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80



60
Population Size




40

Neanderthals

20

Moderns
0
-42 -40 -38 -36 -34 -32 -30 -28 -26 -24


Time (kyr BP)


(d) 140


120


100


80


60
Population Size




40
Neanderthals

20
Moderns
0
-42 -40 -38 -36 -34 -32 -30 -28 -26 -24


Time (kyr BP)

Figure 7.13. (cont.)
184 Neanderthals and Modern Humans


(e) 200




100
Population Size




Neanderthals



Moderns
0
-42 -40 -38 -36 -34 -32 -30 -28 -26 -24


Time (kyr BP)


(f) 300




200
Population Size




100

Neanderthals



Moderns
0
-42 -40 -38 -36 -34 -32 -30 -28 -26 -24


Time (kyr BP)

Figure 7.13. (cont.)
Modern Human colonisation and Neanderthal extinction 185

the Euro-Siberian bioclimatic stage, is very distinct (Figure 7.13e). The Nean-
derthals are extinct by 38.5 kyr and Moderns are present from 40 kyr, always at
higher population levels than the Neanderthals. The model predicts a regional
overlap of 1.5 kyr.
Figure 7.14 illustrates the geographical distribution patterns of Neanderthal
and Modern Human populations in Iberia at 39 kyr and 33 kyr as generated


(a)




Figure 7.14. Simulated geographical distribution of Neanderthal and Modern
populations. (a) Neanderthals at 39 kyr; (b) Moderns at 39 kyr; (c) Neanderthals at
33 kyr; and (d) Moderns at 33 kyr. Darkness and circle size re¬‚ect modelled densities.
186 Neanderthals and Modern Humans




Figure 7.14. (cont.)


by the model. The pattern shows a well-established Neanderthal population
in Iberia at 39 kyr with the main areas of population density being within the
thermo-Mediterranean stages (Figure 7.14a). The Moderns are restricted to the
north and appear thinly spread on the ground (Figure 7.14b). At 33 kyr, only
6 kyr later, the Neanderthals are thinly spread and restricted to the thermo-
Mediterranean stages (Figure 7.14c). The Moderns are ¬rmly established in the
north and Moderns have spread to almost the whole of the peninsula except the
southernmost regions (Figure 7.14d).
Modern Human colonisation and Neanderthal extinction 187




Figure 7.15. Predicted periods of overlap between Neanderthals and Moderns. Black,
2.5 ’ 4 kyr; grey, 0 ’ 2.5 kyr; white, no overlap.



The predicted lengths of regional overlap are illustrated in Figure 7.15. The
mosaic nature of the distribution results from the heterogeneous bioclimatic
landscapes of Iberia and the time lags associated with the progressive south-
ward spread of Moderns. Overlaps ranged from no overlap to maxima of 4 kyr
at the regional spatial scale even though the two forms were present together in
Iberia, as we have seen, for 9 kyr. There are large areas of the central Meseta
and of southern Iberia where no overlap is expected and in which the extinction
of the Neanderthal populations cannot be related to the presence of Moderns. I
have commented elsewhere (Finlayson, 1999) that the probability of hybridis-
ation between Neanderthals and Moderns ought to be greatest in areas of high
temporal overlap. The conclusion is that the effect of competition at the simu-
lated level is minor and only changed the outcome of the Neanderthal population
by a small degree. The model suggests that a huge advantage of the Modern
population over the Neanderthal would be required for competition to have
an overriding effect over the climate. Clearly, there is no competitive effect in
certain areas (e.g. Gibraltar and Carihuela) where the two populations do not
meet at all.
A possible criticism of the decline of Neanderthal populations in response to
cold, as we have seen in this chapter, is that the same populations would have
had to undergo even colder episodes earlier in their history which they survived.
In order to answer this question I extrapolated the Neanderthal warm-adapted,
cold-intolerant model back in time from the established population sizes of the
intial model while maintaining the same criteria for rate of population change in
188 Neanderthals and Modern Humans

80000




60000




40000
Population Size




Variable Growth Rate
20000


Constant Growth Rate
0
-140 -120 -100 -80 -60 -40 -20


Time (kyr BP)

Figure 7.16. Simulated evolution of the Iberian Neanderthal population since the last
interglacial.



response to climatic factors. I then upgraded the model further by introducing
variations in recovery rate during warm intervals depending on the bioclimatic
stage (i.e. from increments of 5 in the Euro-Siberian stage to 20 in the thermo-
Mediterranean). The model thus supplants a uniform growth rate by a vari-
able growth rate dependent on the bioclimatic stage. The difference between
the ˜standard™ model and the upgrade was one of degree but both produced
a very similar result (Figure 7.16). The model shows that the Iberian popula-
tion would have reached its heyday at the end of the last interglacial (OIS 5a)
which would have been when ecotonal landscapes would have peaked. At its
height the Iberian modelled population was of the order of 70 000. This would
translate to a population density of 0.1/km2 , well within the hunter“gatherer
range described earlier. With some intervals of minor recovery, from then on
the population was in a steady and consistent decline towards extinction (Fig-
ure 7.16). The decline was particularly abrupt during the severe conditions of
OIS 4 (Mellars, 1996) but the population was able to recover from this crash,
and consequent bottleneck, although the Stage 5a population level could not be
attained again. The relatively mild and variable climate of early OIS 3 permit-
ted a partial recovery. The cooling towards the end of this stage, though not as
severe as earlier events, was enough to seal the fate of the population since the
population level at the start of the crash was already depressed from the effects
Modern Human colonisation and Neanderthal extinction 189

of OIS 4. It is the cumulative effect of a number of cold events that appears
to account for the extinction of the Neanderthals. A comparison of the pattern
for the southern site of Gibraltar and the northern site of El Castillo illustrated
how much closer the northern population came to extinction at the end of OIS
4 than the southern population. By extrapolation it can be inferred that more
northerly European populations would have been severely affected by cooling
events after the last interglacial and many probably became extinct. Subsequent
spreads into western Europe would have involved populations from the Iberian
refugium. The south-western region of France is interesting in this respect.
Some Neanderthal populations may have been able to survive here through-
out as they did in northern Spain. The proximity of this region to the Iberian
refugium would have also meant rapid recolonisation during warm events. Its
more northerly position and proximity to the European Plain would have also
made it more vulnerable to climatic instability and would have also permit-
ted an earlier colonisation by Moderns than further south. The relationship
between Chatelperronian technology, associated with interstadial conditions
around 36 kyr, followed by Aurignacian with the onset of glacial conditions at
34 kyr (Mellars, 1996) is in accord with these predictions.
Of the series of models that were run the one that I have presented in detail
appears to be the one which best ¬ts the existing empirical data. The basis
of any good model is that it should make testable predictions which should
allow progress to be made through falsi¬cation or con¬rmation. The following
testable predictions are derived from the model:

(1) The geographical pattern of Neanderthal extinction in Iberia is not a
straight north to south process but is related to bioclimatic stages. Speci¬c
predictions can be made for each 50 — 50-km UTM unit.
(2) The spread of Moderns from the north to the southernmost points of the
Iberian Peninsula takes 10 kyr.
(3) It is unnecessary to postulate a static barrier (e.g. an Ebro Frontier; Zilhao,
1996) which divides Neanderthal and Modern Human populations for a
protracted period. Delays between arrival of Moderns in different parts of
the Iberian Peninsula are predicted by the model.
(4) Neanderthals and Moderns never met in some parts of the Iberian Penin-
sula. The pattern of overlaps is not north“south but is in the form of a
mosaic. Geographical areas are de¬ned by degree of overlap.
(5) The effect of stochastic extinction and re-colonisation of local populations
within regions, independently of the situation of the global population, can
create apparently opposing patterns in speci¬c localities.
(6) The Neanderthal population of Iberia, and by extrapolation that of western
Europe, reached its maximum level during OIS 5a after which numbers
progressively dwindled towards eventual extinction.

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