A Fistful of Teeth – Do the Qesem Cave Fossils Really Change Our Understanding of Human Evolution?

A handful of fossil teeth found in Israel’s Qesem Cave, described in the American Journal of Physical Anthropology and attributed to 400,000 year old members of our own species in multiple news reports, are said to rewrite the story of human evolution. This discovery doubles the antiquity of Homo sapiens, the articles say, and identify a new point of origin for our species. “Find in Israeli cave may change evolution story” proclaims The Australian, while the Daily Mail asks and answers “Did first humans come out of Middle East and not Africa? Israeli discovery forces scientists to re-examine evolution of modern man.” (The Jerusalem Post, by comparison, went with the tamer “Homo sapiens lived in Eretz Yisrael 400,000 years ago.”) As is often the case, though, the hype surrounding this find far outstrips its actual significance.

Since the reality of human prehistory was recognized in 1859*, the time and place of our origins has been one of the most contentious subjects in anthropology. The details of human evolution were almost entirely unknown during the latter half of the 19th century. Determining whether the remains of prehistoric humans represented ancestors, new species, or ancient cousins was difficult enough alone!

As discoveries of fossil humans trickled in from Europe and Asia, however, a general consensus developed that humans had originated in the east. A smattering of fossil discoveries from Java and China, primarily of the species we now call Homo erectus, were interpreted within theoretical frameworks influenced by racism, nationalism, and other social factors, causing anthropologists to push aside discoveries of even earlier humans in South Africa as too ape-like to be ancestral to us. (Although it was generally agreed that humans had evolved from prehistoric apes, many anthropologists envisioned a “pre-sapiens” line of ancestry that showed indications of reaching upward towards our present form from an early date.) It was not until the 1950’s that Africa was finally recognized as the place of origin for the human family, and by the waning years of the 20th century a combination of genetic and paleontological techniques allowed scientists to confirm that our own peculiar species originated in Africa. According to the current spread of data, anatomically-modern representatives of Homo sapiens originated in Africa about 200,000 years ago, and populations of our species began to disperse out of Africa by about 70,000 years ago.

Discoveries announced within the past year have complicated this picture. Though these findings are provisional, there is significant support for the hypothesis that some non-African populations of Homo sapiens interbred with Neanderthals, and a paper published just last week indicates that some Melanesian people carry genetic indications of interbreeding between their ancestors and a poorly-known group of prehistoric humans closely related to Neanderthals called Denisovans.** (Razib has just posted an insightful review of what these new discoveries mean at Gene Expression.) The origin of our species can still be traced to Africa, but things were not as simple as our species replacing populations of other humans across the globe one-by-one until we were the last hominin standing. The ballyhooed conclusions derived from the Qesem Cave fossils would further upset matters. If Homo sapiens existed in Israel by 400,000 years ago then there might have been an earlier dispersal from Africa which fizzled out, or, as has been heralded by sensationalist reports, our species might have originated in Israel.

Echoing the disparity between hype and scientific support displayed during the debut of the fossil primate Darwinius last year, however, this latter, controversial hypothesis is not mooted in the actual scientific paper about the Qesem Cave fossils. What is an inconclusive report on a handful of teeth has been artificially inflated into a textbook-changing discovery (a sensationalistic journalist’s most favorite kind). The truth of the matter is that the identity of these humans remains unresolved.

Described by Tel Aviv University archaeologists Ran Barkai, Avi Gopher, and colleagues, the Qesem Cave teeth were found in two sets. One set, found lower down in the cave layers, consisted of a lower canine and the two lower premolars. The second set, from a geologically younger part of the cave sequence, was comprised of an upper second incisor, canine, and third molar (one of the “wisdom teeth”) in addition to two milk teeth, with the milk teeth obviously representing a juvenile individual. Exact dates for these two levels are difficult to determine. The authors of the study propose that the older set represents people who occupied the cave between 300,000 and 400,000 years ago while the bearers of the more recent sets occupied the cave between 200,000 and 300,000 years ago.

What sort of human – or humans – do these teeth represent? That is unknown. When compared to the teeth of Neanderthals and prehistoric Homo sapiens, the Qesem Cave teeth consistently fell at the overlap of variation between the groups. In fact, both the Neanderthals and prehistoric human populations exhibit a significant amount of variation in tooth shape, making the task of identifying the owners of the Qesem teeth extremely difficult. The graphs printed in the paper show the spread of variation in human teeth based upon their dimensions from front-to-back (mesiodistal) and side-to-side (buccolingual). There is a great degree of overlap between Neanderthals and early Homo sapiens in terms of these parameters. The only teeth that appear to group more closely with modern humans are the geologically older lower premolars, and even those fell along the border of Neanderthal variation.

Nor were the minute anatomical details of the teeth especially helpful in solving this case. The Qesem Cave teeth are generally Neanderthal-like, the authors state, but lack some of the features thought to definitively characterize Neanderthals. Instead the authors prefer to associate the Qesem Cave teeth with the Skhul/Qafzeh hominins – indeterminate prehistoric humans from other cave sites in Israel dated between about 80,000 and 120,000 years ago. Although known from much more complete material, these individuals have variously been described as part of an early migration of Homo sapiens out of Africa, hybrids between our lineage and Neanderthals, and a unique type of extinct human. Since we cannot confidently identify these humans, the proposal that the Qesem Cave humans may have been akin to the geologically younger Skhul/Qafzeh individuals does not tell us very much at all.

Left with such inconclusive fossils, the authors of the new study propose three different scenarios:

A) “The first one is of a local archaic Homo population occupying southwest Asia during the Middle Pleistocene, to which the Qesem specimens would be attributed.”

Citing the resemblance of the Qesem teeth to those of the Skhul/Qafzeh hominins and the characteristic nature of the stone tools found at these sites, the authors favor the idea that the prehistoric inhabitants of Israel were part of an isolated pocket of human evolution. The questions of what species these humans belonged, how they got there, and what happened to them, however, are left unresolved. This hypothesis appears to be the closest to the media hype around this discovery, but if this is so then the popular reports went far beyond the paltry details of the hypothesis presented in the paper.

B) “The second scenario is one of long-term in situ evolution of Neanderthals in southwest Asia.”

In the second hypothesis, the Qesem teeth would represent a population of Neanderthals which evolved some unique features independently of their relatives elsewhere. This alternative did not make it into the popular reports.

C) “The third scenario is that more than one Pleistocene human taxon is represented within the Qesem dental sample.”

Despite being geologically younger, the teeth from the upper Qesem Cave layers have more in common with lineages of archaic humans than the older ones. This difference may indicate that the older variety of humans – whether a unique population, subspecies, or species – was eventually replaced sometime after 300,000 years ago. Given that the teeth in each set are from different parts of the jaw and cannot be directly compared, however, this hypothesis is somewhat tenuous. A better sampling of teeth would be needed to be sure, especially given the wide range of variation shown among the teeth. This hypothesis was also ignored in popular reports.

As the authors themselves state, “Resolution of these alternative scenarios must await further discoveries of additional and more complete Middle Pleistocene remains from southwest Asia.” The identity of the Qesem Cave humans remains unclear, as do their origins. Even if they turn out to be early members of Homo sapiens, this does not automatically mean that our species evolved in Israel first. Instead, such a conclusion would raise several alternative scenarios, including the possibility that there are as-yet-undiscovered deposits of early Homo sapiens fossils in Africa which document an earlier dispersal from Africa distinct from the one around 70,000 years ago. For now, though, the identity of the Qesem Cave humans cannot be conclusively determined. All the grandiose statements about their relevance to the origin of our species reach beyond what the actual fossil material will allow.

If they teach us anything, the Qesem Cave teeth remind us how much remains to be discovered about human evolution during the past 500,000 years. Multiple human lineages left Africa and dispersed throughout Europe and Asia, and a combination of fossil and genetic data has thrown significant support to the notion that many of these disparate populations interacted rather intimately with each other. In truth, we are only just beginning to understand how this happened, and there are many specimens – like the Qesem Cave teeth and Skhul/Qafzeh specimens – which will remain suspended in the realm of scientific uncertainty until further discoveries provide us with the proper context to understand them. These complementary discoveries will require months, years, and decades of additional work, but anyone even superficially familiar with the process of science will not be surprised by this. It has only been a century and a half since human prehistory was even regarded as a reality – there is still much work to be done and many amazing discoveries yet to be made.

*This was not attributable to the publication of Charles Darwin’s On the Origin of Species in that year, but several serendipitous fossil finds in England and France. As geology, paleontology, and archaeology developed during the early 19th century, many naturalists believed that there was a temporal dividing line between the modern world and that of the great Pleistocene beasts. The 1858 excavation of the UK’s Brixham Cave and examinations of fossil and archaeological sites in France’s Somme Valley, however, confirmed that humans had overlapped with the mammoths, giant hyenas, cave bears, and saber-toothed cats of the Ice Age era. The consensus about human antiquity changed almost overnight. During the 1859 meeting of the British Association for the Advancement of Science the eminent geologist Charles Lyell announced that there was finally compelling evidence for human prehistory, hinting that Darwin’s forthcoming book – published in November of that year – would further foster inquiries into the subject. For more on this episode in the history of science, see Chapter 10 of Written in Stone and Men Among the Mammoths by A. Bowdoin Van Riper.

** Given these new discoveries, we will have to reexamine how we define our species and our relationship to extinct lineages like the Neanderthals and Denisovans. At the moment, it is unclear whether these lineages should be classified as separate species, as subspecies of Homo sapiens, or in some other category. In the interest of keeping this post as accessible as possible, however, I am using Homo sapiens to refer to the lineage which split from our last common ancestor with the Neanderthals (regardless of later gene-swapping).

References:

Hershkovitz, I., Smith, P., Sarig, R., Quam, R., Rodríguez, L., García, R., Arsuaga, J., Barkai, R., & Gopher, A. (2010). Middle pleistocene dental remains from Qesem Cave (Israel) American Journal of Physical Anthropology DOI: 10.1002/ajpa.21446