Why we changed our minds

John Skoyles and Dorion Sagan's title consciously echoes Carl Sagan's The Dragons of Eden , an account of the evolution of human intelligence now more than a quarter of a century old. Its sequel updates, expands and reinterprets some of the earlier book's central premises in light of developments in neuroscience since the original publication.

The key concept in this new treatment is neural plasticity - the capacity of neurons to undertake new, reordered functions and activities during development, or in response to changed environmental stimuli (for example, injury or disease). This leads to a view of cortical function not as hard-wired as in the cyto-architectonic charts of many biology texts, but composed of software routines, programmed - and continually reprogrammed - by each of us, which the authors label "mindware". Prefrontal cortex mindware is particularly important in modulating, coordinating and organising routines originating elsewhere in the cortex. Individually, mindware "booting" may occur through play or, for language acquisition, through the infant repeating overheard sounds, the meaning of which can be explored in later verbal exchanges.

This emphasis on cognitive fluidity contrasts markedly with some other interpretations of mind evolution, such as the "Swiss penknife" model of domain-specific modules proposed by Leda Cosmides and John Tooby and, to lesser extent, the "cathedral architecture" analogue developed by Steven Mithen for interpreting the archaeological record. Curiously, these alternative models and their authors are missing from both text and the almost 80-page bibliography.

Early hominid environments were dangerous and food resources patchy and irregular, which placed a premium on individuals able to exploit kin relations and extend social links beyond the immediate present. Such pressures promoted symbolism, originally to stand for kin recognition and social relationships, enabling these to be maintained over time and space even when the relevant individuals were absent. These developments in turn lead to more complex social networks and the cognitive abilities to exploit these.

Symbolism is here seen as cognition's prime emancipator, while the flux of mindware routines and neural plasticity provides the nutrient medium within which emergent symbolism is seeded, and which it in turn feeds back to and further enriches. Language arose from proto-language (as seen, for example, in chimpanzees) through such developing symbolism, perhaps promoted by sexual selection on the part of those individuals who were the most effective communicators and so the most adept social manipulators. Broadly speaking, this capability characterised Lower and Middle Pleistocene Homo erectus groups.

Around 100,000 years ago, symbolism and the mindware networks associated with it evolved beyond this phase to encompass abstract processes, operations and extended symbolic systems, the manipulation of which enables us to carry out complex operations, greatly enhancing cognitive power. For Skoyles and Sagan, symbols are not just "propagated arbitrary associations" but "active shapers of the very substrate by which we act, think and feel.

Symbols, working together with our prefrontal cortex and neural plasticity, transform our minds and the nature of our consciousness. Our symbols are brain and mind changers." So while anatomically identical to our Aurignacian predecessors, cognitively we are different (as we are from our Tudor or Victorian forebears).

Evidence for this cognitive transformation first appears with the technological specialisation, art and decoration of the Upper Palaeolithic, about 40,000 years ago. The authors omit recent African discoveries of evidence that may be twice that age or more, but they stress that the potential for such developments could be much older, citing earlier isolated and ephemeral occurrences of individual elements that only regularly and repeatedly combine in the Upper Palaeolithic. There has to be a sustained need for such symbolic innovation, as well as the capacity to develop it.

Did the broadly contemporaneous Neanderthals possess similar cognitive capacities? Perhaps, but for whatever reason, they did not exploit them, so becoming extinct after about 60,000 years of co-existence with anatomically modern humans. Here Skoyles and Sagan disappoint, for they tamely conclude that Neanderthals failed to make these cognitive advances because their greater physical strength meant they had no need to: "Brains rather than brawn allowed our ancestors to edge out the Neanderthals." After all that has gone before, this simplistic conclusion is a major let-down.

Each chapter opens with an arresting anecdote or image - though sometimes its relevance is not always immediately clear, and this can impede continuity between chapters. Apart from this, the book is generally clearly and engagingly written.

But the authors labour under a major disadvantage in attempting to convey to a wide readership the findings of contemporary neuroscience and their implications, without using a single illustration. For those not deterred by such an omission, this account can be recommended as vivid advocacy for a particular interpretation of cognitive evolution; but for alternative perspectives they will also need to consult other authors, including those noted above.

Alan Bilsborough is professor of anthropology and pro vice-chancellor, University of Durham.