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70 Neanderthals and Modern Humans

geographical range away from the tropics, surmounting previous ecolog-
ical barriers, with increasing chances of success.
(2) The climatic variability of the Pleistocene produced repeating scenarios of
changing habitats, landscapes and barriers that enabled African hominids
to expand the geographical range, or shift it northwards, and colonise
Eurasia on multiple occasions as they tracked these changing environ-
ments. This climatic variability also created situations, especially marked
in northern areas such as Europe, of high extinction probability of regional
hominid populations. As geographical range expanded, the probability of
populations becoming isolated when climate introduced barriers, would
have increased. In the case of African and Eurasian populations there
would have been instances of genetic isolation of populations but the pre-
dominant theme would have been one of gene ¬‚ow particularly among
geographically proximate populations.
(3) In this chapter I have established four potential time periods of colonisation
from Africa: pre-1.7 Myr, that may have involved a number of colonisation
events “ 800“750 kyr (OIS 21), 600“550 kyr (OIS 15) and 250“200 kyr
(OIS 7). These need not be strict colonisations and may instead involve
the spread of cultural innovations. We have also established ¬ve further
periods during which gene ¬‚ow would have existed between Africa and
Eurasia. These periods could also have received African hominids or cul-
tural innovations but largely to south-west Asia (from where they could
have subsequently dispersed). These periods were: 700“650 kyr (OIS 17);
550“500 kyr (OIS 13); 450“250 kyr (OIS 11“9); 200“100 (OIS 7“5); and
50“0 kyr (OIS 2“1). In the latter case I predict such events only during
the warmer OIS 1 (10“0 kyr). Finally, four periods when non-tropical
Eurasian extinctions were most likely were identi¬ed: 750“700 kyr (OIS
20“18); 650“600 kyr (OIS 16); 500“450 kyr (OIS 12); and 100“50 kyr
(OIS 4).
4 The Modern Human“Neanderthal
problem

Current theories of Modern Human origins are divisible into two groups. There
are those that promote regional continuity and hybridisation and those that ad-
vocate a recent African origin to all Moderns (Klein, 1999). In the ¬rst category
is the strict Regional Continuity Model (Wolpoff, 1989) which proposes that
ancestral populations of an archaic hominid dispersed from Africa across the
Old World around 1.9 Myr and that the populations that settled in different
parts of the world independently evolved into Moderns. For this to have hap-
pened, without the different populations becoming distinct species, the model
predicts that there was regular gene ¬‚ow between populations. Subsidiaries of
the Regional Continuity Model have been advanced. Brauer (1992) proposed
that there was a degree of regional continuity between populations but that there
was a signi¬cant African genetic contribution to European and western Asian
populations through hybridisation and assimilation. Smith (1992) proposed a
similar model but reduced the importance of the African contribution with a
smaller number of genes being assimilated by European and western Asian
populations. The ˜intermediate™ models would seem to have some support from
the genetic evidence (Templeton, 2002).
The late Pleistocene Out-of-Africa Model (Cann et al., 1987; Stringer &
Andrews, 1988) is the parent of the rival group. It proposes that Moderns
evolved in Africa between 130 and 200 kyr ago, spread out of Africa and
replaced all other archaic human populations after 100 kyr ago. There would
therefore be no genetic contribution from any archaic group (e.g. the Nean-
derthals) to the Modern Human gene pool. A variant is the Weak Garden of
Eden Model (Harpending et al., 1993; Sherry et al., 1994; Ambrose, 1998).
It differs from the ˜classic™ Out-of-Africa in that it proposes that there is no
population increase after an initial expansion from Africa around 100 kyr and
a major demographic expansion between 70 and 40 kyr. The populations that
established themselves in different regions of the Old World were small and
widely dispersed and suffered genetic bottlenecks (Haigh & Maynard Smith,
1972; Jones & Rouhani, 1986; Harpending et al., 1993; Sherry et al., 1994;
Rogers & Jorde, 1995). There was a subsequent population expansion of these
genetically isolated populations between 70 and 50 kyr, which was related
to new technologies (Upper Palaeolithic/Late Stone Age) that increased the


71
72 Neanderthals and Modern Humans

environmental carrying capacity for human populations. The Multiple Disper-
sals, Bottlenecks and Replacement Model attempts to provide a mechanism for
ex-African expansions. This model sees the environment as the driving force
(Lahr & Foley, 1994, 1998; Foley & Lahr, 1997) and the Middle to Upper
Palaeolithic technological transition as a key factor. According to this model
the ancestral African population was reduced in size, experiencing a genetic
bottleneck, due to climate-driven habitat fragmentation. A series of population
increases with dispersal followed by further bottlenecks characterised human
expansion. According to Lahr & Foley (1998) the ancestor of the Neanderthals,
that they name Homo helmei, would have dispersed within and out of Africa
during Oxygen Isotope Stage (OIS) 7 or 8. The ¬rst dispersal, into the Middle
East, occurred during the mild OIS 5. Subsequent cooling caused a population
retreat, these Modern Humans being presumed not to be behaviourally or phys-
iologically adapted (or at least not as well as the contemporary Neanderthals)
to the cold of Eurasia. A second dispersal, in OIS 4 or early OIS 3, enabled
the dispersal of a population into Asia and a ¬nal one, around 45 kyr, coincid-
ing with the Middle“Upper Palaeolithic transition, into the Middle East was
rapidly followed by the colonisation of Europe. This model recognises that, as
Moderns spread they replaced archaic populations including the Neanderthals.
Lahr & Foley (1998) attempt to provide a mechanism that is based on exist-
ing theoretical frameworks of evolutionary ecology and biogeography. They
also recognise that evolutionary events, such as Modern Human origins, have
a strong geographical component and highlight the vital link between demog-
raphy and spatial distribution.
The alternatives available to us until now have therefore required that the fate
of archaic groups is determined either by ˜eviction™ or ˜continuity/replacement™
(Lahr & Foley, 1998; Tattersall & Schwartz, 2000). An alternative hypothesis
has recently been proposed that does not require the intervention of Moderns
in the extinction of the archaics which is seen as part of a natural and recurring
process of habitat fragmentation during glacial cycles that severely affected
non-tropical hominid populations (Finlayson, 1999; Finlayson et al., 2000a).
This model, which is developed in this book, differs from the traditional and,
until now apparently mutually exclusive, alternatives of replacement (usually
by competition) or continuity that do not consider non-human related extinction
of archaic populations, including the Neanderthals, to be important. Patterns
of hominid evolution and the key elements of Modern Human behaviour can
be explained within the framework of the general principles of evolutionary
ecology (Foley, 1992). This alternative model is precisely based on theoretical
evolutionary ecology and geography (Hutchinson, 1959; MacArthur & Wilson,
1967; MacArthur, 1984; Brown, 1995).
Modern Human“Neanderthal problem 73

The use of culture as an all embracing and all pervading explanation to the
evolution of Moderns has obscured the processes by which Moderns evolved
(Foley, 1989). Throughout this book I view humans as components of ecological
communities with the driving force behind change being natural selection acting
to ˜keep up™ with the spatio-temporal heterogeneities of Pleistocene Earth. As
these heterogeneities became more marked so populations that were adapted
to cope with change fared best. Behavioural attributes that permitted rapid
adjustments to change were selected. Humans increasingly became re¬ned risk
managers.


The species problem

Before discussing the biology of Neanderthals and Modern Humans we should
establish who they were and what their relationship to each other was. The de-
bate concerning Modern Human origins often seems to revolve around whether
or not followers of a particular camp regard the two to be distinct species or
not. The point about the de¬nition of Neanderthals and Moderns, or indeed any
other human, is that it is a taxonomic concept. The discussion about human
origins must be an evolutionary one and, whether or not we are advocates of
cladistics, taxonomy should only be seen as a convenient tool in packaging and
not as a proxy for evolutionary thinking. In evolutionary terms it does not matter
what we call Neanderthals or Moderns. The point is that the genetic evidence,
which is the only reliable tool that we have today, indicates that Neanderthals
and Moderns had a common ancestry that can be approximately dated at around
500“400 kyr and that the two lineages apparently went along separate paths,
one in Eurasia and the other in Africa. Physical barriers, aided by climate, ap-
parently kept the two lineages apart until they re-met in Eurasia some time after
100 kyr ago (depending on location). We presume they did not have genetic
contact in the interim but it is only a presumption. As we saw in the previous
chapter it is a presumption that is unlikely to have held across the entire geo-
graphical range throughout the period 500“40 kyr. What happened when the
two lineages met? We cannot be certain because the evidence is so meagre.
One thing seems clear from the genetic evidence “ no Neanderthal genes sur-
vived (Krings et al., 1997, 1999, 2000; Ovchinnikov et al., 2000). We should
not be surprised if at some point in the future contrasting evidence is found.
Why could Neanderthals and Moderns not interbreed and leave mixed traits?
There is no biological reason whatsoever but clearly, on present evidence, the
Neanderthal genetic contribution is nil and may have at best been very small.
There is no reason either to suspect a uniform pattern across space. Put in simple
74 Neanderthals and Modern Humans

terms “ what happened in France need not have been the same as what happened
in Java.
The origins of humans (members of the genus Homo) date approximately to
the Plio-Pleistocene boundary around 2 Myr (Wood & Collard, 1999; Hawks
et al., 2000). Whether we choose to consider H. erectus and H. sapiens to be
a single, evolving, species (Hawks et al., 2000; Wolpoff & Caspari, 2000) or
separate species (Stringer, 2002a; Tattersall, 2002) does not alter the nature of
the discussion of this book that is concerned with the evolutionary ecology of
populations and not with taxonomic de¬nitions. For the purpose of this book it
is enough to recognise a speciation event, probably subsequently unparalleled,
somewhere near the Plio-Pleistocene boundary that led to the ¬rst member of
the genus Homo (Mayr, 1950; Wolpoff & Caspari, 2000). Thereafter, we lack the
resolution to allow precision in the identi¬cation of lineages as the Pleistocene
picture is likely to have been so complicated spatially and temporally. In this
context we should note that, in North American songbirds at least, the paradigm
that many species originated as a consequence of the late Pleistocene glaciations
has been shown to be ¬‚awed. Instead the glaciations were an ecological obstacle
through which only some species were able to persist (Klicka & Zink, 1997).
There is no doubt that, among humans, there would have been many cases
of geographical separation leading to the emergence of distinct populations.
Where isolation was suf¬ciently long the trajectories, as in the case of the
Neanderthals, would have led to distinct morphological and related features.
This, on its own, does not make the Neanderthals a distinct species as some
authors seem to suggest (Lieberman et al., 2002). Whether such differences
were of a kind that precluded interbreeding when populations met once more,
thus con¬rming the presence of distinct biological species (Cain, 1971), is
something that we cannot answer today. In any case we have to be aware
that reproductive isolation even between good species may, in some cases, be
imperfect (Schluter & Nagel, 1995). Morphological distinctness, the basis for
allocating fossils into species, is only a general, and not infallible, guide in the
delimitation of species (Simpson, 1951; Cain, 1971). The weakness of relying
on morphology is especially evident if we consider the phenotypic plasticity of
most organisms (Geist, 1998). Genetic differences are, equally, subject to our
own protocols and de¬nitions. The splitting of the Homo phylogeny is therefore
subjective and not directly relevant to the question of Modern Human origins
and the extinction of archaic populations.
So there would have been multiple branches in the evolution of Homo in
the Pleistocene, especially as the geographical range expanded and the chances
of isolation became greater. There must also have been continuity in at least
one population, that which led to the Moderns. We may therefore best regard
modern H. sapiens to be the terminator, for now, of an ancestral-descendant
Modern Human“Neanderthal problem 75

sequence of interbreeding populations that evolved independently of others “
the gens described by Simpson (1951). A number of authors have attempted to
link the emergence of Moderns to a speciation event (Crow, 2002). The evidence
in favour is inconclusive. A study of a highly variable sub-terminal non-coding
region from human chromosome 16p13.3 did not reveal a signal for population
growth in Africa that would be expected if such a speciation event had taken
place (Alonso & Armour, 2001).
Evidence of widespread hybridisation between Neanderthals and Moderns
would certainly be suggestive but so far we only have the claim from Lagar
Velho in Portugal (Duarte et al., 1999; Zilhao & Trinkaus, 2002), based on
morphology, and that is it. This recent discovery of a skeleton in Portugal,
claimed to be a Modern“Neanderthal hybrid and dated at 25 kyr (Duarte
et al., 1999), is in apparent con¬‚ict with the genetic evidence. The skeleton
was found buried in a distinctively Upper Palaeolithic pattern, implying be-
havioural modernity, but its anatomy was claimed to be a mosaic of Neanderthal
and early Modern Human features. The claim has been vigorously contested by
some who feel that the skeleton lacks distinctive Neanderthal features (Tattersall
& Schwartz, 1999). Duarte et al. (1999) and Zilhao & Trinkaus (2002) claim
the skeleton as evidence in support of interbreeding between early Moderns and
Neanderthals. The authors recognised the inappropriateness of applying a strict
biological species distinction, based on reproductive isolation, to Neanderthals
and early Moderns. They also rejected hypotheses of full replacement of late
archaic humans by early Moderns everywhere outside Africa and instead saw
the need for an approach that brought together regional complexities, temporal,
human biological and cultural processes as well as the historical trajectories
that took place. If this child was a hybrid, then the claim for widespread hy-
bridisation between Neanderthals and Moderns rests on the dating evidence
that suggests that the hybrid was in existence up to 5 kyr after the extinction
of the last Neanderthal in Portugal. We have to accept that transposing what
happened in a single valley to the whole world is risky. In the Middle East,
Neanderthals and Moderns supposedly occupied the same geographical area
for longer than anywhere else (Arensburg & Belfer-Cohen, 1998). It is an area
that has produced fossils of Neanderthals and Moderns but so far no hybrids. So
we cannot, presently, use the biological species concept to determine whether
we are dealing with one or two species. There is something that is even more
worrying and for that I must now turn to the question of convergent and parallel
evolution.
I have already said that the only available solid evidence that we can draw
upon is the genetic evidence. The reason is that I seriously question the valid-
ity of arguments based solely on morphological comparisons. The problem is
exacerbated by the small sample sizes available, which oblige researchers to
76 Neanderthals and Modern Humans

combine specimens from distant parts of the geographical range and from differ-
ent time periods, often making statistically unsatisfactory inferences. Through-
out the animal kingdom we ¬nd numerous cases of unrelated species converging
biologically in response to similar ecological problems (Cody, 1974, 1975). The
point is that the probability of convergence in response to similar pressures has
to, logically, be even greater among closely related forms because they are start-
ing off from templates that are quite similar to each other. This means that we
need genetic evidence to support any evolutionary conclusions that we draw
from morphology because genes tell us a history that is independent. So can
we differentiate, especially when we only have single or even small groups
of specimens, between lineages and convergence on morphology alone? The
answer is that we cannot.
At any point during the late Pleistocene, Neanderthals, Moderns and other
contemporary human populations are best regarded as a sapiens polytypic
species (Cain, 1971; Aguirre, 1994; Smith et al., 1995). A time slice at a point
in the late Pleistocene would reveal a range of human populations spread across
parts of Africa, Eurasia and Oceania. Some would have been genetically linked
to each other, behaving as sub-species, while the more extreme populations may
well have behaved as good species with minimal or no inter-breeding. The two
extremes were probably in operation at different times and in different parts of
the world. The human array at any point should best be regarded as a polytypic
species of common descent and varying degrees of subsequent isolation. This
view is in keeping with the increasing evidence that demonstrates that species
across their range are often divided into patchworks of parapatric sub-species
and races with intervening hybrid zones (Hewitt, 1989).
When a species is separated by a geographic barrier and the terminal forms
gradually diverge and eventually behave as two distinct species when they meet
on the other side we have an example of a polytypic species that is known as a

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