Most people who have considered how humans evolved from apes have concluded that the development of language was crucial to the process. Unfortunately it's impossible to know how or when language appeared in our species, although this has not prevented a large amount of speculation. McCrone has to go in for a fair amount of this: for example, he refers to the "halting proto-speech" of Homo erectus and tells us that the Neanderthals could probably tell someone to shift over at the fireside or to watch out for the sabretooth creeping up in the bushes, but not much more. However, all this is really very uncertain; for all we know, the Neanderthals may have passed the winter evenings round the camp fire reciting long epic poems to one another.
The replacement of the Neanderthals by modern man between about 40,000 and 30,000 years ago is taken here to signal the arrival of a modern type of consciousness, complete with art, religion, and culture. But we know that anatomically modern humans were present alongside Neanderthals long before this, and for many thousands of years there was little cultural difference between the two groups, so the evidence that Cro-Magnon man had advanced language skills is a little equivocal. The appearance of art in the form of palaeololithic cave paintings is capable of more than one interpretation and does not necessarily prove that the artists in question possessed full linguistic abilities—a view recently argued by Nicholas Humphrey.
McCrone attaches a lot of importance to the "inner voice", an internal running commentary which he thinks is the origin of self-awareness and which he says is present in all of us, but he offers no evidence for his claim that this is a universal human experience. Do forest-dwelling hunter-gatherers (the few who are left), for example, have this inner voice, and is it exactly the same with them as with us? For that matter, do all members of our own society have it? And what happens to people who lose the power of speech because of a stroke: does their inner voice then fall silent, and, if so, are they no longer self-aware? The whole subject needs a longer and fuller treatment than it gets here. McCrone says that little has been written about the inner voice, which makes it rather risky on his part to generalize about it. (He fails to mention Julian Jaynes, who did write about it at length: see my essay, Julian Jaynes Revisited.)
This book is a compact and readable introduction to the subject for anyone who is new to it, though the lack of an index is regrettable. However, it's the kind of book that make one keep muttering to oneself: "Yes, but…", and few of one's potential objections are discussed at all, let alone in any detail. The relation between brain and language, for example, is a great deal more complicated than you would suspect from reading this book. Most of the questions that it touches on, often rather briefly, are now addressed in considerably more depth by Terrence Deacon in The Symbolic Species.