Saturday, February 23, 2008

Evolutionary biology

A basic principle of homebirth advocacy is that women are "designed to give birth". A variety of further claims flow from this: birth shoulded be "trusted" because it is designed to work; the incidence of death in childbirth must be low or "we wouldn't be here"; complications are caused by intervening with the natural process instead of respecting it. These claims are spectacularly wrong because the basic principle is spectacularly wrong.

Women are not "designed". Their bodies are the result of evolution. Evolution does not create perfection. It produces a wide variation in many traits and then selects for those that improve survival in the existing environment. The scientific discipline of evolutionary biology seeks to apply our knowledge of evolution to understanding the basis of human health and health problems. It has important implications for our understanding of childbirth and it reveals why claims that women are "designed" to give birth vaginally are both unscientific and foolish.

There is a basic medical explanation for many diseases. For example, sickle cell anemia is caused by a genetic mutation that leads to "sickling" of the red blood cells. The abormally shaped blood cells clog the small vessels producing the characteristic painful symptoms. Evolutionary biology provides us with a reason why sickle cell anemia is so prevalent. Individuals who carry sickle cell trait (the unexpressed mutation of sickle cell anemia) are more likely to survive malaria and therefore, the trait is actually protective against a disease that is endemic in many parts of the world. When two individuals with sickle cell trait mate with each other and produce children, one quarter of the children will get a "double dose" of the trait and, therefore, suffer from sickle cell anemia. The overall benefit of sickle cell trait outweighs the cases of sickle cell anemia. Hence the trait (and the disease) have persisted.

Randolph Nesse, a pioneer in evolutionary biology, in an article in the Bulletin of the World Health Organization, asks:
Why are our coronary arteries so narrow and prone to plaque? Why hasn’t natural selection found a different, safer route for childbirth, or at least enlarged the pelvic opening? Why do we have third molars, given the trouble they cause? Why can’t our bodies create better defences against bacteria? Why is the endothelium so vulnerable to inflammation?
We don't yet know the answers to these questions, but evolutionary biology shows us that many diseases and complications, in childbirth and in other aspects of human health, have a genetic advantage in certain situations. For example:
... work on diseases of pregnancy continues to accumulate evidence that diabetes and hypertension of pregnancy may be related to evolutionary conflicts between interests of the maternal and paternal genomes and the fetus. Imprinted genes that influence these conflicts have profound implications for in vitro fertilization policies.
... If a rare allele slows ageing, can we assume it is superior? Not necessarily, as evolutionary patterns have shown that such benefits usually have other costs. If an allele accounts for much of the vulnerability to myopia, obesity or alcoholism, does that mean it is a genetic defect? No, it is more likely to be a quirk, a variation harmless in ancestral environments that results in disease only when it interacts with aspects of the modern environment. If a genotype is associated with allergic sensitivity to dust mites and asthma, does that mean it is abnormal? No, it may well protect against schistosomiasis, and that protection may influence both the prevalence of that infection and asthma in certain groups.
The implications of evolutionary biology for childbirth and homebirth advocacy are clear. Childbirth is not perfect; it is not even trustworthy. It represents a compromise between various evolutionary forces. Complications and death are quite common in childbirth, as in other aspects of human health. To deny this is to deny fundamental principles of biology.

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