French Connection to China Syndrome, dentally
I've read through the new paper by Martinón-Torres et al., on Eurasian continuity in the Middle Pleistocene. They've put out an interesting hypothesis, with some support from previous work, but ultimately I think their methods are too weak to test it.
The press coverage of the paper so far (e.g., this AP article) has been a little confusing, because it misses this point: this paper is not about modern human origins, it's about much earlier evolutionary relationships. National Geographic News resorts to the always-safe:
The finding suggests that the hominid family tree could be much more complex than previously thought.
Ah, so that's what it means! More complex than previously thought! Why isn't there ever a story that makes things simpler than previously thought? I mean, isn't it a sign of a failed science if you have to add complexity to your hypothesis every time you make a new observation? It's like Ptolemaic paleoanthropology!
Anyway, enough of that rant. Let's look at what the paper really says, which is much more interesting than the press! Here's the abstract:
A common assumption in the evolutionary scenario of the first Eurasian hominin populations is that they all had an African origin. This assumption also seems to apply for the Early and Middle Pleistocene populations, whose presence in Europe has been largely explained by a discontinuous flow of African emigrant waves. Only recently, some voices have speculated about the possibility of Asia being a center of speciation. However, no hard evidence has been presented to support this hypothesis. We present evidence from the most complete and up-to-date analysis of the hominin permanent dentition from Africa and Eurasia. The results show important morphological differences between the hominins found in both continents during the Pleistocene, suggesting that their evolutionary courses were relatively independent. We propose that the genetic impact of Asia in the colonization of Europe during the Early and Middle Pleistocene was stronger than that of Africa.
OK, so this is about the initial colonization of Europe and the subsequent evolutionary trends in Europe, Asia, and Africa. The observation is that European teeth show a continued similarity to Asians during the Middle Pleistocene, and there is no evidence that European teeth evolved in the direction of Africans during that time period.
Why is that interesting? Two reasons:
1. The hypothesis directly conflicts with the idea that Middle Pleistocene Europeans were linked to Africans. A large number of anthropologists have been pushing the European-African link, under the old hypothesis that these ancient people belonged to a species that was distinct from East Asians. The European-African clade in this hypothesis is often called Homo heidelbergensis; the Asian clade remains Homo erectus.
2. The hypothesis also seems to conflict with genetic data, which suggest that the relationship of European and African hominids is more recent than the early Middle Pleistocene. In particular, the genetic divergence time between human and Neandertal genomes appears to date to more recently than 700,000 years ago (Green et al. 2006, Noonan et al. 2006), which means that the population divergence must be still more recent. Also, Alan Templeton's papers (e.g., 2002, 2006) claim evidence for migrations from Africa into Europe and Asia during the Middle Pleistocene. Those claims are consistent with the Neandertal genome data, as far as we know it, and they suggest gene flow from Africa into Eurasia.
So, the authors ought to deal with these issues. They do so in their discussion, which in this short paper is one long paragraph. I'm quoting it here in full to comment on the details:
If the population of the Eurasian continent during the Early and
Middle Pleistocene was mainly the result of several out-of-Africa incursions, we should have found African influences in the morphology of the Eurasian populations. However, the continuity of the "Eurasian dental pattern" from the Early Pleistocene until the appearance of the Upper Pleistocene Neanderthals suggests that the evolutionary courses of the Eurasian and the African continents were relatively independent for a long period and that the impact of Asia in the colonization of Europe was stronger than that of Africa.
That is the conclusion of the analysis, in brief. The strength of the conclusion depends on the power of the analytical methods to detect gene flow based on morphological similarities. More on that below.
This finding does not necessarily imply that there was not genetic flow between continents, but emphasizes that this interchange could have been both ways (25, 26).
This seems a little misleading. They have no particular evidence of gene flow from Eurasia to Africa (that would be the "both ways"). Nor do they have evidence in their analysis of gene flow from Africa to Eurasia, after the initial colonization. So they don't have any evidence for gene flow at all. So the finding doesn't emphasize anything about gene flow, other than that the teeth don't show obvious evidence for it.
Around 1 Ma, hominins appear to have dispersed into temperate latitudes as far north as 40 - 45° N (27-29), not only from Africa, but also within Eurasia (29 - 31). These populations were probably descendants of an ancient out-of-Africa exodus, rather than a later one at the end of the Early Pleistocene (30).
This is an important assertion. Other workers have emphasized the similarities of some African fossils to East Asian fossils (mainly from Java, plus Gongwangling in China) in the late Early Pleistocene. That has always been the case with OH 9, and it influenced the description of the Daka and Buia crania as well. The question is how early Asian populations became morphologically distinctive. Here, the authors argue that it was very early, without substantial signs for later interaction, which in the context of the cranial comparisons is now an extreme claim.
In addition, a recent study on the European Lower Pleistocene hominin populations has revealed a possible Eurasian origin for these groups (32).
This refers to the description of the ATD6-96 mandible, which contains an earlier assertion about Asian-European connections. I return to this below.
Furthermore, it has been pointed out that during the Middle Pleistocene there was hardly any faunal exchange bet ween East Africa and the Levant (33) and that the desert between the Sahara and Arabia was an important barrier at that time (26), therefore contributing to the isolation of both continents.
This is an important argument in support of their hypothesis. If movement between Africa and Eurasia was difficult during this time span, that reinforces their claim, and makes it less plausible that there were large-scale dispersals out of Africa during the Middle Pleistocene. That leaves us with a mention of a major exception to their proposed pattern: the evolution of humans in the Late Pleistocene:
With the exception of the SAP [i.e., H. sapiens] out-of Africa dispersion based mainly on genetic data (2), the history of human populations in Eurasia may not have been the result of a few high-impact replacement waves of dispersals from Africa, but a much more complex puzzle of dispersals and contacts among populations within and outside continents. In the light of these results, we propose that Asia has played an important role in the colonization of Europe, and that future studies on this issue are obliged to pay serious attention to the "unknown" continent (Martinón-Torres et al. 2007:3).
The citation of the ATD6-96 mandible leads us to a passage from that earlier paper (Carbonell et al. 2005), which also describes the hypothesis that the founding population of Europe was Asian. Remember that this research group calls the Gran Dolina sample, Homo antecessor, and they initially had written that this species probably colonized Europe from Africa in the late Lower Pleistocene. Here's the relevant paragraph from the cited paper (Carbonell et al. 2005):
The differences in dimensions and robustness between the TD6 mandibles and the East and North African mandibles cast doubt on the African origin of H. antecessor. In contrast, our comparative analysis suggests looking toward the Asian continent. In this respect, it is relevant to mention some data that remained unpublished in 1997, when the new species was named (10), and that are relevant to this discussion. The partial cranium Nanjing I, recovered in 1993-1994 from the Hulu Cave (Tangshan Hill, eastern central China), shows clear modern midfacial traits similar to those observed in the specimen ATD6-69 (19). Wang and Tobias (20) also found similarities between Nanjing I and the Zhoukoudian hominins. Geochronological dates, combined with ecological and paleoclimatic evidence, indicate that the Nanjing skull is ~600 thousand years old (21). Furthermore, the Locality 1 levels at Zhoukoudian, which yielded most hominin specimens, are now considered at least 800 thousand years old (22). Thus, these Chinese hominins may be contemporaneous with or slightly younger than the TD6 hominins. If the Gran Dolina and Chinese populations are phylogenetically related, they should share a common ancestor that also had a modern midfacial pattern and a gracile mandible. In the cranium, this hypothetical common ancestor would have had a low and flat temporal squama, and an unfused styloid process. These traits would have been retained in the Asian hominins but lost in the TD6 hominins, who exhibit a fused styloid process, a convex temporal squama, and probably a significant increase in cranial capacity (19). The Ceprano calvaria (Italy), which has been tentatively assigned to H. antecessor (23), exhibits a convex temporal squama and a cranial capacity of about 1,057 ml (24). Interestingly, TD6 and Zhoukoudian are the only hominins that have a zygomaxillary tubercle before the Upper Pleistocene (19).
So that provides cranial and mandibular evidence of an Asia-Europe connection, supporting the dental evidence provided in the current paper. Still, that evidence is from the initial founding of Europe in the Early Pleistocene and doesn't necessarily apply to the trends during the Middle Pleistocene.
After working through the data supplements for the paper, I think that the analysis is much weaker in statistical power than it could be. In their analysis, they disregard much of the variation within these ancient samples and focus on the differences between samples according to their scoring methods. This may reveal the broad relationships among samples -- if we disregard the possibility of selected parallelisms -- but it does not say anything about the possibility of gene flow among the samples.
Indeed, the result of their analysis (a dendrogram, or branching tree) is quite incapable of showing genetic exchanges at all. It can only show branching events, which means that the result will show either an exclusive relationship between Europeans and Asians, or an exclusive relationship between Europeans and Africans, but never a mixed relationship.
The only result in the paper that indicates a European-Asian relationship is from their cladistic analysis of a subset of the data. And it isn't especially strong evidence, since the Middle Pleistocene Africans are limited to the relatively early sites of Rabat and Tighenif (Ternifine). Granted, the later sample is also small in number, but this isn't really a test of relationships; it's more of a suggestion.
The phenogram inexplicably omits Middle and Lower Pleistocene Africans entirely, and considers only australopithecines and habilines as the African sample.
So, at the moment I consider this to be a very interesting hypothesis in search of a good test. There is no test of gene flow here, just an assertion. Yet, the cranial comparisons give the assertion some plausibility -- and remember, another idea out there is the hypothesis that early Homo originated in Asia and migrated to Africa later.
I think that these topics together constitute the important problem in early human relationships right now, so I'll be writing some more about them. There are many additional interesting facts to consider...
References:
Martinón-Torres M, Bermúdez de Castro JM, Gómez-Robles A, Arsuaga JL, Carbonell E, Lordkipanidze D, Manzi, G, Margvelashvili A. 2007. Dental evidence on the hominin dispersals during the Pleistocene. Proc Nat Acad Sci USA (early) doi:10.1073/pnas.0706152104
Stringer C. 2002. Modern human origins: progress and prospects. Phil Trans Roy Soc Lond B 357:563-579. doi:10.1098/rstb.2001.1057
Rightmire GP. 1998. Human evolution in the Middle Pleistocene: the role of Homo heidelbergensis. Evol Anthropol 6:218-227. doi:10.1002/(SICI)1520-6505(1998)6:6<218::AID-EVAN4>3.0.CO;2-6
Carbonell E and 19 others. 2005. An Early Pleistocene hominin mandible from Atapuerca-TD6, Spain. Proc Nat Acad Sci USA 102:5674-5678. doi:10.1073/pnas.0501841102
Bruner E, Manzi G. 2005. CT-based description and phyletic evaluation of the archaic human calvarium from Ceprano, Italy. Anat Rec A 285A:643-657. doi:10.1002/ar.a.20205
Bringing down "Goliath"
A number of readers have been asking what the deal is with the "Goliath" specimen discussed by Lee Berger (and reconstructed by him and Steve Churchill) in the National Geographic program, "Searching for the Ultimate Survivor." The femoral fragment found by Berger himself was apparently from Hoedjiespunt, around 300,000 years old. The specimen itself has not yet been reported.
The reconstruction shown on the program is based on the Kabwe cranial and postcranial remains. The Kabwe skull is the best known specimen from the site, but there are also another maxilla and postcrania representing three or more individuals. One (E719) of two innominate bones (os coxae) and one femur (E 907) are quite large, although they probably do not belong to the same individual as the skull (Wolpoff 1999). I would assume that the full-body reconstruction on the program used these to estimate and model a very large body size.
Kabwe (E 686) cranium, lateral view
"Ultimate Survivor" discusses the "Goliaths" living in Europe, which means that they are talking about Homo heidelbergensis. There is a clear division of opinion about this species in the field. Some researchers, myself included, think it is a superfluous name that doesn't describe a real ancient reproductive community, and so we tend not to use it at all. But among those who believe that H. heidelbergensis is valid, there are essentially two viewpoints. Some would limit its application to European fossils only, which is where the type specimen, the Mauer mandible, was found. Others would apply H. heidelbergensis much more broadly to essentially all Middle Pleistocene European and African fossils, and some specimens from China as well. In this usage, H. heidelbergensis is basically inclusive of all specimens that have been called "archaic Homo sapiens, on the basis of enlarged brain size compared to earlier humans combined with the lack of most of the distinctive features of Neandertals.
So the question is, were Middle Pleistocene humans a race of giants? There is no question that there were some individuals with large mass. The large Kabwe specimen is one; the individual represented by the very broad Sima de los Huesos pelvis is another -- probably the most massive individual in the Middle Pleistocene record. These large specimens had masses upward of 80 to 90 kg, and are more massive than any Early Pleistocene humans, who averaged only between 60 and 70 kg.
But these large specimens provide only a small part of the overall picture of body size. The multiple skeletal remains from the single site of Kabwe alone indicate a range of body sizes. Not only in Africa but elsewhere there is clear evidence of a mixture of smaller and larger specimens. As shown by Ruff et al. (1997), this range of variation is not more extensive than in living human populations. Part of the variation is related to climate (higher latitude populations are more massive), part is probably due to sex (the largest specimens are undoubtedly males, meaning that there must have been a range of smaller female individuals also). But whetever the sources of variation they were substantial and did not greatly change after the beginning of the Middle Pleistocene.
Body mass vs. time, from Ruff et al. 1997. Note the large body sizes of a few individuals after 600,000 years ago, and the subsequent stasis.
The large body size of some Middle Pleistocene fossil individuals, as well as the Late Pleistocene Neandertals, has led to considerable speculation about their adaptation. Much of this centers around the assertion that early humans were greatly powerful and muscular compared to recent people. This assertion is supported by the increased shaft thickness of the long bones of many early humans. These shafts generally have quite thick cortical bone and reduced medullary cavities; not only compared to recent people but also to early Holocene skeletal remains. Since early farmers certainly worked hard and did not lead lives of luxury, the archaic humans stand out as robust.
Perhaps world-class athletes -- at least those before the widespread use of anabolic steroids -- offer a closer approximation to the body build and mass of archaic Homo sapiens. Tanner (1964) reported on the mass and proportions of athletes in the 1958 British Empire and Commonwealth Games in Cardiff, and the 1960 Olympic Games in Rome. The competitors who most closely approximate the build of Neanderthals were the throwers, weight-lifters and wrestlers. Some of these men weighed as much as 91 kg, even though they were narrower across the hips than most Neanderthals. Using a larger, more muscular living human reference sample could produce even larger and perhaps more realistic body-mass estimates (Kappelman 1997:127).
Of course, I weigh as much as 91 kg, and without making any claims about how narrow I am at the hips, my body proportions are not especially Neandertal-like. I doubt that an Olympic weightlifter would make a better model of a Neandertal than I do. The specialized muscle building regimen necessary for performance athletes is not part of the standard mode of human growth and development. Almost certainly a Neandertal with the lean body mass of a performance athlete would be at a huge energetic disadvantage without a substantial fat store, since the availability of food for hunter-gatherers is neither uniform nor uninterrupted. Today's hunter-gatherers are not particularly muscle-bound -- although they are strong and lean, they are not "cut," and when healthy they have noticeable fat stores. So while I would not suggest that archaic humans were by any means portly, I would suggest that if they had high mass estimates, then a substantial part of that must be modeled as fat rather than pure muscle.
The relatively rapid decrease in modern human body mass during the past 90,000 yr is a dramatic contrast to the large body mass of archaic Homo during the preceding two million years. What selection pressures could have resulted in both smaller body mass and larger relative brain size in modern humans? These changes do not seem to be tightly linked to technological innovation, although the less sturdily constructed skeleton implicates different behaviours, suggesting that modern humans adopted increasingly less active lifestyles. Now, rather than focusing solely on models that favour selection for ever-larger brains, we should examine the possibility that the pattern in modern humans was driven by selection for smaller bodies, perhaps favoured by a social structure that relied on more cooperative foraging and better communication skills (Kappelman 1997:127).
We can add an additional possibility: that increased dietary constraints resulted from population size increases, and that Late Pleistocene humans decreased in body size as a secular trend. It is almost certainly true that a secular trend toward lower mass occurred during the Holocene with the advent of agricultural subsistence. The lower protein and other nutritional content of early agricultural diets combined with the increased incidence of epidemic diseases during childhood both resulted in smaller adult body sizes. Since the industrial revolution, this secular trend has reversed in societies with increasingly Westernized diets.
Moreover evidence from the past 40 years has indicated that the body size differences among human populations have begun to decrease as nutrition has improved in developing nations:
Current analyses indicate that body mass varies inversely with mean annual temperature in males (r=-0.27, P < 0.001) and females (r=-0.28, P < 0.001), as does the BMI (males: r=-0.22, P=0.001; females: r=-0.30, P < 0.001). The surface area/body mass ratio is positively correlated with temperature in both sexes (males: r=0.29, P < 0.001; females: r=0.34, P < 0.001), whereas the relationship between RSH and temperature is negative (males: r=-0.37, P < 0.001; females: r=-0.46, P < 0.001). These results are consistent with previous work showing that humans follow the ecological rules of Bergmann and Allen. However, the slope of the best-fit regressions between measures of body mass (i.e., mass, BMI, and surface area/mass) and temperature are more modest than those presented by Roberts. These differences appear to be attributable to secular trends in mass, particularly among tropical populations. Body mass and the BMI have increased over the last 40 years, whereas the surface area/body mass ratio has decreased. These findings indicate that, although climatic factors continue to be significant correlates of world-wide variation in human body size and morphology, differential changes in nutrition among tropical, developing world populations have moderated their influence (Katzmarzyk and Leonard 1998:483).
This means that mass is approaching the same situation as stature, where any prediction of Allen's rule appears to be partly cancelled by the tall present-day stature of Northern Europeans, which is in large part a recent, post-industrial development.
So in my view, the body size of Middle Pleistocene humans was influenced by not only their activity pattern and adaptation to locomotion, but also their diet and body composition. They cannot be described as giants, or "Goliaths" compared to recent humans. In particular, the mean body sizes of people living today in industrialized societies are very similar to those of Middle Pleistocene humans.
The body size in Western countries today is a function of genes acting in an environment with nearly maximal nutrition and minimal disease and parasite load. In archaic humans, evidence suggests a diet very high in animal protein, and the small population sizes and low densities would likely maintain a low rate of acute communicable diseases and parasites. Unlike recent hunter-gatherers who occupy lands historically unused by agriculturalists, archaic humans could live and forage in the most productive habitats with the most abundant food sources. In short, archaic humans were probably healthier and better-fed than their later Upper Paleolithic and Holocene counterparts.
The Middle Pleistocene saw the most extensive increases in human brain sizes during all of human evolution. It is interesting to consider the role of diet and population density in creating the circumstances during which this increase happened.
References:
Kappelman J. 1997. They might be giants. Nature 387:126-127.
Katzmarzyk PT and Leonard WR. 1998. Climatic influences on human body size and proportions: ecological adaptations and secular trends. Am J Phys Anthropol 106(4):483-503.
Ruff CB, Trinkaus E, Holliday TW. 1997. Body mass and encephalization in Pleistocene Homo. Nature 387:173-176.
Wolpoff MH. 1999. Paleoanthropology. McGraw-Hill, New York.
French team studies Narmada fossil
These days it's increasingly easy to keep track of what everybody else is doing. Consider this, from newKerala.com:
A French paleontology team is currently in Kolkata to study the only homo erectus skull found in India to find new insights into the over half million year old fossil, which has baffled Indian scientists for over 21 years.
The team joined by their Indian counterparts took a CT scan of the skull here, where it has been housed since it was first discovered by geologist Arun Sonakia in the village of Hathnora, near Bhopal in Madhya Pradesh in December 1982.
The scientists hope the CT scan data will allow then to complete the severely damaged and delicate skull by using advanced compute [sic] simulation.
Having gotten valuable insights into brain formation and structures like inner ear and sinuses, scientists could even be able to make complete 3D image of the specimen, which initially was mistaken for years as of a man known as the "Narmada Man".
...
"It is interesting for us and we want to compare it with discoveries of the same species in Europe ...we have discovered items during our excavations and we want to compare with fossil discovered in China, in Java and some fossil in Africa also," said Madam De Lumley, who is leading the French team.
This should make for some interesting work. It would be exciting if the EU starts putting some of this data on The Neanderthal Tools!
New Middle Pleistocene hominid found in Ethiopia
Reuters is running a short story describing the find of a 200,000 - 500,000 year old skull by Sileshi Semaw's field crew.
The skull appeared "to be intermediate between the earlier Homo erectus and the later Homo sapiens," Sileshi Semaw, an Ethiopian research scientist at the Stone Age Institute at Indiana University, told a news conference in Addis Ababa.
Every press account I've seen is a variant of the Reuters story, but MSNBC is running it with a picture of the discoverer holding the skull or a cast. Looks like a low frontal with a marked postorbital constriction, but a relatively thin supraorbital torus. Hard to tell if the frontal and parietals are put together at the right orientation, but the parietal walls look to slope inward from vertical. A lot depends on how it's reconstructed, and it's a gamy angle in the photo, but it doesn't impress me as looking much like the other Middle Pleistocene African skulls.
UPDATE (3/26/2006): A helpful reader points me to the press release from the Stone Age Institute. The name "Gawis" is being applied to the site where the skull was found.
The specimen includes the braincase, upper face, and upper jaw. Several surface exposed stone tools were found at the hominid site. Additional contemporary stone tools excavated from the same stratigraphic level near the hominid site include Late Acheulian artifacts.
This description indicates more material than the photograph shows.
Tuberculosis in an archaic human
Based on a press release from John Kappelman, this is pretty interesting:
Although most scientists believe tuberculosis emerged only several thousand years ago, new research from The University of Texas at Austin reveals the most ancient evidence of the disease has been found in a 500,000-year-old human fossil from Turkey.
...
The researchers identified this specimen of Homo erectus as a young male based on aspects of the cranial suture closure, sinus formation and the size of the ridges of the brow. They also found a series of small lesions etched into the bone of the cranium whose shape and location are characteristic of the Leptomeningitis tuberculosa, a form of tuberculosis that attacks the meninges of the brain.
I'll have quite a bit more on this when the paper becomes available (in AJPA); most tuberculosis strains in living people originated within the last 35,000 years, but a couple of years ago it was suggested that these stem from a much older bacillus species in hominids.
There's a lot of fluff in the press release about skin pigmentation, vitamin D and depressed immune systems. It's just fluff -- we don't know what color these hominids were, and there are plenty of light-skinned people in the world with tuberculosis. I don't see why finding tuberculosis roughly 500,000 years earlier than ever before isn't interesting enough!
A new Middle Pleistocene hominid from Turkey
John Kappelman was kind enough to send me a preprint of the report on the new Turkish Middle Pleistocene specimen. The specimen consists of frontal and parietal fragments from a single skull, and comes from a travertine quarry outside the town of Kacabaş, western Turkey. The skull was found in the course of quarrying activities, and may be the first hominid specimen to have been "reduced to a standard rough-cut tile thickness of about 35 mm."
The paper reports a date estimate of 490 ± 0.05 to 510 ± 0.05 kyr, based on thermoluminescence of travertine. The paper contains some pictures of the specimen, description of its anatomy, and evidence for the presence of tuberculosis.
I review these elements below.
John Hawks Department of Anthropology
University of Wisconsin—Madison
Copyright © 2007 John Hawks