Who Were the Oldest Homo sapiens?

Guest Blogger: Rene Studer-Halbach

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Jebel Irhoud 1
The cranium of Jebel Irhoud 1, the original specimen discovered by miners in 1961. Modified from Bruner & Pearson (2012)

On June 8 a team of researchers headed by Jean-Jacques Hublin published a pair of papers describing a new set of fossils excavated from Jebel Irhoud, Morocco. The authors argue that these new discoveries are the earliest known Homo sapiens found anywhere in the world. This leads naturally to two simple questions: was this individual a human, and did it really live roughly 315,000 years ago?

To answer the first question, Hublin et al. used digitized 3D landmarks (or, a consistent set of points on all of the skulls) to statistically analyze the shape of the Jebel Irhoud specimens and compare them to a set of other hominin fossils. This allows you to compare shape differences independent of size differences. This analysis suggests that these specimens are more similar to Homo sapiens than any other species. That being said, this method is far from conclusive. Several of the major features that we use to identify Homo sapiens in the fossil record, including a vertical forehead, globular braincase, and protruding chin, are absent from the Moroccan fossils. Are these Homo sapiens because they are more similar to us than anything else, or do we need to rely on the presence of those specific traits to define the species? If they are humans, then we need to update our definition of what it means to be a human, morphologically. Even if not, it’s clearly something extremely human-like living in a time and place where we never expected to find one.

The second question has its own set of complications. The team (Richter et al.) used thermoluminescence dating of artifacts and electron spin resonance (ESR) dating of teeth to arrive at the date of the fossils. Thermoluminescence and ESR dating both measure radiation exposure (or accumulated dose) to determine the age of an artifact or fossil. The ESR dating suggested a date of 252 – 318 ka, but with a p-value that was not low enough to be statistically significant. In and of itself, that would be a tenuous basis for such an extraordinary claim, but the thermoluminescence dating of burned artifacts found in association with those fossils revealed a date of roughly 315 ka for the geological layer as a whole. This was repeated many times over. It’s not perfect, but the date seems reasonably secure.

What does this all mean? Why has this been reported everywhere, from social media to TV news? Most of the coverage has focused on the date. These may be the earliest members of our species ever discovered. That’s cool, and especially since it pushed back the first appearance date so far, from ~200,000 to ~315,000 years ago. But I think that misses the most interesting aspect of this discovery. It makes us reconsider what it means to be human in an evolutionary sense.

As the authors note in the title of their article, this find makes the case for a pan-African evolution of Homo sapiens. Whatever these individuals were, they were different from us, that much is clear, but they were more similar than anything else we’ve found outside of Homo sapiens. Did the traits that we use to define ourselves evolve piecemeal, across Africa? The discoverers of these new fossils suggest as much, arguing that the clear delineations between archaic and modern Homo sapiens no longer apply. It might be that these specimens represent a bridge between those two groups. If so, what we call them is largely a question of what definition you like to use for a species. That’s a question for another time, and maybe one that’s best to answer by looking at other species, where the stakes don’t seem so high.

One way you could characterize the last several decades of research in human evolution is to say that our understanding has changed from a linear evolution to a bushy one. We’ve filled out the tree a little more, and we see more of the branches and evolutionary dead-ends in our lineage. These finds are doing the same thing, but for the evolution of our own species, regardless of what they’re called. Hopefully this will inspire a new set of excavations across Africa, looking for more fossils to confound us and upend our expectations.

References
Hublin, J. J., Ben-Ncer, A., Bailey, S. E., Freidline, S. E., Neubauer, S., Skinner, M. M., Bergmann, I., Le Cabec, A., Benazzi, S., Harvati, K. & Gunz, P. (2017). New fossils from Jebel Irhoud, Morocco and the pan-African origin of Homo sapiens. Nature, 546(7657), 289-292.

Richter, D., Grün, R., Joannes-Boyau, R., Steele, T.E., Amani, F., Rué, M., Fernandes, P., Raynal, J.P., Geraads, D., Ben-Ncer, A. & Hublin, J.J. (2017). The age of the hominin fossils from Jebel Irhoud, Morocco, and the origins of the Middle Stone Age. Nature, 546(7657), 293-296.

Rene Studer-Halbach is a PhD candidate in the Department of Anthropology at Rutgers University. He works on ecological niche modeling and community structure in South African Plio-Pleistocene primates.  

BS&M Does Homo naledi

On May 9th, Lee Berger and colleagues published three new papers on Homo naledi, the most recent South African hominin fossil find to make media waves. The original H. naledi fossil material was discovered in 2013 by two cavers working with Berger; it comes from the Dinaledi Chamber of the Rising Star cave system, from which it takes its species name (Berger et al. 2015). The first H. naledi discovery was remarkable because there are at least 15 individuals (from juveniles to adults of both sexes) represented in the assemblage and there are often multiple copies of each skeletal element present, which allows paleoanthropologists to look at variation within the species, and to see how it grew and developed. In total, there are about 1550 hominin bones and teeth in the assemblage – the largest single species assemblage found anywhere in Africa (Berger et al. 2015).

The three new articles covered the discovery of additional skeletal material (Hawks et al. 2017) and the age of the fossils (which had been a major source of speculation) (Dirks et al. 2017), and proposed a hypothesis for understanding Pleistocene hominin diversity in subequatorial Africa as part of a larger mammalian biogeographic pattern (Berger et al. 2017).

The new fossil material comes from a second chamber within the Rising Star system, the Lesedi Chamber, and represents at least three individuals (though in actuality the material likely comes from more than three individuals, based on where the bones were recovered from within the Chamber). The most spectacular of these remains is a relatively complete skull with associated skeletal material; the researchers have named this individual Neo. The new material looks a lot like the previously discovered H. naledi bones and teeth, and also includes both adult and juvenile material. The major thing that differentiates the Lesedi Chamber finds from the Dinaledi Chamber finds is that the Lesedi Chamber also contains animal skeletal material (Hawks et al. 2017).

The paper on the age of the fossils used several different methods (including dating geological features of the cave itself as well as directly dating some of the fossil teeth) to produce an age range for the material of 236,000-335,000 years old (Dirks et al. 2017). This means that the material is later Middle Pleistocene in age, much younger than would have been predicted based on looking at features of the skull and skeleton (like brain size).

In a previous paper, Hawks and Berger (2016) discussed what three different hypothesized ages (including a scenario that does match the new date) for the original H. naledi material would mean for its place in human evolution, and they follow up on this in the third new paper – now that we have a date, what does it mean? The date is younger than the first appearance of Homo erectus around 1.8 million years ago. As H. erectus is generally thought to be part of the lineage that is directly ancestral to us, this would seem to preclude H. naledi as a member of our direct line, unless it represents a sister group to our species that preserves a lot of the primitive morphology of a shared ancestor. This interpretation bumps H. erectus to a side branch of our family tree. A different type of analysis of the features of various species of Homo suggests instead a more bushy view of earliest Homo – whatever that ancestor was split into a number of species, each having only some of the ancestor’s features. What we can say is that H. naledi is likely only part of a branch that originated earlier in time, with the authors going so far as to suggest that we might already have fossils from earlier on this branch that we have not recognized due to their fragmentary nature (Berger et al. 2017).

Regardless of where Homo naledi ends up on our family tree, it’s still an incredible discovery that contributes to our understanding of the human fossil record. It provides a window into a time period in our history from which we don’t have great data and underscores two important ideas: first, that for most of our evolutionary history, Homo sapiens was not the only hominin species on the planet; and second, that there are still spectacular fossils waiting to be discovered.

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