Nesse: Because what we want in fatty foods,
and sugary foods, and sedentary lives,
it was very useful back in Africa, but now
those habits, which are products of natural
selection shaping our motivational systems,
they're killing us. You go to your doctor and say
"Doctor, what should I do to be healthy?"
Your doctor says "Exercise a lot, and
basically eat a lot of foods that don't
taste that good." And, sadly, we were
designed to pursue the foods that were in
short supply in Africa. And now that we can
get lots of them, they're really bad for us.
Dawkins: Also, if we go back a bit
further in history, we walked on all fours,
and, I mean that's a pretty dramatic change,
you know, to walk on two legs
instead of four. You'd expect that
that would have ramifications.
Nesse: It certainly does. You know, in
children especially, but sometimes in adults,
our bowels get tangled up with each other.
And they essentially tie themselves in knots.
You'd think that, you know, natural selection
would have fixed that. And if we look over
here, at a skeleton of a donkey, you can see
how the backbone is here in a nice arch
first of all, and you can also see the cavity
where the intestines are in there. They hang
all of the intestines from a blade of tissue
that goes right down the middle so that
they don't get tangled up. If you take this
donkey and turned it on it's hind legs,
like this, all of a sudden all of the intestines
Nesse: which were hanging very nicely here,
Dawkins: Hanging from the back bone there.
Nesse: They're all of a sudden draping in
a big tangled mess, like a pile of tangled
fishing line. Again the amazing thing is
they don't get tangled up more than they do.
Dawkins: Yeah.
Nesse: And of course the other problem
Nesse: just about everybody has in their
life at some time or another is back pain.
And it's just, you know, why didn't natural
selection fix that one? And again we can,
this is a great example of what you were
just saying Richard, about, you know,
natural selection can't start fresh and we
have legacies from our new way of life
standing up. Notice here with the donkey,
it has a nice arched, curved backbone,
just like a bridge, a very nice way of
supporting everything. If you look at the
chimpanzee here, you see that there's still
mostly that bridge, with the beginnings of
a little spot here where the back turns out.
In fully upright humans, you see that the
back, that you have a nice curve here, and
then you have a sharp, a sudden sharp bend.
And that's so that we can stand upright.
A marvelous thing that we can do, and I'm
not sure why we stand upright.
Nesse: That's a good...a good question.
Dawkins: That's a good question.
Nesse: But I do know that this bend in the
backbone, is something that causes terrible
problems for many people. That's the
evolutionary explanation for back pain.
There's the physical explanation, the
mechanical explanation is bone pinching nerves.
But that's the mechanical explanation.
The questions we want to answer are
Nesse: why did natural selection (unintelligible)?
Dawkins: That, that's a very good illustration
Dawkins because doctors will always tell
you what the problem is. "Your bones are
pinching your nerves," but what you're asking
is "Well why is that happening? What's the,
Dawkins: historical reasons for that?"
Nesse: Exactly. Those of us who are
trying to advance evolutionary applications
of medicine would like to see every
medical textbook for every disease have
one more paragraph. Instead of just an
explanation for how the body works and
how it breaks, we want one more paragraph
about why natural selection has left it
vulnerable to breaking. And we've gone into
some of the reasons for that already.
Dawkins: Yes. And what about cancer?
That's a special problem, isn't it?
Nesse: Oh it's a terrible problem,
because it gets worse and worse
as you get older of course.
It too turns out to be a tradeoff though.
We can't regrow all of our tissues. We can
regrow some of them, but not all of them.
Our skin, if you cut it, it heals, and your
liver even. If you lose some of it, it'll regrow.
Your heart? No. Your brain? No.
So why can't we regrow a heart, and regrow
a brain? And the reason for that is that in
ancestral times people who injure their heart
or their brains had no chance of surviving.
So there was no selection benefit from
being able to repair those kinds of tissues.
But you asked about cancer. It's so hard
to talk about cancer. The cells divide,
and there are very fancy mechanisms in
natural selection that shape to keep their
division under control. In fact, one can
look at the whole process of development
within the body, is largely one that
specializes cells so they divide just the
Nesse: right amount and then stop.
Dawkins: And then stop, yeah.
Nesse: One of the mechanisms that keeps
cells from dividing out of control looks
like it's something called "telomeres."
Now on the end of each chromosome, the
chromosomes are the little bits of DNA
that are all strung together. At the end of
each chromosome there's a bit of DNA
hanging off the end. And every single time
that the cell divides, a little bit of that
DNA gets clipped off. And the next time it
divides a little bit more gets clipped off,
and basically it's just the unraveling of
the rope at the end, that it needs to be a
little spare at the end. But it's also a
wonderful safety mechanism, because if
this line of cells starts dividing out of
control, then snip, snip, snip, snip, snip,
and then the cell dies. 'Cause once that
telomere, that little bit of extra DNA
hanging of the end is gone, the cell is
going to die. A marvelous safety device,
but it has a cost too, doesn't it? Because
then, as you get older, and the cells divide,
at a certain point you will have used up
all telomere and you will have cells dying
because of that. And in a recent study,
it was found that people who had longer
telomeres, indeed were living longer lives.
The next part of the story is to show
that they are at a greater risk of cancer.
And so here we see in a very
dramatic form the tradeoffs between
advantages and disadvantages.
Dawkins: Some people might say if
natural selection is so good, how come
we do get old and die anyway?
Nesse: Oh my goodness. That's how I got
into this business Richard. I don't know
if we've ever talked about that. But when I
was a sophmore as an undergraduate, I asked
myself the question, "Why didn't natural
selection make us live longer?" And it should,
I was convinced as an undergraduate, because
there's a lot of genetic variation in how
long you live. And since we know that
different genes can make you live longer,
why didn't natural selection increase the
frequency of the genes that make you
live longer? But instead, no. Most people
are dead in their eighties, and everybody's
basically dead by age one-hundred.
I came up with what I thought was a
fabulous explanation. I thought that it was
Nesse: very important for the species
Dawkins: (laughter)
Nesse: to get rid of some individuals so
that the whole species could evolve and
adapt to a changing environment. And my
professor said "That's brilliant Nesse!
You have a good biology brain."
Dawkins: He didn't. (laughter)
Nesse: He did say this, but it was,
Nesse: it was very shortly, actually
George Williams' published his book.
Dawkins: I can't help feeling that if you'd
gone to Charles Darwin rather than to your
professor, he would have said something
like this. But it's reproduction that really
matters, individual survival is only a,
a means to the end of reproduction.
Nesse: You know, I didn't ever learn that,
even in my evolutionary biology course
at college. I don't know why, people talk
as if living healthy lives was what natural
selection shaped. But of course it doesn't.
It shapes maximizing reproduction.
And if that makes you live a shorter
lifespan, too bad, that's what happens.
Dawkins: So if you reproduce like crazy for
for a short time and then die, you've done
better than somebody who doesn't reproduce
very much, and lives a long time?
Nesse: Indeed. Indeed. So when I was in
medical school I stuck up my hand and
I said "So professor, why do we age?"
and they said, "Well it's because machines
break, that's why!" And I said "No, why
wasn't natural selection better about this?"
and they said "You don't understand, it's
genetic. There are mutations, things break."
I said "Well maybe..." After I finished
medical school, I started hanging out with
evolutionary biologists at the University of
Michigan, including actually Bill Hamilton
and Bobbi Low, and a lot of other
great people, and they immediately said,
"You're a doctor, and you don't know
George Williams' theory about aging?
You're ignorant!" I said "No, I've just
gotten the best medical education you
can get." And they said "No, no. You
don't know anything." So I read that paper
of his from 1957, and I realized that I'd
been completely wrong about aging, even
though I was deeply interested in it for
fifteen years, and then I did a study,
looking at how strongly aging influences
survival and reproduction for humans now,
and here's a simple calculation I did;
if there weren't any aging, how long
would we live? It's very easy to do.
You just assume that everybody, you know
your likelihood of dying goes up as you
get older, that's really what aging is.
What if your likelihood of death stayed
the same throughout your whole life as
it is as age twenty? About a third of us
would live to a thousand years old.
That makes it really dramatic. So now we
come back to George Williams' explanation.
Take a gene, he said, that makes your bones
heal more quickly. It changes calcium just
a little bit. What would it be selected for?
Because you break bones early in life a
fair amount, healing them quicker would
be a good thing. What if that exact same
gene had a different effect that deposited
calcium in your coronary arteries? What if
that killed everybody by age 120? And he
noted, "Too bad," if it gives a selective
advantage early in life and lots of
people are alive, that gene is going to
become more frequent, even though that
same gene eventually kills everyone.
Once I realized that, I started realizing
that I wanted to understand every
aspect of the body from an evolutionary
viewpoint, trying to figure out why
natural selection didn't do it better.
