeleton of ‘Neo’ is one of the most complete ever discovered, technically more complete than the famous Lucy fossil given the preservation of the skull and mandible,” says Berger.*** The skeleton included the preserved fragile parts of the face, as well as the arms, legs, wrist and hand, spine, and ribs. Moreover, “the vertebrae are just wonderfully preserved, and unique,” says John Hawks of the University of Wisconsin-Madison, a leader of the Rising Star research team and lead author of the paper describing the new fossils. “They have a shape we’ve only seen in Neanderthals.”***
But for many reasons, they were not remotely Neanderthal. And like the Dinaledi finds, Neo’s skull exhibited a relatively small brain case — about a third the size of modern humans and Neanderthals.
Were these individuals the same species found in the Dinaledi Chamber?
Yes, say the researchers. The features are consistent with the Dinaledi finds — they are Homo naledi. Perhaps not entirely surprising, as they were found within the same cave system.
But what about their age?
Schematic of the newly-discovered Lesedi chamber within the Rising Star cave system. Courtesy Marina Elliott/Wits University
“Neo” skeleton of Homo naledi from the Lesedi Chamber. Courtesy Wits University/John Hawks
“Neo” skull of Homo naledi from the Lesedi Chamber. Courtesy Wits University/John Hawks
Frontal view of the “Neo” skull of Homo naledi from the Lesedi Chamber. Courtesy Wits University/John Hawks
Dating the Lesedi finds
It almost goes without saying that one of the most tantalizing questions among the interested public and scholars about the hominin fossil finds in both the Dinaledi and Lesedi chambers centered on their age. Even by the time the great discovery in the Dinaledi was published news in 2015, however, scientists were not yet in a position to suggest their age for a variety of reasons (see below****) — a frustrating situation, given how important this element is within the context of human evolution research.
We now finally know, based on a long and deliberate process, the date range of the fossils — at least for the Dinaledi finds. Researchers suggest an age range of between 236 and 335,000 years ago, a surprising result given their initial hypothesis that, based on the morphology of the finds, they could be as old as 2 million years.
“The dating of naledi was extremely challenging,” said Paul Dirks of James Cook University, the lead author of the study that reported the results of the lengthy multi-institutional effort to date the fossils. “Eventually, six independent dating methods allowed us to constrain the age of this population of Homo naledi to a period known as the late Middle Pleistocene.”***
The researchers applied optically stimulated luminescence (OSL) dating of the sediments within the Dinaledi Chamber, as well as Uranium-Thorium dating and paleomagnetic analyses of the chamber flowstones, to establish the minimum and maximum geological age boundaries of the fossils. To come up with the final age range, they dated the teeth of Homo naledi using Uranium series dating (U-series) and electron spin resonance dating (ESR). The combination of techniques and independent results from multiple institutions and specialists has provided a high confidence in the results.
This was clearly startling news about the Dinaledi finds — but what about the Lesedi fossils?
To date, researchers have not determined the age of the Lesedi finds. But the Lesedi Chamber skeletal remains are very similar to those from the Dinaledi Chamber — so similar, in fact, that there is little doubt by the experts who have examined both sets of specimens that the individuals in the Lesedi Chamber would be considered members of the same species as those of the Dinaledi Chamber. They have yet to actually date the Lesedi fossils because, among other things, the dating process would mean destroying some of the material, and there is still more work to do examining the fossils and officially describing them in detail. “Once described, we will look at the way forward for establishing the age of these particular fossils,” says Dirks. Given the striking similarity between the two sets of fossil specimens from separate chambers, including the similarity in their respective preservation state, however, the researchers hypothesize for starters that their age is the same.
Dr Hannah Hilbert-Wolf (left) and Dr. Eric Roberts (right) evaluating data being recorded by the Gamma Ray Spectrometer in the Rising Star Cave, which was later used to refine dating models. Courtesy Wits University
A hominin throwback?
Given the apparent age, a major quandary about Homo naledi and its relatively small brain case and unique mosaic of primitive and derived characteristics has revolved around its place on the continuum of human evolution. On its face, based on the traditional view of the evolutionary scheme, this hominin species may seem out of place in the general evolutionary timeline most scholars have defined for the ascent of humans. Hominins with more primitive characteristics have tended to be older, those the with more ‘derived’ or ‘advanced’ characteristics much later or younger on the timescale. But now we know that a variety of hominin species actually coexisted in subequatorial Africa, including evidence that Australopithecines, the suggested ancestral precursor genus to humans, and members of Homo, the hominin genus that includes such species as Homo erectus, archaic humans and modern humans, likely overlapped on the time scale of human evolution. And now in the south African region where we find Homo naledi, we have evidence and dating that naledi may have coexisted with archaic humans and Homo sapiens — modern humans — a suggestion that is consistent with the emerging picture of an Africa as home to a variety of hominin species that were living, as it were, side-by-side at the same time.
This is not to say that Homo naledi did not belong to a much earlier time during the Pleistocene epoch in Africa.
“Some populations of H. naledi lived between 235,000 and 336,000 years ago, but other populations may have lived much earlier, as suggested by the many features that H. naledi shares with much earlier fossils,” state the Rising Star researchers in a fact brief. “It is possible that these earlier populations may have given rise to either H. erectus or H. sapiens, or these species may have emerged from common ancestors.”**
There is no evidence to date to suggest that, even though their time periods may have overlapped, members of Homo naledi and Homo sapiens ever really crossed paths on the same stage. “The fossil record suggests that some archaic human populations existed in subequatorial Africa during much of the existence of H. naledi,” write the study authors.“This evidence includes the Kabwe skeletal remains from Zambia, and the Florisbad skull from South Africa.” But they say it is not certain that they made direct contact with each other.
“Neo” skull from Lesedi Chamber (left) with DH1 Homo naledi skull from Dinaledi Chamber (right). Courtesy Wits University/John Hawks
“Lucy” and “Neo”. Left: “Lucy” skeleton of Australopithecus afarensis, 3.2 million years old. Right: “Neo” skeleton of Homo naledi, roughly 250,000 years old. Courtesy Wits University/John Hawks
Homo naledi was very different from archaic humans that lived around the same time. Left: Kabwe skull from Zambia, an archaic human. Right: “Neo” skull of Homo naledi. Courtesy Wits University/John Hawks
The significance of Rising Star
The Rising Star team of researchers point to a number of reasons why the naledi fossils mark an important chapter in the ongoing search to understand the human place on the evolutionary spectrum.
First, the fossils represent yet a new species of Homo, adding to the mosaic of early human species within the hominin taxonomical tribe. It further buttresses the emerging and arguably puzzling portrait of a complex human evolutionary history, raising more questions about the “how” of the ancestral journey that eventually resulted in a sole surviving human species — Homo sapiens.
Second, the apparent late age range of the fossil finds adds to the accumulating evidence that a variety of hominins likely coexisted in subequatorial Africa. Like the Homo floresiensis (“hobbits”) of Indonesia, they may have tread the same ground at the same time as our species, modern humans — which had its origins between 300,000 and 200,000 years ago, based on studies and currently available evidence.
Third, Homo naledi lived during a time when the archaeological record in South Africa shows the presence of Middle Stone Age toolmaking and use of natural pigments, an emerging symbolic material culture with much greater sophistication than the Acheulean (usually attributed to Homo erectus and other early humans) and the Oldowan (the earliest widely recognized stone tool industry). Although modern humans are generally thought to have created and used MSA artifacts, Homo naledi, with an apparent population that existed between 236 and 335,000 years ago, cannot be unequivocally ruled out as makers of this later technology.
Fourth, the fossil finds were curiously concentrated in two separate difficult-to-access chambers within the dark zones of a cave system. Careful examination by the researchers, thus far, seems to rule out the accumulation of the bones in these locations by natural causes, including predators. To deposit the bones in these difficult-to-access chambers, according to the researchers, would have taken extraordinary effort. “This likely adds weight to the hypothesis that Homo naledi was using dark, remote places to cache its dead,” says Hawks.*** It suggests some tantalizing implications about behavior that add to the list of questions about when, how and where hominins were becoming “human” in the behavioral sense.
Fifth, in order to access the chambers, they likely would have needed some form of light. Could they have mastered the use of fire? “It is a reasonable hypothesis that H. naledi must have controlled fire in order to repeatedly use areas deep within the Rising Star cave system,” write the researchers. “However, the team has not yet confirmed any physical or chemical evidence of ancient fires in either the Dinaledi or Lesedi Chambers.” But archaeologists have found possible evidence of the controlled use of fire in other sites in South Africa going back as far as 1 million years ago, such as at Swartkrans Cave, which is only 800 meters from Rising Star.
A search far from over
One thing is certain, the ongoing global search to understand the ‘how’, ‘where’ and ‘when’ of human origins and evolution has taken a complex path of twists, turns and rethinking over decades of research, and more especially in recent years. If anything, the Rising Star discoveries have raised more questions than answers:
— How does Homo naledi really fit into the mosaic of the expanding list of hominin candidates in the human family origins bush?
— When did the first or earliest populations of Homo naledi actually exist, and where, assuming the Rising Star population was not among the first populations of the species?
— Was the Rising Star group of fossils representative of one of the last Homo naledi populations, or were there later populations, the fossil remains of which have not yet been discovered, pointing to an even later extinction date range?
— Was there contact between Homo naledi and modern humans, or other archaic humans, and if so, what new questions and investigations would this raise?
— Do the Rising Star discoveries hint at an even more complex picture of hominin diversity in southern Africa than what the current evidence already suggests, and by extension, all of Africa?
These are but a few of the many questions that need answers, and the scientific push for a better understanding has been non-stop. Concerning the Rising Star fossils themselves, the research study authors have been very clear about one thing: There is much more work to do. “Their anatomy suggests that their ancestors diverged from the ancestors of modern humans much earlier in time,” states Berger and the team. “Some scientific results have suggested that the H. naledi lineage may have originated as early as 2.5 million years ago, others as late as one million years ago. The wide range of uncertainty means that we will need to discover much more about other primitive members of our genus before we can answer how long H. naledi may have existed.” Thus, continue Berger, et al.: “The team has been surveying more potential fossil sites in the Cradle of Humankind, and there are excellent prospects for discovering more new hominin-bearing caves. We hope to make an announcement of exciting new naledi fossils very soon. In addition, it is possible that fragments of H. naledi have already been found at sites in other parts of Africa but not recognized. A re-examination of fossil evidence across the continent may provide more discoveries.”**
Berger hints at more to come from Rising Star.
“The age of Homo naledi is not going to be the last surprise that comes out of these caves, I suspect.”*
Standing left to right: 2017 Explorers, Mathabela Tsikoane, Maropeng Ramalepa, Dirk van Rooyen, Steven Tucker (seated), and Rick Hunter (seated) inside the Rising Star cave system. Courtesy Wits University/Marina Elliott
** Homo naledi Fact Sheet, Wits University
**** It has been very challenging to date the H. naledi fossils accurately for several reasons. Firstly, whilst it is possible to date the fossils directly (and in the end we did date the fossils directly), this is a destructive approach which we only want to follow in exceptional cases, and only after the fossils had been fully described and analysed. But even then, only few direct dating techniques are available (in this case combined U-Th and ESR), and these techniques have relatively large error margins and they rely on model assumptions so everything needed to be checked, preferably with independent techniques.
Secondly, fossils can be dated by targeting the sediments in which they are embedded. The problem in the case of H. naledi is that the sediments are not yet consolidated, and that there is clear evidence for repeated reworking of the sediments including the fossils. This means that it has been extremely difficult to determine the exact context of the fossils. There were also a number of technical issues relating to the specific conditions in the cave (for example excessive radon loss in the sediments, and unknowns around the burial history because of the reworking), which made it hard to acquire reliable ages.
Thirdly, we used a large number of techniques and a double blind approach of some of the most important techniques, which has meant that the efforts of 10 separate laboratories in Australia, Europe and South Africa had to be coordinated. Some of the techniques have long lead times, for others careful coordination and sample preparation was required. This in combination with the technical difficulties in the cave meant that it took time to get good results.
— Homo naledi Fact Sheet, Wits University
For more articles on human evolution, see Popular Archaeology’s Human Evolution Issue.