In his "Afterthoughts", William Calvin explains that it took him several years to work out what to do with an article he had written titled the "The great climate flip-flop". His solution was to republish it as the last third of a book that otherwise deals with the earliest stages of human evolution in Africa. Calvin's rationale lies in his claim that repeated occurrences of rapid climate change during the Pleistocene period created the selective pressures for the evolution of human intelligence. This may be the case, but the abrupt change in subject matter and style between the first two-thirds and the remainder of this book is as great as any climatic flip-flop of the past.
Since the mid-1970s, a remarkable record of Pleistocene climate change has been acquired, initially from marine sediments and then from ice cores, that provides an astonishing level of detail. We now know that complete glacial-interglacial cycles have occurred approximately every 100,000 years during the past million years; that the transition between cold and warm periods was often remarkably rapid; and that many climatic fluctuations have occurred within each cycle. Exploring the relationship between such change and human and cultural evolution is a key task for palaeo-anthropologists. It is, however, frustratingly difficult to relate the poorly dated fossils and archaeological sites to the detailed climatic sequence.
A Brain for All Seasons is written as 35 short chapters, each supposedly an email communication from Calvin to his seminar group as he travels through Europe and Africa, and flies home to Seattle across the North Atlantic. The first 20 provide us with his thoughts about the impact of rapid climate change on human evolution and are "sent" from locations as varied as Darwin's Down House in Kent and Olorgesailie in Kenya. They are written in an informal, almost folksy style that meanders from one topic to another, and frequently digresses into anecdotes. There is little development of academic argument to support a great deal of assertion.
Calvin is drawn towards several ideas that the majority of "pros" - as he refers to palaeo-anthropologists - frown upon, including the "aquatic ape" hypothesis and the possibility of natural selection operating on groups. He claims that climatic change and human evolution are connected because a succession of "boom and bust" environments involving widespread droughts created population bottlenecks through which only the most intelligent hominids survived. Although this is not a new idea - Rick Potts's Humanity's Descent (1996) provides a more thorough study - Calvin puts greater emphasis on the abrupt and short-term climatic fluctuations than have previous writers.
A neuroscientist by training, his knowledge about human evolution, the brain and language is broad, and he writes engagingly on these topics, often summarising ideas developed in his previous books. He returns, for example, to his "handaxe as Frisbee" idea - the notion that the ovate and pear-shaped bifaces made by Homo erectus (among others) were designed for throwing at herds of animals - now relating this to the significance of waterholes at times of drought. Although accurate throwing is a remarkable human achievement, Calvin's treatment of the archaeological data is cavalier, making it sound simpler than it really is.
Indeed, Calvin should be more careful about his archaeological claims: contrary to his text, there is no evidence that handaxes appeared 1.8 million years ago, that hominids were in Europe by 1.7 million years ago, and that sites in South America date back 20,000 to 30,000 years. Endnotes are provided to substantiate some of his claims (none of the above) and for further reading, but these are not actually denoted in the text, supposedly "to avoid superscript clutter". Hence their value is hugely diminished.
Calvin uses long quotes throughout his book, often dedicating a page to one or more passages from the works of climatologists and palaeo-anthropologists. At times the book seems more like an anthology, and the prose in these quoted passages often outshines his own.
The final 12 instalments of Calvin's email course deal with the causes of climatic change, focusing on the abrupt shifts of the Late Pleistocene. His emphasis is on how gradual global warming can lead to a sudden and dramatic cooling, owing to alteration in ocean circulation. This part of the book is the most successful in terms of popular-science writing, as he draws on the latest research to provide a thorough and accessible explanation of the causes of recent changes in climate. He predicts that future abrupt climatic cooling, paradoxically induced by global warming, will tear apart civilisation's institutions, armies will fight for the remaining resources and a world full of despotic governments will arise. He suggests that pre-emptive strikes to block Scandinavian fjords that would prevent the influx of fresh water into the North Atlantic, and thus change ocean circulation, should be seriously considered. It is all very American - looking for the dramatic big fix rather than ways to minimise global warming and hence reduce the risk of catastrophic cooling.
For readers not familiar with Pleistocene climate change, the book will seem the wrong way round - the evidence and explanations for abrupt shifts in Late Pleistocene climate should come before the possible impact on human evolution of similar changes earlier in the Pleistocene and Pliocene. The book is better suited to those with some knowledge of the fossil and archaeological records and able to appreciate Calvin's simplification of, and often failure to acknowledge, the controversies over their interpretation. If read as a polemic it is certainly enjoyable, often thoughtful, sometimes entertaining, but always with an important message for those concerned with both the past and future of our species.
Steven Mithen is professor of early prehistory, University of Reading.
A Brain for All Seasons: Human Evolution and Abrupt Climate Change
Author - William Calvin
ISBN - 0 226 09201 1
Publisher - University of Chicago Press
Price - £17.50
Pages - 341