In 2015, a huge cache of human fossils was discovered in the very back of a cave in South Africa. This included the remains of a diminutive, small-brained hominin that had the characteristics of the earliest members of our genus, Homo. Dubbed Homo naledi, these fossils closely resemble other hominins from East Africa that have been reliably dated to around 2 million years ago. Homo naledi, however, was missing a date—until recently. Nearly everyone in the scientific community thought that the date of the Homo naledi fossils, when calculated, would fall within the same general time period as other primitive early Homo remains. We were wrong. The radiometric dates—recorded using several methods carried out at different laboratories—yielded almost identical ages of between 236 and 300 thousand years before the present (BP). This was an order of magnitude younger than we expected. To say we were surprised would be an understatement.
This new information has raised some important and difficult questions within the scientific community. First, how do the new dates change our understanding of the progression of human evolution? Second, given that palaeoanthropologists (scientists who study human evolution) were so incredibly wrong about the age of these remains, why should the non-scientific world believe us when we make pronouncements in the future?
What do the new dates tell us that we did not know before?
This was a lesson we palaeoanthropologists should have learned from the Homo floresiensis remains from Liang Bua: Our notions of human evolution proceeding in a linear fashion do not comport with reality.
Since the beginning of the 1900s, it has been largely assumed that human evolution proceeded in a straight line, from primitive forms to more advanced ones, culminating in our own species. This was known as phyletic gradualism. To be sure, there were occasional side branches, but the main trunk was always there. This perspective focused on the concept of species change in the fossil record, which, in turn, encouraged an obsessive focus on “missing links.”
It was not until the 1970s that the discipline of evolutionary science began to address evolutionary change in terms of characteristics or traits, rather than species. This shift came about because it was reasoned that individual traits evolve at different rates. Consequently, the focus has shifted from trying to find a single “missing link” to trying to determine the evolutionary relationships among related species by examining similarities and differences between their characteristics. This perspective has slowly moved its way into palaeoanthropology and has been changing how we think species have evolved and interacted.
It is clear, now, that human evolution has progressed almost nothing like our original preconceptions. Instead of a trunk of a tree that has occasional side branches, our history is one of a very large bush with many different branches, some of which extend a great length. Speciation appears to be the norm in our history. Further, it is becoming clear that some species persist long after their chronological successors have arrived on the scene.
How does this relate directly to Homo naledi? Homo naledi appears (at least at this point) to possibly have been one of a number of species of early Homo that appeared on the landscape sometime around 2 million years ago. From this group, a species that may have been our ancestor, Homo ergaster, eventually arose. Some of these species died out, but it is clear that some persisted until comparatively recently. Currently, we only have a dim understanding of how they were related and how they might have interacted. Until the discovery of Homo naledi, we had no evidence of early members of the genus Homo in South Africa at all. Now, we have evidence of these species in North, East and South Africa. The appearance of Homo in the Afar Triangle at 2.7 to 2.8 million years ago seems to suggest that an early form arose in North Africa first. Additionally, we know from the evidence in the country of Georgia that the range of early Homo was vast both geographically and chronologically. Recent human fossil evidence dated to around 700 thousand years ago from the island of Flores in Indonesia suggests a strong link between the inhabitants of that island and a species very similar to Homo naledi.
A lot of work needs to be done in order for us to clarify the relationships between these hominins.
Why should the non-scientific world believe us when we make pronouncements about our ancestry?
Critics of evolutionary theory have contended that our understanding of life’s history is flawed by our preconceived and dogmatic acceptance of a theory, in the absence of compelling evidence. They also contend, not without some merit, that our understanding of evolutionary history is constantly being revised and, thus, suspect. ID supporter Jonathan Wells lays out these exact concerns in a recent blog post, and it’s not hard to find other examples.
But it is important to realize that the revised understanding of Homo naledi is not based on a failure of scientific methods, either in excavation or examination. It is based on a failure to correctly understand the complexities of sub-Saharan African human prehistory.
Human evolutionary studies operates in the same way that other sciences operate: through the practice of investigation, uncovering evidence, asking questions of the evidence, and formulating and testing possible explanations to the phenomenon. These possible explanations are hypotheses, which—if not falsified—form the basis of theory. In this case, the theory is an overarching explanation of how humans have evolved. It is not perfect, and it is not without the preconceptions, biases, and misunderstandings that plague other hard sciences. It is, however, self-correcting.
It was not someone outside the discipline of palaeoanthropology that pointed out the discrepancy between the perceived and actual age of the Homo naledi fossils. That happened within the discipline. Does this episode leave us with egg on our faces? Maybe. Or maybe it just reminds us that our understanding of the world around us is ever-changing. Maybe it reminds us that our understanding of that world is incomplete because of our own human limitations. Scientific discovery is a humbling process.
Does this episode cast doubt on the enterprise of studying human evolution? Not hardly. We have strong evidence from multiple independent sources, most notably fossils and genetics, that human beings developed through an evolutionary process. This remains true even if the details of the process are still under investigation.
Think of it this way: if a forensic investigator has compelling evidence from multiple independent sources that a suspect is tied to a certain murder scene, this evidence is still compelling even if the investigator is wrong about some of the details of exactly how the crime unfolded. The small mistakes and corrections do not invalidate the larger conclusion.
The same is true of human evolution. There is no other theory that has anywhere near the explanatory power of evolution. Nothing in this article changes this fact. And regardless of how Homo naledi challenges our understanding of how humans evolved, the discovery of these amazing fossils is yet another confirmation of the basic idea of common ancestry between humans and primates.