Human evolution: on the neandertals

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The following points are made by Jean-Jacques Hublin (Current Biology 2006 16:R113):

1) A Neandertal (Homo neanderthalensis or Homo sapiens neanderthalensis) is a kind of human that lived in Europe and Western Asia until around 30,000 years ago. The first Neandertal skeleton was found by quarry workers in 1856 in the Neander Valley in Germany. At first people thought the odd-looking skeleton was that of a diseased person or of a Cossack from the Napoleonic War. It was only later, when similar remains were found in Belgium, that Neandertals became fully accepted as an extinct type of human. Since then, a debate about their relationship to modern humans has raged among paleontologists and geneticists, and inspired many writers of fiction.

2) Around 400 fossils of Neandertals or their direct ancestors are known, and several almost complete skeletons have been discovered, including immature individuals. They show that Neandertals looked very different from extant humans. In fact, morphometric comparisons show that Neandertals were as different from modern humans as are closely related species of great apes from each other, such as bonobos and common chimpanzees. They had large brains in elongated braincases, receding foreheads and supraorbital buttresses. Their faces were long and very projecting in the mid-part, with large noses and no chins. They had large husky bodies weighing around 80 kg. Their bones and muscles were extremely robust, maybe partly because of their behavior and level of activity. However, some of the typical features of Neandertals, for example the shape of the inner ear, were established even before birth. To sustain their muscular bodies they required lots of calories per day. Isotopic studies show that they did this by eating a lot of meat and fat.

3) Neandertals evolved mostly in Europe, where populations suffered several demographic crashes and some level of isolation due to dramatic climatic fluctuations during the last half million years. During this time, Neandertals accumulated more and more differences from humans living in Africa, where direct ancestors of modern humans evolved. Neandertal populations expanded into the Near East and Central Asia. In other periods, modern humans expanded into the southern Near East. Between 50-40,000 years ago, modern populations expanded out of Africa, eventually replacing other human forms, including Neandertals. Image

4) Did Neandertals and modern humans coexist in the same regions? The general global answer to this question has become much clearer thanks to paleoanthropology and molecular genetics. Paleoanthropological evidence indicates that Neandertals were mostly replaced, with very rare, if any, interbreeding. However, the most recent Neandertals developed behaviors and technology that resemble that of the African immigrants. For example, some of them started using body ornaments for the first time after the arrival of modern humans who had practiced this behavior for a long time. Such interactions suggest some period of coexistence, although the replacement of the Neandertals may have happened differently in different areas. However, so far, there is no direct evidence that the two groups lived in close contact.[1-5]

References (abridged):

1. K. Harvati, S.R. Frost and K.P. McNulty, Neanderthal taxonomy reconsidered: Implications of 3D primate models of intra- and interspecific differences, Proc. Natl. Acad. Sci. USA 101 (2004), pp. 1147-1152.

2. J.-J. Hublin, Climatic Changes, Paleogeography, and the Evolution of the Neandertals. In: T. Akazawa, K. Aoki and O. Bar-Yosef, Editors, Neandertals and Modern Humans in Western Asia, Plenum Press, New York (1998), pp. 295-310

3. M. Krings, A. Stone, R.W. Schmitz, H. Krainitzki, M. Stoneking and S. Pääbo, Neandertal DNA sequences and the origin of modern humans, Cell 90 (1997), pp. 19-30

4. P. Mellars, Neanderthals and the modern human colonization of Europe, Nature 432 (2004), pp. 461-465

5. D. Serre, A. Langaney, M. Chech, Nicola M. Teschler, M. Paunovic, P. Mennecier, M. Hofreiter, G. Possnert and S. Pääbo, No evidence of Neandertal mtDNA contribution to early modern humans, PLoS Biol. 2 (2004), pp. 313-317

Current Biology




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)





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
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