Breast Cancer: evolutionary implications

I’ve not long finished researching and writing an article on Rachel Carson for the national Sunday newspaper in New Zealand. The article, naturally enough, was about pesticides, but it roamed into the area of breast cancer inasmuch as there is a suggested link between pesticides and breast cancer and, as fate would have it, Carson died of breast cancer. While I did not include this in the article, it seems apparent that our changing lifestyles are having a big impact on the dramatic increase in the rate of breast cancer over the last 100 years.

         I don’t know – nor do I want to know – when Rachel Carson had her first period, but at the turn of the 20th Century (she was born in 1907) it was likely that she would not have started menstruating until her late teens or, even, early twenties. If she were born now, she’d start somewhere around 11. Women are also delaying the age at which they start a family, if at all, and reducing the size of their families. What all this means is that women today experience many more periods than they would have in the past (during pregnancy and breast feeding, the menstrual cycle is suppressed), exposing them to many more monthly surges of oestrogen, the female hormone. And, one thing we know for certain is that in high doses oestrogens are carcinogenic.

And this is where I started to see how Natural Selection could come back to bite us. In the state in which human females evolved they would live decades less than we do now, they would become reproductively viable (ie start menstruating) only when they had garnered enough resources and fat reserves to ensure reproduction was likely to be successful, and once they started reproducing they would reproduce as often as they possibly could. But now in humans, reproduction is decoupled from survival. And what this all means is that women today may experience a 200% - 400% increase in the number of menstrual cycles they go through in their lifetimes, which is a heap more oestrogen to deal with in a system that evolved to operate under conditions where they experienced only a fraction of that. It’s like designing a tire to operate effectively on my Ford Mondeo and then putting it on a Formula One car: something’s going to give.

In a sense then, if Darwin is right and our bodies have evolved under a specific set of conditions, then we, through our ability to rise above the selective pressures that forged them, have exposed ourselves to dangers for which we have been unprepared by selection, thereby making us vulnerable.
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