- [Rhonda]: Yeah, please do talk about the, because to me, what would be the advantage
to developing depression from an evolutionary standpoint?
- [Charles]: Well, the argument here is that depression, human depression, may have evolved
out of sickness.
And there’s actually a fair amount of evidence to suggest that that may be the case.
So for one thing, if you make a list of the symptoms that you have when you’re acutely
sick, and you cross those with the symptoms you have when you’re depressed, there’s
a really significant overlap.
Depressed people are a little more likely to wanna kill themselves, are a little more
likely to have sort of beat down on themselves.
That doesn’t happen as often in sickness, but many symptoms, and some very surprising
ones, are shared by depression and sickness.
The example I often given is hyperthermia, right?
So, you know, when you’re really sick, when inflammation is activated, one of the things
it does is induce a fever.
We’ve known for many years that if you take medically healthy depressed people, they have
chronic elevations in their body temperature.
And it follows the same sort of diurnal pattern as you see in sickness.
So you see more of this elevation at night.
There’s some very interesting data actually done up the road in Los Angeles, at UCLA in
the 90s, that if you measure core body temperature of people that are not depressed versus people
that are, the depressed people’s body temperature is higher.
Then if you treat them, in this case they used electroconvulsive therapy, which is,
you know, very, very rapidly acting powerful treatment, you treat the depressed people,
you measure the body temperature again, bang, it goes right down to the level of control
people.
So we’ve known for a long time that depression is a hyperthermic state, but, you know, if
you look at the things that happen physiologically when you get sick, one of the things that
happens is something called an “Acute Phase Reaction.”
So you get a change in the chemicals that your liver makes, right?
So you get a downgrading of sort of housekeeping chemicals like albumin, and you get an up
rise in the things like CRP.
You tend to also do things like lose iron, lose zinc, and you say, why, you know, when
you get sick, why do you slump your iron?
Well, the answer is because the microbes, especially intracellular bacteria, need the
iron more than you do.
They have to have that iron to replicate.
You could live for a while without iron, but they can’t.
And there’s all sorts of data showing that for instance, iron supplementation kills you
if you’re infected, or kids are given iron in high pathogen third world areas are much
more likely to die of infection.
So, you know, what happens with evolution is you get a suite of reactions that although
sometimes costly to the person, or to the person’s body, are generally more costly
to the bacteria.
So the reason that we get sick when inflammation gets activated is not just because nature
wants to torture us, it’s because if you make a list of all the things that happen,
many of them have been shown to be pathogen protective.
So for instance, hyperthermia, fever is a powerful pathogen protector for a couple reasons.
First off, it sort of ramps up immune functioning.
But most microorganisms are not built to last, they’re built cheap, that’s why they can
actually mutate so quickly.
So they tend to unwind at higher temperatures.
So, you know, a fever is an anti-pathogenesis, an antibiotic strategy, right?
So it’s very striking that depression is so reliably associated with things like shunting
your iron, or shunting your zinc, raising your body temperature, you know.
So if you think about, you know, why would those characteristics occur in a condition
like depression that we think of as being largely psychosocial, you know, if your girlfriend
dumps you and you get depressed, it might make sense why you weep and cry.
And, you know, maybe it be you sit down, and it makes you reevaluate your life, but why
should it elevate your body temperature?
Why should it cause your iron stores to be reduced?
On the other hand, you know, if depression evolved out of sickness as a strategy for
pathogen defense, all those things make a lot of sense.
And so the argument is that if you look at the things that killed hominids and human
beings before about 10,000 years ago, they were largely not the infectious agents that
killed us across history, right?
Most people died of things like malaria, and smallpox, and measles, these horrible crowd
infections over the last 10,000 years since the invention of agriculture.
Before that, most people died from trauma.
It’s interesting, sometimes afflicted by other people just, you know, just get scraped,
getting cut up.
The things that kill you from trauma are much more likely to be extracellular bacterial
things.
Those are the types of organisms that are especially likely to be wiped out by the kind
of sickness reactions that get activated both in depression and sickness.
So we and others have made the argument that this is the way to think about it.
That until modern times, stress was a reliable indicator that you were at significantly increased
risk of wounding, and wounding is going to kill you because you’re going to get infection.
So stress becomes linked with a pre potent inflammatory activation so that your immune
system kind of run to their guard stations, it’s like smoke alarm principle, you know.
Stress means you’re at an increased risk of dying from a wounding-based infection.
So stress becomes reliably associated with inflammation.
Inflammation induces sickness.
But sickness and depression share a lot in common, so over time, what happens is that
anything that signals a need for increased inflammation activates a suite of behaviors
that in humans over time, also sort of evolves into depression.
So it’s an even deeper way of saying that in fact, the link between inflammation and
depression may be deeper across evolutionary time and for adaptive purposes, than it actually
is in terms of mechanism.