Lewis Wolpert: 'Despite the success of evolution, our bodies have flaws'

Theodosius Dobzhansky, the geneticist, said: "Nothing in biology makes sense except in the light of evolution." How right he was. But this has only quite recently been recognised to be true of illness and medicine. Darwinian medicine is an exciting new field. Yet when I say this to clinical colleagues they look rather blank, so I refer them to the key book Why We Get Sick by Randolph Nesse and George Williams.

Despite the brilliant, blind success of evolution, our bodies have flaws that cause illness. Particularly important is the role of genes, which can predispose us to many illnesses, from heart attacks to schizophrenia. Often the mutations are in genes that predispose us to illness because their benefits outweigh their costs. This is true of illnesses such as depression, where the faulty genes are related to emotions like sadness that are highly adaptive; they help us to survive, and so have been selected for in evolution.

The best example is sickle-cell anaemia, found in significant numbers of people in malaria areas. The haemoglobin in red blood cells folds abnormally, giving the cell a sickle shape, making its passage through tiny blood vessels difficult, causing bleeding, pain and early death. This occurs when the individual has two copies of the defective gene, but if they have only one such gene they have a great advantage; they are protected from malaria. Other cases are less clear; the cystic fibrosis mutation may have protected against diarrhoea, and Tay-Sachs disease, common among Jews, may have protected against TB.

Infections and cancer can only be properly understood in evolutionary terms. With infections, bacteria and viruses are attacked by the immune system, and they have evolved in order to avoid this attack. For each weapon the body uses to destroy them, they may evolve a means of escape. The resistance of tuberculosis bacteria is a major problem, and is nothing but the result of bacterial mutations leading to resistant strains.

With cancer, the cells change their character due to errors in the genes and so escape normal controls. The development of a malignant cancer is a tragic example of cells evolving, becoming more and more different and so being selected for survival, but with the ultimate death of the host.

One evolutionary puzzle relates to why we get a fever when we have an infection. Is the fever something that has been selected to help us recover, or is it just something that happens without adaptive advantage? There is evidence that fever is adaptive provided the body's temperature does not get dangerously high. Studies found that children with chicken pox who were not given aspirin recovered about one day sooner, and the same with colds. Yet it remains a puzzle why a fever helps to fight infection. Perhaps the immune system works better at the raised temperature.

Then there is ageing. We, and all animals, have to accept that once we have reproduced and raised our offspring, evolution has no interest our survival. Errors creep into our bodies with time, wear and tear and faulty genes. Repair mechanisms have been selected to keep the body going until reproduction is over. Then, alas, ageing has its way.

And then there are the diseases of civilisation, such as obesity and heart disease. Many dietary problems arise from a mismatch between tastes evolved for Stone Age conditions and their likely effects today – we eat more fat and sugar than we need.

This is an advancing field, and I now teach it. I hope it will become an essential part of medical education.

Lewis Wolpert is professor of biology as applied to medicine at University College London