Science: Babies: A hormonal conundrum

In the thin, cold air 4,000 metres above sea level, the Aymara Indians of the Bolivian Andes tend their livestock and grow their crops as they have done for generations. Yet their continued survival is a paradox for Western science.

Any Aymara woman wandering into a European gynaecology clinic for an examination would almost certainly be classed as infertile. Blood tests would show such low levels of the hormones progesterone and oestrogen that doctors would assume her ovaries were not working properly.

How the Aymara women continue to conceive and carry children, then, is the question Dr Gillian Bentley of Cambridge University's department of biological anthropology is trying to answer. Working with Bolivian scientists and American colleagues, she is comparing hormones in samples from Aymara women following their traditional lifestyle with those of their relations who have migrated to cities like La Paz. This will pinpoint whether the differences are genetically based or a result of environmental factors such as diet.

Trained originally as an archaeologist, Dr Bentley is now a specialist in the relatively new science of reproductive ecology. She studies the environmental factors affecting fertility in different ethnic groups around the world. These are increasingly showing that there is far greater variation in human physiology than the medical text books would claim. And often other groups living simpler lifestyles show lower levels of reproductive hormones. So it may well be western women whose bodies are unusual, and these findings could have important health implications. Breast cancer and other diseases are influenced by levels of steroid hormones.

Dr Bentley's research therefore touches on two areas - having children and keeping healthy - that affect virtually every human being alive. As a consequence she can rarely be assured of being alone with her thoughts. Whether in the dental hygienist's chair or on an aeroplane flight, she often finds herself giving impromptu explanations of her work. "I get a lot of satisfaction from knowing that most people at some point in their lives are interested in this topic, because they want to have children or want not to have them," she says. "Very often when people hear what area of research I do, they give me their whole reproductive history - it shows that these issues strike a deep chord in them.'

However, as an anthropologist Dr Bentley also knows she is working against the clock in trying to understand how human society evolved. Over the next few years the numbers of ethnic groups following their traditional way of life will diminish as they are swallowed up by the modern technological world. For some, these changes may bring benefits - but not for the majority. "They usually become the poorest of the poor, they are dispossessed with little access to income, adequate nutrition or healthcare," she says. "So, many of the populations who can tell us most about the evolutionary strategies that have typified humans throughout most of our history may disappear rapidly during the next millennium."