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paleoanthropology, genetics and evolution

teeth

  • Les Rois revisited, and dental classification of other Aurignacian individuals

    Tue, 2009-06-02 10:43 -- John Hawks

    I pointed a couple of weeks ago to the Les Rois Neandertal paper by Ramirez Rozzi and colleagues.

    In the new article section of Journal of Human Evolution, Shara Bailey, Tim Weaver and Jean-Jacques Hublin have a paper that examines the Les Rois sample (among many others) in terms of dental discrete traits. Basically, they set up a discriminant function that can tell Mousterian Neandertals from later Upper Paleolithic people with around 89 percent classification accuracy, and then they applied it to Aurignacian and Châtelperronian dental remains. Here's what they concluded about Les Rois (p. 13):

    The analysis of the entire Les Rois sample (n = 15) showed that they have an overwhelmingly ‘modern’ signal. Fourteen of the fifteen individuals had high posterior probabilities of belonging to the Upper Paleolithic modern human group (>80%). This is not unexpected considering the teeth are associated with an Aurignacian industry (Dujardin, 2000) and most come from unit B, which has been dated to 28,715 ± 145 BP using AMS C14. One individual (Mandible B), however, was classified as Neandertal with a low posterior probability (54%) based on 13 traits.

    Ramirez Rozzi (pers. comm.) has recently suggested that Mandible B represents a Neandertal, based largely on the asym-
    metrical P4 together with some aspects of the corpus. Trinkaus (2007) has also argued that the specimens from Les Rois are mixed in morphology. While the P4 is asymmetrical, a large sagittal crack in the crown exaggerates this feature and the remaining aspects of the tooth are distinctively not Neandertal-like (it lacks a transverse crest and multiple lingual cusps: Fig. 6). In the end, we do not consider the posterior probability of 54% to be compelling enough to conclude, based on dental traits, that there were Neandertals present at Les Rois.

    Ramirez Rozzi et al. (2009) made their conclusions about the Les Rois specimens mainly based upon perikymata packing patterns, secondarily supported by tooth sizes and the dental nonmetrics used here. So they are really concluding the same thing about mandible B. What's interesting is that Ramirez Rozzi and colleagues find several other dental individuals from the earlier unit with similar enamel formation patterns, and which they claim are also Neandertals.

    In some senses, Les Rois is a good case study for how more complete specimens come to dominate the discussion to the exclusion of other fragmentary remains. Mandible B is itself a tiny piece of a skeleton, but because it has several different anatomical elements on that little piece, the two papers can conduct this kind of in-depth analysis. But the status of the other isolated teeth are equally important -- if Ramirez Rozzi and colleagues are right, they might be enough to establish the earlier Les Rois sample as standing apart from other early Aurignacian-associated samples. If we're talking about a single specimen, however well documented, the situation is somewhat different.

    As everybody knows, my null hypothesis is that the samples are mixed in their morphological pattern. On that topic, the more interesting implication of the study by Bailey and colleagues (2009) is that when they applied their discriminant function to Châtelperronian samples, they got a similar classification frequency to Neandertals as for earlier Neandertal samples. And when they applied their function to Aurigacian samples, they got a similar classification frequency to modern humans as for later Upper Paleolithic samples. Under the hypothesis of a mixed transition from earlier Neandertals through later Châtelperronian Neandertals into early Aurignacian and into the Upper Paleolithic, you'd expect the intermediate time steps to give a lower classification frequency -- more Neandertal-like in Aurignacian than later; more modern-like in Châtelperronian than earlier. Bailey and colleagues found that their function classified 85 percent of Aurignacian dental individuals as modern (29/35) compared to 89 percent of later Upper Paleolithic individuals (56/63). Those numbers aren't significantly different, and given the variability in completeness of assigned specimens, I wouldn't go farther.

    One possible criticism of the paper is that the morphological pattern that makes up the discriminant mostly consists of traits that are shared by both groups but differ in frequencies. It is still quite possible to use the traits to discriminate individual specimens, but somewhat harder to interpret what a change in trait frequencies means in genetic terms. Bailey and colleagues recognize this issue, and raise it in their discussion of the Oase 2 specimen, for example (p. 12):

    Given that the cranium of Oase 2 is clearly not that of a Neandertal (Rougier et al., 2007), the assignment of this individual to the Neandertal group was unexpected. Trinkaus (2007) has suggested that, while essentially ‘modern,’ both Oase 1 and 2 exhibit a mosaic of cranio-dental features, some of which are archaic (e.g., dental proportions, long and flat frontal bone), and others apparently derived towards anatomically modern humans (parietal curvature, absence of supraorbital torus) or towards Neandertals (unilateral lingual bridging of the mandibular canal).

    It is important to note that the dental traits aligning Oase 2 with Neandertals are archaic in nature, as they are observed in other fossil hominins as well (Bailey, 2002b, 2006; Martínon-Torres et al., 2007). It is unfortunate that incisor morphology could not be assessed (teeth are missing), and that the upper M1s are too worn to ascertain occlusal polygon shape and occlusal polygon area, since these are features that are likely derived for the Neandertals/Neandertal lineage (Bailey, 2004; Gómez-Robles et al., 2007). Considering that some of the most diagnostic features of the upper dentition could not be assessed and that our approach is not 100% accurate, we caution against over-interpreting the classification of Oase 2.

    That's the basic problem of comparing Neandertals with humans, which we encounter genetically as well as morphologically. The groups differ in the frequencies of traits, but not often in the exclusive presence of distinctive ones. Sometimes, what once looked like a distinctive trait is then found in the other group -- so it's not distinctive anymore! Some researchers focus on "distinctive combinations" of traits, which tend to include a mixture of primitive and derived morphologies. But differences in trait frequencies automatically lead to differences in the combination of traits. Sometimes combinations are disproportionately represented (that is, putting traits together separates the samples more than considering them individually), but it is unclear how much of a trait combination may be explained by a history of inbreeding (in isolated populations) and how much may be explained by pleiotropy (of a few genes that differ in frequency).

    You'd think this problem would be easier than it is. But look at Les Rois -- a fragmentary but interesting sample, with a blend of morphologies in different specimens. How do we interpret the similarity of Les Rois mandible B to Neandertals? Is it a Neandertal? If we considered the dental nonmetric features alone, as Bailey and colleagues suggest, the specimen looks like a Neandertal but with really rather weak evidence. If we add the perikymata and size data, the similarities with Neandertals are increased, and other teeth from the site also tend to look more Neandertal-like. But we know that perikymata patterns vary in recent human populations. How should we consider this variation as we compare the Les Rois teeth -- should we consider Europeans only, or modern humans more broadly? How did those traits change within the last 30,000 years, and is that relevant to the 10,000 years before?

    Well, it's certainly enough to keep things interesting. I'm raising a lot more questions than offering answers. I like the approach Bailey and colleagues have taken because it includes much of the available sample in a way that can be considered as a unit. But then when we return the the issue of particular specimens and the possible patterns of genetic causation of the traits, we are left with the same problems as before.

    References:

    Bailey SE, Weaver TD, Hublin J-J. 2009. Who made the Aurignacian and other early Upper Paleolithic industries. J Hum Evol (in press) doi:10.1016/j.jhevol.2009.02.003

    Tags: 
    Upper Paleolithic
    Aurignacian
    Europe
    teeth
    Chatelperronian
    Neandertals

  • Another Aurignacian Neandertal, or just dinner?

    Tue, 2009-05-12 11:41 -- John Hawks

    I said I was going to do my best to scoop the press this week. How about this piece of undernews: at one of the few early Aurignacian sites to preserve skeletal remains, Les Rois, France, one of the Aurignacian-associated mandibles looks like it may have been a Neandertal.

    Before I tell the whole story, let me telegraph the bottom line: Do I think this specimen was really an Aurignacian Neandertal?

    My opinion has always been that Europeans in the time span from 40,000 to 25,000 radiocarbon years presented a varying mixture of "Neandertal" and "modern" morphological features. From that standpoint, it is not surprising to find a mandible that has the combination of features reported here. In this case, the most significant mandible (which is really quite a small fragment) shows one very interesting characteristic: a perikymata count and packing pattern similar to Neandertals and different from other Upper Paleolithic European teeth. But as I'll point out below, living humans are variable in their enamel formation in ways that reduce the significance of the differences between Neandertals and later Europeans.

    But the story is significant -- not only do these remains extend the biological variability of known Aurignacian-associated people to include Neandertal-like developmental patterns, but also they help to inform us about the potential of cultural associations at other sites, including Vindija.

    The morphology

    Let's consider what the authors wrote about the specimens. Here's most of the abstract of the paper, in Journal of Anthropological Sciences, by Fernando Ramirez Rozzi and colleagues:

    Here we reassess the taxonomic attribution of the human remains, their cultural affiliation, and provide five new radiocarbon dates for the site. Patterns of tooth growth along with the morphological and morphometric analysis of the human remains indicate that a juvenile mandible showing cutmarks presents some Neandertal features, whereas another mandible is attributed to Anatomically Modern Humans. Reappraisal of the archaeological sequence demonstrates that human remains derive from two layers dated to 28–30 kyr BP attributed to the Aurignacian, the only cultural tradition detected at the site. Three possible explanations may account for this unexpected evidence. The first one is that the Aurignacian was exclusively produced by AMH and that the child mandible from unit A2 represents evidence for consumption or, more likely, symbolic use of a Neandertal child by Aurignacian AMH. The second possible explanation is that Aurignacian technologies were produced at Les Rois by human groups bearing both AMH and Neandertal features. Human remains from Les Rois would be in this case the first evidence of a biological contact between the two human groups. The third possibility is that all human remains from Les Rois represent an AMH population with conserved plesiomorphic characters suggesting a larger variation in modern humans from the Upper Palaeolithic (Ramirez Rozzi et al. 2009:153).

    So what is this "child mandible from unit A2"? Here's a picture showing pretty much every view:

    Les Rois mandible B

    Figure 3B from Ramirez Rozzi et al. 2009, picturing the Les Rois mandible B

    As you can see from the picture, the mandible is far from complete. It has its adult premolars it lacks any posterior teeth and the base of the mandibular corpus. If I were looking for a diagnosis, I would not necessarily expect to find one. In that respect, the mandible is similar to the Kent's Cavern maxilla. As in that case, for Les Rois B I don't think you can do much to substantiate either Neandertal or non-Neandertal affinity based on external morphology alone. The absolute dimensions of the teeth overlap with both Neandertals and modern humans, as do the root dimensions (as determined by scans).

    The text also includes this:

    The change in orientation of the mandibular surface at the canine level evokes a flat or slightly arched anterior mandibular surface, characteristic of Neandertals (Schwartz & Tattersall, 2000) (Ramirez Rozzi et al. 2009:161).

    This is correct but not strongly probative; the morphology is hard to judge and overlaps between these groups of humans.

    Another mandible from the side, mandible A, the authors diagnose as a modern human with no specific Neandertal-like characteristics. Confusingly, mandible A comes from unit B, which overlies the unit A2 where the mandible B was found. And the site has a number of isolated teeth from both these units, some of which figure into the story.

    The radiocarbon dates for the units A1 through B are clustered in a range around 30,000 radiocarbon years. That makes them far from the earliest Aurignacian, and they postdate substantially any Neandertal remains in France -- really, they overlap only with the latest Mousterian sites in southern Spain.

    Perikymata

    So given the scant morphological evidence, why does the paper conclude so strongly in favor of some Neandertal affinity for the specimen?

    The answer has to do with enamel formation. The authors examined the perikymata counts and packing patterns on the Les Rois teeth -- an observation that was simply unavailable to earlier scientists who examined and reported on the remains. Whereas the eyeball-level morphological features of the specimens are relatively undiagnostic, the perikymata patterns appear to be more interesting. The slightly later specimens including mandible A, in unit B of the site, all look like other Upper Paleolithic non-Neandertal specimens. But the teeth in mandible B have enamel development profiles like Neandertals and unlike "anatomically modern" specimens from the Upper Paleolithic of Europe.

    Mandible B is not alone in having a Neandertal-like developmental profile. From the same unit of the site, A2, there is one other canine tooth and three incisors, representing at least two individuals, all of which also have low perikymata counts. The paper represents these teeth as falling within the Neandertal distribution and outside the range represented by modern humans.

    Unlike mandible B, the teeth present in mandible A all have high perikymata counts, there are no nonmetric characters present that would suggest Neanderthal affinity. If you found this at a much later site, you wouldn't notice anything unusual about it. Does it matter that the "modern" looking specimens are the later ones, and the "Neandertal" looking specimens are earlier? Not clear -- there are really too few remains to make this into a significant story, particularly in the context where the two units do not differ significantly in radiocarbon ages.

    Should we believe that the dental remains from unit A2 are Neandertal? The dental development information is directly relevant to the variability of early Upper Paleolithic Europeans -- the Les Rois specimens here extend that variability into significant overlap with Neandertal dental development schedules. I think that's quite important -- there's no clean break denoting the demise of the Neandertals. That observation reflects other early Upper Paleolithic European samples, many of which also present Neandertal-like morphological characters.

    But it's unclear to what extent enamel formation profiles, reflected by perikymata counts, accurately inform about phylogeny. Modern humans are quite variable in these perikymata counts and packing patterns. When it comes to total counts of perikymata, Neandertals cannot be distinguished from the variability among recent human populations (Guatelli-Steinberg et al. 2005; 2007). In this study, molars are not an issue, because they are not preserved for the relevant teeth. Perikymata packing patterns do separate known Neandertal specimens from samples of recent humans (Guatelli-Steinberg et al. 2007), and in that respect the Les Rois A2 teeth are similar to Neandertals.

    How important are these observations of dental development? That's a broader question than I am prepared to answer here, except to note my earlier posts on dental development in Neandertals ("Neandertal teeth: the other shoe", "How modern is 'modern tooth development'?"). I can also point to a current review of the issue by Debbie Guatelli-Steinberg (2009), which introduces the recent literature. The short answer is that nobody really knows.

    Cutmarks

    The mandible B is also cutmarked:

    Cutmarks on mandible B consist of three parallel striations located on the lingual aspect, below the right lateral canine and P3 (Fig. 5 and Fig. 4 Suppl. Mat.). Two of them bear diagnostic features of flint cutting-edge generated marks in form of v-shaped cross sections, “barbs” and, in one case, a typical splitting (Fisher 1995) (Ramirez Rozzi et al. 2009:170).

    The authors point out that many of the faunal remains are also cutmarked, including mandibles apparently smashed open. I suppose this may be construed as evidence for cannibalism -- at the extreme, that the fearsome modern humans were hunting down the last Neandertals. And there's no particular reason to think that this isn't cannibalism at Les Rois, but given the scarcity of the sample, it's not nearly so strong as the evidence at some other sites.

    The authors suggest that this may fit in with a pattern evident at other Upper Paleolithic sites, in which human remains were deliberately altered or processed for symbolic purposes. There is a perforated human tooth at the site, evidently created for use as a pendant. Some kind of mortuary practice is probably just as consistent with the scanty information we have as cannibalism.

    Vindija

    Regardless of whether the Les Rois hominids are Early Aurignacian or somewhat later in date, they appear to represent a population that includes substantial variability not present in later Europeans, but overlapping with earlier Neandertals. That observation of variability is consistent with a mixture of populations, possibly representing a declining fraction of Neandertal-derived genes over time.

    So I would guess that Les Rois represents part of a larger range of variation. Further, we should keep in mind that the morphology of Neandertals in late Mousterian or Châtelperronian contexts also has variability that overlaps with contemporary human populations elsewhere.

    On that topic, the Les Rois dental remains should make us return to the other Aurignacian-associated Neandertals: the Vindija G1 Neandertals. These remains are also fragmentary, but much more substantial and numerous than those from Les Rois. The anatomy of the specimens is covered in the review paper by Karavanic and Smith (1998), which itself reiterates earlier observations made by Fred Smith, Milford Wolpoff and others. This is a younger assemblage than the G3 layer where the Vindija genetic samples were taken, and represents the final Neandertals at the site.

    The dating of the G1 layer has fluctuated back and forth, as I discussed in 2006. The most recent date puts the G1 Vi 207 and Vi 208 specimens at approximately 33,000 radiocarbon years -- possibly overlapping in date with Les Rois specimens, but probably a couple of thousand years older.

    In addition to the physical remains, Vindija G1 is notable for the presence of several bone artifacts, including a split-base bone point, type artifact of the Aurignacian. The layer also includes some leaf-shaped bifacial points. There has been considerable controversy about whether these Aurignacian-like elements of the assemblage are actually associated with the Neandertal remains, or whether some mixing of the layers occurred due to cryoturbation (freezing-induced sediment disturbance). The strongest argument in favor of disturbance is that individuals with Neandertal-like morphologies have never before been clearly associated with Aurignacian tools. It seems to me that the Les Rois remains pretty well demolish that argument.

    Vindija deserves a longer discussion than this short note. Karavanic and Smith (1998) argued that the G1 assemblage should not be called "Aurignacian" but that the Upper Paleolithic elements of it be recognized as novel parts of a regional cultural tradition with roots in the local Mousterian. That would accord with other "transitional" technocomplexes like the Châtelperronian and Bohunician, which appear to combine new Upper Paleolithic tool forms with lithic procurement and processing strategies common in earlier Mousterian assemblages.

    In that light, the concept of "Early Aurignacian" deserves close examination: it seems that almost everywhere in Europe we look, we find evidence of conceptual mixture. That's certainly true of the biological remains, and when we consider that these sites and assemblages cover thousands of years of time -- extending up to hundreds of human generations -- it seems hard to believe that we can't make some more sense out of them.

    Anyway, this post has gone on long enough. I have a lot more notes as background, and I'll see if I can't shape them up for posting over the next couple of weeks.

    References:

    Guatelli-Steinberg D, Reid DJ, Bishop TA, Larsen CS. 2005. Anterior tooth growth periods in Neandertals were comparable to those of modern humans. Proc Nat Acad Sci USA 102:14197-14202. doi:10.1073/pnas.0503108102

    Guatelli-Steinberg D. 2009. Recent studies of dental development in Neandertals: Implications for Neandertal life histories. Evol Anthropol 18:9-20. doi:10.1002/evan.20190

    Ramirez Rozzi FV, d'Errico F, Vanhaeren M, Grootes PM, Kerautret B, Dujardin V. 2009. Cutmarked human remains bearing Neandertal features and modern human remains associated with the Aurignacian at Les Rois. J Anthropol Sci 87:153-185.

    Tags: 
    development
    France
    Neandertals
    Upper Paleolithic
    Aurignacian
    Europe
    Les Rois
    teeth

  • Frankenteeth on the horizon

    Tue, 2009-01-06 09:25 -- John Hawks

    Joel Garreau (Radical Evolution) covers the future genetics beat for the Washington Post. In today's edition, he has an interesting article about tooth regeneration from stem cells:

    As long as there are hockey players, there will be niche markets for false teeth. But the real news about the future of dentures is that there isn't much of one. Toothlessness has declined 60 percent in the United States since 1960. Baby boomers will be the first generation in human history typically to go to their graves with most of their teeth.

    And now comes tooth regeneration: growing teeth in adults, on demand, to replace missing ones. Soon.

    Naturally, as a paleoanthropologist, this topic is near to my heart. The teeth are one of the better-understood developmental systems, at least in mice. Being a couple hundred million years old, the genetic program that builds the teeth is admirably modularized.

    The story traipses through the cultural meaning of tooth loss and its relation to senescence and health risks like smoking and diet. Best line:

    If you are one of those obedient doobies who listened to your dentist and had your wisdom teeth removed for no particularly urgent reason, you are hosed.

    Well, my twin daughters are losing their teeth now, but I feel no urge to save their baby teeth in liquid nitrogen like some of the subjects of the article. That's just crazy talk.

    Tags: 
    regenerative medicine
    stem cells
    teeth
    biotech

  • Quote: Darwin on wisdom teeth

    Sun, 2008-12-28 08:30 -- John Hawks

    Darwin, in The Descent of Man, volume 1, pp. 26-27:

    It appears as if the posterior molar or wisdom-teeth were tending to become rudimentary in the more civilised races of man. These teeth are rather smaller than the other molars, as is likewise the case with the corresponding teeth in the chimpanzee and orang; and they have only two separate fangs. They do not cut through the gums till about the seventeenth year, and I am assured by dentists that they are much more liable to decay, and are earlier lost, than the other teeth. It is also remarkable that they are much more liable to vary both in structure and in the period of their development than the other teeth. In the Melanian races, on the other hand, the wisdom-teeth are usually furnished with three separate fangs, and are generally sound: they also differ from the other molars in size less than in the Caucasian races. Prof. Schaaffhausen accounts for this difference between the races by "the posterior dental portion of the jaw being always shortened" in those that are civilised, and this shortening may, I presume, be safely attributed to civilised men habitually feeding on soft, cooked food, and thus using their jaws less. I am informed by Mr. Brace that it is becoming quite a common practice in the United States to remove some of the molar teeth of children, as the jaw does not grow large enough for the perfect development of the normal number.

    This "Mr. Brace" was Charles Loring Brace, great-grandfather of the anthropologist of the same name who became well-known for studying dental reduction.

    Tags: 
    Darwin
    teeth
    quotes

  • Dental growth in early Homo

    Tue, 2005-03-15 23:58 -- John Hawks

    Dean and colleagues (2001) present a study of perikymata counts of anterior teeth (incisors and canines) in early humans and australopithecines, compared to extant apes and humans. The basic microanatomy of teeth and their process of development is briefly described in this write-up about Afropithecus.

    The operating assumption that makes this study interesting is that "brain size, age at first reproduction, lifespan and other life-history traits correlate tightly with dental development" (628). Of course, this is true not only of dental development but all these other traits with each other and with others (most notably, body mass), since these kinds of "correlations" are really interspecies allometries of one kind or another. But the interesting thing is the apparent deviation of dental development rates in humans from other hominoid species. We take longer to grow our dental enamel, and this difference is likely to reflect a difference in the rate of growth or maturation:

    Modern human enamel develops along a slower trajectory because the earliest-formed enamel, closest to the enamel dentine junction, is secreted in smaller increments for a longer time period. None of the trajectories of enamel growth in apes, australopiths of fossils attributed to Homo habilis, Homo rudolfensis or Homo erectus falls within that of the sample from modern humans (628-629).

    Despite considerable variation in external tooth morphology, early fossil hominins share a common and fundamental enamel growth trajectory with the African ape clade that is derived in modern humans. (629).

    It is now generally held that a prolonged life-history schedule reflects a reduction in the mortality rate of adults, triggered perhaps by behavioural, dietary or other changes. An increase in brain size (and cognitive ability) is associated with this reduction, but does not necessarily drive it, even in the human lineage. The size of key brain components associated with learning and cognition correlates with the timing of dental development in primates as the cost in time needed to grow and learn to use a larger brain increases. In this context a slower trajectory of enamel growth in permanent teeth, one of the basic determinants of tooth formation time, can be regarded as a life-history attribute associated with the extended, or prolonged, growth period of modern humans. The first evidence for a shift in enamel growth rates in the hominin fossil record seems to be with the origin of the larger-brained Neanderthals (at least by 100 kyr ago; see Tabun specimen C1 in Fig. 1) and modern humans (629-630).

    Dean and colleagues also posit that the dental development data for specimens attributed to Homo habilis and/or Homo rudolfensis are consistent with an assignment to Australopithecus. Certainly the data do not contradict such an assignment, since there is no apparent difference between australopithecines, early humans, or any of the habilines in the study. Of course, the data also do not contradict the assignment of KNM-WT 15000 to Australopithecus -- they are just uninformative about taxonomic assignment. The key here is that the enamel formation times of the teeth show that early humans were essentially australopithecine-like in their dental development; and that both kinds of hominids were basically apelike.

    The authors promote an interesting linkage between dental development times and brain size expansions:

    If correct, these estimates of molar emergence times have shifted a little, in step with brain size, from those known for African great apes and australopiths. Nevertheless, it now seems increasingly likely that a period of development truly like that of modern humans arose after the appearance of H. erectus, when both brain size and body size were well within the ranges known for modern humans (630).

    This is not to say that dental development times determine brain sizes or vice versa, but that the processes that promote delayed maturation connect both of these features. I would suggest that this connection might involve a genetic correlation between dental maturation and brain size that altered both characters with selection on overall maturation rate. If a prolongation of juvenile growth was a product of selection acting on social learning, then both slower growth and larger brains may have emerged in concert with each other.

    The interesting part of this potential linkage is that growth may not have reached its modern rate 2 million years ago, but instead may have continued to change as brain sizes increased during the Middle and Late Pleistocene. That scenario is consistent with the dental development patterns in this study. In that perspective, this study can be chalked up in a list of ways early Homo was not very much like modern humans in its adaptive pattern.

    References:

    Dean C, Leakey MG, Reid D, Schrenk F, Schwartz GT, Stringer C, Walker A. 2001. Growth processes in teeth distinguish modern humans from Homo erectus and earlier hominins. Nature 414:628Ð631.
    Nature online

    Tags: 
    Miocene
    Homo erectus
    development
    Neandertals
    teeth
    Late Pleistocene
    Homo rudolfensis
    Homo habilis

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