'Why there's so much cancer'

One in three people will suffer cancer in their lifetime, and Professor Mel Greaves, of the Institute of Cancer Research, believes this may be the penalty we pay for being human and successful. Because of our ancient evolutionary biology, our cells are programmed to proliferate, migrate and occasionally mutate and this provides an inherent risk of cancer for us and other animals. But modern man and woman have adopted lifestyles and diets which are fundamentally at odds with our Stone Age genetics.

We smoke, sunbathe, eat too much and exercise too little. Now Professor Greaves points an accusatory finger at sex, noting first that the virus responsible for cervical cancer is sexually transmitted. More than that, he says men were not designed to copulate beyond their prime, and that women were adapted by evolution to spend most of their younger years pregnant or breast-feeding. These two factors, he believes, linked to hormonal changes, may have contributed significantly to the increases in breast and prostate cancer of recent decades.

Women have a particularly bad deal in the cancer stakes, according to Professor Greaves, who is also director of the Leukaemia Research Fund centre at the Institute. Surgery, drug therapy, screening and early diagnosis have been successful in reducing death rates, "but something is obviously very wrong here," he says in his new book Cancer, the Evolutionary Legacy (Oxford University Press, £19.99). "An awful number of relatively young to middle-aged women do develop breast cancer and even more have silent and incipient cancers that never see the light of day. Why have mammary glands become a minefield of genetic mishaps?"

He believes the most convincing reason for the majority of breast, uterine or ovarian cancers is that they reflect an "own goal" for being human and female. Our hormones are based on a five-million-year-old genetic programme that primes for regular pregnancy and lactation (the production of breast-milk). The female reproductive system is programmed for 35 years of constant, cyclical but non-seasonal activity, just like that of our closest relatives, gorillas, chimpanzees and orangutans. The hormones oestradiol and progesterone are secreted into the blood at certain phases and induce breast tissue to proliferate. Sitting in his offices over-looking the Fulham Road, Professor Greaves draws a stark comparison between modern woman and females of hunter-gatherer societies. Primitive Eve would have been fertile between the ages of 17 and 45, had an average of six births, with breast-feeding lasting for around 18 years.

This means she would have avoided more than 200 menstrual cycles, hormonal stress and risk to the ovaries and the breast.

Modern woman, on the other hand, is likely to become fertile at around 12 and a half, but not have her first baby until nearly 30. She is likely to have two children and breast feed for only around three months each.

In rural African communities, where breast cancer is rare, breast-feeding for 18 months or more is common. Early pregnancy is also known to give protection against the disease, possibly because it affects the important breast "stem" cells at a critical stage, weeding out those which are damaged.

Professor Greaves says: "It may be that in the future, girls reaching puberty will be able to take a hormonal pill that mimics pregnancy to control the oestrogen pump and avoid the constant hormonal stress. I think that is where the real breakthrough will come." For men, it is prostate cancer that represents one of the biggest hazards. It is now the second most common male cancer and is diagnosed in 21,000 patients each year and claimed 9,500 lives in 1998. Professor Greaves says the reasons for its increase are not clear but are not solely to do with better, earlier diagnosis. It may, on the other hand, have something to do with sexual activity. Men are not biologically designed to have sex over the age of 50, nor to live to the age of 70 or more. As Professor Greaves says: "What other mammal, including dogs and our great ape relatives, continues to indulge in sexual activity long after it can successfully compete with younger, fitter males for the favours of the harem?''

The answer is none, and possibly that might give us an insight into the extraordinary increase in prostate cancer. Before the 20th century, the disease was considered rare but it will soon be more common than lung cancer. In hunter-gatherer societies, having a large prostate gland might give a man an advantage over his rivals in the prospect for a successful fertilisation.

Evolution has also equipped men with the capacity for a steady flow of the male hormone testosterone. But yet again this may carry a delayed penalty.

The failure to turn off the "testosterone tap" and continuing to have sex may, as men age, promote the growth of small prostatic cancers or malignant, life-threatening cancers. A few studies, but not all, have linked prostate cancer to a higher than normal level of sexual activity in later life, but Professor Greaves says this merits further study.

He thinks cancer cells have a kind of blind selfishness which harks back to our early evolutionary history. "There is no malice in cancer, only an ancient memory and drive in cells to survive and clone themselves," he says. He feels it is important for people to understand that cancer is, in one respect, the price we pay for our own evolutionary success, paradoxical as it may seem. "I wanted to explain why there is so much cancer, and why we have had such a hard time treating it."

The very properties that make man adaptable, strong, and clever, which drive our fertility and ensure our longevity, also promote the risk of cancer. Most tumours appear long after we have passed our biological prime, so as far as natural selection is concerned, they are invisible.

"But why do humans have so much of it?" asks Professor Greaves. "Old elephants don't have as much cancer as we do. Human behaviour has escalated the risk of cancer and several other 'modern' diseases substantially."