[Paleopsych] SW: Neanderthals and the Colonization of Europe
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Anthropology: Neanderthals and the Colonization of Europe
http://scienceweek.com/2004/sc041231-1.htm
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
--------------------------------
Related Material:
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
--------------------------------
Related Material:
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
Current Biology http://www.current-biology.com
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