Did Hunger Drive the Evolution of Homo sapiens?

This time last year, science news headlines blared a spectacular claim – the first members of our species evolved 200,000 years earlier than previously thought. The evidence consisted of a small collection of teeth. Discovered in roughly 200,000 to 400,000 year old deposits in Israel’s Qesem Cave, these fossils were said to herald the archaic beginnings of our own species. We didn’t evolve in Africa, reports claimed, but got an earlier start along the eastern border of the Mediterranean in the Levant.

I wasn’t convinced. The teeth in question fell within the range of variation for both early Homo sapiens and Neanderthals. Without additional fossil material, there was no way to tell what human species the teeth belonged to or how those individuals might be related to later populations in the same area. The researchers behind the new study recognized this in the paper. There were no firm conclusions about who the teeth belonged to the in actual American Journal of Physical Anthropology study. Carl Zimmer aptly called the media hype surrounding the paper “journalistic vaporware”, though, as he pointed out, study co-author and Tel Aviv University archaeologist Avi Gopher fed the overblown conclusions in press interviews.

Now I feel as if it’s 2010 again. Earlier this month Gopher joined co-authors Miki Ben-Dor, Israel Hershkovitz, and Ran Barkai in publishing a new paper on the Qesem Cave teeth. Only it wasn’t framed that way. Titled “Man the Fat Hunter”, the PLoS One paper suggests that an insatiable need for fat not only contributed to the disappearance of the early human Homo erectus, but led to the evolution of a “lighter, more agile, and cognitively capable” human represented by the Qesem Cave teeth. Last year’s conclusions have been given some shiny new wrapping.

News sites have reiterated what was said last year, with elephants added. “Disappearance of elephants in the Levant 400,000 years ago led to emergence of the Homo sapien” said the Jerusalem Post, and io9 jumped in with “Modern humans may have evolved 200,000 years earlier than we thought.” The paper’s press release – reproduced at PhysOrg – takes a different approach in hinting that Homo sapiens evolved in the Levant without actually putting it in those words:

Not only do their findings on elephants and the Homo erectus diet give a long-awaited explanation for the evolution of modern humans, but they also call what scientists know about the “birth-place” of modern man into question.

Evidence from the Qesem Cave corroborates this revolutionary timeline. Findings from the site dated from as long as 400,000 years ago, clearly indicate the presence of new and innovative human behavior and a new human type. This sets the stage for a new understanding of the human story, says Prof. Gopher.

“Modern humans.” That phrase is being used as synonymous with Homo sapiens, even though we still do not actually know who the Qesem Cave people were. In fact, at the very start of the PLoS One paper Ben-Dor and colleagues write:

As the classification of varieties of the genus Homo is problematic, we refrain in this paper from any taxonomic designations that would indicate species or subspecies affiliation for the hominins of Qesem Cave. The Qesem Cave hominin, based on the analysis of teeth shares dental characteristics with the Skhul/Qafzeh Middle Paleolithic populations [prehistoric humans which have not yet been confidently identified] and to some extent also with Neandertals.

Such statements have done little to quell the frustrating mess of interpretations drawn from the Qesem Cave finds. The papers insist that we can’t know what the Qesem Cave hominins were, but the press releases, news reports, and snippets of the new paper hint that the archaeologists behind these studies think otherwise. In the PLoS One paper, the researchers say “In the Levant, dental remains from the Acheulo-Yabrudian site of Qesem Cave, Israel demonstrate resemblance to dental records of later, Middle Paleolithic populations in the region indicating that H. erectus was replaced some 400 kyr ago by a new hominin ancestral to later populations in the Levant.” But we don’t know that. The Qesem Cave teeth show that a different variety of human succeeded Homo erectus in that one spot. There is no sign that the teeth belonged to ancestors of later populations in the region, nor is there any indication that the Qesem Cave people were direct descendants of Homo erectus in the region. This latter point, in particular, is at the heart of the new paper.

Homo erectus was one of the first truly big-brained humans. But large, complex brains are a huge energy draw and, therefore, Ben-Dor and co-authors insist that Homo erectus in the Levant would have been skilled elephant-hunters. This is partially based on the association between elephant bones and tools thought to have been used by Homo erectus at Levant archaeological sites, but the paper is not so much about fossil evidence as nutrition. Homo erectus needed a lot of energy to nourish their big-brained selves, and, according to Ben-Dor and colleagues, elephants would have been easily-accessible bundles of fat that the humans came to rely upon.

Hard data for this hypothesis is scarce. Using modern humans and the proposed diets of our prehistoric relatives, the proposed nutritional requirements of Homo erectus were estimated from a tangle of information related to protein intake and the ability to process fibrous plant foods. Likewise, the fat content of prehistoric elephants was roughly estimated based upon the attributes of modern African buffalo, and they way Homo erectus would have hunted these animals is based upon a mix of observations from modern hunters and anecdotal evidence. The case rests on rough approximations of how much fat an individual Homo erectus might require, how nutritious an individual elephant might be, and how easy elephants could be hunted.

In the estimation of Ben-Dor and collaborators, elephants would have been a “unique food package, the hunt of which is not particularly physically challenging and does not necessarily require mastery of sophisticated technology.” I can’t imagine that they’re writing from experience. Even if we assume that elephants could be as safely and easily preyed upon as the researchers suggest, though, there is still a whole mess of unknowns:

Albeit the present lack of direct archaeological evidence, we believe that [sharing of elephant meat or longterm preservation by drying] were not beyond the capabilities of H. erectus. We have no knowledge of the size of the group that partook in the consumption of a single elephant during the Middle Pleistocene, but it seems plausible to consider the aggregation of several small groups in the case of a successful hunt. While we have no data regarding the extent to which a hunted elephant was utilized, in our view, the abundance of evidence for elephant utilization in Acheulian sites in itself is testimony that a significant part of the elephant’s potential energetic value was extractable by H. erectus. [emphasis mine]

While there is archaeological evidence – primarily cut-marked bones – that Homo erectus occasionally dined on elephant, the details of how often they did so and how they acquired that food is unknown. And the fossil record of the Levant may decouple the disappearance of the predator and its prey. While Ben-Dor and colleagues affirm that Homo erectus and Elephas antiquus disappeared from the region simultaneously before 400,000 years ago, a 2009 review of Levant archaeological sites by anthropologist Miriam Belmaker noted the presence of the elephant at one archaeological site that may be as little as 200,000 years old called Revadim Quarry. Though elephant remains are absent from Qesem Cave itself, Elephas antiquus may lived in the area during the same time.

The trick is obtaining accurate dates for these sites. As Ben-Dor and co-authors note, an elephant-bearing site called Holon was thought to be 200,000 years old but is probably significantly older. The age of the Revadim Quarry still lacks tight constraint – it could fall in line with the proposed 400,000 year cut-off point, or it might indicate that the elephants survived in the Levant for two-hundred thousand years longer than the PLoS One study accounts for. Either way, it is worth noting that Homo erectus and Elephas antiquus didn’t actually go extinct 400,000 years ago – both species survived for thousands of years afterwards in other locations. In 2005 paleontologist Anthony Stuart proposed that the eventual extinction of the elephant – perhaps as late as 50,000 years ago – may have been primarily driven by the loss of woodland habitats when the global climate became colder at the end of the last interglacial period. Homo erectus survived for about as long within Asia, though why these populations disappeared is a mystery.

For the authors of the PLoS One study, though, it doesn’t matter why the elephants were extirpated from the Levant. What the researchers underscore is that Homo erectus vanished about the same time – a coincidence they take as evidence that big-brained humans relied on elephants for food. Now here’s where the Qesem hominins come in.

With the elephants gone, the hungry Homo erectus had to hunt a greater quantity of smaller, leaner game. Fallow deer would have been one of the mammals on the menu (indicated by the abundant deer bones in Qesem Cave). This created an evolve-or-die situation. According to Ben-Dor and colleagues, Homo erectus hunters expended more energy chasing down small prey, and this was made even worse by the fact that the animals were not as high in fat as the elephants. Eventually – through what the researchers envision as some unknown evolutionary process – the Levant Homo erectus deteriorated and died.  The humans that replaced them – tacitly considered to be like Homo sapiens by the authors – also required lots of fat to fuel their large brains, but had lighter, more agile bodies better-suited to catching large amounts of smaller prey.

Though not stated explicitly, the authors hint that the Qesem hominins would be the descendants of Homo erectus that somehow survived the change – that their hypothesis represents “a local, Levantine emergence of a new hominin lineage” which can be categorized as a “modern human.” As the researchers put it, “Our calculations show that the elephant’s disappearance from the Levant just before 400 kyr was significant enough an event to have triggered the evolution of a species that was more adept, both physically and mentally, to obtain dense energy (such as fat) from a higher number of smaller, more evasive animals.” Hunger, they hint, was the driving force behind our origin.

But I don’t think the paper demonstrates that at all. The fossil and archaeological record indicates that one type of human – Homo erectus – preceded another, as-yet-unknown species in the same region. There is no clear indication of how the two might be connected. Were the Qesem hominins descendants of Homo erectus in the Levant, or did they originate elsewhere and simply move in once the old tenants had disappeared? And, given that the affinities of the Qesem hominins are still ambiguous, we can’t say anything about whether they might be ancestral to later populations or represent a short-lived occupation along the route out of Africa and through Eurasia.

Nor is there any direct evidence that Homo erectus almost entirely relied on prehistoric elephants for fat, or that the Homo sapiens body type was an adaptation for catching lots of small mammals. In my reading, the paper is rife with assumptions and speculations arranged to highlight the proposed – but not proven – importance of the Qesem teeth to the questions of when, where, why, and how our species evolved. We remain poorly acquainted with our prehistoric cousins from Qesem Cave. Until we know more about their story, they will remain some of our most mysterious relatives – known only to us through a fistful of teeth and the piles of deer bones they left in their home.

Top Image: A restoration of the prehistoric elephant Elephas antiquus by Flickr user Maggi_94.

References:

Belmaker, M. (2009). Hominin Adaptability and Patterns of Faunal Turnover in the Early to Middle Pleistocene Transition in the Levant Sourcebook of Paleolithic Transitions, 2, 211-227 DOI: 10.1007/978-0-387-76487-0_12

Ben-Dor, M., Gopher, A., Hershkovitz, I., & Barkai, R. (2011). Man the Fat Hunter: The Demise of Homo erectus and the Emergence of a New Hominin Lineage in the Middle Pleistocene (ca. 400 kyr) Levant PLoS ONE, 6 (12) DOI: 10.1371/journal.pone.0028689

Hershkovitz, I., Smith, P., Sarig, R., Quam, R., Rodríguez, L., García, R., Arsuaga, J., Barkai, R., & Gopher, A. (2011). Middle pleistocene dental remains from Qesem Cave (Israel) American Journal of Physical Anthropology, 144 (4), 575-592 DOI: 10.1002/ajpa.21446

STUART, A. (2005). The extinction of woolly mammoth (Mammuthus primigenius) and straight-tusked elephant (Palaeoloxodon antiquus) in Europe Quaternary International, 126-128, 171-177 DOI: 10.1016/j.quaint.2004.04.021