SciCafe: Stress and Human Evolution

SciCafe: Stress and Human Evolution - Transcript

Zaneta Thayer (Biological Anthropologist, Dartmouth College):

I'm going to be talking to you today about stress and human evolution. It can get a little heavy at times, so I've already had a beer. Hopefully you're all having a beer, and we can all learn and have fun. [Laughs]

So I'm an anthropologist. How many people have ever taken a course in anthropology or studied anthropology in college? Awesome, there's so many of you. That's a wonderful thing to see.

So for those of you who have taken anthropology, those of you who haven't, don't know very much about it, anthropology is a remarkably broad and diverse field, and so there are many anthropologists who study a lot of different things. We have cultural anthropologists who are interested in studying differences in cultures and people living all around the world. We have linguistic anthropologists who are interested in trying to understand variation in human language and what that can tell us about population histories. We have archeologists who study the remains of past civilizations.

And we have biological anthropologists, which is my specific breed of anthropology, and we're really interested in trying to understand human evolutionary history as well as patterns of human biological variation, and what explains that variation. Whether it has to do with our evolutionary history, whether it has to do with the environments that we grow and develop in; all these different factors can influence variation.

And so I guess in sum, anthropology is pretty awesome, because really, we're just all interested in explaining human variation, just on all these different levels.

So in my own research, something I've been really interested in trying to understand, a really remarkably consistent pattern of variation is what's known as a social gradient in health. And this is the observation that both within and between societies, individuals who are socially disadvantaged tend to have poorer health outcomes and shorter life expectancies than individuals who are more socially advantaged.

So as an example, we can see this right here in New York City. So this is an image showing differences in life expectancy across the city, and what's really remarkable is that babies born six subway stations apart in this city have a nine year difference in life expectancy. So let that sink in for a minute.

So the question you should all be asking yourselves, and which I find myself asking a lot in this research is, what gives? What in the world could explain this remarkably consistent pattern of variation, that we see not only in a city like New York, but when we compare health across this country and compare health across different countries.

So an anthropologist who has done a lot to inform our understanding of how the social gradient in health emerges is a gentleman by the name of Robert Sapolsky. He's actually a primatologist, and he does his research not amongst humans, but actually amongst baboons. And in case you're wondering, he's the individual on the left. A lot of hair happening there.

So Sapolsky, again, works with baboons, and baboons, like humans, live in very socially structured societies. So what he's argued is that, depending on where you are in these socially structured societies, depending on where you are in this social hierarchy, the amounts of stress that you're exposed to can be very different. So you could be exposed to a lot of affiliative behaviors, a lot of friendly interactions, or you could have a lot of aggressive interactions with other baboons.

So again, if you're higher on the hierarchy you're more likely to have friendly interactions, and if you're lower on the hierarchy you're exposed to more adverse sort of interactions. And these differences in stress exposures can actually lead to differences in how your stress hormone systems work. So the more stressors you're exposed to, the more often you release stress hormones. And this is important because when your stress hormone levels are very high it could actually lead to the development of poor health.

So what Sapolsky finds is, again, individuals who are lower on the social hierarchy amongst baboons are exposed to more stressors, they have higher stress hormones, and then they're more likely to develop poor health. So since humans also live in socially structured societies, it's just possible that a similar process may explain why we see different patterns of health in human societies as well.

So one thing that I think is important to get out there right away is that humans are really good at getting stressed out. And one of the unique things about our species is that the mere anticipation of stress is enough to activate the stress hormone system. So in our evolutionary history, the stress hormone response evolved to be adaptive, right? You see a predator and all of a sudden you get all this energy and it helps you to evade the predator, run away and be successful.

But our stress system isn't only activated when we see the predator. We stay up at night and we think about upcoming exams. We think about financial problems. And we think about the discrimination we experienced. And all these things can actually lead to chronic activation of our stress hormone response and have really maladaptive effects on our biology and our health.

And again, because the stress system, by design, is meant to influence your body in so many different ways, when over activated it can have really broad effects. So overactive stress hormones can contribute to things like anxiety, depression, digestive problems, heart disease, sleep problems, weight gain, memory and concentration impairment, as well as infertility.

So the point I want you to take away from this is that chronic stress is really bad for your health. So if you don't learn anything else tonight and you think it's all in your head, no. It's actually bad for your health, and there's a lot of science to back this up.

So in my own work what I've tried to do is take what's called a life force perspective. I'm really interested in trying to understand how stress across the life course can influence health. If we think about it, stress experience in adulthood, in childhood, in infancy, and even prenatally, can all shape your risk for developing poor health in adulthood.

So what I want to talk to you about tonight are just some different studies I've worked on looking at stress experiences at these different times in life course, and talk about how they influence health.

So one of the first studies I want to talk about relates to the effects of stress in adulthood. There are many different ways to measure health. There are many different ways to measure stress. And again, this is very broad literature. But I just wanted to provide you some examples that I worked on recently.

So in one study what we've looked at is blood pressure. So blood pressure is an important indicator of heart health, but it's also influenced by many different factors. It can be influenced by things like exercise, by alcohol use, as well as stress experience.

Another important outcome that we've looked at is what's known as heartrate variability. So this is basically the amount of time between heartbeats, and this can also tell us about heart health. And again, it's influenced by things like exercise, substance abuse, as well as things like stress.

So in general, we consider higher blood pressure to be considered unhealthy, and lower heartrate variability to also be considered unhealthy.

So one of the samples that we worked with is actually a sample of Native Americans from a couple of northern plains communities. And what we were interested in understanding is whether differences in stress, specifically experiences of racial discrimination, may explain variability in these outcomes, above and beyond the impacts of things like income, education, exercise and substance abuse, for example.

What we found was that, indeed, racial discrimination was associated with higher blood pressure and lower heartrate variability above and beyond all of these other health behaviors and indicators that we expect to be associated with them. So what this means is that racial discrimination impacts heart health worse than all of these other factors.

So in addition to looking at stress and health in adulthood, we can also look in childhood. So working in the same sample, a measure we've looked at is called allostatic load. So allostatic load is basically meant to be this composite measure of physiological function across different systems.

So blood pressure is included in the cardiovascular measure. But in addition to looking at things like blood pressure, we also look at anthropometric measurements, so things like body mass index and waist circumference. We look at inflammation to try and get an index of how well immune function is working. We look at metabolism by looking at different blood lipids, and we can also look at different neuroendocrine markers, which are basically stress physiology markers.

And what we found is that early life trauma and developing PTSD is associated with higher allostatic load. So basically, poor functioning across all of these different systems. Again, above and beyond the effects of education, of income, of substance abuse and exercise.

And then when we look at the different components of allostatic load, we find that the neuroendocrine systems, the stress hormone system, is actually the most sensitive to early life trauma experience, which is what we might expect since we think that trauma and stress impacts the stress system first.

So a lot of my research is actually focused on the impacts of infancy and prenatal stress on health. It might surprise you to know that the stress experienced by your mother actually influences your health now. So if you can, find your mother, hug her and say, thank you for all the things she does for you.

But anyway, before talking about my own research, I actually wanted to highlight a study from here in New York City because I thought it might be of interest to you. One really interesting study looking at the effects of prenatal stress on offspring health was done here in response to 9/11.

What the researchers did was they actually recruited women who were pregnant during 9/11, and then compared those who developed PTSD in response to 9/11 with those that didn't. And then after a year they followed up with the offspring of those women and saw whether or how the development of PTSD in mothers affected the stress hormone response in offspring.

What they found was that infants of women who developed PTSD following 9/11 had lower stress hormones at a year of age than the offspring of women who didn't develop PTSD, suggesting that stress hormones in the offspring are sensitive to the mental health of their mothers.

So in my own research I haven't been working in New York. I've actually been working in New Zealand. So New Zealand, I know, why would you study stress and health there? It's like the most wonderful place ever. If you've seen Lord of the Rings, a very war-torn region.

So in New Zealand I created this birth cohort. I went to these prenatal clinics and walked up to literally every pregnant woman there and asked if they wanted to be in my study. And when they thankfully said yes, I went and visited them in their homes and I interviewed them, and I asked them all about their lives. I asked them about the things that make them happy, the things that make them sad. I asked them about what their jobs are like, what their relationships with their partners are like, how much support do they get from people, what's their neighborhood like?

And so from these questionnaires I could get a little bit of sense of what sorts of stressors people are exposed to, as well as their sources of support. I then collected saliva samples from the mothers from which I can look at stress hormones. So then I can look at how the stressors that people tell me about in their lives relate to their stress hormones.

After birth I followed up with the women again and looked at their baby's stress hormones and saliva, as well as differences in gene expression patterns. So what I was interested in trying to understand, again, was whether maternal stress experience would be associated with offspring stress response.

So before talking about the specific results of my study I wanted to give a little sense in the day in the life of the field, just to give you an idea of the variability in terms of people's experiences. Because I think one of the most interesting things about doing stress research is that the types of things people stress about are very variable, right? But not all of them impact physiology in the same way.

So one day, I started in a suburb called Mission Bay, which is a very lovely suburb overlooking the water in the nice part of Auckland, and I was interviewing the wife of a pastor who, in terms of socioeconomic circumstances, she was in a pretty good position. The home they lived in was actually owned by the church.

But actually, the biggest source of stress for this woman was going to be the outcome of her birth. She had had a very difficult first birth and she was really concerned about complications that may arise. So she told me that she had been You Tubing cesarean section videos and was really getting herself stressed out about it.

Later that day I went to South Auckland and met with a Samoan immigrant. So this woman had four children and she had been educated in Samoa. But unfortunately, when she moved to New Zealand, they didn't recognize her degree, so she wasn't able to work, which was very hard for her. She wanted to go to work. She wanted to be productive. She wanted to be able to make money, and she couldn't. But she said that she wanted to stay in New Zealand because she felt like the educational opportunities there for her children were much better than they would be in the islands.

The last woman I interviewed that day was a Tongan immigrant. And she actually lived in a house full of 24 people, and confessed to me that at mealtimes, the household size often swelled to 29.

The point being that the experiences for women varied greatly. They're all pregnant, but the types of stressors that they're experiencing are really quite broad. And this got me interested in trying to understand, do different stressors impact physiology in different ways?

So in terms of my results, what was really interesting was that when I look at all these different types of stressors, the two stressors that came out as being most important were actually poverty and racial discrimination. And so maternal experience of poverty and racial discrimination were associated with higher stress hormones in moms in pregnancy, and after birth those same women gave birth to infants with higher stress reactivity.

So again, I want to go back and talk about what we talked about in the beginning, this idea that higher stress hormones are associated with worse health. What these findings suggest is that some individuals may be predisposed to developing poor health at birth based on their mom's experiences before they were even born.

But I think what's also really interesting is I also asked women about PTSD symptoms, and infants of women who had higher PTSD symptoms actually had lower stress reactivity after birth, which is actually very similar to those New York 9/11 studies that I talked about a few minutes ago. So there is obviously something very different about the stress experience associated with PTSD development in terms of how it impacts our physiology. And I'm happy to talk about that more in the Q&A if you guys are interested.

So these findings really take on an interesting light when we consider what they mean from an evolutionary perspective. Why is it that offspring should be sensitive to maternal stress experience?

So what I think is really interesting to consider is the fact that maternal environmental experience across her life course influences maternal biology. And maternal biology in turn shapes the environment in which her baby is growing and developing, and that's her baby's resultant biology.

So we can think about maternal biology as basically an integrated signal linking maternal environmental quality across her life course with offspring biology. So again, why would this be adaptive?

I think a really illuminative example to show why this might be adaptive actually comes from the snowshoe hare. Even though I'm an anthropologist, I like animals, too. So here is a snowshoe hare who lives in Canada. Snowshoe hare's natural predator is the lynx. So if you look at population dynamics across time, as snowshoe hares increase in one year, the next year lynx populations increase. Then snowshoe hare populations decrease because there's more lynxes going after them.

So what's really interesting is that in years where there's more lynxes, there's a greater predator density, pregnant snowshoe hares actually have higher stress hormones in pregnancy. And they, in turn, give birth to offspring with higher stress reactivity.

So if we think about this from adaptive perspective, it may make sense for baby hares living in an environment with lots of predators to have a more active stress response, because this allows them to be more vigilant and have more anti-predator like behaviors in order to increase their survival.

What's really interesting is that these effects aren't limited to humans and hares. If we look across species comparing humans and non-human mammals, birds, fish and reptiles, we see that across these different species, there's lots of examples of offspring stress response and behaviors being influenced by prenatal stress experience.

So what's really interesting is that the human stress response system is actually common across vertebrates. So it's this very evolutionarily conserve system, and that suggests that it's been very strongly selected for across time, and probably has some sort of adaptive function.

But what's really interesting is to compare the different types of stress in these studies, this comparative study I was talking about. So in these animal ecological observational studies, the types of stressors were often things like predation risk. And again, in such instances where there are predators around, it makes sense to have a higher stress reactivity in order to have more anti-predator behaviors and increase your survival.

If you look at the types of stressors in human studies, it's things like discrimination, poverty, pollution, trauma, intimate partner violence, 9/11, the Holocaust. These are all, first of all, human-induced stressors. And also, it doesn't necessarily increase our survival to have a higher stress response in response to these stressors.

So what I would argue is that we have this stress architecture that's meant to be adaptive, right? It obviously evolved to be adaptive of a particular environmental context. But it's now being activated in a qualitatively different context; in a chronic way that can really negatively impact our health.

So I want to do a quick checklist of what we've learned. One, chronic stress is bad for you. Two, stress can have intergenerational effects. Three, the stress response evolved in response to qualitatively different types of stressors.

So now that I sufficiently bummed you out for the evening, what do we actually do about it? I want to take a minute, again, to think about the context in which the system evolved. And let's imagine a gazelle not so peacefully bounding across the field as a lion is right on its heels. And the activation of the stress response for this gazelle is meant to be a wonderful thing. It's making the gazelle think clearer. It's giving it more energy than it's ever had. And it's helping that gazelle to survive.

I don't know about you guys but what do you do when you're stressed? If you're anything like me, you have a 10 hour binge fest of Stranger Things whilst drinking beer and eating white cheddar Cheez-Its. And while we're doing that, we have all this cortisol and all this energy just surging through our bodies running amok, making a mess if our hippocampus and our arteries. And basically, our combination of being willing to stress out about everything, and our unwillingness to want to do anything productive with it really contributes to the development of poor health.

And again, this is something that—sorry, New York, I don't mean to pick on you—but it's something you guys should care about, because there's lots of surveys talking about how stressed we all are. The American Psychological Association does studies every year across the country and also highlights residents from different cities. And according to them, 78 percent of New York City residents are somewhat or very concerned about things like work or money, and this is higher than the national average. So it's something that we all need to care about.

So what should we do about it? Well, I think that one thing to think about is what happens when we get stressed and we get all this energy? So we need to use that energy and channel it in productive ways. So if you work a desk job like I do, the best way to do that is to do things like exercise. So then we're using this energy in a productive way that it was meant to be used in.

Another point I want to make is that primates are remarkably social. We are incredibly social creatures, and this sociality has allowed our species to do amazing things like have really dependent offspring really close together; all these things that shouldn't actually happen as a species. So we need to chain all that sociality and use that to help diminish our stress response. So actually, talking it out with people and getting support from others is really beneficial for reducing our stress.

The last thing is that to the extent to which it's possible, it's important to be proactive in terms of how we handle our stress. So I think one of the really interesting things from the stress literature is that not all types of stress impact physiology in the same way. And in particular, stressor that are unpredictable have more maladaptive impacts on your stress physiology system than stressors that are predictable.

And in part, I think this relates to the fact that stressors that are predictable we can have some agency over. So if we think about some sort of upcoming exam or assignment, it's really stressful to think that you have a talk coming up, for example. But you can start preparing for that talk a couple of weeks in advance, and then it's a little less stressful than if you're trying to cram for it the night before. So in doing that, you've got a bit of agency and control, and it helps you to manage that stress response.

So to end, I wanted to go back to this image that we started with in the beginning and make the point that there's nothing natural about this pattern that we're observing. And that in fact, differences in exposure to chronic stress can actually shape patterns of health and disease within society, vis-à-vis chronic activation of stress physiology systems that evolved in a very different context.

But it doesn't have to be this way. So by supporting mothers, by supporting children, by supporting each other, we can actually improve population health and make things a lot better.

So thank you very much.