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Vers une anthropologie des catastrophesActes des 9e Journes Anthropologiques de Valbonne

Sous la direction de Luc Buchet, Catherine Rigeade,Isabelle Sguyet Michel Signoli - ditionsapdca, Antibes /ined, Paris, 2009

Catastrophes or sudden changes.The need to review ourtime perspective in Prehistory

JordiEStvEz

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

Abstract Catastrophes and sudden changes were denied by prehistorians, as they are difficult to integrateinto a scientific processual explanation. However, sudden changes are part of normal changes in nature andin society and cannot be ignored even to explain gradual changes. Sudden changes can switch to catastro-phes depending not only on their character, but also on some apparently random features, as well as on thecapacity of reaction of the society affected. The development of sciences allows one to begin detecting suchsudden changes and their effects, and the epistemological advance in Science facilitates the integration and

investigation of these complex phenomena. Here I explore some examples, such as the probable impact ofthe Storegga tsunami on the coastal Mesolithic settlements of Scandinavia, the gap in the populationsequence of the Cantabrian coast that followed the beginning of the Holocene, the impact of the Laachervolcanic eruption, and lastly, the relatively sudden extinction of the American megafauna.

Catastrophes ou changements brusques.La ncessit de revoir notre perspective en Prhistoire

Mots-cls Catastrophes, thorie des changements subits, Prhistoire.

Rsum La science prhistorique a ni limportance des changements brusques dans lexplication dudveloppement des socits. Cependant, ces mutations soudaines font partie de la transformation normale

de lUnivers et sont mme indispensables pour comprendre les changements graduels. Le fait quun chan-gement brusque se transforme en catastrophe ou non ne dpend pas seulement de sa propre nature, maisde la capacit de raction de la socit sur laquelle il agit. Le dveloppement pistmologique et les techni-ques archologiques des sciences auxiliaires permettent aujourdhui de documenter des changementsbrusques et de commencer explorer les effets de ces vnements complexes sur les socits prhistoriques.Nous prsentons quelques exemples : limpact possible du tsunami de Storegga sur les populations mso-lithiques de la Scandinavie, la chute du peuplement de la cte cantabrique au dbut de lHolocne, limpactde lrupion des volcans Laacher et, enfin, lextinction relativement soudaine de la mgafaune amri-caine.


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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 mytho-logical thinking gave a rise to a heavy debate with the defenders of the explanations given byreligions, particularly in the Bible. Catastrophes have been assimilated into most societies andexplained as tales and myths. Illustrated classes have used those events and sometimes thepossibility to prevent low-frequency but extraordinary large wave phenomena to manipulatethe society for their own benefits in the name of divinities. Therefore it is not surprising thata scientific and objective perspective tends to deny the relevance of catastrophes in rationalistexplanations.

Scientific geology grew stimulated by the needs of critical raw materials like metals andcharcoal. 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 pheno-mena developing through time (like Paleontology or Prehistory) needed another basic prin-ciple Gradualism to obtain satisfactory explanations. Darwins Theory of Evolution byNatural Selection could not have been formulated without those two supporting principles.

The theory of Catastrophes of the prestigious paleontologist Cuvier was a a goodreference point to collate the account in the Bible with fossil record in the struggle with thescientific perspective. In this fighting context Science emphasized the need of long time andgradual processes to cope with the cause-effect relationships. This long time and gradualperspective 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 Hegels idealist position into a realist andscientific perspective. Meanwhile, at the end of the 19th century, advances in the theory ofPhysics and Mathematics (specially differential calculation and non linear equations) beganto allow surpassing mechanical and simplistic explanations. Despite those facts, those kindof explanation predominate since the end of the following century.

The defenders of the ancient myths of origin adopted the adequate socio-political strate-gies against the scientific explanations to allow the continuity of religions (Estvez, Vila,1999). This strategy was centered (and succeeded specially) on the more sensitive case of theexplanation 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 onlyapparently declares the acceptance of Gradualism and Actualism. This organic model ofdevelopment sustained that civilizations were born, grew, collapsed and died off, beensubstituted by younger, more powerful cultures, that spread their ideas and people.Nevertheless the gradualist and evolutionist paradigm at the end of the 20th century hasreduced this previous explanation, and the remains of Catastrophism were swept out ofScience after having been stigmatized by Acadaemia. The significance of sudden changes forsocial change has been refused. Paradoxically, at this moment, other Sciences, such asEvolutive Biology or Geology began to review the importance of those sudden changes (see


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cataStRopheS oR Sudden changeS. the needto Reviewou Rtime peRSpective

for instance Leroy and Stewart, eds. 2002). That is the case of the Theory of PunctuatedEquilibrium in the new Evolutionist Synthesis. The scientific study of Change has begun toincorporate 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 andintermittent 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 ofthe Academic community (by the principle of resilience and conservatism). But it is also dueto the loss of prestige of Catastrophism and to the technical impossibility to focus to theadequate chronological scale to detect the phenomena of sudden changes and to correlatethem with social changes in Prehistory.

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

drive these societies to crises through their same mechanism of reproduction and continuitysafety systems.

In any case, even the first prehistoric hunter-gatherer societies, the most defenselessbecause of their lesser foresight capability (due to their lower technological development andless accumulated experience) could be differently affected depending on their economic andsocial strategies. The archaeological study of these interactions between the environment andthe social development can contribute with relevant information to fully understand socialprocesses (Estvez, 2005).

A new theory of change

The development of dating systems and of the different paleoenvironmental proxies canallow approaching the study of sudden changes in Prehistory. But to revert the situation it isnecessary to define better the concept of sudden change and to distinguish it from the conceptof catastrophe. It is necessary to strictly delimit what is a catastrophe and how to proceed toits study from a scientific perspective, that is not only in a descriptive way, but so far aspossible 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 consistentcomplex system of explanations.

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

Catastrophe can be defined as a negative effect triggered commonly by a sudden or radicalchange 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. Followingthis description, it is not appropriated to speak about naturalcatastrophe like we often do innatural language. This is true firstly, because the concept actually refers to an effect and notto 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 negativenessis relative anyway. Something negative to a partiality can benefit the totality. In Nature Sciencesfor 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 suste-nance 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 AncienRgime. But there is no doubt that it benefited the development of rationalism, freethinkingand the rule of the right.

Thus the problem that arises is to determine what the partiality is that has to be affectedin order to consider the consequences of an unforeseen event of change as catastrophe. But itis not a simple question of the relativity of the damage. It is also a question of the indefinitelimits 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 andhow to differentiate between a sudden and a gradual change. In our case the change betweenhunter-gatherers and farmers has been described as the Neolithic Revolution, but we haveenvisaged it as a very slow change.

We should define the speed, frequency and magnitude of the changes that can beconsidered 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 changesthat 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 theaffected 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 notsubmitted to regular cycles or when the wavelength is so long that prior similar events didnot remain registered in the memory of the society. Finally, sudden changes can produce acatastrophe when they do not follow a process of a single cause but when they are the resultof a non-linear process. In summary, the catastrophic effect of sudden changes dependspartially on the hazard, on the non-linear exponential multiplying results of their effects, aswell as on the own structure of the society: on its capacity to remember, to prevent and toassimilate 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 ofchange. In that sense it would be very interesting also in Social Sciences to focus thesephenomena 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. Wecould gain in this way a wider and comprehensive view of the development of human soci-eties, because those sudden changes have accompanied humankind from the beginning ofour trail.

The length of this record on social behavior of more than two million years allows oneto verify when different kinds and scales of sudden changes occur (making longer or


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complex cycles more evident, calculating probabilities, looking for multi-causality andtrigger effects) and how they affect human societies. When and why do they produce a cata-strophic effect? We may get a deeper and a more equilibrated perspective of the significance

of catastrophes.

Capability of Prehistoric Archaeology to explore different kindsof sudden changes

We are aware that the internal dynamics of the societies themselves, and the effect of theaccumulation 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 naturalchanges. I will follow the different scales and ranges of changes analyzing to what extent theycan 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 theseexternal events with scepticism, because of their apparently hazardous character and theirenormous scale and disconnection to events that develop on Earth.

Nevertheless, the evidence collected of their effects are now enough to force to considertheir impact and subsume them as trigger or causes of paleontological events. Since theseventies (Alvarez etal., 1980) different, direct and indirect, evidence of impacts of meteorites

have been collected which researchers have tried to correlate with mass extinctions. Impactsof different ranges from a local to a global magnitude have been detected and the effects donot seam linearly correlated to the size of the meteorite. But the capability to detect directlythe effect of a collision is very low: only a minority are solids and are of the right size toimpact on the Earth surface. Furthermore, this probability is even less if we consider thatmore 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 differencesin the way of crystallization and in the structure.

Therefore, to try to detect these events, we have to look better for other indicators such astrace 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 iceand sediment cores, that are used in paleoclimatology in which they have not been speciallystudied. Some difficulty arises from the abundance of some noble gases like Argon can alsobe 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 craterof 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 Quaternaryand coincided with the broad replacement of species that took place. The Barringer Craterthat bears the name of its discoverer is closer in time and can be dated around 30 ky BP. That

approaches the possibility that this event could affect indirectly hominid populations.Apart from collisions of large extraterrestrial masses, subatomic particles also arrive andradiate 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 astro-nomical causes such as explosions of supernovas (McHargue etal., 1995). But it depends alsoon global changes in the Earth. Sudden changes in the Earths magnetic field and its effect onthe high atmosphere protection belt may influence the incidence of these particles. There issome hypothesis to explain by this process the great anomaly in the radiocarbon content(Jris, 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 twomaximum peaks in 10-Berilium and 36-Chlorine of cosmogenic origin and a minimum inpalaeomagnetism (Beck et al., 2001).

The data obtained by the models show that an alternative explanation based only in solaractivity together with paleomagnetism do not explain even half of the variability. Thus, thereis a possibility that radical changes in the termohialine marine circulation regime of theoceans were responsible for the anomaly. Relating all these possibilities and the more or lesscontemporaneous Barringer Crater, an intriguing and attractive hypothesis of meteorite crashcould 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 forother causes and triggers (climate changes, volcanoes) with higher probabilities because oftheir greater frequency. At the same time we can be calm, because the more frequent eventsof 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 befound 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 interestingfor us.

We are going first to focus on abiotic changes. The big changes in climate were attributedfirst 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 inpaleoclimatology (Bjrk et al., 1996; 1998). These studies have show that the climate did oscil-late more frequently than we knew just some years previously. The Dansgaard-Oscher andthe Heinrich episodes at the end of the Pleistocene are a good examples. But climate alsooperates as sudden changes in the way we have exposed: the astronomical variables act asdominant causes, while global factors like thermohialine flow are the determinant factor andlocal events can be the triggering effects (Clark et al., 2001). A good example of these local


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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 Dryasand Preboreal Oscillation (Barber et al., 1999; Hostetler etal., 2000). These sudden changesoccured over a very short time span. Some of the outbursts of those big lakes could injectfrom 4,600 to 163,000 km3 freshwater into the sea in about a year (Teller et al., 2002). TheAgassiz lake drainage into the St. Lawrence for instance at the beginning of the YoungerDryas (about 12,900 calibrated calendar years BP, before the present) probably lasted a fewmonths to a few years (Colman, 2002).

So it seams clear that the main climate oscillation happened in a human life span andcould have catastrophic effects on humans. Nevertheless, they had a different impact onprehistoric hunter-gatherer societies. Despite the resemblance of the climate changes andtheir 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 andon 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 socialreproduction (Estvez 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 10 kyyears BP a total change becomes evident, both in the technology and in the strategies ofexploitation of resources (Gonzlez, Daz, 1991, p. 60). This event is correlated with a signifi-cant reduction of sites dating between 10.1 and 9.6 ky BP (Gassiot, Estvez, 2004).

Fig. 1. Dates of the main sudden outbursts of the large glacial lakes during the Late Glacial

(after Clarket al., 2001; Bjorcket al., 1996; Barberet al., 1999; Hostetleret al., 2000).

Larger outbursts of the periglacial lakes that triggered cold periods

(dates in 14C years BP)

LAKE AGASSIZ

16.5ky BP12.8ky BP11.0ky BP outBurSt of MorE thAn 9,500 km3 cld wae

igges Yge Dyas

9.6ky BP pebeal cld scillai PBo 7.7ky BP outBurSt 163,000 km3 > 40 m ig waeall (4-5 Sv lx i a yea)

BALtIC LAKE

10.95-10.85ky BP Billige bsempeae dps 15 C i Swedis lakes i 70 yeas

9.9-9.5ky BP bs 1.5 3 hm3 evey secd > 25 m ig waeallempeae dps 1.5-3 C i n Alaic ad 8 C e Geelad


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

Nevertheless, the event is not marked with the same intensity everywhere. In an areaneighboring 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 anddid not bring the same consequences to all hunter-gatherer societies of the time. We have alsoto admit that these societies had the capacity to respond with strategic conscientious deci-sions to the stress of those sudden changes. The climate change was a difficulty that could besurpassed; 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 thePeninsula with Catalonia on the Northeastern mediterranean basin of the Peninsula (sites likeParco, Filador, Forad, Gai, Guineu, Cingle Vermell, Forcas and Margineda show datescorresponding to the hiatus recorded in Cantabria).

Monticchio

Mediterraneo

Valle del Ebroe interior

Cantabric

GISP

d180 MEASURMENTS

Core GISP2 Greenland

Grootes et al., 1993

Stuiver et al., 1993

Moose et al., 1994

Sowers et al., 1993

cal BC

cal BC

cal BC

7000

13000 12000 11000 10000 9000 8000 7000 6000 5000

13000 12000 11000 10000 9000 8000 7000 6000 5000

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13000 12000 11000 10000 9000 8000 7000 6000 5000

6000

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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.


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To explain this catastrophe in the Cantabrian region it would be enough to consider thepossibility for instance of a series of extraordinary copious snowfalls. These could be the righttrigger 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 andfor the hiatus in settlement.The effects of a previous climate change of a similar range (the called Allerd warming)

did not have a similar effect. There was a continuous process of development (from industriesof the Magdalenian to those of the Azilian phases). Therefore we can argue that the level ofsocial development (meaning both population and strategies) is the directional vector thatdetermines 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 beconsidered as sudden change too. They are out of the normal frequency and range of gradualphenomena of the same type.

Earthquakes are documented enough in prehistory and their effect can be seen in cavesoccupied by people. It should be possible to synchronize the major events of this kind on aregional 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 nowthere is no clear evidence of a catastrophe of that kind affecting people living at the precisetime of the collapse.

Tsunamis are sudden changes of regional or supra-regional range that could heavily affectlittoral societies. In some cases they can reach tens of meters of height. So it is easy to conceivetheir impact on societies living in close relation to the shores, for instance Tierra del FuegoCanoers, whose settlements and activities were developed 90% no higher than 5 m above thesea level.

The problem is the ability to detect this evidence. Studies on past tsunamis started todevelop only since 1990, when researchers began to identify and discriminate their effectsfrom 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 archaeologicalcoastal 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 atsunami wave over 5 m high every 100 years. Since the oldest evidence is dated 7150 BP thereis a mean of a giant wave of more than 30 m high every thousand years, penetrating morethan 19 km into the landscape (Pinegina et al., 2003).

One of the most interesting tsunamis of European Prehistory because its intensity was theStoregga tsunami dated by 14C to 7.3-7.2ky BP. Waves reached over 30 m high. It could havewashed out the coasts of the North Sea, North England, eastern Scotland and Scandinaviaand 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 thisevent: in Norway where the wave could be 16 m high the traces could be misinterpreted asthe results of the so-called Tapes transgression. However, I found some very suggestive data

that led to suggest on a catastrophic event (Estvez, 2005): a series of settlements that occu-pied a sensitive localization in Norway and Western Sweden interrupted their life close orjust before this moment. Disordered marine sediments that gave abnormal dates in series ofcoherent absolute dates cover some of the abandoned sites.

The site called Lego on the south of Stavanger has been dated to 7.5ky BP and is coveredby 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 siteof Balltorp (Nordquist, 2000). It was situated on an ancient island of the Gota fiord, one ortwo meters above the then sea level. That meant that it was exposed to a wave of only 3 mhigh. The latest date is 7240 + 130 BP, after which a thick level of coarse-grained sand coveredthe 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 BuaVsterGrd, situated on a little ancient island less than one meter above sea-level ends at7425 BP. lmanas was inhabited until 7405 BP, Dammen until 7600 BP. Likewise for the sitesof Grand Belt: Tude/Frlunde 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 14Cyears after the event.

Globally, from 83 Mesolithic dates of the Swedish coast there is no date between 7400 and7055 BP. This suggests a real important hiatus. It coincides with a moment that has beendefined as an atypical of transition between two well-established cultural periods ofSadarna and Lilhut. The magnitude of the catastrophe is difficult to measure. It could dependon the seasonality and the time of arrival of the wave during the day combined with thetopography, the settlement pattern and the capability of the population to react fast. Detailedresearch on the coastal sediment profiles (on land and underwater) and their correlation withthe sedimentation in the sites, however, could reveal the impact of this and other events ofthis type.

Volcanoes

The contemporary eruption of the Pinatubo volcano has enabled scientists to understandhow volcanoes influence the global climate. Big explosions like the Tobas occurring around71 ky BP or like the South Italian eruption around 37 ky BP can provoke persistent wintereffects that could have had catastrophic consequences for hominids and even trigger climateglobal changes (Stuiver etal., 1995) ruling over human evolution (Taberlet, Cheddadi, 1999;Fedele etal., 2002). A smaller eruption may also have an enormous impact on a regional scaleas the contemporary Hudson event in Patagonia has demonstrated. This effect on prehistoric


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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

ofashes and other materials were lifted. A thick layer of eruptive materials covered 1,300 km 2of 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 brokethrough, provoking a big wave along its lower course. There is dendrochronological evidencein southern Germany and Switzerland of a period of seven years of volcanic winter as aconsequence of this eruption. The global climate, however, was not much influenced. Thewarm Allerd period lasted 200 hundred years more. The same human groups (calledFedermesser culture) settled very quickly again in the area, as indicated by the site of BadBreisig (Baales, 2000).

Biotic changes

The recent studies of proxies in cores more precise analysis and finer chronologicaldiscrimination enable one to state that changes in prehistoric ecosystems were not beennecessarily gradual, but much faster that we had thought previously. For instance, the meanrange of the maximal extremes in the frequency of pollen spectra is of about 142 years duringthe Upper Pleistocene in the central Mediterranean (Allen et al., 1999). This change triggeredby fast climatic events could affect Paleolithic societies in a way that we can not evaluateuntil 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 selectiondoes not operate on an individual level (intraspecific selection), but on a massive level ofspecies 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. Aspecies 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 tothe 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 animalswith a long reproductive cycle (big animals), short reproductive lives, high fluctuating popu-lation, non social behavior, lesser population density (like the big carnivores placed on thesummit of the trophic level), and narrow ecological niche.

Chain effects and the chaotic structure of extinctions may allow non-scaled fractalapproaches (Mandelbrot, 1982; Sugihara, May, 1990; Sol etal., 1997). However, the extinc-tions of the end of the Pleistocene may be not comparable to previous ones. It is not a simplequestion of the time scale (unless we have a coarser chronological picture of the older extinc-tions). The extinctions at the end of the Pleistocene are contemporaneous with the presenceof modern humans (MacPhee, 1999). So if they can be considered as sudden changes, thenthey could have affected in a catastrophic way human populations depending on those


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animals for their subsistence. We have to consider that the modern human factor is a newemerging issue, germane to these extinctions, that is a factor that could have been a relevantcause.

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 naturalfactors (climatic or biotic changes) or were not necessarily sudden. These explanations arebiased because they relay on an a priori idea about human incompetence (because the insuf-

ficient 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 contem-poraneous nor are they synchronized with the global change towards the Holocene (comparefig. 3 and 4). Furthermore, the hypotheses of strictly natural causes of extinction driven by theclimate are unlikely, because most of the extinct species were survivors of tens of similarclimate changes throughout the Pleistocene (most species of the extinct American megafaunahad for instance a very old pre-quaternary origin).

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

Holocene

180 GRIP

West Europe

Russia

Baltic area

kilo years cal. BC

Wrangel island

Eurasia Global.

16 14 12 10 8 6 4 2 0

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 (slash)

the Holocene boundary. The graph shows that the fnal extinction is biased by regions.


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with the mass extinction of the native fauna led scholars to other explanations. Somehypotheses link indirectly the human presence as a secondary factor for naturally-triggeredextinctions: humans could be the final concurrent factor for environmental stress that affectedspecies (Haynes, 1991 or Stuart, 1991 and 1999); humans could act indirectly changingvegetation, or introducing new competitive species (Grayson, 2001), or new devasting hyper-diseases (MacPhee, Marx, 1997).

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

The problem with this hypothesis is that it proposes a very short time lapse for the entirehuman 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 thecontinent. The hypothesis does not consider that the extinction of some of the commonspecies had a precedent in Europe and Asia where the modern human presence is perfectlyregistered for tens of thousands years before.

Almost all of these hypotheses, however, need to purge the undesired (too modern or tooold) radiocarbon dates to match with the record. To solve these contradictions it is possible toargue another alternative model that matches better with the set of existing absolute dateswithout 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

Holocene

VOSTOK

SAJAMA

HIATUS

7000

8000

9000

10000

11000

12000

13000

14000

15000CalPAI (version Mar 2003)

6000

5000

210

-1-2-3-4-5-6-7-8

-9-10-11

BP

14

16

18

20

22

24

26

28

years cal BC

1300014000 12000 11000 10000 9000 8000 7000 6000 5000 4000

1300014000 12000 11000 10000 9000 8000 7000 6000 5000 4000

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.


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Mediterranean islands like Cyprus or the Balearic islands (Simmons, 1999; Geddes et al., 1986or Masseti, Darlas, 1999); a second consolidation step (with deep impact and much ofevidence) 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 littlebiotic impact, changes in technology and regionalization) when people readapt to the conse-quences of overexploitation and extinction of a significant spectrum of the previously existingfaunas.

In this case, catastrophic extinction should be considered as a result in changes in societyand technology. This happens not in a gradual but in a non-linear and sudden process. In thecase of mammoths for instance, we may explain the extinctions as a case of this kind ofsudden changes: the dominant cause could be in some cases the climate change and the lowrate of reproduction. The determinant cause could have been the constant increase in humanpopulation 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 surpassexisting barriers and to access easier to this prey (through new weapons and techniques, likeClovis/Cola de Pescado/Jobo complex spear-points) or entering (through for instance themanagement of thermal technologies) into preserved and uncolonized areas where theanimals, undisturbed, could reproduce until that time.

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

Conclusion

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

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

The differentiation between sudden changes and catastrophes that they may generate isa 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 catastropheor not is just a question of how we do manage our knowledge and on the actions that we willundertake to prevent the effects.


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