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ANTHROPOLOGY: NEANDERTHALS AND THE COLONIZATION OF EUROPE

The following points are made by Paul Mellars (Nature 2004 432:461):

1) The most significant contributions over the past decade to the study of the fate of the Neanderthals have come from detailed studies of the DNA structure of present-day human populations in different areas of the world, combined with the gradually accumulating recovery of residual traces of "ancient" DNA extracted from a number of Neanderthal and early anatomically modern human remains. Studies of both mitochondrial and Y-chromosome DNA patterns in modern world populations (inherited respectively through the female and male lineages) point to the genetic origins of all present-day populations within one limited area of Africa somewhere in the region of 150,000 years before present (yr BP), followed by their dispersal to other regions of the world between about 60,000 and 40,000 yr BP(1-5).

2) These results are further reinforced by recent discoveries of skeletal remains of anatomically modern populations in different areas. Discoveries at Herto in Ethiopia reported in 2003 confirm the presence of early forms of anatomically modern humans in Africa by about 160,000 yr BP, whereas the earliest discoveries of distinctively modern populations in both Europe and most parts of Asia can be dated no earlier than 40,000-45,000 yr BP. The one exception is in Israel, where the rich skeletal remains from the Skhul and Qafzeh caves indicate a precocious, and apparently short-lived, incursion of early anatomically modern populations into this region (presumably via the Nile valley) at an early stage in the last glaciation, around 100,000 yr BP.

3) In Europe, the most dramatic support for these patterns has come from the recovery of a number of relatively well-preserved sequences of mitochondrial DNA from a number of actual skeletal finds of Neanderthals and early anatomically modern humans. Analyses of seven separate Neanderthal specimens (including those from the Neanderthal type-site itself) yielded segments of mitochondrial DNA that are radically different from those of all known present-day populations in either Europe or other parts of the world, and that are equally different from those recovered from five early specimens of anatomically modern populations from European sites. The conclusion is clear that there was either very little -- if any -- interbreeding between the local Neanderthals and the intrusive modern populations in Europe, or that if such interbreeding did take place, all genetic traces of this interbreeding were subsequently eliminated from the European gene pool.

4) The mitochondrial DNA evidence recovered from the Neanderthal specimens further suggests that the initial evolutionary separation of the Neanderthals from the populations which eventually gave rise to genetically modern populations must reach back at least 300,000 yr -- a finding that is in good agreement with the surviving fossil evidence from Africa and Europe1. Whether this evidence is sufficient to indicate that the Neanderthals belonged to an entirely separate biological species from modern humans is at present more controversial(1,2).

5) The fate of the Neanderthal populations of Europe and western Asia has gripped the popular and scientific imaginations for the past century. Following at least 200,000 years of successful adaptation to the glacial climates of northwestern Eurasia, they disappeared abruptly between 30,000 and 40,000 years ago, to be replaced by populations all but identical to modern humans. Recent research suggests that the roots of this dramatic population replacement can be traced far back to events on another continent, with the appearance of distinctively modern human remains and artefacts in eastern and southern Africa.

6) That the Neanderthals were replaced by populations that had evolved biologically, and no doubt behaviorally, in the very different environments of southern Africa makes the rapid demise of the Neanderthals even more remarkable, and forces us to ask what cultural or cognitive developments may have made this replacement possible. The rapidly accumulating archaeological evidence for highly symbolic patterns of culture and technology within African populations dating back to at least 70,000 yr BP (marked by the appearance of complex bone technology, multiple-component missile heads, perforated sea-shell ornaments, complex abstract "artistic" designs and abundant use of red ochre --recently recorded from the Blombos Cave and other sites in southern Africa) may provide the critical clue to new patterns of cognition, and probably complex linguistic communication, linked directly with the biological evolution of anatomically and genetically modern populations(1,3). Perhaps it was the emergence of more complex language and other forms of symbolic communication that gave the crucial adaptive advantage to fully modern populations and led to their subsequent dispersal across Asia and Europe and the demise of the European Neanderthals. The precise mechanisms and timing of this dramatic population dispersal from southern Africa to the rest of the world remains to be investigated(1,3,4).

References (abridged):

1. Stringer, C. Modern human origins: progress and prospects. Phil. Trans. R. Soc. Lond. B 357, 563-579 (2002)

2. Tattersall, I. in The Speciation of Modern Homo sapiens (ed. Crow, T. J.) 49-59 (British Academy, London, 2002)

3. Forster, P. Ice ages and the mitochondrial DNA chronology of human dispersals: a review. Phil. Trans. R. Soc. Lond. B 359, 255-264 (2004)

4. Lahr, M. M. & Foley, R. Towards a theory of modern human origins: geography, demography and diversity in modern human evolution. Yb. Physical Anthropol. 41, 127-176 (1998)

5. Richards, M. et al. Tracing European founder lineages in the near Eastern mitochondrial gene pool. Am. J. Hum. Genet. 67, 1251-1276 (2000)

Nature http://www.nature.com/nature

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ANTHROPOLOGY: ON THE NEANDERTHALS

The following points are made by Pat Shipman (American Scientist 2004 92:506):

1) Neandertals (Neanderthals) were probably not members of our own species, judging from recent analyses of mitochondrial DNA. Nonetheless, Neandertals were clearly built on a human-like plan (or vice versa) with some crucial modifications. A glance at the fossil remains of these hominids shows that Neandertal bones are much more robust than those of modern Homo sapiens. The skulls of the two species also show several striking differences. One of the most noticeable Neandertal features is the unmistakably large, bony browridges that stick out over the eyes. Below the orbits, the face is more prognathic -- the nose and jaw protrude farther in front of the braincase -- than a human face. The prominent nasal bones in Neandertal skulls top wide nasal openings, suggesting that they sported large, aquiline noses. Unlike the smoother, rounded contour of the human skull, the back of the Neandertal skull has a distinctive bulge, often referred to as a chignon or bun. Overall, the Neandertal skull resembles what you might expect if someone took a human skull made of rubber, grabbed it by the face and back of the head, and pulled.

2) These comparisons have attracted the attention of researchers who study the interactions between evolution and development from birth to adulthood -- so-called "evo-devo." Put simply, they wanted to know: How do you grow up Neandertal? In the spring of 2004, several studies offered answers to this question. F. Ramirez Rozzi and J.M. Bermudez de Castro (1) compared the rates of dental growth in several species within the genus Homo, including Neandertals. They examined the perikymata -- small enamel ridges on the tooth surface -- of incisor and canine teeth from 55 Neandertals, 25 Homo antecessor and Homo heidelbergensis individuals (two species that some anthropologists group together) and 39 ancient but anatomically modern humans.

3) Perikymata are created as a tooth grows. In humans and their close kin (such as Homo erectus), one ridge is created approximately every nine days during tooth development. The ridges of more distant relatives, including chimpanzees and gorillas, are formed at shorter intervals. By counting the number of perikymata, investigators can calculate how long the tooth took to form. Ramirez Rozzi and Bermudez de Castro (1) found that Neandertals formed their teeth in fewer days than did H. antecessor and H. heidelbergensis. If Neandertals had been the most ancient of the lot, one might expect them to be the most ape-like. But although the other fossil species are older still, they already show the human pattern. The finding is also a surprise because some researchers still propose that Neandertals are basically just strange-looking humans -- a judgment challenged by this fundamental difference.

4) Dental maturity is a common proxy for overall maturity because neurological, skeletal and sexual milestones are correlated with the pace of tooth mineralization. Ramirez Rozzi and Bermudez de Castro (1) concluded that faster dental development meant that Neandertals reached adulthood 15 percent sooner than humans, on average. To state this finding in practical terms, if humans attain physical maturity at 18 years, Neandertals were similarly grown at 15 years. The study also examined the spacing of perikymata across the front surfaces of incisors and canines. Dental enamel forms first at the tip of the crown -- the first point to emerge from the gum -- and then proceeds toward the roots.

5) In modern humans, the perikymata are widely spaced in the half of the tooth that formed first, indicating that lots of enamel was deposited during each nine-day increment. On the second half of each human tooth, the ridges are more closely spaced, showing a slower daily rate of enamel formation. Like human teeth, Neandertal teeth look as if they grew rapidly at first and then slowed down. However, on the part of each Neandertal tooth that grew later, the perikymata are more spread out than in their human counterparts. In other words, although the rate of enamel formation also decreased with age in Neandertals, the slowdown was less pronounced. This pattern of dental growth resembles that of apes. We know that the apes of today reach physical maturity much faster than humans. So, presumably, did Neandertals.(2-4)

References (abridged):

1. Krovitz, G. 2003. Shape and growth differences between Neandertals and modern humans: Grounds for species-level distinction? In Patterns of Growth and Development in the Genus Homo, ed. J. L. Thompson, G. E. Krovitz and A. J. Nelson. Cambridge, UK: Cambridge University Press

2. Ramirez Rozzi, F., and J. M. Bermudez de Castro. 2004. Surprisingly rapid growth in Neanderthals. Nature 428:936-939

3. Trinkaus, E. 1995. Neandertal mortality patterns. Journal of Archaeological Science 22:121-142

4. Williams, F. L., L. R. Godfrey and M. R. Sutherland. 2003. Diagnosing heterochronic perturbations in the craniofacial evolution of Homo (Neandertals and modern humans) and Pan (P. troglodytes and P. paniscus). In Patterns of Growth and Development in the Genus Homo, ed. J. L. Thompson, G. E. Krovitz and A. J. Nelson. Cambridge, UK: Cambridge University Press

American Scientist http://www.americanscientist.org

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ANTHROPOLOGY: ON NEANDERTHAL MITOCHONDRIAL DNA

The following points are made by Alan Cooper et al (Current Biology 2004 14:R431):

1) The genetic affinities of the earliest modern humans of Europe and the earlier hominid occupants of the area, the Neandertals, has remained a hotly debated topic since the discovery of the extraordinarily robust skull cap and limb bones in the Neander Valley in 1856. While it is impossible to rule out a surreptitious coupling of the two groups in the more than 10,000 years they apparently co-occupied Europe, recent research and population genetic theory suggest that any genetic interchange was limited.

2) This issue is central to the two main theories of modern human origins: the replacement model, where modern humans rapidly replaced archaic forms, such as Neandertals, as they began to spread from Africa through Eurasia and the rest of the world sometime around 100,000 years ago [1]; and the multi-regional model, where genetic exchange or even continuity exists between archaic and modern humans [2,3]. Two years ago, a review [4] reported that characteristic mitochondrial DNA (mtDNA) sequences retrieved from remains of four Neandertals are absent from modern human populations. It remained possible, however, that these sequences had been present in early modern humans, but had been lost through genetic drift or the continuous influx of modern human DNA in the intervening 28,000 years since Neandertals became extinct.

3) The difficulty in testing these ideas using ancient DNA is that most ancient human remains are contaminated with modern human DNA, which deeply penetrates bone and teeth samples during the washing and routine handling that takes place after excavation. This modern DNA will either out-compete authentic ancient sequences in PCR reactions, or recombine with them to produce artificial, but authentic looking genetic sequences [5]. Consequently, even when strict criteria for authenticating ancient DNA results are followed, it can be impossible to determine the authenticity of results.

4) The approach taken recently by Serre et al [2004] avoided this problem by searching only for the presence of Neandertal mtDNA sequences in both early modern human and Neandertal fossils, while ignoring modern human sequences because they are potentially contaminants. Four additional Neandertal specimens tested positive, but Neandertal sequences could not be detected in five early modern human fossils with biochemical preservation consistent with DNA survival from the Czech Republic and France. This appears to confirm that sequences characteristic to Neandertal remains were not widespread in early modern humans.

5) In summary: Mitochondrial DNA sequences recovered from eight Neandertal specimens cannot be detected in either early fossil Europeans or in modern populations. This indicates that if Neandertals made any genetic contribution at all to modern humans, it must have been limited, though the extent of the contribution cannot be resolved at present.

References (abridged):

1. Stringer, C.B. and Andrews, P. (1998). Genetic and fossil evidence for the origin of modern humans. Science 239, 1263-1268

2. Hawks, J.D. and Wolpoff, M.H. (2001). The accretion model of Neandertal evolution. Evol. Int. J. Org. Evol. 55, 1474-1485

3. Templeton, A. (2002). Out of Africa again and again. Nature 416, 45-51

4. Schmitz, R.W., Serre, D., Bonani, G., Feine, S., Hillgruber, F., Krainitzki, H., Poobo, S., and Smith, F.H. (2002). The Neandertal type site revisited: interdisciplinary investigations of skeletal remains from the Neander Valley, Germany. Proc. Natl. Acad. Sci. USA 99, 13342-13347

5. Poobo, S., Higuchi, R.G., and Wilson, A.C. (1989). Ancient DNA and the polymerase chain reaction. J. Biol. Chem. 264, 9709-9712

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