Stress and Human Evolution – AMNH SciCafe

Stress and Human Evolution – AMNH SciCafe


>>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. [Laughter] 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, and 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 course 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. Maybe you’ve seen Lord of the Rings, a very
war-torn region. [Laughter] 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 what one of the most interesting
things about doing stress research is, 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. 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 thus 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 an 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 conserved
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? [Laughs] I want to take a minute, again, to think about
the context in which this 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? [Laughter] 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. [Applause]

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