A total of 280 million humans suffer from depression. Depression is expected to be the leading cause of disease burden in high-income countries by 2030, according to the World Health Organization. Why are so many struggling with their mental health, and what can be done? As a psychiatrist, I have pondered these questions my entire professional life. They must be looked at from many perspectives. One that is often forgotten is evolution.
What has shaped human psychology and mental health more than any culture, religion, or ideology is that during 99.9 percent of our species’ history, half of all humans died before adulthood. They did not die from what kills us today—cardiovascular disease, cancer, and stroke—but from starvation, infections, dehydration, bleeding, accidents, and murder.
Since you and I are the last generation in an unbroken line of survivors who, against all odds, did not succumb prematurely to the dangers of our ancestral environment, we have in us defense mechanisms that protect us against what killed our ancestors. One such mechanism is to be hypervigilant to cues of threat, to see danger everywhere, and to plan for the worst. To see the world as dangerous and always plan for the worst is what we today call anxiety. Another mechanism is to want to withdraw, save energy, and avoid danger. To want to avoid is what we call depression. From an evolutionary perspective, it is not surprising that some people have anxiety and depression. What is surprising is that some people don’t.
I always tell my patients that depression and anxiety have nothing to do with bad character or “weak nerves”; it is quite the opposite. Strong arms can lift heavy objects. Strong legs can run fast. But a strong brain is not a brain that goes through periods of stress unaffected. It is a brain that does all it can to help you survive. Even if that means that you see too much danger (anxiety) or withdraw (depression).
Why is this important? We constantly tell stories about ourselves, and these stories become roadmaps for how we view our future. If we view ourselves as sick, broken, or lacking a certain neurotransmitter, we see ourselves as damaged goods. I have seen how that can become a self-fulfilling prophecy. I cannot have a normal relationship because I have an anxiety disorder. I tell my patients that depression and anxiety are evolutionary defense mechanisms without which humans would not exist, that these mechanisms can be fixed, and that seeking help is a sign of strength.
Antidepressant Effects of Exercise
An evolutionary lens on mental health does not only provide a perspective that in itself can be therapeutic; it also gives important clues on how to treat and prevent mental health problems. To the surprise of many—psychiatrists included—regular exercise is a powerful antidepressant.1 A vast number of studies show exercise is an effective antidepressant method for treating mild and moderate depression; it also protects against future depression. Many of my patients who have had a previous depression want to continue with medication to avoid a new depression. I always tell them that exercise will give them as much protection as medication. Of course, it is not either-or; the combination of medication and exercise is particularly powerful.
Role of Inflammation
But how can regular running or cycling have such a powerful effect on our mood? One of several clues comes from our immune system, which does not just include the T- and B-cells you heard about during COVID-19 but also our behavior. When we have an infection, the level of inflammation rises in the body. The brain responds by downregulating mood and drive. We feel sick and want to rest. This makes sense. During an infection, the body must preserve energy since it costs energy to have the intersystem going at full speed. For you and me, extra calories are easy to find at the grocery store, but for almost all previous generations, this was not the case. Calories were hard to come by. Downregulation of mood made sure the body saved energy to fight the infection.
However, inflammation does not just rise during infections but also when we are physically inactive and sleep-deprived and when we experience long-term stress. This means that our modern lifestyle, in which we constantly sit and don’t get sufficient sleep, also leads to increased levels of inflammation. The brain therefore gets the same signal as if we were infected. Now, here comes the main point: The brain cannot distinguish what caused the increased inflammation. It therefore misinterprets our lifestyle as “I am infected; I should hide under the blanket to save energy.” And since our lifestyle goes on and on for long periods, the brain downregulates mood for long periods. We feel constantly low—a depression.
An evolutionary defense mechanism that helped humans survive and save energy to fight infections has thus become a trap when it is triggered by modern lifestyle factors. This may sound like “just another theory,” but in the last decades, we have learned that around a third of all depressions are linked to increased levels of inflammation.2,3 From an evolutionary viewpoint, it is difficult to think of a more depression-genic lifestyle than our current one.
Depression Essential Reads
The 280 million depressed individuals in today’s world are all of a sudden not such a mystery.
You have heard a zillion times that you should exercise, prioritize sleep, and make sure to meet your friends in real life. But when you learn how these things affect the brain and why the neurobiological machinery of our mental health functions the way it does, you will become more motivated to do them. More than that, you will stop looking at yourself as broken and become kinder to yourself. It certainly has been that way for me and for many of my patients.