Psychological Notes: The Stone Age mental toolbox we inherit

THE HUMAN brain is an extraordinary organ. It has allowed us to walk on the moon, to discover the roots of matter and life, and to play chess almost as well as a computer. But this virtuosity raises a puzzle. The brain of Homo sapiens achieved its modern form and size between 50,000 and 100,000 years ago, well before the invention of agriculture, civilisations and writing in the last 10,000 years. Our foraging ancestors had no occasions to do astrophysics or play chess, and natural selection would not have rewarded them with more babies if they had. How, then, did our outsize, science-ready brain evolve?

This puzzle has frustrated our attempts to understand the mind as part of the natural world. But the puzzle can be solved with a key idea: the process of natural selection equipped our ancestors with a mental toolbox of intuitive theories about the world, which they used to master rocks, tools, plants, animals, and one another. We use the same toolbox today to handle the intellectual challenges of modern societies, including the most abstruse concepts of science and mathematics.

Humans evolved mental machinery that allowed us to co-operate with and outsmart the local flora and fauna. Vital to that machinery - what makes it so powerful and essential to foragers and neuroscientists alike - is its ability to analyse the world into useful categories. The world is a heterogeneous place. To generalise from our experiences properly and make good predictions about events unseen, we need to understand something of the causal structure of the world - its contents and the laws that make it tick. Thanks to our ancestors' mental toolbox, we seem to be endowed with several kinds of intuitions that do just that.

And this brings us to how Stone Age minds grasp modern science. Formal sciences grew out of their intuitive counterparts. The conviction that living things have an essence, for example, is what impelled the first professional biologists to try to understand the nature of plants and animals by cutting them open and putting bits of them under a microscope. Anyone who announced he was trying to understand the nature of chairs by bringing them into a laboratory and putting bits of them under a scope would be dismissed as mad, not given a grant.

But modern science forces us to make some changes in our thinking, including turning off parts of the intuitions out of which it grew. Newton's first law states that a moving object continues in a straight line unless acted on by a force. Ask college students what happens to a whirling tetherball that is cut loose, however, and a depressingly large minority say it would continue in a circular path. The students explain that the object acquires a "force" or "momentum" that powers it along the curve until the momentum gets "used up" and the path straightens out. Although erroneous, the students' beliefs are completely understandable since we evolved in a world with substantial friction that makes moving objects slow down and stop.

Modern science also pries our intuitive faculties loose from the objects they usually apply to and aims them at seemingly inappropriate ones. To do mathematics, we primates - visual animals - invented graphs. These allow abstruse concepts to present themselves to our mind's eyes as reassuringly familiar shapes. To do chemistry, we stretch our intuitive physics and treat the essence of a natural substance as a collection of tiny, bouncy, sticky objects. To do biology, we take our way of understanding artefacts and apply it to living things - organs as machines "engineered" by natural selection - and then to their essences, the molecule of life. To do psychology, we treat the mind as an organ of a living creature, as an artefact designed by natural selection, and as a collection of physical objects, neurons.

According to a saying, if you give a boy a hammer, the whole world becomes a nail. If you give a species an elementary grasp of psychology, biology, and mechanics, then for better and worse, the whole world becomes a society, a zoo, and a machine.

Adapted from Steven Pinker's `How the Mind Works' (Penguin, pounds 9.99)