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The dynamics of colonisation and extinction

Straus & Bar-Yosef (2001) have de¬ned hominins as ˜purpose-driven species™.
The notion of purpose-driven human migrations is pervasive in the literature.
For many authors, the migrations into Australia must have required craft and
navigation skills (Klein, 1999). These same authors seem to ignore the abil-
ity of other primates (e.g. macaques) that, without boats, regularly colonised
deep water islands in South-east Asia that were never linked to the mainland
(Brandon-Jones, 1996; Abegg & Thierry, 2002). Dispersals out of Africa are
also referred to as migrations. This way of de¬ning changes in the geographical
range of humans through time confuses proximate factors, such as curiosity,
with the ultimate factors responsible for range changes. Behind the arguments
is the notion that humans are apart from all other living organisms and that ˜spe-
cial™ mechanisms can be found to explain their behaviour. If this had indeed
been the case then we would have to postulate a non-biological model of hu-
man geographical expansion, one that would have been independent of natural
Human range expansions, contractions and extinctions 41

selection. Given that humans behaved as components of the ecosystems of which
they were a part, it is far more likely that they were ultimately very much gov-
erned by selective pressures even if their socio-cultural attributes (themselves
phenotypic expressions of an evolved genetic plasticity) gave them signi¬cant
advantages over other species in the same ecosystems.
So how do changes in geographical range occur? They are the response to
demographic pressure within the existing range and favourable environmental
changes in the periphery (Foley, 1997; Dynesius & Jansson, 2000). The speed
of invasion into a new area is the product of the interaction between local
adaptation and genetic and demographic parameters (Kirkpatrick & Barton,
1997; G´ rcia-Ramos & Rodriguez, 2002). A population may be increasing as
a
a result of favourable conditions and intra-speci¬c competition forces some
individuals to disperse away from the dense core area. Most dispersers will
not ¬nd a suitable area for settling or may end up in an area already occupied
by the same species. New marginal populations may, however, occupy optimal
habitats or they may occupy sub-optimal ones in which they are nevertheless
able to make a living. A successful colonisation depends on the capacity to
adjust genetically (G´ rcia-Ramos & Rodriguez, 2002) or behaviourally to a
a
spatially varying environment and results in a longer lasting population that
is independent of arrivals of additional dispersers (Gyllenberg et al., 1997).
It is worth noting in this context that sink populations that are maintained by
immigration from source populations, even when birth rate is below death rate,
may show varying degrees of permanence (Brown & Kodric-Brown, 1977;
Dytham, 2000). Thus, the presence of a population in an area is not necessarily
proof of its success in that area. We should be conscious of this when considering
individual archaeological sites. At the other end of the spectrum, colonisation“
extinction models predict that at any point in time there will be a proportion
of habitable patches that will be empty because of demographic stochastic
extinctions (Hanski & Gilpin, 1997; Tilman & Karieva, 1997; Hutchings et al.,
2000). So absence is not proof of unsuitability either!
Returning to a successful colonisation, as the population grows so its range
expands until unsuitable habitats are encountered, unless individuals are able
to adapt to the new circumstances. Ranges may also shift. If conditions on
one side of the range are deteriorating then the population contracts in those
areas, either through local extinction or by movements of individuals into core
areas with consequent increase in intra-speci¬c competition. In such cases the
advantage is likely to be with the residents and so the local marginal popula-
tions may become extinct anyway. If, at the same time, favourable conditions
are becoming available (perhaps due to a climate change) then there will be
expansion into those new areas by the same process described before. The out-
come is a range shift. These are generalised models. There are other ways of
42 Neanderthals and Modern Humans

expanding geographical range. Central areas in the range need not necessarily be
the core population areas and individuals may ˜jump™ from one optimal habitat
to another even if there is unsuitable habitat in between (Lewis, 1997). Hewitt
(1999, 2000) considers that populations on the northern edge of a refugium
would have rapidly recolonised empty territory during climatic amelioration,
with the leading-edge expansion being led by long-distance dispersers rapidly
setting up colonies and expanding. Such expansions would necessarily lead to
loss of genetic diversity among these small founder populations.
Highly vagile animals are able to integrate heterogeneity over broader scales
and therefore perceive the environment with a coarser ¬lter (Wiens, 1997).
Dispersal ability and dispersal rate are therefore important internal population
parameters (Lehman & Tilman, 1997; Lewis, 1997). Colonists can arrive ac-
tively or passively and there may be a number of reasons why they arrive in
a new area: (a) following a change of conditions; (b) following removal of a
barrier; (c) following the creation of a passageway; or (d) following a genetic
change which adapted them to conditions in the colonised area. In cases of
environmental instability, as in Pleistocene Eurasia, the time delay of popula-
tion response, relative to the period of the environmental cycle, is crucial for
persistence. Populations with fast response that track cycles will reach periodic
lows and risk extinction. The Neanderthals are a good example. Populations
with slow response may be able to keep a more or less stable population size.
The Moderns may well be an example (Chapters 5 and 7).
One way of reducing the effects of environmental ¬‚uctuations is to prolong
the response time to environmental changes (Hutchings et al., 2000) “ i.e.
to invest in environmental resistance. This can be achieved through ˜escape
responses™. Dormancy or hibernation are examples. I argue in this book that
the complex social systems of Moderns, their extended networks and their
systems of operating at large scales and storing and caching resources effectively
prolonged their response to environmental changes, that is Moderns invested
in environmental resistance.
Temporal and spatial heterogeneity are likely to be perceived by a colonist
population as being greater than in the source area. This means that during an
initial phase of colonisation a population needs to rapidly colonise many patches
to reduce the risk of extinction. In spatial terms, dispersal ability can be an escape
mechanism. A high instantaneous rate of increase (r), an avoidance of density-
dependence and high dispersability all guarantee successful colonisation in
environments expected to ¬‚uctuate either systematically, randomly or spatially.
Competition can alter the success of colonisation. A species with potential to
change its position along the resource spectrum is likely to be a good coloniser.
We see these attributes in the characteristics of Moderns and we also have a
theoretical basis for understanding their eventual success in areas like the Middle
East where they may have faced competition from Neanderthals (Chapter 7).
Human range expansions, contractions and extinctions 43

Finlayson et al. (2000a) have proposed the generalised conditions that would
have lead to geographical range expansions and contractions on a global level
during the Quaternary. These range changes have to be viewed against the
climatic backdrop that characterises the Quaternary and differentiates it from
earlier periods (Denton, 1999). Throughout the Quaternary we observe cyclical
climatic changes, their frequency intensifying towards the latter stages (Imbrie
et al., 1984; Ruddiman et al., 1986). We observe, at different scales, variability
even in equatorial and tropical regions (deMenocal, 1995). It is this climatic
variability that, through consequent habitat variability, drove the dynamics of
geographical range in humans and indeed in many other species (Potts, 1996a, b,
1998). Given that the number of such major and minor oscillations was very high
over the last two million years (Shackleton & Opdyke, 1973, 1976; Shackleton
et al., 1984) we would predict many geographical expansion and contraction
events, not just one or two. The intensity and duration of each event, coupled
with the demographic situation of the initial population in the core area, would
have been the key elements in the extent and direction of the range expansion
(Finlayson et al., 2000a). Once populations became established away from the
initial core area then, assuming they survived subsequent unfavourable events,
these secondary core populations would have acted as new sources of expansion
when favourable conditions resumed.
This leads me to the all-important question of extinctions. As with range ex-
pansions we have to view extinctions at different scales. At the smallest scales,
extinctions of local populations would have been a regular feature of human
populations throughout the Quaternary. Such extinctions would have proba-
bly affected marginal populations most severely and small effective population
sizes would have meant that many extinctions would have been the result of
stochastic processes (see Chapter 7). Regional extinctions would have been less
frequent, though not uncommon, and would have occurred when more signif-
icant alterations in favourable conditions happened, suf¬cient for all the local
populations within a region to have been affected. Finally, global extinctions
would have been the least likely given that regional populations somewhere
would have been buffered against unfavourable conditions elsewhere.
Human populations in tropical and equatorial regions would have been least
prone to extinction given that the range of resource options in such regions
would have been greatest and the effects of climatic oscillations on habitats
least felt (Figure 3.1; Finlayson et al., 2000a). In addition, these areas would
have enjoyed a fairly constant day length (and therefore year-round foraging
and hunting) throughout the year. So populations in equatorial and tropical
Africa, and subsequently in South-east Asia, would have enjoyed the greatest
degree of regional permanence. Next would be the proximal warm temperate
regions and the least conducive to regional permanence would have been the
cool temperate and boreal regions. As humans evolved physical and behavioural
44 Neanderthals and Modern Humans




Figure 3.1. Source and sink regions in human evolution. Arrows indicate probable
strength and direction of geographical expansion. The Strait of Gibraltar as an entry
point is only partly supported by the available evidence (see text). The boxes represent
major regions of the world. Largely tropical areas are in black: AFR, Africa; SEA,
South-east Asia; AUS, Australia; SAM, South America. Temperate areas are in white:
MLB, Mid-latitude belt of Eurasia; CHI, China; NEP, North Eurasian Plain; NAM,
North America. Numbers indicate the approximate process of initial colonisation by
Homo. For any given stage in the colonisation process of Homo, persistence is
predicted to be highest in black (source) areas and lowest in white (sink) areas.
Australia and South America were colonised too recently to have been important
source areas in the Pleistocene. Mid-latitude Eurasia and China act as refugia and
secondary sources of colonisations of areas to the north. Only Africa and South-east
Asia would have had continuous occupation after 1.9 Myr bp. After Finlayson et al.
(2000a).



adaptations that improved colonisation and persistence so areas further away
from the tropics could be successfully colonised, Moderns being the best at
doing so.
Viewed in this manner the extinction of the Neanderthals (Chapter 7) is
not unusual or even surprising. It is the extinction of a complex of regional
populations in Europe and western Asia. It is an example of events that probably
occurred repeatedly earlier in the Quaternary and tells us that we must exercise
care in taking for granted cases of regional continuity in non-tropical areas.
Take the case of H. antecessor at Atapuerca (Spain) 800 kyr ago (Carbonell
et al., 1995). Were these the ancestors of subsequent European humans or did
they simply go extinct? The serious answer to this question is that we do not
know. Yet, on morphology (in spite of the inherent problems with morphological
criteria, Chapter 4) a direct ancestry is proposed. But even in Atapuerca itself we
cannot convincingly show continuity. The fossils from Gran Dolina and Sima de
los Huesos (Spain; Arsuaga et al., 1993) are separated by half-a-million years
and we simply do not know what happened in between. Hopefully, with time we
may know as excavations proceed but today we cannot say one way or the other
with certainty. In ecological terms it is of interest to note that when humans lived
Human range expansions, contractions and extinctions 45

in Atapuerca, climatic conditions were milder than at present (Cuenca-Besc´ s o
et al., 1999; Cuenca-Besc´ s, 2003; van der Made, 1999). Today, Atapuerca is
o
a harsh environment in the winter and it must have been even harsher during
glacials. To suggest continuity is, to my mind, a very bold assertion in the light
of the limited data available.
So if there were multiple colonisations and extinctions in Eurasia, how many
were there? At present, that is an impossible question to answer. The evidence
from Orce (Spain) is unclear but suggests a possible earlier colonisation that
may have occurred via the Strait of Gibraltar (Arribas & Palmqvist, 1999; Oms
et al., 2000). That is open to debate and must await further evidence. We would
then have to see if these humans were part of the same colonisation that lead
to Atapuerca or something else. Post-Atapuerca there may have been several
colonisations of Europe, each time with greater success. The pre-Neanderthals
and the Moderns were the last two of a chain.
The colonisations would have been part of a continuum of range expansions
of varying extent, local and regional extinctions, subsequent re-colonisations
and even re-colonisations into areas occupied by a previous colonisation that
persisted. The latter, I would predict, would have been most frequent close
to the tropical core areas. In such areas of contact the outcome would have
been determined by a variety of factors including the time and degree to
which the two meeting populations had been previously isolated, and thus the
degree of genetic, morphological and behavioural isolation, the densities of the
two populations relative to environmental carrying capacity and the degree of
ecological isolation. In cases where the conditions for competition would have
been right, then population attributes that gave one population the edge over
the other would have been critical. In the rapidly ¬‚uctuating conditions of the
Quaternary, the conditions for such competition would have been rare, more so
as one went away from the tropics.


The global pattern of colonisation and extinction

The patterns of faunal interchange between tropical and boreal regions have a
deep history within the Neogene (Pickford & Morales, 1994). Latitudinal ¬‚uc-
tuations in the boundary zone between the tropical and boreal biogeographical
realms have marked the past 22.5 Myr. The difference in receipt of solar energy
on the Earth™s surface and the inclination, at a steep angle to its orbital plane, of
the axis of the Earth™s rotation have meant that the zone of maximum receipt of
solar energy shifted latitudinally across the globe causing seasonality. Season-
ality at high latitudes is overwhelmed by daylength and temperature changes
(Pickford & Morales, 1994). Migration, hibernation and summer reproduction
46 Neanderthals and Modern Humans

are typical responses of animals to these predictable changes. Humidity changes
dominate the low latitudes where temperature and daylength variations are of
lesser importance. Wet and dry seasons thus dominate tropical seasonality pat-
terns. Aestivation and wet season reproduction are typical responses.
Throughout the Pleistocene the populations of humans across the world
underwent ¬‚uctuations, range expansions and contractions. In this respect they
differed little from a whole range of organisms (Hewitt, 2000). Those at great-
est risk of extinction were those furthest away from the tropics, the habitat
fragmentation caused by increasing cooling and aridity contracting the north-
ern parts of the range and also compressing the altitude range. The length
of such adverse climatic periods, occurring as single events or series of such
events with brief interludes, was probably more signi¬cant than the intensity
of the adverse pulses. Range contraction would have taken the form of regional
population extinctions especially when climate variations were rapid (Hewitt,
1996, 1999, 2000), a situation that caused the extinction of, for example, tree
species (McGlone, 1996), reptiles (Busack, 1986) and mammals (Martin &
Klein, 1984). During improved climatic conditions, northward extensions of
the range of populations that had managed to survive commenced from south-
ern refugia (Hewitt, 1999, 2000). The risk of becoming extinct would have
depended on: (a) the ability to colonise suf¬cient sites during periods of peak
abundance so as to permit survival when they became rare; (b) stochastic ef-
fects that might have eliminated populations that spent long periods in small
isolated sites; and (c) the ability to track suitable climates during periods of rapid
change (McGlone, 1996). In the case of trees, for example, differences in source
areas and migration rates continuously changed the forest composition north
of the Alps (Zagwijn, 1992). Faunal composition would have varied similarly
as animals behaved in a Gleasonian manner, that is individually responding to
environmental variables (FAUNMAP, 1996; Hewitt, 1999; Chapter 2).
Tropical African hominid populations would have bene¬ted from increased
cooling and aridity and their range would have expanded within the tropics.
Subsequent amelioration immediately after cold/arid periods (when populations
were at their highest) would have permitted northward expansions as the Sahara
Desert became savannah and grassland (Finlayson et al., 2000a). In this way,

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