In writing … I have tried to steer a middle course between what I see as two extreme approaches—one in which the data are simply a springboard for stimulating speculation about what might have happened in the past, and another in which they are meaningless except to test and eliminate all but one competing explanation of what really happened. The difficulty with the first perspective is that it emphasizes imagination over validity. The difficulty with the second, whose roots lie in a perception of how the physical sciences have advanced, is that it assumes an unrealistic degree of control over data quality and quantity.The book contains an exceptionally large number of excellent line drawings of fossils and tools by Kathryn Cruz-Uribe, which are accompanied by explanatory text paragraphs that bring out the significance of the features illustrated. The various topics in the text are discussed in a lot of detail, not all of which will necessarily be of interest to every reader. But the topics are well organised and there is often a short paragraph summarising the significance of what has gone before, so judicious skipping is easy. Another nice feature is that, within each chapter, Klein usually leads into the subject by outlining the historical steps that led to our present state of knowledge; this adds to the interest and also helps comprehension.
The book covers the whole span of human evolution. The first two chapters provide some basic information that will be needed to understand the main text, including natural selection, species classification and nomenclature, and the main methods used in modern archaeology, including dating methods. There then follow chapters dealing with primate evolution from the Late Cretaceous to the Late Miocene; the Australopithecines and Homo habilis; the evolution of the genus Homo; the Neanderthals and their contemporaries; and anatomically modern humans. A short final chapter draws all the threads together in a helpful summary.
In his preface to the second edition Klein tells us that he has moved from describing human evolution as having taken place in a series of stages to now favouring a branching scheme, in which at least three different human lineages emerged: Homo sapiens in Africa, Homo neanderthalensis in Europe, and Homo erectus in East Asia. (Since he wrote still other lineages have been discovered.)
Our earliest ancestors separated from the ancestors of modern chimpanzees between 7 and 5 million yeas (my) ago. They differed from the apes mainly in their lower limbs, which allowed them to a walk upright at least part of the time. They are given the name australopithecines, although, as Klein remarks, if they were around today we would probably call them bipedal apes. They were quite small, as were the earliest Homo, H. habilis. I hadn't fully realised that this was so. The same applies to the 'robust' australopithecines, whom I had vaguely imagined as being about the size of a gorilla; in fact, they too were small although very strongly built. H. habilis produced the first recognisable stone tools, although they were crude.
There is a good deal of disagreement about how to classify the early Homo species but in the version Klein prefers Homo ergaster is the species from which later Homo descended.
H. ergaster was the first species to colonize Eurasia, and by 1 my ago it had given rise to H. erectus in the Far East. By 600-500 ky [thousand years] ago it had produced additional lines leading to H. neanderthalensis in Europe and to H. sapiens in Africa. Arguably, about 600 ky ago H. neanderthalensis and H. sapiens shared a common ancestor to which the name H. heidelbergensis could be applied. H. heidelbergensis contains many fossils that were previously central to "archaic" H. sapiens.This "working phylogeny" certainly makes sense of what is often a confusing picture, although Klein warns us: "The reality may be far more complex and the sparse fossil record may obscure an even greater number of fossil species." He also points out that his scheme is controversial: many specialists classify later 'H. ergaster' fossils as H. erectus. This implies either that H. erectus evolved in Africa and then spread to eastern Asia or that H. erectus evolved in Asia and then spread back to Africa. Klein holds that H. erectus evolved from H. ergaster people who had migrated to Asia but that H. ergaster continued to evolve in Africa and gave rise to subsequent types of Homo.
Like all its predecessor, H. ergaster evolved in equatorial Africa. It was as large as modern humans and formed part of a trend to the possession of bigger brains. It made tools that were more sophisticated than those made by H. habilis, giving rise to the Acheulian "Industrial Tradition" which began roughly 1.7 to 1.6 my ago.
Palaeontologists disagree about whether the appearance of H. ergaster, and that of H. sapiens later, was a gradual process or occurred more or less abruptly. Klein favours the 'abrupt' view. It implies that there was some genetic change in the structure or function of the brain, although it is not at present possible to test this hypothesis from the fossil evidence.
The same kind of abrupt change, Klein believes, happened as recently as 50 ky years ago, at about the time when modern humans left Africa to colonise the rest of the world. Up to this time human technology had been relatively static, with few advances over many thousands of years. The Neanderthals and their contemporaries were apparently content to continue to make the same kinds of tools that had served them for millennia and showed no propensity to make art or develop a complex social life. After 50 ky ago modern humans, both in Africa and outside it, rapidly acquired the ability to manufacture more and more complex tools and weapons and produced an astonishing range of art. This enhanced innovatory capacity on the part of modern humans is probably what precipitated the eventual extinction of the Neanderthals.
This is a pretty comprehensive hook that is full of all kinds of fascinating facts. The author has his own views, but he gives fair play to others and everything is backed up with evidence. As one would expect in a book of this kind, ample references are provided. These are arranged in a way that makes them particularly easy to refer to; rather than appearing at the end of each chapter they are given in alphabetical order at the end of the book and are numbered sequentially; the corresponding numbers in the text allow quick cross-refencencing without breaking up the flow of reading too much.
I found that although the book contained more information than I always needed it is so well arranged and so balanced that I felt I was obtaining a clearer picture of human evolution than I had had before. And it could also be useful in the future to refer back to section, such as those on the evolution of the primates, which it seemed appropriate to skim over on a first reading.
All in all, I think this is an excellent piece of science writing. The only thing that prevents me from giving it a two-star rating is that, having been published in 1999, it is inevitably somewhat out of date. For example, Klein doubts that there was significant interbreeding between modern humans and the Neanderthals, but thanks to Svante Pääbo's sequencing of the Neanderthal genome we now know that this did occur. And Klein's view that there were probably more branches from the human tree has been confirmed by the discovery of the Denisovans. In general, genetic studies have gone considerably further in the intervening years than was the case when Klein was writing. We really need a third edition of the book.