When did Homo sapiens first leave Africa? This is a topic that has been undergoing many revisions in recent years, as new fossils and genetic evidence have been piling up that contradict longstanding models. That’s a feature, not a bug, of the scientific process, and it makes it very exciting time to be a biological anthropologist!
Yesterday a new piece of evidence was presented to the scientific community that means we’ll be revising our textbooks once more. A paper published in Nature by Dr. Katerina Harvati and colleagues, Apidima Cave fossils provides earliest evidence of Homo sapiens in Eurasia, announced that new analyses of a skull from Greece have revealed that it belonged to a H. sapiens individual dating to 210,000 years ago—“over 150,000 years earlier than previous records suggested,” as paleontologist and co-author of the study Chris Stringer noted in a statement on Twitter.
The fossil in question was one of a pair of skulls excavated in the late 1970s from the Apidima cave complex, located in southern Greece. They were encased in breccia, found high up between two cave walls near the ceiling. The depositional context made it very difficult to date and study the two skulls. Archaeologist Ivana Radovanović (University of Kansas), who was not involved with this study, told me in an email that the Apidima cave complex has been “investigated and continuously published (on) since the late 1970s, but the chronology of the two skulls from cave A and their attribution to a particular hominin was uncertain. Both skulls were damaged in a way that didn’t allow measurements necessary for such attribution to be performed.”
For this study, the authors took a new approach. They digitally reconstructed the two skulls, creating mirrored images of their missing parts with the corresponding fragment from the other side. This allowed them to then statistically compare the crania to those of other hominins. In doing so, they found that Apidima 2 grouped most closely with Neanderthals, while Apidima 1 looked most like modern humans like us.
They dated fragments removed from the skulls using Uranium-series dating, a method which calculates the ratios of isotopes within a sample to provide estimates of the date of deposition of the material. They found that despite the fact that the two crania were located next to each other, the Neanderthal-like human had lived about 170,000 years ago, and the H. sapiens-like human had lived around 210,000 years ago. The breccia they were encased in was only about 150,000 years old. Dr. Radovanović explained to me that these results mean that “prior to final hardening of this deposit both skulls were washed out along with the sediments they were part of. The sediment containing the skulls hardened over time and was subsequently exposed to erosion. The different ages for each skull are evidence that they belong to individuals that lived at very different times. They were found in a secondary position and therefore any evidence of intentional burial or ritual practices that each of these skulls could be part of is inconclusive.”
Older prevailing models of human evolution suggested an evolutionary origin for hominins anatomically and behaviorally similar to us (sometimes referred to as modern Homo sapiens) at about 300,000 years ago throughout Africa followed by a single and recent migration out of Africa about 60,000-50,000 years ago. This finding, along with younger H. sapiens fossils (dated to 177,000-194,000 years ago) found in Israel last year suggests that there must have been some very early dispersals of H. sapiens into Eurasia. The later Neanderthal fossil evidence at this and other Eurasian sites suggests that these early modern humans didn’t persist but were replaced by Neanderthals, who themselves were eventually replaced by Upper Paleolithic Homo sapiens around 40,000 years ago.
One interesting issue that this article highlights is how difficult it can sometimes be to reconcile different lines of evidence on understanding past events. For human migrations out of Africa, specifically, these fossil results don’t quite match what all of the genetic evidence tells us. Although there is some evidence in Neanderthal genomes of a very early gene flow event predating 200,000 years ago, according to Aylwyn Scally, a geneticist at the University of Cambridge, “the curious thing is that almost all the genetic ancestry in modern humans looks like it derives from a dispersal out of Africa only 60,000-100,000 years ago. We haven’t yet found significant evidence of genetic ancestry descending from much earlier dispersals, whereas we do detect the signals of mating between our ancestors and some of the Neanderthal and Denisovans they encountered while expanding across Europe and Asia. It might simply be that all of these earlier sapiens groups had already gone extinct, and hence left no genetic legacy. But if there were many of them, that starts to seem a bit odd, since as even a very small number of individual mating events can in principle be detectable today. Alternatively, it might be that our methods are missing the evidence – perhaps incorrectly identifying some of it as Neanderthal ancestry. Without direct ancient DNA data, these signals are difficult to discern. Nevertheless, models of human evolution should assign a greater weight to the possibility of multiple or semi-continuous migration of humans out of Africa.”
Critics of this study, as reported by Lizzie Wade for Science, argue that the fossil isn’t H. sapiens at all, but more likely belongs to a group within the Neanderthal lineage. In a statement posted on Twitter, Chris Stringer agreed that “As with any challenging new find, the appropriate initial reaction should be a healthy skepticism, even when my own name is on the paper,” and noted several issues of incompleteness with the fossil crania and what they did to overcome them. He concludes with some predictions: “if we have interpreted the Apidima evidence correctly, the handiwork of these early H. sapiens must be present elsewhere in the European record.”