Catapulted into the depths of unconscionable pain, the unlucky souls who find themselves afflicted with migraine may ask themselves, “why me?”. It turns out that because genetic factors are a determinant for migraine susceptibility, and genetics are in turn subject to natural selection, the fact that migraines are still so prevalent in society points to a possible evolutionary advantage for their existence.
The following blog post will discuss what migraine is, what evolution is, and five evolutionary explanations that may account for the persistence of migraine.
What is a migraine?
A broad definition of migraine is that it’s a complex sequence of physiological events that unfold over hours to days inside the brain after being triggered (1). They are a reaction of sorts, similar to that of a fever or a sneeze.
However, unlike sneezes and fevers, it’s not entirely clear what problems migraines are solving. Instead, this common disorder has been likened to a biological glitch rather than serving any biological purpose, which is quite the evolutionary conundrum given that migraine is such a common human affliction (conditions that impair function generally lead to poorer survival of an individual and eventual elimination of the trait in the population).
Helping to curtail this confusion, science has put forward some evolutionary explanations as to why migraines persist in society.
What is evolution?
Evolution describes changes to the heritable characteristics of a species over time. According to Charles Darwin, if a heritable trait helps an individual to survive and reproduce in the midst of environmental pressures (think food scarcity, temperature, weather conditions, predators, pathogens, etc.), that genetic variation is more likely to be passed to the next generation. In contrast, traits which reduce reproductive fitness are eventually eliminated. This process is known as natural selection – advantageous traits are selected for, whilst disadvantageous traits are selected against.
Adaptations are features that support an organism’s survival by making them better suited to their environment. Organisms can adapt to an environment in a variety of ways including through structural, behavioural, physiological and biochemical means.
Evolution of a species occurs over many generations due to a change in the frequency of adaptive traits in the population. It is therefore not necessarily the strongest or most intelligent species that survive, but often the most responsive to change.
It’s difficult to comprehend that such monstrous pain suffered by migraneurs could provide an evolutionary advantage. However the fact that migraines still exist in the human population means they may indeed provide an upper hand, allowing us to survive long enough in our environment to reproduce and pass on the “migraine trait”.
Evolutionary explanations for migraine
Scientists involved in Darwinian medicine – a field of study that investigates human disease from an evolutionary perspective – have put forward five explanations as to why migraines have been favoured by natural selection (2):
Migraine as a defence mechanism
Common symptoms of migraine such as pain, nausea and vomiting whilst at first may seem like design defects of the body, are in fact biological defences that the body has evolved over time.
Let’s look first at pain. There could be two possible explanations for the painful headache that results from an input of strong sensory stimuli:
1.) Pain isn’t an evolutionary advantage in and of itself, but instead a secondary system of other central nervous system processes that themselves provide evolutionary advantages
2.) The pain provides some benefit to the body; most notably as a mechanism to avoid potentially harmful situations. Pain in migraine may encourage the sufferer to withdraw from triggering or complex environments, an advantage in times throughout history where overlooking predators or other dangers due to stimulus overload may have led to death.
Another defence mechanism attributed to migraine is the sensitivity migraineurs have toward strong smells. The hypothesis is that migraine is an attempt to interrupt the entry of toxins into the brain. Similarly, symptoms of nausea and vomiting many migraineurs report may be a part of a mechanism which the body uses to eliminate toxins in the diet (food intolerance/sensitivity is prevalent in migraineurs).
Finally…
One question to ask is why natural selection would favour such an easily activated defence mechanism system? It’s thought that the body’s defence mechanisms, shaped by natural selection, have adapted to react not only to life-threatening events, but insignificant threats also (much like a smoke detector waking you if there’s a fire, but also if your toast is burning too; literally a false alarm), which means we experience pain that is normal but often unnecessary. The fact that most defensive reactions occur in response to seemingly trivial threats explains why when the defences in the body are blocked, there’s no real consequence to the body – that is until the real deal occurs. Even a small chance of protection from life-threatening factors is thought to make the evolutionary trade-off of awful migraine side effects worth it if it means living to see another day.
Migraine as a result of conflict with other organisms
A central nervous system that is prone to migraine is favoured by natural selection if it offers an advantage in helping to fight off infections. Evidence has shown that patients with migraine are more likely to develop severe headaches in response to infections. It’s thought that perhaps headache in some cases could be a mechanism that helps kill or expel pathogens. For example increased vasodilation – a prominent reaction in the migraineur – helps bring extra blood flow and immune cells to the brain to fight pathogens. This shows that migraineurs may deal more effectively with central nervous infections than non-migraineurs – a selective advantage.
Migraine as result of new environmental factors
Modern times call for drastic measures. It’s thought that compared to much of human evolution where genes predisposing to migraine might have been less likely to lead to terrible headache, modern environmental factors instead have triggered increased suffering.
Our nervous system evolved in times of less sensory overload – less bright light, less loud noises, less disturbed sleep, less emotional stress – and it’s not had time to catch up.
Interestingly, cases where the inability of the nervous system to deal with such environmental onslaughts even back in cavemen times, may have prompted people to migrate from the triggering environment thus promoting the spread of their genes.
Migraine as a trade-off between genetic harms and benefits
Like many aspects of life, the body, due to being a product of natural selection, has to deal with trade-offs and vulnerabilities that can often lead to disease.
A prime example is sickle cell disease, a condition in which blood cells are abnormally shaped leading to health issues and even death for those who inherit two copies of the gene. The reason it persists in society is because those who inherit one copy of the gene confer some resistance to malaria – a serious and sometimes fatal disease to those carrying no sickle cell genes. As we can see, the advantages sickle cell offers individuals is sufficiently powerful for it to persist.
Migraine appears to have similar trade-offs where benefits of an overstimulated nervous system outweigh the cons of dealing with debilitating headaches. In the population there are few people on one end of the migraine spectrum who experience no headaches, few on the other end who suffer severe headaches and the majority of people who fall somewhere in the middle.
Based on the sickle cell example, those who suffer very frequent headaches (mainly women in their reproductive years) would be disadvantaged – this area of science is neglected and more research is needed to understand why those whose nervous systems are incapable of generating migraine might be hindered.
Those in the middle of the spectrum suffering mild or infrequent migraine, akin to those who have one of sickle cell genes, could possibly have enhanced survival. For example, some of the symptoms of migraine, such as visual sensitivity, that provide a heightened alertness to the environment might be an advantage.
Another example is the influence of sex steroids in enhancing expression of migraine (more predominant in women), which may be explained by the fact that certain migraine-linked traits such a aversion to life threatening environments were more important adaptions for women throughout history than men. Women who have historically been responsible for ‘homemaking tasks’ such as child rearing and food preparation would be at an advantage if they had migraine due to the associated increase in sensitivity to environmental cues which would allow the woman to in turn have increased attention to the safety of her children and home. Conversely, men were historically attributed the role of hunters and gathers, the job of which would be severely hindered by the expression of migraine.
It’s important to remember that migraines are polygenic (influenced by more than one gene) and are complex so while using a single gene disorder as a comparative example is helpful in understanding the concept, it is somewhat oversimplified.
Migraine as a design constraint
Mismatches between ancient brain structures and more recently evolved brain structures could be at the root of migraine. One postulation is that acute migraine attacks may result from the inability of brain-stem regions (the oldest, innermost region of the brain) to suppress excessive stimuli from higher brain centres.
Other evidence for how changes in the brain may account for migraine can be seen using the comparison of human DNA with that of great ape DNA. Human DNA differs only a fraction from ape DNA, however, there is very little evidence to suggest that apes also suffer from migraine. The uniqueness of migraine to humans highlights its possible link to recent evolutionary changes in the brain.
Conclusion
Whilst it’s evident that migraine is in no way a desirable human experience, it’s comforting somewhat to know that the pain migraineurs do experience may not be in vain. Easy activation of the trigeminovascular system (thought to be responsible for migraine agony) is pain with purpose, helping us to understand why humans have evolved to endure the dreaded head throb.
That being said, if evolution could one day figure out how to do its job minus the unbearable pain of migraines, that would be most wonderful. Please and thank you.
Brigid xx
References
(1) Pescador Ruschel MA, De Jesus O. Migraine Headache. 2021 Aug 30. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan–. PMID: 32809622.
(2) Loder E. What is the evolutionary advantage of migraine? Cephalalgia. 2002 Oct;22(8):624-32. doi: 10.1046/j.1468-2982.2002.00437.x. PMID: 12383059
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