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This week, I have been writing about a new way of looking at the devastating brain condition Huntington’s disease. The genetic mutation responsible may also lead to higher IQ in childhood and early adulthood – and so could have been selected for in our evolutionary past.
While this idea isn’t proven, it might require a rethink of therapies being developed to treat Huntington’s. That’s because most of the leading candidates are based on the idea that the mutated protein that results from the genetic change is solely harmful and so needs to be eradicated from the brain at an early age.
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Huntington’s disease seems to be an example of a trend seen in many different areas of medicine, where we consider evolutionary explanations for illnesses and symptoms, an approach also called Darwinian medicine.
Instead of assuming that medical conditions happen because something in the body has “gone wrong”, we may gain a deeper understanding by viewing symptoms as a trade-off from genes that also improve our reproductive success.
You can read about how this may apply in Huntington’s disease here and below are some other examples.
Sickle cell disease
Sickle cell disease occurs when people have two mutated versions of the gene for haemoglobin, the protein in red blood cells that carries oxygen. The abnormal form of haemoglobin causes cells to deform into a curved shape, which means they die prematurely, leading to anaemia. They also sometimes get stuck in blood vessels, causing episodes of intense pain.
People with only one copy of the mutated gene have no symptoms, but if they catch malaria from a mosquito bite, red blood cells harbouring the parasite also deform into a sickle shape and are eliminated by the spleen. This gives people who are sickle cell carriers a great advantage in parts of the world where malaria is rife.
The evolutionary explanation for why the sickle cell mutation has persisted hasn’t changed treatments for the condition, one of which now includes an innovative gene therapy, recently approved in the UK. But it does explain why people whose ancestors came from hotter parts of the world, where malaria is more common, are more likely to carry the gene, which helps guide who should be offered genetic screening during pregnancy.
Fever
A raised temperature is usually caused by infections. The standard medical advice for anyone whose temperature goes above about 38°C (100°F) has been to use over-the-counter medicines such as ibuprofen and paracetamol (known as acetaminophen in the US) to bring it down. But in the past few years, there has been a rethink.
The body’s temperature is carefully controlled by the hypothalamus at the base of the brain. During an infection, our temperature rises for a reason: many bacteria and viruses reproduce more slowly at higher temperatures, while our immune cells seem to work better.
This means that taking drugs to bring down temperature could actually be counterproductive. Even intensive care doctors are rethinking how much they should intervene with high temperatures.
Medical advice to the public has also started changing in some countries. In the UK, for instance, National Health Service websites say that drugs such as paracetamol should be taken only to combat discomfort or distress from the high temperature, not just for the sake of it, saying it is “the body’s natural response to fighting infections”.
Pharmacists can give further advice on over-the-counter medication.
Cancer
Cancer has long been understood as a condition where evolution is taking place within an individual. Tumour cells arise because they have genetic mutations that make them better at surviving, reproducing and spreading to new sites in the body.
But so far, this knowledge hasn’t influenced the main cancer treatments: surgery, radiotherapy and chemotherapy. Now, some researchers believe they can develop better therapies by tracing the “evolutionary tree” of cancer cells within any one person.
This could allow the creation of drug therapies targeted to some of the earliest mutations in the cancer tree’s “trunk”, which are more likely to kill all the cancer cells. Existing drug therapies could also be made more potent by giving them intermittently or in lower doses, in the same way farmers may fine-tune their use of pesticides to avoid selecting for insects with resistance to these chemicals. You can read more about these strategies in this long read here.
Anxiety
The causes of mental health conditions seem harder to unravel than with most physical illnesses as we have no genetic tests, blood tests or brain scans that can diagnose or define them.
Nevertheless, , a psychiatrist at Arizona State University, believes that seeing anxiety disorders such as panic attacks through the evolutionary lens can be helpful. He says in his book Good Reasons for Bad Feelings that anxiety could be seen as having a “danger sensor” with a setting that is too sensitive.
As Nesse says in this , imagine early humans approaching a water hole, on the look out for signs of predators. If their “danger sensor” is too sensitive, they will flee at the first rustle in the bushes and consequently go thirsty. If it isn’t sensitive enough, however, they may be killed. As the cost of being too insensitive is death, it makes sense for our sensors to be on a setting where they sometimes give false alarms, or unwarranted anxiety.
And in the same way that we may put up with a smoke alarm that is too sensitive and occasionally goes off when we burn toast, evolution could have put up with a too-sensitive danger sensor to raise our chances of survival in a more risky environment. Nesse says these kinds of metaphors have helped some of the people he treats to understand their anxiety and see their symptoms improve.
