catastrophes or sudden changes. the need to review our...

Vers une anthropologie des catastrophes Actes des 9° journees Anthropologiques de Valbonne

Sous la direction de Luc BUCHET, Catherine RIGEADE, Isabelle SEGUY et Michel SIGNOl! - Editions APDCA, Antibes/ INED, Paris, 2009

Catastrophes or sudden changes. The need to review our time perspective in Prehistory lord; EsrEVfZ

Key-words - Catastrophes, theory of sudden changes, Prehistory.

Abstract - Catastrophes and sudden changes were denied by prehistorians, as they are difficult to integrate into a scientific processual explanation. However, sudden changes are part of normal changes in nature and in society and cannot be ignored even to explain gradual changes. Sudden changes can switch to catastrophes depending not only on their character, but also on some apparently random features, as well as on the capacity of reaction of the society affected. The development of sciences allows one to begin detecting such sudden changes and their effects, and the epistemological advance in Science facilitates the integration and investigation of these complex phenomena. Here r explore some examples, such as the probable impact of the Storegga tsunami on the coastal Mesolithic settlements of Scandinavia, the gap in the population sequence of the Cantabrian coast that followed the beginning of the Holocene, the impact of the Laacher volcanic eruption, and lastiy, the relatively sudden extinction of the American megafauna.

Catastrophes ou changements brusques. La necessite de revoir notre perspective en Prehistoire

Mots-ch~s - Catastrophes, theorie des changements subits, Prehistoire.

Resume - La science prehistorique a nie I'importance des changements brusques dans l'explication du developpement des societes. Cependant, ces mutations soudaines font partie de la transformation normale de I'Univers et sont meme indispensables pour comprendre les changements graduels. Le fait qu'un changement brusque se transforme en catastrophe ou non ne depend pas seulement de sa propre nature, mais de la capacite de reaction de la societe sur laquelle il agit. Le developpement epistemologique et les techniques archeologiques des sciences auxiliaires permettent aujourd'hui de documenter des changements brusques et de commencer a explorer les effets de ces evenements complexes sur les societes prehistoriques. Nous presentons quelques exemples: !'impact possible du tsunami de Storegga sur les populations mesolithiques de la Scandinavie, la chute du peuplement de la cote cantabrique au debut de I'Holocene, !'impact de I'erupion des volcans Laacher et, enfin, l'extinction relativement soudaine de la megafaune americaine.

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Introduction

The beginnings of scientific perspective in Prehistory had to overrun more difficulties than in other Sciences. The scientific explanation of issues that were the hard core of mythological thinking gave a rise to a heavy debate with the defenders of the explanations given by religions, particularly in the Bible. Catastrophes have been assimilated into most societies and explained as tales and myths. Illustrated classes have used those events and sometimes the possibility to prevent low-frequency but extraordinary large wave phenomena to manipulate the society for their own benefits in the name of divinities. Therefore it is not surprising that a scientific and objective perspective tends to deny the relevance of catastrophes in rationalist

explanations. Scientific geology grew stimulated by the needs of critical raw materials like metals and

charcoal. It was the first Science to adopt Actualism and a realistic perspective as funda

mental principles. But geology and the other Sciences dealing with the explanation of phenomena developing through time (like Paleontology or Prehistory) needed another basic principle - Gradualism - to obtain satisfactory explanations. Darwin's Theory of Evolution by Natural Selection could not have been formulated without those two supporting principles.

The theory of Catastrophes of the prestigious paleontologist Cuvier was a a good reference point to collate the account in the Bible with fossil record in the struggle with the scientific perspective. In this fighting context Science emphasized the need of "long time" and

gradual processes to cope with the cause-effect relationships. This "long time" and gradual perspective prevailed lastly in Biology and Geology.

At the same time, the development of western Philosophy, since Hegel, led to the retrieval of Dialectics. Marx and mainly Engels reconverted Hegel's idealist position into a realist and scientific perspective. Meanwhile, at the end of the 19th century, advances in the theory of

Physics and Mathematics (specially differential calculation and non linear equations) began to allow surpassing mechanical and simplistic explanations. Despite those facts, those kind of explanation predominate since the end of the following century.

The defenders of the ancient myths of origin adopted the adequate socio-political strategies against the scientific explanations to allow the continuity of religions (Estevez, Vila, 1999). This strategy was centered (and succeeded specially) on the more sensitive case of the explanation of the human origins. The success of this politics explains why Cultural History lasted the predominant paradigm during almost all the 20th century in Western Europe.

Cultural History can be considered a profoundly catastrophist approach, but only apparently declares the acceptance of Gradualism and Actualism. This organic model of development sustained that civilizations were born, grew, collapsed and died off, been substituted by younger, more powerful cultures, that spread their ideas and people. Nevertheless the gradualist and evolutionist paradigm at the end of the 20th century has reduced this previous explanation, and the remains of Catastrophism were swept out of Science after having been stigmatized by Acadaemia. The significance of sudden changes for social change has been refused. Paradoxically, at this moment, other Sciences, such as Evolutive Biology or Geology began to review the importance of those sudden changes (see

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CATASTROPHES OR SUDDEN CHANGES. THE NEED TO REVIEW OUR TIME PERSPECTIVE

for instance Leroy and Stewart, eds. 2002). That is the case of the Theory of Punctuated

Equilibrium in the new Evolutionist Synthesis. The scientific study of Change has begun to incorporate the dialectical thinking of the skips and leaps of conditions, the Theory of Chaos,

the emergence of complexity, the criticality of auto-organized systems, the non-linear and

intermittent dynamics and the potency laws.

Nevertheless, this is not the case of prehistoric Archaeology. It has refused, with few

exceptions until now, to incorporate these new elements. This is, in part due to the inertia of

the Academic community (by the principle of resilience and conservatism). But it is also due

to the loss of prestige of Catastrophism and to the technical impossibility to focus to the

adequate chronological scale to detect the phenomena of sudden changes and to correlate

them with social changes in Prehistory.

Sudden changes, however, must have had a crucial role in the development of prehistoric societies. Sudden changes, as the modern history of hunter-gatherer societies shows, can

drive these societies to crises through their same mechanism of reproduction and continuity

safety systems.

In any case, even the first prehistoric hunter-gatherer societies, the most defenseless

because of their lesser foresight capability (due to their lower technological development and

less accumulated experience) could be differently affected depending on their economic and

social strategies. The archaeological study of these interactions between the environment and

the social development can contribute with relevant information to fully understand social

processes (Estevez, 2005).

A new theory of change

The development of dating systems and of the different paleoenvironmental proxies can

allow approaching the study of sudden changes in Prehistory. But to revert the situation it is

necessary to define better the concept of sudden change and to distinguish it from the concept

of catastrophe. It is necessary to strictly delimit what is a catastrophe and how to proceed to

its study from a scientific perspective, that is not only in a descriptive way, but so far as possible in a nomothetic and predictive way.

We have to display, as in a fractal view, the different time scales and the different causali

ties (see a preceding paper in Bailey, 1983) and observe how they integrate a consistent

complex system of explanations.

The use of the term catastrophe may be inadequate or insufficient to describe certain changes in the environment and in societies.

Catastrophe can be defined as a negative effect triggered commonly by a sudden or radical

change that modifies or breaks a historical (social) gradual dynamic or that simply causes,

through a non-previewed event, major damages to humans or to material goods. Following

this description, it is not appropriated to speak about natural catastrophe like we often do in

"natural language". This is true firstly, because the concept actually refers to an effect and not

to its cause. Secondly, because the concept is linked to a negative aspect.

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A natural sudden change does not necessarily bear such negative effect. This negativeness is relative anyway. Something negative to a partiality can benefit the totality. In Nature Sciences for instance there is no doubt that the extinction of dinosaurs did benefit the evolution of mammals. It is accepted at last that disturbances are necessary for the development and sustenance of ecosystems. Even wood fire is necessary for some species of trees to regenerate.

We can also agree with this issue if we think in terms of Social Sciences. We can argue for instance that the French Revolution was a catastrophe for aristocracy and for the "Ancien Regime". But there is no doubt that it benefited the development of rationalism, freethinking and the rule of the right.

Thus the problem that arises is to determine what the partiality is that has to be affected in order to consider the consequences of an unforeseen event of change as catastrophe. But it is not a simple question of the relativity of the damage. It is also a question of the indefinite limits of its magnitude. How many people have to be affected, to consider these effects as a catastrophe? Another problem is to delimit the speed of the change, how long its effects and how to differentiate between a sudden and a gradual change. In our case the change between hunter-gatherers and farmers has been described as the "Neolithic Revolution", but we have envisaged it as a very slow change.

We should define the speed, frequency and magnitude of the changes that can be considered a sudden change and that can cause an event that we could define as catastrophe. But again we will find a source of subjectivity and ambiguity. Most of the sudden changes that we know are not anomalous or abnormal. We have the evidence that they are repeated. They may have a stochastic or cyclical character, but they have to be unforeseen for the affected society. This means that their cycles or the breadth of their wave are beyond range of the memory of the affected society. This can happen when their frequency is apparently not submitted to regular cycles or when the wavelength is so long that prior similar events did not remain registered in the memory of the society. Finally, sudden changes can produce a

catastrophe when they do not follow a process of a single cause but when they are the result of a non-linear process. In summary, the catastrophic effect of sudden changes depends partially on the hazard, on the non-linear exponential multiplying results of their effects, as well as on the own structure of the society: on its capacity to remember, to prevent and to

assimilate the effects. Consequently, an effective approach can be made, adopting a perspective and developing

the concepts and instruments of the Chaos Theory, non-linear equations and the dialectics of change. In that sense it would be very interesting also in Social Sciences to focus these phenomena on distinguishing the dominant from the determinant cause and from the trig

gering effect, and eventually from the persistence factor. Adopting this perspective would be without any doubt very productive in Prehistory. We

could gain in this way a wider and comprehensive view of the development of human societies, because those sudden changes have accompanied humankind from the beginning of

our trail. The length of this record on social behavior of more than two million years allows one

to verify when different kinds and scales of sudden changes occur (making longer or


complex cycles more evident, calculating probabilities, looking for multi-causality and

trigger effects) and how they affect human societies. When and why do they produce a catastrophic effect? We may get a deeper and a more equilibrated perspective of the significance

of catastrophes.

Capability of Prehistoric Archaeology to explore different kinds of sudden changes

We are aware that the internal dynamics of the societies themselves, and the effect of the accumulation of experiences can lead to sudden changes in the SOciety itself and in its envi

ronment. This is well-described by the equations of declining returns. In the present paper, however, I will discuss in the first instance the effects of natural

changes. I will follow the different scales and ranges of changes analyzing to what extent they can be made evident and how can we analyze them in Prehistoric Archaeology.

Sudden changes of extra terrestrial origin

These are astronomical changes. Social Science has look regarded the influence of these

external events with scepticism, because of their apparently hazardous character and their enormous scale and disconnection to events that develop on Earth.

Nevertheless, the evidence collected of their effects are now enough to force to consider their impact and subsume them as trigger or causes of paleontological events. Since the

seventies (Alvarez et al., 1980) different, direct and indirect, evidence of impacts of meteorites have been collected which researchers have tried to correlate with mass extinctions. Impacts of different ranges from a local to a global magnitude have been detected and the effects do

not seam linearly correlated to the size of the meteorite. But the capability to detect directly

the effect of a collision is very low: only a minority are solids and are of the right size to impact on the Earth surface. Furthermore, this probability is even less if we consider that more than two-thirds of the surface is covered by water masses.

Geomorphological processes have erased the traces of smaller impacts on the surface.

Smelted rocks could be easily confused with volcanic minerals, although there are differences in the way of crystallization and in the structure.

Therefore, to try to detect these events, we have to look better for other indicators such as trace elements, notably a high proportion of siderophyle minerals like iridium, nickel, cobalt,

platinum or iridium which researchers began to search for more systematically since 1970 (MacLaren, 1970). The technical solution could be to search for those trace elements in the ice and sediment cores, that are used in paleoclimatology in which they have not been specially studied. Some difficulty arises from the abundance of some noble gases like Argon can also

be attributed to other causes, for instance to solar activity. Nevertheless, the evidence collected until now allows one to estimate some probabilities.

It has been argued that there is a possibility of one to three crashes that could produce a crater of more than 20 km every million years. Thus, we know that from the beginning of the human

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trail a big impact was not improbable. Perhaps, it occurred at the beginning of the Quaternary

and coincided with the broad replacement of species that took place. The Barringer Crater that bears the name of its discoverer is closer in time and can be dated around 30ky BP. That

approaches the possibility that this event could affect indirectly hominid populations. Apart from collisions of large extraterrestrial masses, subatomic particles also arrive and

radiate the Earth continuously, producing fundamental changes in the atmosphere but also (to a degree not yet evaluated) in the chemical composition (vg. DNA) of living organisms.

There is evidence that such bombardment is not regular but that it fluctuates owing to astronomical causes such as explosions of supernovas (McHargue et al., 1995). But it depends also on global changes in the Earth. Sudden changes in the Earth's magnetic field and its effect on

the high atmosphere protection belt may influence the incidence of these particles. There is

some hypothesis to explain by this process the great anomaly in the radiocarbon content

(Joris, Weninger, 1996) of the atmosphere (occurred between 44 and 33 ky BP that coincides with the end of the predominance of Paleoanthropines). This anomaly is followed by two maximum peaks in 10-Berilium and 36-Chlorine of cosmogenic origin and a minimum in

palaeomagnetism (Beck et al., 2001) . The data obtained by the models show that an alternative explanation based only in solar

activity together with paleomagnetism do not explain even half of the variability. Thus, there

is a possibility that radical changes in the termohialine marine circulation regime of the

oceans were responsible for the anomaly. Relating all these possibilities and the more or less contemporaneous Barringer Crater, an intriguing and attractive hypothesis of meteorite crash

could finally arise as a possibility. Nevertheless, there is no verification yet for such idea. Meanwhile to explain social changes we have to lay aside these causes and search first for

other causes and triggers (climate changes, volcanoes ... ) with higher probabilities because of

their greater frequency. At the same time we can be calm, because the more frequent events of collisions are smaller in size and their effects cannot be considered significant for the deve

lopment of societies other than a less longer-lasting register in ideology that could only be found in historical societies, and is therefore irrelevant for Prehistory.

Changes in the Earth

These are more frequent and easy to detect. Therefore they should be more interesting

for us. We are going first to focus on abiotic changes. The big changes in climate were attributed

first to low-frequency events, such as the astronomical movements of our planet. However, the evidence of the global change has triggered an enormous development of the studies in

paleoclimatology (Bjork et al., 1996; 1998). These studies have show that the climate did oscillate more frequently than we knew just some years previously. The Dansgaard-Oscher and

the Heinrich episodes at the end of the Pleistocene are a good examples. But climate also

operates as sudden changes in the way we have exposed: the astronomical variables act as dominant causes, while global factors like thermohialine flow are the determinant factor and

local events can be the triggering effects (Clark et al., 2001). A good example of these local


LAKE AGASSIZ

16.5 ky BP 12.8 ky BP 11.0 ky BP

9.6 ky BP ±7.7ky BP

BALTIC LAKE

Larger outbursts of the periglacial lakes that triggered cold periods (dates in 14C years BP)

OUTBURST OF MORE THAN 9,500 km3 of cold water triggers Younger Dryas preboreal cold oscillation PBO OUTBURST 163,000 km3 -> 40 m high waterfall (4-5 Sv flux in a year)

10.95-10.85 ky BP Billingen outburst temperature drops 15°C in Swedish lakes in 70 years

9.9-9.5 ky BP outburst of 1.5 to 3 Hm3 every second -> 25 m high waterfall temperature drops 1.5-3 °C in North Atlantic and 8°C en Greenland

Fig. 1_ Dates of the main sudden outbursts of the large glacial lakes during the Late Glacial (after Clark et aI., 2007; Bjorck et aI., 7996; Barber et aI., 7999; Hostetler et aI., 2000) .

changes that trigger a big climatic change are the great glacial lakes' (Agassiz and Baltic)

sudden outbursts (fig. 1), that should have detonated the cold periods, the Younger Dryas

and Preboreal Oscillation (Barber et al., 1999; Hostetler et al., 2000). These sudden changes

occured over a very short time span. Some of the outbursts of those big lakes could inject

from 4,600 to 163,000 km3 freshwater into the sea in about a year (Teller et al., 2002). The

Agassiz lake drainage into the St. Lawrence for instance at the beginning of the Younger

Dryas (about 12,900 calibrated calendar years BP, before the present) probably lasted a few

months to a few years (Col man, 2002).

So it seams clear that the main climate oscillation happened in a human life span and

could have catastrophic effects on humans. Nevertheless, they had a different impact on

prehistoric hunter-gatherer societies. Despite the resemblance of the climate changes and

their global effect, societies had different degrees of tolerance and capacity for adaptation.

This was dependant on the organization of their strategies, on their economic cycle, on their

degree of specialization, mobility and in a determinant way on the density of population and

on the degree of stress in their relationship with the resources exploited in the environment.

That is on the level achieved in the contradiction between production of goods and social

reproduction (Estevez et al., 1998). The case of the Cantabrian coast in northern Spain during the last cold pulse is very inter

esting. During the Late-glacial period we witness a continuous development, but after 10ky

years BP a total change becomes evident, both in the technology and in the strategies of

exploitation of resources (Gonzalez, Dfaz, 1991, p. 60). This event is correlated with a signifi

cant reduction of sites dating between 10.1 and 9.6ky BP (Gassiot, Estevez, 2004).

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·]2

-34

-36

-" -40

·42

·44

Monticchio

.=~----= 1\ .. Medi terraneo ~~l.l..,~,1iJII..\"'~ . ...+.-------

1]000 12000 11000 10000 9000 8000 7000 6000 5000

Valle del Ebro e interior

Cantabric

GISP

d l eo MfA$URMENT$ Cor~ GJSP2 Greenl.mrl

Grootes et al., 1993 StuiVt'f et.11., 1993 Moose et al. , 1994 $o\'Vt!rs el .11., 1993

-46 +-~13"":"OOO~~1200"'0~1~'()()("')~)~11OOO-"'91"'1OO""""'aO-OO-7"'OO"'O ~6000"""~5000~""""""------cal BC

Fig. 2. Cumulated Probability frequencies for the 14C dates in different regions of the Iberian Peninsula compared against the temperature curve of GISP in Greenland and Monticchio Lake in Italy. The arrows indicate the hiatus in Cantabrian Spain compared with the continuity in the Mediterranean basin area.

This change and this hiatus (fig. 2) are significant because they are synchronous with the sudden climate change observed in the ice cores of Greenland and in the sediments of the European lakes. The event is also correlated with other similar regional events (Estevez, 2005): there is a scarceness of archaeological sites and a hiatus in southeastern and northern France, in Belgium, northern Germany and Poland. For instance, in the site Calowanie between level V (10.5 ky BP) and level VII (9.2 ky BP) there are levels of dunes, gyttja and peat. This phenomenon can be traced until the Argolide (in Franchthi Cave site in Greece there is also a hiatus between 10,460 and 9430 years BP).

Nevertheless, the event is not marked with the same intensity everywhere. In an area neighboring the Cantabrian coast, the Ebro valley as well as on the Iberian Mediterranean coast there is a reduction of dates for this period, but there are some that cover this lapse. Therefore, we have to conclude that this global change in climate did not affect evenly and did not bring the same consequences to all hunter-gatherer societies of the time. We have also to admit that these societies had the capacity to respond with strategic conscientious decisions to the stress of those sudden changes. The climate change was a difficulty that could be surpassed; but other groups could not get through this catastrophic handicap (Cueto et al., 2006). These differences can be seen comparing the Cantabrian region in the north of the Peninsula with Catalonia on the Northeastern mediterranean basin of the Peninsula (sites like Parco, Filador, Forada, Gai, Guineu, Cingle Vermell, Forcas and Margineda show dates corresponding to the hiatus recorded in Cantabria).


To explain this catastrophe in the Cantabrian region it would be enough to consider the

possibility for instance of a series of extraordinary copious snowfalls. These could be the right

trigger in a situation of specialized exploitation of red deer driven to its potential limits. The

delay in the generation of alternatives could be the cause for the persistence of the effect and

for the hiatus in settlement.

The effects of a previous climate change of a similar range (the called Allemd warming)

did not have a similar effect. There was a continuous process of development (from industries

of the Magdalenian to those of the Azilian phases). Therefore we can argue that the level of

social development (meaning both population and strategies) is the directional vector that

determines the evolution in the face of these sudden changes.

Earthquakes and tsunamis

These are the paradigms of catastrophe-bearing events. By some aspects they could be

considered as sudden change too. They are out of the normal frequency and range of gradual

phenomena of the same type.

Earthquakes are documented enough in prehistory and their effect can be seen in caves

occupied by people. It should be possible to synchronize the major events of this kind on a

regional scale. As far as we know, this research has not yet been conducted. The caves regu

larly visited and occupied by hunter-gatherer groups collapsed frequently, but until now

there is no clear evidence of a catastrophe of that kind affecting people living at the precise

time of the collapse.

Tsunamis are sudden changes of regional or supra-regional range that could heavily affect

littoral societies. In some cases they can reach tens of meters of height. So it is easy to conceive

their impact on societies living in close relation to the shores, for instance Tierra del Fuego

Canoers, whose settlements and activities were developed 90% no higher than 5 m above the

sea level.

The problem is the ability to detect this evidence. Studies on past tsunamis started to

develop only since 1990, when researchers began to identify and discriminate their effects

from other geomorphologic features provoked by transgressions, heavy storms and cyclones.

Its is not surprising that they were not recorded at all in the excavations of archaeological

coastal sites conducted before 1995. In Kamtchatka, an especially sensible zone, the mean of

their incidence has been calculated. There is a mean of one tsunami every 30 years and a

tsunami wave over 5 m high every 100 years. Since the oldest evidence is dated 7150 BP there

is a mean of a giant wave of more than 30 m high every thousand years, penetrating more

than 19 km into the landscape (Pinegina et al., 2003) . One of the most interesting tsunamis of European Prehistory because its intensity was the

Storegga tsunami dated by 14C to 7.3-7.2 ky BP. Waves reached over 30 m high. It could have

washed out the coasts of the North Sea, North England, eastern Scotland and Scandinavia

and perhaps even penetrated the Baltic coasts. Some of these coasts were populated by fisher

hunter-gatherers with a settlement pattern that exposed them to damage.

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The complicated geodynamics of all these coasts render difficult the identification of this

event: in Norway where the wave could be 16 m high the traces could be misinterpreted as

the results of the so-called Tapes transgression. However, I found some very suggestive data

that led to suggest on a catastrophic event (Estevez, 2005): a series of settlements that occu

pied a sensitive localization in Norway and Western Sweden interrupted their life close or

just before this moment. Disordered marine sediments that gave abnormal dates in series of

coherent absolute dates cover some of the abandoned sites.

The site called Lego on the south of Stavanger has been dated to 7.5 ky BP and is covered

by sediments that were described as different from the transgressive sediments of other sites

(for instance Jare and Dunde). It has been suggested that they were the result of a tsunami

(Bang Andersen, 1995). Something similar happened on the West Coast of Sweden on the site

of Balltorp (Nordquist, 2000). It was situated on an ancient island of the Gota fiord, one or

two meters above the then sea level. That meant that it was exposed to a wave of only 3 m

high. The latest date is 7240:!J30 BP, after which a thick level of coarse-grained sand covered

the site. The materials on this last level are jumbled. A wrong date of 7570±.150 BP (too old)

was obtained for this level. It is the only one of a series of 11 dates that is out of sequence.

There are other possible similar instances: the last occupation of the site called Bua

VasterGard, situated on a little ancient island less than one meter above sea-level ends at

7425 BP. Olmanas was inhabited until 7405 BP, Dammen until 7600 BP. Likewise for the sites

of Grand Belt: TudejFr0lunde is dated 7245 BP, Musholm 7490-7410 BP and Kalo Vig 7500 BP.

The last site to have been affected is Halsskov (6830 BP) which began over four hundred 14C

years after the event.

Globally, from 83 Mesolithic dates of the Swedish coast there is no date between 7400 and

7055 BP. This suggests a real important hiatus. It coincides with a moment that has been

defined as an "atypical of transition" between two well-established cultural periods of

Sadarna and Lilhut. The magnitude of the catastrophe is difficult to measure. It could depend

on the seasonality and the time of arrival of the wave during the day combined with the

topography, the settlement pattern and the capability of the population to react fast. Detailed

research on the coastal sediment profiles (on land and underwater) and their correlation with

the sedimentation in the sites, however, could reveal the impact of this and other events of

this type.

Volcanoes

The contemporary eruption of the Pinatubo volcano has enabled scientists to understand

how volcanoes influence the global climate. Big explosions like the Toba's occurring around

71 ky BP or like the South Italian eruption around 37 ky BP can provoke persistent winter

effects that could have had catastrophic consequences for hominids and even trigger climate

global changes (Stuiver et al., 1995) ruling over human evolution (Taberlet, Cheddadi, 1999; Fedele et al., 2002). A smaller eruption may also have an enormous impact on a regional scale

as the contemporary Hudson event in Patagonia has demonstrated. This effect on prehistoric


hunter-gatherer societies would have depended on random factors and again on the organi

zational strategies of groups. The eruption of the two Laacher See volcanoes on the Eifel in Germany took place in

spring, 12,916 BP. Three eruptive episodes lasted for some weeks; 6 km3 of lava, 20 km3 of ashes and other materials were lifted. A thick layer of eruptive materials covered 1,300 km2

of the middle Rhine valley. This layer which reached from southern Scandinavia to the Alps, could be 50 m thick near the eruption point. The river Rhine was dammed until it broke

through, provoking a big wave along its lower course. There is dendrochronological evidence in southern Germany and Switzerland of a period of seven years of volcanic winter as a

consequence of this eruption. The global climate, however, was not much influenced. The warm Aller0d period lasted 200 hundred years more. The same human groups (called

"Federmesser culture") settled very quickly again in the area, as indicated by the site of Bad

Breisig (Baales, 2000).

Biotic changes

The recent studies of proxies in cores more precise analysis and finer chronological discrimination enable one to state that changes in prehistoric ecosystems were not been

necessarily gradual, but much faster that we had thought previously. For instance, the mean

range of the maximal extremes in the frequency of pollen spectra is of about 142 years during

the Upper Pleistocene in the central Mediterranean (Allen et al., 1999). This change triggered by fast climatic events could affect Paleolithic societies in a way that we can not evaluate

until now. Animal species are submitted to fast changes: mutations, extinctions, and pandemics.

Extinctions may be considered as an extreme case of evolution in which natural selection does not operate on an individual level (intraspecific selection), but on a massive level of

species if the whole ecosystem level is taken into account (Beutler, 2003). I will focus now on the sudden changes that can be produced strictly by biotic causes. A

species can be reduced to extinction when its population falls under a non-renewable level.

The population density varies depending on the survival taxa of the newborns (submitted to

the reproductive potential) more than on the survival of adults (submitted to epizooties)

(Gaillard et al., 1998). Therefore, sudden changes have more incidence in the case of animals

with a long reproductive cycle (big animals), short reproductive lives, high fluctuating population, non social behavior, lesser population density (like the big carnivores placed on the

summit of the trophic level), and narrow ecological niche.

Chain effects and the chaotic structure of extinctions may allow non-scaled fractal approaches (Mandelbrot, 1982; Sugihara, May, 1990; Sole et al., 1997). However, the extinc

tions of the end of the Pleistocene may be not comparable to previous ones. It is not a simple

question of the time scale (unless we have a coarser chronological picture of the older extinctions) . The extinctions at the end of the Pleistocene are contemporaneous with the presence

of modern humans (MacPhee, 1999). So if they can be considered as sudden changes, then

they could have affected in a catastrophic way human populations depending on those

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West Europe

Russia

Baltic area

Wrangel island

Eurasia Global.

f) 180 GRIP

I I I I I I • I

16 14 12 la 8 6 4 o ki lo years ca l. BC

Fig. 3. Cumulated Probability frequencies for the 14C dates on mammoth bones in different regions of Eurasia including the dates of the late mammoths of the Holocene in Wrangel Island. On bottom the temperature curve as registered in the stable oxygen isotope contents of the GRIP core and showing (s lash) the Holocene boundary. The graph shows that the final extinction is biased by regions.

animals for their subsistence. We have to consider that the "modern human factor" is a new emerging issue, germane to these extinctions, that is a factor that could have been a relevant cause.

For some researchers (Graham, Lundelius, 1984; Graham, 1990; Guthrie, 1984; 1990; Lister, Sher, 1995; Grayson, 2001) the extinctions at the end of the Pleistocene were driven by natural factors (climatic or biotic changes) or were not necessarily sudden. These explanations are biased because they relay on an a priori idea about human incompetence (because the insufficient technological and demographic level existing at that time) to provoke the massive kill. The argument against these hypothesis is that the extinction of these species are nor contemporaneous nor are they synchronized with the global change towards the Holocene (compare fig. 3 and 4). Furthermore, the hypotheses of strictly natural causes of extinction driven by the climate are unlikely, because most of the extinct species were survivors of tens of similar climate changes throughout the Pleistocene (most species of the extinct American megafauna had for instance a very old pre-quaternary origin).

The islands may be considered in this respect as laboratories of larger-scale processes like extinctions, and there we can easily correlate the human first arrival with the extinctions of the native species. The synchrony of the first massive record of human activity in America


14000

13000

12000 -

11 000

10000 ._--;;:::::"' __ _

9000

BOOO F=========:::::> 7000

'000

5000'~~~~~~~~~~~~~~~~7~O~~~~5~~~--~ , , ., ., ., ., ., ., ., .,

Holocene

::: +. ~~~~~~~~..."."~~~.....,.,~,.....,.",="'"~",-~.".,,...~....-{" 14000 13000 12CXJO i llW 10000 'moo 8000 . 7000 6000 5000 4000

years car BC

Fig. 4. Cumulated Probability frequencies for the 14C dates on last megafauna bones of the extreme south

of South America plotted against (on bottom) the paleotemperature curves of the Vostok and the Sajama cores. There is a remarkable hiatus in the beginnings of the Holocene but extinction does not occur at the

moment of the climate change.

with the mass extinction of the native fauna led scholars to other explanations. Some hypotheses link indirectly the human presence as a secondary factor for naturally-triggered extinctions: humans could be the final concurrent factor for environmental stress that affected species (Haynes, 1991 or Stuart, 1991 and 1999); humans could act indirectly changing vegetation, or introducing new competitive species (Grayson, 2001), or new devasting hyperdiseases (MacPhee, Marx, 1997).

In to the face of these positions, the hypothesis of overkill directly driven by human newcomers was formulated and modelized (Martin, Wright, 1967; Martin, Klein, 1984; Nitecki, Nitecki, 1984; Martin, Steadman, 1999; Schuster, Schtile, 2000 ... ). This has been called the "Blitzkrieg" or "advance wave" hypothesis (Young, Bettinger, 1995).

The problem with this hypothesis is that it proposes a very short time lapse for the entire human spread over America coinciding with the Clovis technology (the so called "Clovis first" hypothesis), whereas there are some traces of very much older occupation all over the continent. The hypothesis does not consider that the extinction of some of the common species had a precedent in Europe and Asia where the modern human presence is perfectly registered for tens of thousands years before.

Almost all of these hypotheses, however, need to purge the undesired (too modern or too old) radiocarbon dates to match with the record. To solve these contradictions it is possible to argue another alternative model that matches better with the set of existing absolute dates without conducting a subjective process of eliminating those considered "inconvenient".

The colonization of new spaces can follow three steps: a preliminary surveying phase (with a great impact on the fauna but few direct traces left). This can be the case of some

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32

Mediterranean islands like Cyprus or the Balearic islands (Simmons, 1999; Geddes et al., 1986 or Masseti, Darlas, 1999); a second consolidation step (with deep impact and much of evidence) in which the people adjust the technology to the exploitation of the local fauna and terminate the more sensible species in a catastrophic way; and lastly a final phase (with little biotic impact, changes in technology and regionalization) when people readapt to the consequences of overexploitation and extinction of a significant spectrum of the previously existing faunas .

In this case, catastrophic extinction should be considered as a result in changes in society and technology. This happens not in a gradual but in a non-linear and sudden process. In the case of mammoths for instance, we may explain the extinctions as a case of this kind of sudden changes: the dominant cause could be in some cases the climate change and the low rate of reproduction. The determinant cause could have been the constant increase in human population pressure and the trend towards the exploitation of a resource of maximal return. The trigger may have been a sudden change in the technologies allowing people to surpass existing barriers and to access easier to this prey (through new weapons and techniques, like Clovis/Cola de PescadolJobo complex spear-points) or entering (through for instance the management of thermal technologies) into preserved and uncolonized areas where the animals, undisturbed, could reproduce until that time.

We can explain in that way extinctions without recurring to dramatic synchronous universal environmental changes or to the massive interference of humans. In any case, whatever the cause and the actual time span of the process, the biotic change of species at the end of the Pleistocene can be considered as a sudden change that has had a deep impact and even catastrophic effect on some human populations.

Conclusion

We will have to accept and to incorporate the impact of sudden changes in Prehistory, as Paleontology has done. Paleontological events such as the extinction of the dinosaurs are not incompatible with a scientific approach, and does not contradict the Theory of Evolution, but complement the explanation, on another level or scale, the effect of Natural Selection. At the same time, the gradual action of Natural Selection cannot be understood without sudden changes on a molecular DNA level, caused perhaps by astronomical radiation.

Naturally, we have to continue studying gradual long-term major changes, although we have to ask also how can they be linked with short-term small changes, or how small very gradual changes become (by accumulation of their effects or triggered by other independent phenomena) a qualitative change or a sudden phase change (Estevez, 2005).

The differentiation between sudden changes and catastrophes that they may generate is a fundamental issue. Their study can contribute to demonstrate, in the more optimistic view, that societies can react in front of sudden changes. That some of these became a catastrophe or not is just a question of how we do manage our knowledge and on the actions that we will

undertake to prevent the effects.


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