Science: Steven Mithen says the way to understand how the human mind has developed is for psychologists and archaeologists to finally join forces. Marek Kohn investigates
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A plump, glowing PowerMac computer sits on the desk in Steven Mithen's office at Reading University. It's a desirable piece of kit, and Mithen would like to buy a second for his Department of Archaeology. But he can't, because it's obsolete. It's a year old.

The contrast with the stuff of Mithen's work could not be more extreme. As well as being what he calls "a humble dirt archaeologist", he has taken a series of bold steps in the construction of a "cognitive archaeology", which allies itself with other disciplines in order to understand how the mind evolved. Among all the mysteries it needs to illuminate, none is more enigmatic than that of the hand-axes. These stone tools varied a little in size and shape, but essentially conformed to a single pattern, with two faces, and shaped like a pear in outline. General-purpose tools rather than axes as such, they were a transcontinental product, now left scattered in a trail from the southern limits of Africa to the gravel beds of the Thames Valley. Nearly 1.5 million years elapsed before they became obsolete.

Prior to early humans making hand-axes, they simply split rocks apart to create sharp-edged tools that were crude but serviceable. They continued to do so after the appearance of the hand-axe, indicating that they still found rough and ready tools useful. Hand-axes are undoubtedly better tools, as archaeologists can confirm by making their own and trying a spot of Stone Age butchery for themselves. But they aren't that much better. The improvement in performance they offer is not a convincing

explanation for the extra effort they require - or the extra brainpower. Not does it explain the fact that some are too big or too small to be of any practical use, or the apparent obsession with symmetry that some of them seem to express.

What hand-axes demand is that we try to understand the minds of their makers. As Steven Mithen points out in his new book, The Prehistory of the Mind, these minds were human, but fundamentally different from our own. "In our culture, normally anything that has an imposed form has symbolic meanings," he says. "We're dealing with something that has imposed form but has not got symbolic meaning, and we have great difficulty in getting our minds around that. The challenge of archaeology is to conceive of a different past, and conceive of a human mind - an intelligent mind - that hasn't got a symbolic capacity."

Mithen insists that in our ancestors, some forms of intelligence were highly developed, but not the sort measured by IQ tests. And he admits that, try as he might, he can't get the hang of hand-axe knapping.

To illustrate his vision of the ancient mind, Mithen draws an analogy from a different strand of his professional experience. He spent three seasons of fieldwork at the Italian abbey of San Vicenzo, in Molise, discovering a complex process of architectural evolution that spanned the first millennium of the Christian era. The mind, he later realised, has evolved in a similar way, with older structures modified and new ones added.

Mithen is both inspired and provoked by the rise, over the past 15 years or so, of evolutionary psychology. One of the central themes of this ambitious young discipline is that the mind comprises separate units, known as modules, devoted to specific

tasks. Darwinian theorists, notably Leda Cosmides and John Tooby, believe that evolutionary pressure would have placed a premium on specialisation. Cosmides and Tooby argue that systems dedicated to solving specific types of problem would reduce errors, and speed up decision-making. They extend the argument made long ago by Noam Chomsky in respect of language, that the speed at which young children learn certain sorts of things implies that they must have neural structures already configured to process certain types of information.

Mithen agrees that, besides a language module, we probably have an intuitive capacity for psychology. It is vital to have a "theory of mind", an understanding that other people have minds like one's own, in order to understand their behaviour and to

engage in relationships - whether co-operative or manipulative - with them. The lack of a theory of mind, dubbed "mind-blindness" by the Cambridge psychologist Simon Baron-Cohen, is autism.

We are also intuitive physicists, Mithen suggests: children rapidly learn to distinguish between animate and inanimate objects, and readily grasp the properties of the latter. He believes we may be natural biologists too: contemporary hunter-gatherers have encyclopaedic knowledge of the flora and fauna around them, and humans have been hunter-gatherers for all but the last few moments of their evolutionary history.

Adding putative modules to the list is a favourite pastime of evolutionary psychologists, whose suggestions range from maps to a sense of justice. The result is a model of the mind as, in Leda Cosmides's analogy, a Swiss Army knife. At this point, Mithen objects. It's not that he feels the list-making has got out of hand, but that the analogy refutes itself.

The very fact that a human being can compare the mind to a Swiss Army knife, packed with specialised devices which cannot be used in combination, is a striking illustration of the fact that the mind is radically unlike the Swiss Army's equivalent ofthe hand-axe. The early human brain was not organised in such a way that knowledge about tools could be applied to knowledge about minds. But the distinguishing characteristic of the modern mind is its ability to carry information across domains. Withinthe domain of language, the prime dynamic is metaphor. The root of much of our vocabulary and all of our verbal creativity, it often leaps off into a sort of applied surrealism. Cognitive evolutionists can now choose between the Swiss Army knife and Mithen's alternative, the cathedral.

Actually, Mithen doesn't object to Cosmides's vision of modularity. Evolutionary psychologists do not, of course, imagine that modules remain opaque to each other. Rather, they argue that modularity enhances our thinking in some dimensions, limits it in others. "A substantial amount of our thought is still domain-specific," Mithen agrees. "We think much better about some issues than we can others, and sometimes we don't integrate our knowledge fully in the way that we should." These biases are signs of the ancient foundations on which the cathedral of the modern mind has been built.

He is speaking about what goes on in individual minds, but his real complaint about evolutionary psychologists is that their discipline has failed to integrate a vital body of knowledge. "What irritates me profoundly is how they ignore the archaeological record," he says. In his book, he emphasises that cognitive archaeology and evolutionary psychology need each other.

Any account of cognitive evolution has to account for aspects of the archaeological record, such as the hand-axe, which appear bizarre to the modern mind. Mithen points out that the stone tools made by early humans demanded as much intelligence of their makers as those produced by anatomically modern humans, who appeared about 100,000 years ago. Yet, when they used bones or antlers as tools, they modified them only slightly, if at all. Modern human hunters developed tool-kits comprising ranges of weapons designed for specific tasks, but Neanderthals did not, though their brains were as large as ours.

His own reading of the record, combined with his reading of modularity theory, has produced an account of the prehistory of the mind as a dialectic between generalised and specialised forms of intelligence. The human lineage descended from primates which had evolved relatively high levels of general intelligence. Mithen argues that it is this type of intelligence which enables chimpanzees to make very simple tools, such as the sticks with the leaves stripped off that the primatologist Jane Goodall saw her subjects use to "fish" for termites. On the other hand, he considers that chimpanzee social behaviour is sufficiently complex to suggest the existence of a special module governing it.

While complex, however, chimpanzee social behaviour does not incorporate a material culture. They appear to teach each other about tool use only rarely, and they do not use objects to signal social status. Mithen argues that the tool domain and the social domain are closed to each other.

Similar cognitive barriers, he suggests, prevented the Neanderthals from making connections between the domain of stone tools and the domain of hunting. They were unable to apply what they knew about stone to what they knew about animals. Mithen compares the early mind to a Romanesque cathedral, in which several chapels are separated by thick walls and low vaults, rendering the sound of prayer in one chapel almost inaudible in the rest of the building.

A key point of Mithen's argument is that if you want to create powerful general-purpose systems, you need to base them upon specialised ones. Computer programmers develop the components of large programs separately, then integrate them into the whole package. So, once the early human mind had built a group of specialised modules around the core of basic primate general intelligence, it was then ready to develop more powerful general faculties.

The distinguishing characteristic of the modern mind, in Mithen's phrase, is "cognitive fluidity". If it were a cathedral, the modern mind would be a Gothic one. In such designs, Mithen writes, "sound, space and light interact to produce a sense of almost limitless space". In the modern mind, the barriers between the modules have been knocked down, creating a sense of almost limitless mental possibilities. This is the mind that can create art, science, and religion.

Perhaps the greatest marvel of the archaeological record is the sudden dawn of diversity, about 50,000 years ago. An unprecedented creativity appears to descend upon humankind, attested by tools and the cave paintings of France and Spain. Eventually, around 10,000 years ago, people began to practise agriculture.

This sudden ferment appears to come out of nowhere. Humans had acquired their modern anatomical form 50,000 years earlier. Archaeologist Clive Gamble and radical anthropologist Chris Knight have noted that this looks like a social revolution, not a biological evolution. Mithen thinks the cultural explosion resulted from increasing cognitive fluidity, a process that started 100,000 years ago with the integration of the social intelligence module and that devoted to natural history. "Thought about the social world, thought about the natural world; they're working together. But what remains" - to be integrated - "is thought about the material world; technical intelligence."

He dips a toe into dangerous waters with his suggestion that different populations completed the modern architecture of fluidity at different stages, between 60,000 and 30,000 years ago. The notion that independent populations are likely to follow the same evolutionary path is a shaky one, and carries echoes of past furores about claims that some living populations are more cognitively developed than others.

Whatever his peers' verdict may be on the details of his vision they seem likely to applaud his strategy of integrating different intellectual domains. Such praise was a feature of comments upon an earlier essay on the hand-axe mystery. Mithen points out that at the far end of the era they study, palaeolithic archaeologists meet up with evolutionary biologists, while at the near end, they make contact with the social anthropologists. "I see an archaeologist as somebody who not only brings them together chronologically, but also tries to bring them together intellectually," he says.

It isn't a straightforward task. He is organising a degree course in cognitive evolution, at the master rather than the bachelor level. It's too interdisciplinary for the structures of undergraduate teaching, he explains. You can't integrate the modules enough.

Steven Mithen's `The Prehistory of the Mind' (Thames & Hudson, pounds 18.95) is published 7 October. Marek Kohn's `The Race Gallery: The Return of Racial Science' is published in paperback by Vintage, pounds 7.99.