Homo naledi hints at the wonders of what we have yet to learn about human evolution.
A common teaching analogy in paleoanthropology is that of the drunk looking for her keys in the light of a streetlamp. When a passerby asks if she dropped something, the drunk responds, “actually I dropped my keys on the other side of the road, but it is too dark for me to see over there.” The correspondence in paleoanthropology is clear; we can only be certain of what we know in relation to where we look. Our understanding of modern human origins has historically been biased toward a narrative of hominin evolution in Europe. Our appreciation of the African hominin record is likely skewed towards the East African Rift Valley. The discovery, analysis, and interpretation of more than 1,500 hominin fossil remains from the Rising Star Cave system in South Africa—given the taxonomic name of Homo naledi—open up an altogether more exciting and challenging perspective on this analogy (Berger et al. 2015). Maybe there are holes, conceptual as well as epistemological, even within the known areas of paleoanthropological vision?
The Southern African fossil record over the past 300,000 years—the H. naledi material has been provisionally dated to between 236,000 and 335,000 years old (Dirks et al. 2017)— is not devoid of hominin or archaeological representation. Fossils such as the Kabwe (Zambia) and Florisbad (South Africa) crania, both believed to date from a similar time range, have been part of the hominin fossil record for more than 80 years. South Africa has long been a center for research on the appearance and diversification of Middle Stone Age technologies dating back 500,000 years. Despite this, the materials recovered to date from the Dinaledi and Lessedi chambers of the cave system are striking and altogether surprising.
Part of what makes H. naledi remarkable is its clear distinction from these previous discoveries and the asymmetry of its distinctiveness. As one example, consider the reconstructed endocasts of the H. naledi material (Holloway et al. 2018). These specimens are uniform in their diminutive size, particularly in relation to the contemporary Southern African fossil record, but they also possess a number of features that are comparable to recent humans. The endocranial volume of the Kabwe cranium is more than double that observed in the H. naledi specimens. When it comes to the specimens’ geological age, only the remains identified as a unique species, Homo florisiensis, from the cave of Liang Bua on the island of Flores, Indonesia, present a smaller endocranial volume this late in the hominin evolutionary record. And yet, analyses of the endocranial morphology highlight a number of features that gesture toward modern neurocranial architecture, despite the size of the remains. Throughout the morphology of H. naledi, represented now by hundreds of fossil fragments, there are interesting combinations of putatively archaic and derived features. In other words, the H. naledi assemblage defies simple classification as “more modern” or “more primitive” than contemporary hominin material elsewhere.
The H. naledi assemblage represents a stark challenge to simplistic, linear narratives of the origin of modernity in the hominin fossil record. And it is a challenge right in the middle of a long-studied, seemingly well-understood, time and place in the hominin fossil record. Paleoanthropology has “known” the Middle–Late Pleistocene is a time period in our evolution marked by continued encephalization linked to the gradual accumulation of behavioral complexity. H. naledi puts an asterisk on that knowledge, and does so in the center of the Cradle of Humankind, not on the margins of our temporal and geographic sampling of the fossil record.
How did we miss this? What has been missing within our evolutionary framework for hominins thus far?
The initial discovery, recovery, and analysis of the H. naledi material has been one of the most widely disseminated stories from paleoanthropology over the past five years. But alongside this work has been remarkable progress in our understanding of modern human origins across Eurasia (and increasingly Africa) based on an ever-growing record of ancient DNA (Gokcumen 2018). While we don’t yet have ancient DNA to provide insights into H. naledi’s ancestry and relationships, ancient hominin DNA attests to a complex demographic and evolutionary relationship between lineages of Neandertals, Denisovans, “modern” Homo sapiens, and potentially additional archaic groups in the Late Pleistocene of Eurasia. The ancient DNA record provides direct evidence of multiple periods of genetic exchange between these populations, coupled with a strong role played by genetic drift (i.e., small population sizes), complicating our understanding of the basic evolutionary framework of modern human origins. As one research group recently put it, “It is likely that gene flow occurred between many, or even most, hominin groups in the Late Pleistocene, and that more such events will be detected as more ancient genomes of high quality become available” (Prüfer et al. 2017, 657).
While the cave of Denisova, in the foothills of the Altai mountains in southern Siberia, is a long way from the Rising Star Cave system in South Africa, I would argue the two are linked in our understanding of later Pleistocene human evolution.
The challenge to a linear narrative of human evolution presented by the H. naledi material, as well as the increasing ancient DNA record, is not a question of species names and phylogenetic trees. Instead, it sits in the murky middle ground of biology between populations distributed across a landscape and species distinguished throughout the fossil record. The history of paleoanthropology has often acted as a conversation across two languages. On one side, we have an understanding of human evolution based on the distribution of variation across living people today. On the other side, is the tangible evidence of our evolutionary past, scattered across time and space, localized at particular fossil and archaeological sites and within specific assemblages. Translations between these two languages present challenges. Variation in living humans is, in many ways, instantaneous in nature, either lacking a temporal depth altogether or more influenced by events in our recent past than is ideal for comparisons with the Pleistocene. Meanwhile, variation in the fossil record lends itself, probably too readily, to interpretations based on discontinuity.
The increasingly mosaic nature of the Late Pleistocene hominin evolutionary record attests to a complex array of evolutionary lineages—populations dispersed across time and space—both unique and reticulate with the broader pattern of human evolution. It is a testament to the growing body of amazing field work over the past several decades that such complexities have become a tangible part of our scientific discourse in human evolution. But it is likely that the limited geographic expanse of historical work in archaeology and paleoanthropology means that much of the diversity of our past, particularly within Africa, remains unsampled (Scerri et al. 2018).
H. naledi is remarkable. It is, in the classic anthropological sense, both strange and familiar. Well within the historical lamplight of paleoanthropology in Southern Africa, and yet not at all what we, as a discipline, were expecting to find. A decade ago, neither Denisovans nor H. naledi existed in our understanding of human evolution. What surprises might the next decade hold? I expect similar discoveries will be made in different areas of our paleoanthropological lamplight in the years to come—but perhaps with less surprise—as the basic evolutionary narrative surrounding human evolution in the Pleistocene continues to shift.
It should not go unsaid that one unparalleled aspect of the H. naledi story is the extent to which the findings are accessible to both the lay and professional public. Most of the H. naledi primary research reports are open access. The research group has made an effort to make as much of the primary fossil material as possible available for examination by uploading high-quality and downloadable 3D images to MorphoSource, an open access digital hub of 3D fossil data. In addition to the open access efforts surrounding the H. naledi material, the work itself has been extensively documented for a public audience, often in real time and with a particular emphasis on education. For those of us who feel that human evolution and its understanding are important for how we understand the world around us today, such work offers the potential to not just reveal new insights in our evolutionary history, but also to shine a brighter light outward, illuminating more questions to study and areas to explore.
Cite as: Van Arsdale, Adam. 2018. “Through Dappled Light.” Anthropology News website, September 18, 2018. DOI: 10.1111/AN.969