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THE advent of agriculture 9000 years ago, which many people believe should have brought human evolution to a halt, in fact did no such thing. A long-term study of a farming community in Mali has revealed that human fertility there is still being shaped by evolution, despite the more reliable flow of food to the family table.

Some researchers think that the abundance of food produced by agriculture should have spared us from the rigours of natural selection. That means, they argue, that the rules governing our behaviour鈥攚hich affect how many children a woman has鈥攚ould not have adapted to the farming lifestyle but stayed tuned to the hunter-gatherer era. 鈥淚t鈥檚 hard to imagine that complex behavioural adaptations were constructed very recently,鈥� says Margo Wilson, an evolutionary psychologist at McMaster University near Toronto.

But Beverly Strassmann and Brenda Gillespie at the University of Michigan, Ann Arbor, have found that women in a West African farming society have evolved so that their birth rate is fine-tuned to their modern conditions. Strassmann spent three years living with the Dogon people in Sangui, southern Mali. She collected data from 167 women on the number and survival of their children. There is a trade-off between the two in any society, she says. If you go on having more children, eventually you no longer have the time or resources to rear them all.

For the Dogon, the data revealed that the optimum point comes at about 10.5 children. At this birth rate, a woman can expect around four children to survive to adulthood, and having more does not increase that number. For the first time in any human society, Strassmann has shown that more than 8 out of 10 Dogon women had a fertility rate statistically indistinguishable from this optimal figure. The figure of 10.5 is probably much higher than the likely optimum birth rate among hunter-gatherer women.

Strassmann suggests that instead of remaining static, the number of children a woman has is probably governed by physiological mechanisms that respond to the environment. She contends that there鈥檚 no reason to suppose the transition from hunter-gatherer to farmer would remove the pressures of selection.

Strassmann has also investigated what behavioural changes have caused the adaptation. Her analysis points to factors such as the age of a woman and of her husband, how long they were married and how often they had sex. But interestingly, the children鈥檚 nutrition doesn鈥檛 seem to have any significant effect, which suggests that other selection pressures are still at work. This leaves some intriguing questions, says Wilson. 鈥淚t would really be nice to know how it is achieved,鈥� she adds.