- Shopping Bag ( 0 items )
How did humankind deal with the extreme challenges of the last Ice Age? How have the relatively benign post-Ice Age conditions affected the evolution and spread of humanity across the globe? By setting our genetic history in the context of climate change during prehistory, the origin of many features of our modern world are identified and presented in this illuminating book. It reviews the aspects of our physiology and intellectual development that have been influenced by climatic factors, and how features of our lives - diet, language and the domestication of animals - are also the product of the climate in which we evolved. In short: climate change in prehistory has in many ways made us what we are today. Climate Change in Prehistory weaves together studies of the climate with anthropological, archaeological and historical studies, and will fascinate all those interested in the effects of climate on human development and history.
Cambridge University Press
0521824095 - Climate Change in Prehistory - by William James Burroughs
Chaos umpire sits,
And by decision more embroils the fray
By which he reigns: next him high arbiter
Chance governs all.
John Milton (1608-1674), Paradise Lost
There is a cosy notion that progress is a natural consequence of the development of human social structures. Reinforced by the rise of Europe from the Middle Ages and the subsequent exploitation of the New World, it is all too easy to forget past setbacks. 'Dark Ages' have punctuated the recorded history of our species. The period following the decline and fall of the Roman Empire is probably the best-known example, but sudden and catastrophic declines of earlier ancient civilisations are important reminders that progress is not an automatic part of the human condition. In popular culture this simple onward and upward view of human development extends back into the Palaeolithic: as the Earth gradually emerged from the ice age the human race stumbled from its caves and started its ascent to civilisation as we know it. While this is a parody of our current understanding about what really happened, it still lurks deep within our cultural subconscious. What it loses sight of is the extent of intellectual development that had been established in prehistory (Rudgley, 1998). In some instances discoveries were made independently at different places and at different times. These punctuated developments may have been a consequence of climatic events, and this tortuous process is part of the story explored.
Surviving the rigours of the ice age also profoundly influenced the evolution of modern humans. The fluctuations within this glacial period dictated how we spread out across the globe. They are hard-wired into us in respect of our genes, stature and health, and integrated into our attitudes to gender, warfare, animals and much more.
In exploring so many aspects of human life there is further complication. A surprising number of the areas of scientific research discussed here involve bitter academic feuds. At every turn throughout this extended interdisciplinary discussion we will find highly respected professionals slugging it out in august journals. The objective of the book is to present a balanced account of how the various debates fit into the wider picture, always recognising that this is a matter of tiptoeing through a series of intellectual minefields.
In the context of understanding prehistory, and how climate change, in particular, played a part in stimulating progress or bringing it to a grinding halt, several developments in the 1990s acted as the inspiration for this book. The first was the discovery of the images found in the Chauvet cave in 1994. When carbon dating (see Appendix) of the charcoal used in these breath-takingly beautiful drawings of animals showed that they were over 30 000 years old, the archaeological world was taken aback (Clottes et al., 1995).
This dating was some 15 000 years (15 kyr) earlier than had been expected, as the images bore a striking resemblance to the much better known drawings in the caves in Lascaux and Altimira that date back to around 17 000 years ago (17 kya). So rather than being the product of the developments that were seen as part of Europe emerging from the last ice age, these images were drawn by our forebears whose descendants had yet to survive the extreme stages of the last ice age, which plunged all of Europe north of the Alps and the Pyrenees into cold storage for over 10 kyr. The only significant difference in the images was that those from the earlier era depicted a world inhabited by more dangerous animals. In particular, the many images of lions (Fig. 1.1) are something that rarely appears in later artwork.
Figure 1.1 A painting of lions from the Chauvet cave, which has been dated as being over 30 000 years old. (With the kind permission of Jean Clottes.)
Image not available in HTML version
Inevitably, the question of the validity of the dating was raised. These doubts took time to address. In addition, the sensational nature of the Chauvet discovery diverted attention from the growing evidence of a much longer artistic tradition in Europe. In defending the Chauvet dates, improved measurements were obtained from a number of other French caves (Valladas et al., 2001), including recent exciting discoveries at Cosquer and Cussac. This analysis confirmed what was becoming increasingly evident from a wide range of sites that palaeolithic cave art was part of an artistic continuum dating back to before 30 kya.
The draughting skills of the people who had created these images transfix the viewer. Any frustrated artist, who has struggled to master the essentials on line and weight in drawing, can only genuflect to someone who had got it in one. This reaction was encapsulated in the earlier observation of an artist who had these skills in abundance - Pablo Picasso - who, on seeing the paintings in the Lascaux cave, observed, 'We have invented nothing!'
What are the implications of these skills surviving for so long? The oldest dates found so far are in the Chauvet cave (between 32 and 30 kya), while the most recent are found in the cave at Le Portel (11.6 kya; Clottes, 2002). Stop and think just how long this period is. It is some 800 generations, or more than ten times the period since the fall of the Roman Empire. This immense period of time suggests that in order for such a tradition to persist, there must have been an effective form of passing on this knowledge. Without it, the fundamental unity of this art could not have survived for so long. Possibly of even greater importance is that the assumption that was made before Chauvet was discovered - that the evolution of art had been gradual, from coarse beginnings rising to an apogee at Lascaux - cannot be sustained. The recent discoveries have shown that as early 30 kya sophisticated artistic skills had already been invented. So, even if the exercise of these skills lapsed from time to time, there was a social consciousness that enabled them to be sustained.
This sense of continuity raises fascinating questions about what was happening to the world during this immense period of time. Here we have the benefit of a second more consequential scientific development. Since the 1960s scientists have been drilling ice cores and making measurements of their properties. Snow deposited on the ice sheets of Antarctica and Greenland, and in glaciers in mountain ranges around the world, contains a remarkable range of information about the climate at the time it fell. Where there is no appreciable melting in summer, the accumulation of snow, which is compressed to form ice, contains a continuous record of various aspects of climate variability and climate change. This includes evidence of changes in temperature, the amount of snow that fell each year, the amount of dust transported from lower latitudes, fall-out from major volcanoes, the composition of air bubbles trapped in the ice and variations in solar activity. The best results are, however, restricted to Antarctica and Greenland, with more limited results from glaciers and ice caps elsewhere around the world.
The dramatic advance with ice cores came with the publication in the early 1990s of the first results of two major international projects: the Greenland Ice Sheet Project Two (GISP2) (Grootes et al., 1993) which successfully completed drilling a 3053-m-long ice core down to the bedrock in the Summit region of central Greenland in July 1993; and its European companion project, the Greenland Ice Core Project (GRIP) (Greenland Ice Core Project Members, 1993), which one year earlier penetrated the ice sheet to a depth of 3029 m, 30 km to the east of GISP2. These cores provided a completely new picture of the chaotic climate throughout the last ice age, the turbulent changes that occurred at the end of this glacial period and the stability of the climate during the last 10 kyr (a period known as the Holocene).
These chaotic changes were evident in many of the ice-core parameters, including rapid fluctuations in the snowfall from year to year and sudden changes in the amount of dust swept up from lower latitudes. The most spectacular results were obtained, however, by measuring the ratio of oxygen isotopes (oxygen-16 and -18), which provided an accurate record of regional temperature over the entire length of the ice core. The amount of the heavy kind of oxygen atoms, oxygen-18 (18O), compared with the lighter far more common isotope oxygen-16 (16O), is a measure of the temperature involved in the precipitation processes. But this is not a simple process. The snow is formed from water vapour that evaporates from oceans at lower latitudes and travels to higher latitudes. The water molecules containing 16O are lighter, and evaporate slightly more readily and are a little less likely to be precipitated in snowflakes than those containing 18O. Both effects are related to the temperature, so the warmer the oceans and the warmer the air over the ice caps the higher the proportion of 16O in the snow that fell. So during warm episodes in the global climate the proportion of the 18O in the ice core is lower.
These cores presented an entirely different picture of the climate during and following the last ice age. Added to the glacial slowness of changes that led to the building and decline of the huge ice sheets was a whole new array of dramatic changes (Fig. 1.2). While these long-term consequences remained, two exciting features emerged from the detailed record of the ice cores. First, they provided much improved evidence of the frequent fluctuations in the climate on the timescales of millennia that ranged from periods of intense cold to times of relative warmth. Second, and even more interesting, these longer-term variations were overlain with evidence of dramatic short-term fluctuations: over Greenland, annual average temperatures rose and fell by up to 10 °C in just a few years, while annual snowfall trebled or declined by a third. As the research team memorably described the patterns (Taylor et al., 1993), the climate across the North Atlantic behaved like a 'flickering switch'.
Figure 1.2 The changes in 18O/16O isotope ratio observed in the GRIP ice core for 200-year intervals over the last 100 000 years (0 to 100 kya), together with an approximate estimate of the changes in temperature that have taken place over this period. (Data archived at the World Data Center for Paleoclimatology, Boulder, Colorado, USA.)
Image not available in HTML version
As for looking farther back in time, just how much could be extracted from ice cores became clear from work in Antarctica. Results obtained from high on the ice sheet, by the European Project for Ice Coring in Antarctica (EPICA) (Fig. 1.3), extended records back through more than 730 kyr, covering eight ice ages (EPICA Community Members, 2004). When combined with earlier ice-core data and other records obtained from around world from ocean sediments, lakebeds and peat deposits, and stalactites and stalagmites, these now give us a remarkably detailed picture of the climate of the last few hundred thousand years. This is offering archaeologists the opportunity to look with far greater precision at the conditions that controlled the development of humankind during the last ice age and the warming that followed it.
Figure 1.3 The measurement of the deuterium/hydrogen isotope ratio (D/H) in an ice core drilled by the European Project for Ice Coring in Antarctica (EPICA) at Dome C in Antarctica, showing how the ice-core record extends back 736 000 years (736 kyr) covering the last eight glacial cycles. The black line is the average values for every 3000 years and the white line is the seven-point running mean of these data. (Data from EPICA community members (2004), supplementary information.)
Image not available in HTML version
The impact of climate change on social and economic development has been a part of historical analysis for many years. There is a tendency to think that human capacity to create the intellectual accomplishments that are so much a part of recorded history did not really blossom until the establishment of identifiable civilisations. In fact, the kernel of these processes formed while grappling with the hardships of the ice age. The evidence of cave paintings and other forms of artistic activity suggests that these intellectual capacities were well developed long before agriculture and the establishment of sizeable human settlements (Rudgley, 1998). It shows clearly that the desire to record accurately observations about the world around us, and to pass that information on to both contemporaries and subsequent generations, dates back to these times. That these images also exhibit sublime aesthetic and spiritual components resonates even more with our own experience.
These images contain something of ourselves and the ideas we seek to represent in art. This does not mean that our distant ancestors produced these pictures for the same reasons that we might today or that we are capable of explaining what their purpose was. Indeed, the danger of inflicting our current perceptions on their imagery takes us into complicated social and psychological analysis (Lewis-Williams, 2002, pp. 41-68). In particular, we run the risk of seeking to impose on palaeolithic ancestors our contemporary concerns about sex, social equality and gender roles. As one writer memorably observed, 'palaeolithic art has often been a "Rorschach [inkblot] Test" in that modern-day observers have tried to read into the mind and spirit of primitive humans, but they have perhaps learned more about their own psyches than about the primitives' (Wenke, 1999, p. 209). For the moment, all that needs to be said is that they already had highly developed understandings of the flora and fauna around them, and were superb draughtsmen or draughtswomen.
The fundamental question is how these creative features of the minds of humans so long ago, with which we can so readily identify, helped them overcome the challenges of the worst of the ice age. These intellectual capabilities were an integral component of their survival. They influenced how they evolved through the long dark night of the ice age and the chaotic dawn of the Holocene. Furthermore, the fact that there is a thread that links us to these people provides particular insight into how modern humans were able to seize the opportunities presented by the climatic amelioration when the ice age ended.
Another development of recent decades has transformed our thinking about human prehistory. This is the whole new science of genetic mapping that brings an entirely different perspective to our past. The discovery of the structure of DNA 50 years ago has altered how we view human evolution. It established the amazing concept that each of us, within our DNA, has a record of the entire evolution of humankind. The development of rapid and inexpensive methods of probing DNA sequences has led, since around 1980, to its application to evolutionary studies and to the creation of the subject of molecular anthropology. The most complete way of sequencing the human genome is to determine the exact order of the 3 billion chemical building blocks (called bases and abbreviated A, T, C and G) that make up the DNA of the 24 different human chromosomes. The sequencing of the entire genome, which contains some 30 000 genes, is the central challenge in the Human Genome Project.
The essential feature of DNA is that it carries the replication instructions for cell division. This process is carried out with extreme reliability, but in about one in a billion divisions mutations occur. This has the consequence that between successive generations these mutations slowly accumulate in the DNA of any species. It is the accumulation of mutations in the DNA that provides both the grist in the mill of natural selection and the metronome underlying the molecular clock. By comparing DNA sequences and measuring the incidence of genetic markers in human and animal populations around the world, it is possible to draw conclusions about the timing of separation of different species and different groups of humans.1
If this process required the seuencing of the entire genome and examining changes in all 30 000 genes it would be impossibly difficult. The breakthrough in the mapping process has come with the discovery that by sequencing two specific parts of human genome it is possible to explore the slow ticking of the human molecular clock. The first of these is mitochondrial DNA (mtDNA; for this and other technical terms, see the Glossary), which is a section that is some 16 000 base pairs long. This is only passed through the female line of the species and also has the benefit that mutations occur more rapidly there than elsewhere in the genome. Nevertheless, the process is very slow. If two people had a common maternal ancestor 10 000 years ago, then there would be one difference in their genetic sequences. The parallel development has been to study the differences in the Y-chromosome, the only purpose of which is to create males.
Studying the genetic variation of mtDNA and the Y-chromosome across human populations can produce objective data that provide new insights into human history over many thousands of years, such as the colonisation of previously uninhabited areas and subsequent migrations. Prior to this the only equivalent analysis relied on the study of languages to infer a pattern of human development. This work, sometimes termed glottochronology, cannot delve as far back into the past as genetic studies, and will not be considered in any detail in this book. Here we will concentrate on genetic mapping, which provides an independent picture of how modern humans peopled the world during and after the last ice age. This analysis must, however, be combined with other sources of knowledge, such as archaeological or historical records, to form a balanced view.
The next stage in this introduction is to confront the challenge of archaeology. It is often easier to write with confidence on fast-developing and relatively new areas of research, such as climate change and genetic mapping, than to review the implications of such new developments for a mature discipline like archaeology. Because the latter consists of an immensely complicated edifice that has been built up over a long time by the painstaking accumulation of fragmentary evidence from a vast array of sources, it is hard to define those aspects of the subject that are most affected by results obtained in a completely different discipline. Furthermore, when it comes to many aspects of prehistory, the field is full of controversy, into which the new data are not easily introduced. As a consequence, there is an inevitable tendency to gloss over these pitfalls and rely on secondary and even tertiary literature to provide an accessible backdrop against which new developments can be more easily projected.
© Cambridge University Press
1. Introduction; 2. The climate of the last 100,000 years; 3. Life in the Ice Age; 4. The evolutionary implications of living with the Ice Age; 5. Emerging from the Ice Age; 6. Recorded history; 7. Our climatic inheritance; 8. The future; Appendix; Bibliography; References.