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1

Observed

0 Cubic Model
-5 -4 -3 -2 -1 0

Time (Myr)

Figure 1.1. (a) Number of hominid species during the last 5 Myr using a conservative
number of species. A cubic model best ¬ts the observed pattern: y = 0.2328 ’
2.5022x ’ 0.9973x2 ’ 0.1059x3 ; R2 = 0.293; P = 0.002. (b) Decline in hominid
species in the last 2 Myr. A cubic model best ¬ts the pattern: y = 0.8187 ’ 2.5122x ’
5.6201x2 ’ 3.1246x3 ; R2 = 0.923; P < 0.0001. (c) Relationship between number of
hominid species and climate variability (coef¬cient of variation of temperature) in the
last 2 Myr. The pattern is best described by a quadratic model: y = 10.9797 ’ 1.9269x
+ 0.914x2 ; R2 = 0.366; P = 0.033.
Human evolution in the Pleistocene 3

5.0
(b)


4.0



3.0
Number of Species




2.0



1.0

Observed

0.0 Cubic
-2.0 -1.5 -1.0 -.5 0.0

Time (Myr)

5.0
(c)


4.0



3.0
Number of Species




2.0



1.0

Observed

0.0 Quadratic
6 7 8 9 10 11 12 13

Climate Variability

Figure 1.1. (cont.)
4 Neanderthals and Modern Humans

become isolated from others. I have introduced scale here and it is an issue that
is central to understanding ecology (Levin, 1992) and will appear frequently in
this book. In this case we can see how small-scale population isolation events
would be expected to be frequent relative to regional events involving many
populations.
Populations most distant from each other would be expected to be genetically
most distinct but linked to each other by intermediate forms. Where isolation
of extreme populations was long, populations at the extremes of the range may
have diverged to the extent that they subsequently behaved as good species.
In the case of Pleistocene Homo, geographical comparisons have to be made
among contemporary forms. As we are studying phenomena through time, it
is also important that geographical patterns from different time periods are
not merged. It is common, for example, to ¬nd generalised distribution maps
of Neanderthal geographical range in the literature (e.g. Stringer & Gamble,
1993). These should only be regarded as maps of the extremes of the range
reached according to currently available evidence. In reality the Neanderthal
range, as that of other forms of Homo and indeed all other animals, shifted,
expanded and contracted through time and it is these range changes that are
likely to be most informative about Neanderthal behaviour, as we shall see in
Chapter 3 (Fig. 1.2). If we follow this approach, bearing in mind the limitations
of the available data, we observe a changing pattern of global distribution of
Homo in the Pleistocene.
There are two apparently contrasting models that, as we shall see in this
book, are in effect extremes of a continuum. Much of the debate that has raged
in the last two decades in this respect has been due to differences in the under-
standing of the evolutionary process and confusion with taxonomic techniques,
particularly cladistics. I will start with a brief statement of the two contrasting
models.
On the one hand, we have the multiregional model that has been championed
by Wolpoff and his school (Wolpoff, 1989). According to this model H. erectus“
H. sapiens is a single species (hence H. sapiens). The variations that are observed
among fossils simply re¬‚ect natural variation as the species has evolved through
time. As populations became isolated, so geographical variations arose between
them just as they do in most widely distributed organisms. According to this
model and its variants, genetic barriers between the populations were never
severe enough to cause speciation. Thus present-day human populations re¬‚ect
a combination of regional variation that dates back to the earliest colonisations
and relatively continuous gene ¬‚ow among the populations. The intensity and
frequency of gene ¬‚ow would be greatest among neighbouring populations and
lowest among those geographically most distant.
Human evolution in the Pleistocene 5




Figure 1.2. Maximum limits (grey area) of the Neanderthal geographical range in
Europe and western and central Asia. Bioclimate boundaries as in Figure 5.3.



On the other hand, we have the ˜Out-of-Africa 2™ model that has been asso-
ciated most strongly with Stringer (Stringer & Andrews, 1988). According to
this model all natural variation that existed among populations of Homo was
removed very rapidly after 100 000 years (kyr) ago by the geographical expan-
sion of ˜Modern Humans™ that evolved somewhere in eastern or north-eastern
Africa. As these ˜Modern Humans™ spread out of Africa they replaced all exist-
ing populations of Homo across Africa and Eurasia. These ˜Archaic™ African and
Eurasian populations had evolved regionally after an earlier ˜Out-of-Africa 1™
expansion of H. ergaster around 1.9“1.8 Myr ago. The model, in its current
form, does not negate the possibility of interbreeding among ˜Modern™ and
˜Archaic™ forms on contact but it does assume that no ˜Archaic™ genes persisted
into present-day populations.
These ideas may seem very different and irreconcilable but in reality this is
not the case. To a large extent the two views re¬‚ect a different understanding of
the evolutionary process. The multiregional model follows the neo-Darwinian
school that sees evolution proceeding through small, cumulative, changes within
6 Neanderthals and Modern Humans

a species. The macro-evolutionary changes observed in the fossil record are sim-
ply the accumulation of many micro-evolutionary changes. Thus H. ergaster/
erectus gradually evolves into H. sapiens. Any division of the lineage into
species is of necessity arbitrary. This interpretation is correct. New species
arise when populations of a species are isolated from each other suf¬ciently
so that when they secondarily meet they do not hybridise to an extent that the
two populations eventually become one (Cain, 1971). Thus the multiregional
model, whether correct or not, is consistent with neo-Darwinian evolutionary
theory.
In the 1970s and subsequently, Gould & Eldredge (1977) proposed a differ-
ent evolutionary process. Coming from a palaeontological background these
authors had dif¬culty in understanding how the major steps (such as apparently
sudden adaptive radiations) observed in the fossil record could arise through the
accumulation of many micro-evolutionary changes. They saw the evolutionary
process as a series of major steps punctuated by long periods of stasis during
which species shifted their adaptive positions within de¬ned parameters but
without signi¬cant speciation taking place. No clear mechanism has been satis-
factorily defended for such a process. At about the same time a new taxonomic
methodology was being developed. Cladistics was seen as a quantitative and
objective method of classifying species that signi¬cantly improved on existing
phylogenetic procedures. By measuring a suite of variables (usually metric),
taxonomists were able to separate those that were common to a lineage from
those that were speci¬c to a lineage. Whenever such speci¬c differences were
observed in a form it was given speci¬c status. Thus, if we understand evolution
as being driven by speciation events we move to a situation in which, as new
species arise (or are de¬ned cladistically which is not the same thing!), the
ancestral ones de facto cease to exist. We can now begin to understand why the
replacement school (that relies heavily on cladistics) has dif¬culty in accepting
a H. ergaster/erectus “ H. sapiens continuum. Instead, it sees every new fossil
that is discovered and has features speci¬c to its lineage as a new species.
In reality the evolutionary process proceeds in two ways: through the gradual
accumulation of small changes within a species and through the formation of
new species, in vertebrates at least in geographical isolation, through a process
known as allopatric speciation. Recent studies seem to be providing evidence
for speciation within a common geographical area through the combination
of ecological and behavioural differences within a population (sympatric and
parapatric speciation) (Maynard Smith, 1966; Rice & Hostert, 1993; Gavrilets
et al., 1998; Dieckmann & Doebeli, 1999; Kondrashov & Kondrashov, 1999;
Tregenza & Butlin, 1999; Danley et al., 2000; Filchak et al., 2000; Johannesson,
2001; Porter & Johnson, 2002).
Human evolution in the Pleistocene 7

There are inconsistencies in the ˜Out-of-Africa 2™ model that are attributable
to not giving importance to gradual micro-evolutionary processes. Thus, if
˜modern humans™ emerged in Africa they must have done so, according to this
view, via a speciation event. An alternative, that is more parsimonious and
equally valid, is that ˜modern humans™ evolved differences gradually over the
last 2 Myr from the ancestor of the hominids that spread to other parts of Africa
and into Eurasia. To accept this position would imply acceptance of regional
continuity in that part of Africa at least. It is these humans that I term mainstream
H. sapiens, the ˜Moderns™, in this book.
The next dif¬culty arises in the de¬nition of species that, as we have seen
already, is fraught with dif¬culties because we are unable to apply the bio-
logical species concept to fossils. It is presumably one reason why palaeo-
anthropologists and archaeologists are so hotly debating the Lagar Velho fossil
from Portugal that is purported to be a Neanderthal“Modern hybrid (Duarte
et al., 1999; Zilhao & Trinkaus, 2002). There is no doubt that the Neanderthals
at least were a separate lineage in human evolution. Using cladistics that makes
them a separate species. This need not be the case. The Neanderthals may have
embarked on a separate evolutionary course from mainstream H. sapiens but
the degree and time of isolation when the two lineages re-met in the Middle
East and later in Europe would have determined whether or not they were a
good species. It is largely a question of detail that has little bearing on the study
of the two populations other than on the question of interbreeding which will be
very hard to resolve in any case. For these reasons I will develop the arguments
in this book along the lines of populations as this will be a more productive
approach. I will utilise nomenclature only in so far as it aids the reader. Nothing
more should be made of the use of particular names.
The multiregional model, on the other hand, does not appear to attach impor-
tance to the geographical replacement of one population by another. Yet, there
are many examples in the literature of the spread of populations and species,
which is a part of the dynamics of the natural world. It seems unlikely that, in
the history of the genus Homo, there should only have been a single successful
˜Out-of-Africa™ expansion. Implicit in the multiregional model is the failure of
any subsequent population expansion other than through genetic assimilation.
In the case of the Moderns and the Neanderthals in Europe, it would seem that
current evidence clearly indicates the replacement of the Neanderthals by the
Moderns. It is a different expectation, and to my mind an unrealistic one, to as-
sume that such replacement need have been worldwide. In any case, as we shall
see later, the colonisation of Europe by Moderns need not have been strictly a
replacement, if by that we mean an active displacement of Neanderthals by the
new arrivals.
8 Neanderthals and Modern Humans

The thrust of this book will, I hope, shed a new light on the processes and
the mechanisms that have marked the course of human evolution. The basis
of the argument has been marked out by Finlayson et al. (2000a) who have
adopted a biogeographical approach that sets off from an evolutionary ecology
stance. According to this view the growth of Modern Human populations and the
decline and extinction of the Neanderthals were independent, climate-linked,
events. Modern superiority, leading to the disappearance of the Neanderthals
through competition, was considered implausible. The initial colonisation of
the world by Moderns was related to a coincidence of climatic and historical
events that favoured a population that was adapted to the exploitation of plains
mammalian herbivores. The geography of the northern hemisphere and climate-
induced vegetation changes coincided to make the colonisation successful.
One of the criticisms of the contrasting models set out above (especially the
˜Out-of-Africa 2™) is that a mechanism has not been put forward to explain the
model. Equally, testable predictions have not been generated. In this book I will
develop an ecological and evolutionary perspective that attempts to understand
human evolution through that of its constituent populations. Climate is seen as a
central element that has been critical in human evolution, not necessarily directly
as some have postulated (Ruff, 1994; Holliday, 1997a, b) but rather through its
effects on the distribution and abundance of plants and animals. I highlight, in
particular, the increasing climatic instability during the Pleistocene as a critical
factor that has been largely ignored (but see Potts, 1996a, b; 1998), although in
my view a new mechanism of ˜variability selection™ is not required, as I will
explain later. Running in parallel with the climatic and ecological vicissitudes
of the Pleistocene, humans have evolved mechanisms to deal better with these
uncertainties. These mechanisms have, in the end, permitted the colonisation
of the entire planet.
2 Biogeographical patterns

The distribution and abundance of plants and animals during the Quaternary is
of great interest in the understanding of the pattern for any particular species.
In our case it is fundamental to understanding the way in which humans were
distributed at different times during the Quaternary.
It is important to start our discussion at the macro-ecological scale. The broad
biogeographic picture will give us important insights at the scale which is most
relevant to our study. We will zoom into lower spatio-temporal scales in later
chapters where it is relevant to the discussion. I will not spend time discussing
well-established biogeographic patterns that I do not regard to be especially
relevant to this book. I am more concerned with the distribution and shifts in
distribution of environments that would have in¬‚uenced human distribution and

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