AN EVOLUTIONARY PERSPECTIVE ON HUMAN HEALTH AND DISEASE
WHAT'S IT ABOUT?
Overview from TED
How does your genetic inheritance, culture and history influence your health? Biological anthropologist Lara Durgavich discusses the field of evolutionary medicine as a gateway to understanding the quirks of human biology - including why a genetic mutation can sometimes have beneficial effects - and emphasizes how unraveling your own evolutionary past could glean insights into your current and future health.
"In the time it takes us to evolve one mechanism of resistance, a pathogenic species will go through millions of generations, giving it ample time to evolve, so it can continue using our bodies as a host."
Lara Durgavich brings historical context to human biology, sharing the importance of evolutionary medicine in personal life and beyond. She teaches university courses on topics including evolutionary medicine, primate evolution and behavior, human biological variation and female reproductive physiology.
Lara Durgavich has a really cool-sounding job title: Biological Anthropologist. Also known as physical anthropology, biological anthropology is the study of the past and present evolution of the human species, particularly focusing on the causes of human diversity.
Durgavich’s TED Talk from March 2019 implores viewers to consider the historical context surrounding their own health conditions, and overall genetics – a field known as “evolutionary medicine”.
She relays her own experience; discovering at nine weeks pregnant that she was a carrier of the fatal genetic disorder, Tay-Sachs disease. If the baby were to inherit the mutation from both parents, both copies of chromosome 15 wouldn’t function properly, leading to the progressive shut down of the central nervous system and death by the age of five.
Thankfully, Durgavich’s husband was not a carrier.
Durgavich was aware of the unusually high rate of Tay-Zach in the Ashkenazi Jewish population, but it wasn’t until some years after the birth of her daughter – whilst teaching a seminar in evolutionary medicine at Harvard - that she began to question why:
“The process of evolution by natural selection typically eliminates harmful mutations. So how did this defective gene persist at all? And why is it found at such a high frequency within this particular population?”
I too have been guilty of questioning why certain genetic mutations haven’t “died out” by now, failing to see any benefit for their existence. Durgavich’s talk has turned that on its head, challenging my perceptions (or misconceptions) of health, evolution and natural selection in the process.
Durgavich’s initial hypothesis was a thought-provoking one: the unusually high rate of the Tay-Sachs mutation in Ashkenazi Jews today may relate to advantages the mutation gave this population in the past.
Of course, she’s not referring to the unfortunate individuals who inherited two copies of the mutation, but to those with only one faulty gene copy – such as herself. Could that one faulty copy have made the population more likely to survive, reproduce and pass on their genetic material?
This phenomenon is called “heterozygote advantage”, and it explains why sickle cell anaemia is more prevalent in African and Asian populations. Durgavich explains:
“In these geographic regions, malaria poses significant risks to health. The parasite that causes malaria, though, can only complete its life cycle in normal, round red blood cells. By changing the shape of a person's red blood cells, the sickle cell mutation confers protection against malaria. People with the mutation aren't less likely to get bitten by the mosquitoes that transmit the disease, but they are less likely to get sick or die as a result. Being a carrier for sickle cell anaemia is therefore the best possible genetic option in a malarial environment.
“Carriers are less susceptible to malaria, because they make some sickled red blood cells, but they make enough normal red blood cells that they aren't negatively affected by sickle cell anaemia.”
In the case of the Ashkenazi population, Durgavich posits that the Tay-Sachs mutation may have offered increased resistance to tuberculosis in past generations. According to research conducted in the 1970s, the Eastern European-born grandparents of a sample of American Ashkenazi children born with Tay-Sachs were much less likely to die from TB than non-Jews from the same area. Only one of the 306 grandparents in the study died of TB, despite TB causing up to 20% of deaths in large Eastern European cities during the early 20th century.
That’s quite a statistic, and one which offers a compelling case for why problematic versions of genes persist at high frequencies in certain populations. But why did this resistance mechanism proliferate among the Ashkenazi population in particular? Durgavich points to historical and cultural factors as one possible explanation:
“At various points in history this population was forced to live in crowded urban ghettos with poor sanitation. Ideal conditions for the tuberculosis bacterium to thrive. In these environments, where TB posed an especially high threat, those individuals who were not carriers of any genetic protection would have been more likely to die.
“This winnowing effect together with a strong cultural predilection for marrying and reproducing only within the Ashkenazi community, would have amplified the relative frequency of carriers, boosting TB resistance but increasing the incidence of Tay-Sachs as an unfortunate side effect.”
From this example, it’s clear to see how unravelling your own evolutionary past can glean useful insights. “Big picture” analysis equips us with the knowledge to make better-informed decisions, not only about our own health and lifestyles, but that of our offspring too. If, as a teenager, you knew for a fact that you were genetically disposed to developing life-ending lung cancer, would you still light that first cigarette?
THE ULTIMATE PERSPECTIVE
There’s a lot we can’t know about our own bodies, but adopting an evolutionary perspective – or “ultimate perspective” as it’s also known - is an important first step. Typically, the biomedical community focuses on what are known as “proximate” explanations: ones that consider immediate factors (“how is x causing problem y right now”). Adopting an ultimate perspective is to consider the broader question of why is there a problem to begin with?
“The next time you or a loved one are faced with a health challenge, whether it's obesity or diabetes, an autoimmune disorder, or a knee or back injury, I encourage you to think about what an ultimate perspective can contribute. Understanding that your health is affected not just by what's going on in your body right now, but also by your genetic inheritance, culture and history, can help you make more informed decisions about predispositions, risks and treatments.”
But that is not the only contribution evolutionary medicine makes in understanding human health.
Durgavich makes it abundantly clear that many of our assumptions about natural selection, and evolution as a whole, are false. The notion that our bodies should have improved over time, for instance, stems from a misconception of how evolution works:
“There are three basic reasons why human bodies, including yours and mine, remain vulnerable to diseases and other health problems today. Natural selection acts slowly, there are limitations to the changes it can make and it optimises for reproductive success, not health.”
That last sentence stayed with me. It’s almost like we take the prospect of long and healthy lives for granted, as if we’ve evolved that way. Longer life spans since the 1950s aren’t a result of evolution – they’re thanks to improved living standards, sanitation, technology, scientific knowledge and progress.
Given the severity of the COVID-19 pandemic, her next statement hit home even harder…
"The way the pace of natural selection affects human health is probably most obvious in people's relationship with infectious pathogens. We're in a constant arms race with bacteria and viruses. Our immune system is continuously evolving to limit their ability to infect, and they are continuously developing ways to outmanoeuvre our defences.... In the time it takes us to evolve one mechanism of resistance, a pathogenic species will go through millions of generations, giving it ample time to evolve, so it can continue using our bodies as a host."
If nothing else, this paragraph serves to explain why developing a vaccine has been so mind-blowingly complicated, and why lockdowns, social distancing measures and face masks have been crucial in the intervening period. The virus still has the upper hand; it always has done. The sooner we – and those in power - recognise this, the better off we will all be.