The human skull cap lay in the sediments of the lava hump, gnawed along the brow by the teeth of a predator. It was a small brain case—roughly 700 to 800 cubic centimeters—primitive, nothing like the large-brained hominins that would later dominate the African continent. And it sat in the middle of southern Africa, surrounded by a wealth of archaeology that researchers had long attributed to early Homo sapiens. The question that followed would unsettle a great deal of what paleoanthropologists thought they knew: what if the narrative was wrong?
What if Homo naledi, a species with a brain the size of a chimpanzee’s, had been making these tools all along? For decades, the sedimentary basin of Olorgesailie in southern Kenya had yielded tens of thousands of stone tools and fossilized animal bones. The site lay in the East African Rift Valley, a landscape shaped by volcanic ridges, shifting lakes, and grasslands that came and went with the climate. Excavations there had documented the presence of early humans across an enormous span of time, from roughly 1.
2 million to 300,000 years ago. The sediments recorded environmental upheavals: lakes that formed and dried, river channels that replaced them, volcanic ash that buried entire landscapes. The faunal remains revealed the creatures that once roamed the region—zebras, antelopes, baboons, elephants, pigs, hippopotamuses—and how those species changed as conditions shifted. Yet for all the artifacts and animal bones recovered, hominid remains were almost entirely absent.
Year after year, dig after dig, the team unearthed the evidence of early human activity but not the humans themselves. The tools told one story: that hominins had lived and worked here for nearly a million years. The bones told another: that these people had died elsewhere. It was not until 2003 that the first Olorgesailie hominid fossil was discovered—62 years after studies at the site began.
The fossil consisted of the left temporal bone, the frontal bone and brow ridge of the brain case, and various additional fragments. It was found in sediments close to the summit of a volcanic ridge the excavation crew called the lava hump, which connected the lowland basin where lakes and streams once formed with the peaks of Mount Olorgesailie. The specimen was thought to be about 900,000 years old, recovered from a layer above a volcanic deposit dated to roughly 974,000 years. It was initially identified as a young Homo erectus, though that opinion was gradually evolving.
Approximately 1. 5 kilometers to the east, a substantial collection of hand axes from the same strata had been found. The location of the fossil may have held the key to why hominid remains were so rare at Olorgesailie. Hominins likely hunted for food at lakesides or by streams, left their tools scattered across the lowlands, but perished in other places.
The highlands, away from the water, offered a safe haven—particularly at night, when predators commonly preyed on hominins around drinking holes. The bite marks along the brow of the brain case suggested that this individual, or at least their remains, had been encountered by a predator. In the opinion of researchers John Hawks and Lee Berger, this partially petrified skull cap was a perfect representation of Homo naledi. The implications were striking: if Homo naledi was present in East Africa around 250,000 years ago, roughly 3,000 kilometers—about 2,000 miles—north of the caves where the species had been discovered, then what kinds of artifacts were associated with it?
Were they making the same kinds of tools as Homo sapiens during the same period? If Homo naledi had hands well suited for tool-making, the question of who manufactured the Olorgesailie artifacts became far more complicated. The site itself had long been recognized for its extraordinary obsidian finds. Obsidian is not naturally found in the Olorgesailie basin, so its discovery there was significant on its own.
The volcanic rock had to have originated somewhere else. Chemical analysis of the surrounding area pointed to sources between 15 and 55 miles away. This implied a system of movement and interaction outside the immediate region—a transportation network that demanded relatively sophisticated cognitive skills, especially social networking, which by definition entails interaction between two or more groups of people. The point was not trivial.
If humans evolved from chimpanzee-like ancestors, one had to understand that chimpanzees cannot cross into another animal’s territory without dying. Entering another’s domain required a valid justification, such as trade. The presence of obsidian at Olorgesailie suggested exactly that kind of negotiated exchange. Manganese and iron-oxide rocks, ideal for producing black and red pigments respectively, were also discovered at the site.
While it was difficult to pinpoint the precise reason for coloring the rocks, the evidence supported their use. Archaeologists frequently view coloring as the source of sophisticated symbolic communication. These pigments may have aided people in communicating alliance membership and preserving relationships with distant communities, just as color is used today to indicate identification through clothes or flags. The research team from the Smithsonian had been digging along the main volcanic ridge connecting the lowlands to Mount Olorgesailie after decades of discovery of thousands of animal fossils.
Once they directed their attention to this area, the first Olorgesailie hominid fossil was discovered in a matter of months. The hominins who made the tools at Olorgesailie were evidently adaptable. They weathered at least 16 significant environmental changes between 1. 2 million and 400,000 years ago—large-scale climatic shifts between arid and moist conditions, volcanic eruptions, and earthquakes.
These events would have altered the basin’s landscape, shifting or removing resources for hominins. Yet after at least 12 of these occurrences, the next stratigraphic layer contained archaeological remains, including stone tools, suggesting that the toolmakers either survived the event or quickly recolonized the region. Other mammalian species were less adaptable. Animal species discovered at Olorgesailie and other sites in southern Kenya changed over time in response to varying environmental conditions.
Baboon, elephant, zebra, pig, and hippopotamus species that had been common in the area became extinct. They were replaced by species still present in East Africa today and closely related to them. The hominins of Olorgesailie encountered a wide range of creatures, but the fossil fauna there was distinct from that of several other East African fossil sites. At Olorgesailie, equids—zebras—made up the majority of the fossilized mammals, whereas in other locations, bovids, the antelope-like species, predominated.
Equus oldowayensis and Theropithecus oswaldi, two large primates that were expert grazers, were also present in the strata where the hominid was discovered. Amphibians and other fossilized creatures lived around the lake and were attached to the grasses. The team determined that early people frequented Hyena Hill frequently around 990,000 years ago. A significant number of stone tools and bones that had been hacked and cracked open for sustenance by early humans were discovered during the excavations there.
Hyenas were present among the fossilized creatures found at Hyena Hill as well. The scientists discovered an old den where hyenas had resided at a time when early people were also present in the region. The hyena skeletons were buried in tunnels beneath the ground, and the burrows were placed in a hill that had been dug by a bulldozer through sediment known not to contain any fossils or stone tools. The type of opportunities available to hominins in the Olorgesailie basin was demonstrated by one excavation in the 990,000-year-old stratum: the Elephant Butchery site.
There, the bones of an extinct elephant, Elephas recki, were discovered surrounded by more than 2,300 stone objects. Sharp flakes made up several of these objects. Cut marks on the elephant’s rib, several of its vertebrae, and even the hyoid bone, where the elephant’s tongue muscles attached, indicated that these flakes may have been used to extract the elephant’s flesh. The killing of the elephant took place when a small area of marshy vegetation, including reeds and other marsh plants, started to dry out.
In order to create some of the flakes used in the butchery, hominins gathered stones from at least 17 different sources and made tools on the spot. The dig was expanded to include the neighboring area when the elephant bones were discovered. Further antelope and zebra butchery sites were ultimately discovered. Early people must have liked to hunt for animals to kill for meat and the nourishing bone marrow in this area of drying marshes.
The stone itself told a story of careful selection. In the oldest section of the Olorgesailie sequence, dating back between 1. 2 million and 900,000 years, stone was mined from volcanic outcrops in the highlands, including Mount Olorgesailie. Several hand axes have been discovered within a five-kilometer radius of these outcrops.
Up to 14 different types of volcanic rock were employed by early hominins, all of which have been located nearby. The team actually discovered a quarry site thought to be about 990,000 years old, used by early hominins to break off enormous flakes of volcanic rock. At this location, hominins tested the stone and started crafting hand axes out of the best pieces—those without mechanical defects—right at the stone source. Tens of thousands of objects were discovered, most of which were scraps from the manufacturing of tools.
Quartzite and obsidian formations occurred 48 kilometers and 18 kilometers, respectively, from the basin. Though just a small number of tools constructed of these two kinds of rock have been discovered at Olorgesailie, they showed that hominins traveled at least these distances. The question of who made these tools remained unresolved. Elsewhere in Africa, early Stone Age sites dating back before 400,000 years ago have parts like blades and prepared cores, which are linked to ancient human fossils rather than more recent ones such as those found in the Cave of Hearths.
On the other hand, modern humans continued to produce archaic large cutting tools long into the Middle Stone Age, as seen at the 160,000-year-old site of Herto in Ethiopia. In the past 20 years, many archaeologists have come to realize that modern behaviors like symbolism, innovation, and art did not suddenly appear as many had thought. The old model held that anatomically modern humans appeared in Africa around 100,000 years ago, but behaved like modern humans only after leaving the continent. This discrepancy in human history showed that we only attained modernity after migrating out of Africa as a species.
There was growing opposition to this viewpoint. Not all academics agreed that modernity originated outside of Africa. Some archaeologists argued that this approach resulted from a serious underestimation of the breadth and complexity of the African archaeological record. They claimed that elements of the human revolution could be found in African Middle Stone Age artifacts.
The Middle Stone Age in Africa is distinguished by the lack or scarcity of massive cutting tools and the availability of prepared core technology for the manufacture of complex points and blades. Importantly, the transition from the African early to Middle Stone Age took place as Homo sapiens was expanding across the continent. For this reason, it might be tempting to view the emergence of the earliest Middle Stone Age technology as a cultural indicator connected to the evolution and appearance of our own species—a smoking gun for proof of the modern human mind. Drawing that judgment so soon would probably be premature.
The association between complex behaviors and hominin species between 500,000 and 160,000 years ago—when numerous species of hominins, not only modern humans, inhabited the African continent—was still not fully understood. There were many older examples of every characteristic traditionally used to distinguish modern humans from archaic humans: culture, art, how the deceased are treated, decoration, and abstract symbols. It did appear that the human revolution that gave rise to modernity never actually occurred, because archaeological evidence showed that modern behaviors began to emerge much earlier, in cultures that existed before our own species. At Olorgesailie, the end of hand-ax technology in the area occurred between 500,000 and 350,000 years ago, often at the same time as the extinction of other animal species.
Although the hominins who made hand axes were prosperous for a considerable amount of time, the hand-ax way of life also ended in the Olorgesailie region by about 300,000 years ago. They were supplanted by others who produced toolkits that were more compact and varied. Some insights into the development of humans could be gained from studying the evolution and persistence of species with more adaptable or versatile lifestyles. It seemed plausible that the evolution of adaptable traits in both hominins and other animals was a crucial evolutionary response to such rapidly changing environmental conditions.
The mystery of who made the tools remained. The relationship between pre- and fully modern human fossils and early and Middle Stone Age remains was complicated and unclear. Whether the Olorgesailie artifacts were the work of Homo erectus, Homo naledi, or early Homo sapiens was not settled. Younger sites at Olorgesailie provided evidence for complex and modern behaviors, including the trade and long-distance transportation of resources like obsidian, the use of pigments, and the potential creation of projectile points.
These behaviors appeared in Africa around the same time as the first known Homo sapiens fossil remains. But the small, primitive brain case from the lava hump raised the possibility that the story was far more complicated. In reality, revolution is easier to explain than gradual complex development.
The answer to who had been making these tools, and what it meant for the understanding of human origins, remained open somewhere in the sediments of southern Kenya.
