All language subtitles for Evolution Ep 5 Why Sex

NARRATOR:
Every year peacocks shed
their magnificent tail feathers.

It must be a relief;

they're heavy and they
attract hungry predators.

(peacock crying)

But when they grow back, it's
perhaps an even greater relief.

Peahens only mate
with well-endowed males.

No fancy display, no sex,

no passing of genes
to the next generation.

That's something every living
thing is programmed to do.

It's worth fighting for,
maybe even dying for.

In fact,
from an evolutionary perspective

sex is more important
than life itself.

Female praying mantises
have been known

to turn their dates into dinner.

But every praying mantis
alive today

had a father who took that risk.

Pacific salmon give
everything they have

to get upriver to spawn.

Once they've succeeded,
they wait for death.

And while we won't
trade our lives for sex

most of us will risk death
to protect our children,

the carriers of our genes.

Evolution is a story written
over countless generations.

To inherit and pass on genes
is to be part of that story.

MAN:
That's our immortality.

That's what connects us
to humans on into the future.

That's what's connected us

to all of our ancestors
in the past.

That's what connects us
to the ancestors that were fish,

the ancestors
that were protozoans

and the ancestors
that were bacteria.

It's the single thread

that connects all of life
on this planet.

NARRATOR:
Sex and genes... driving
behavior, driving evolution.

It's over there.

MAN:
Here in Southwest Texas

roundups have been
a big part of history.

I mean, we have
cattle roundups...

Lately we have
rattlesnake roundups.

But my crew's not here to look
for cattle or rattlesnakes.

We're here to look for a
particular species of lizard

that, uh, has a different
reproductive strategy

than most organisms
that live on this earth.

MAN:
Hey, look, there's one!

WOMAN:
Right there! Right there!

There it goes! There it goes!

It's going back towards you!

Ow! Shoot!

NARRATOR:
Jerry Johnson and his students

are after a species
made up only of females

which give birth
without having had sex.

Each egg contains a complete set
of its mother's genes.

It develops and grows without
any contribution from a male.

And so each baby lizard
is an exact copy (a clone)

of its mother.

Some people think

that they actually have to have
some kind of a lesbian behavior

where a female mounts a female
to get the eggs to develop.

That hasn't been
really proven yet

but it's... it's
an interesting hypothesis.

NARRATOR:
Regardless of how they do it

these lizards have mastered
the art of cloning

which raises
a fundamental question.

As far as these lizards go,
the big question, I think

is they do so well as all-female
species, uh, why is there sex?

I mean, are males
really necessary?

NARRATOR:
Immersed as we all are
in a culture

that extols male courage...

grace...

self-confidence...

passion...

questioning the necessity
of males is rare.

Men almost never do it

and women do it most often
in a fit of pique.

Most of the time, we're happy
to say, "Vive la diff�rence"

and get on with things.

But for evolutionary biologists,
the question is real.

Given the efficiency of cloning,
why would any female put up

with the complications
of sexual reproduction?

For starters,
males can't bear offspring

and rarely share the burden
of raising them.

Then there's the fact

she only gets to pass on
50% of her genes.

Not to mention all the time,
energy and bother involved

in courting and mating.

Nevertheless, virtually
every new life on the planet

is a result
of sexual reproduction

and not asexual cloning.

Oh, no, it's good
for you.

Make you big
and strong.

NARRATOR:
So males must play
a critical role

and sex must offer an advantage.

Whatever it is,
it's buried deep within us.

WOMAN:
All my life,
I've wanted to be a mom.

It's an instinct,
it's a feeling,

it's something that you
just know you want to do.

MAN:
I've done such a 180.

It's... it's amazing.

I never thought
I'd hear myself say that

but I really, really wanted
to have children.

"Itsy-bitsy spider
went up the waterspout."

NARRATOR:
Through seven years,
multiple operations

and more tests and procedures
than either cares to remember

Sharon and J.T. pursued
their dream of parenthood.

J.T.:
She really had this burning
desire to have children.

We just didn't want to give up

without exploring
every possible angle

and every possible way
of doing it.

And there's... there's
a lot of different ways.

I was unaware
of how many, you know

what science can do nowadays.

I mean, I learned a lot
along the way as well.

SHARON TREMITIEDI:
Nyah is the best thing
that's ever happened to me.

I can't imagine
my life before Nyah.

WOMAN:
The biological imperative, as we
all know, is to pass on genes.

Most species on Earth
use sexual reproduction.

Why do this?

There has to be some
fundamental, biological,

evolutionary reason
for sexual reproduction.

This has been one of
the major questions in biology

for a very long time.

NARRATOR:
For 25 years, Robert Vrijenhoek
has been returning

to the remote hills
of Sonora, Mexico

hoping to find clues to the
enduring mystery: Why sex?

In this dry
and forbidding landscape

he seeks those clues
in the lifes of fish.

VRIJENHOEK:
To study the sexual process

which appears to be normal,
predominant in most things,

I study the diseases of sex,
the pathologies of it.

Little fish, in this case,
that live in the Mexican deserts

that have abandoned
the sexual process.

And the beauty of this
is it's a natural experiment

because side-by-side
in these same little puddles

we'll have sexual reproducers,
we'll have asexual reproducers,

all competing in these tiny,
little, simple environments.

I could try to study this
in a... in a massive environment

like an African savanna, but I
can't manage an African savanna.

With my capabilities

I can manage
a few small pools like this

and for 30 years,
I've been coming back

to this part of Mexico to study
what goes on in these pools.

Ah, we got some.

Very good.

VRIJENHOEK:
This is a good sample here.

Look how black these fish are.

Wow.

And look at the pigment on them.

NARRATOR:
Early on, Vrijenhoek discovered

that 40% of the minnows
in these pools

were heavily infected
with a parasite

that causes black spot disease.

The rest seemed
relatively unaffected.

VRIJENHOEK:
When we brought them back
to the laboratory

and started counting the spots

we noticed, well, my goodness,
this is really neat.

The asexual reproducers

were taking much higher loads
of parasites

than the sexual reproducers
on average.

Why should they be
more parasitized

than the sexual reproducers
they were living right beside?

Because they were
all being exposed

to the same parasites
in the water.

There should be no
fundamental reason

for the different parasite loads

on these different kinds
of animals

unless they had something
fundamentally to do

with being asexual
as versus being sexual.

NARRATOR:
But what?

The only thing to do
was to keep walking

and sampling and thinking.

Oh, look it.

Look at this.

NARRATOR:
Finally, it hit him.

He was looking at
a real-world demonstration

of the value of males,

one suggested by an evolutionary
theory called the "Red Queen."

VRIJENHOEK:
The Red Queen
is an elegant idea.

I think it's one
of the great ideas since Darwin

and it goes back to a wonderful
scientist, uh, Leigh Van Valen

who basically asked about,
"Does evolution stop

when things get perfectly well
adapted to their environment?"

He said, "Of course not!"

Evolution is a race

and it's much like the race we
saw in Alice and the Red Queen

in Alice in Wonderland

where they were running
just as fast as they can

and Alice says,
"Isn't this curious?

"As fast as we run,
nothing seems to change.

We're staying
in the same place."

And the Red Queen says, "Yes.

"You have to run just
as fast as you can

to stay in the same place."

You're living
in a complex world,

a world full of parasites,
a world full of viruses

and bacteria, predators,
competitive species

all basically evolving, too.

And the moment any species
stops evolving

in response to these challenges
and threats, it's doomed.

NARRATOR:
The cloned fish
have stopped evolving.

They're an easy target

especially for a short-lived,
fast-evolving parasite.

But the sexual fish
are a moving target.

Each is a new combination

fashioned from
its parents' genes.

So as far as the parasite
is concerned

every individual
in that sexual population

is a unique
and individual challenge.

And that slows down
the transmission of the parasite

through the sexual part
of the population.

NARRATOR:
Here was a solution
to the mystery of sex.

It's the best defense against
rapidly evolving enemies.

Or so it seemed

until a bad drought dried up
the pools, killing the minnows

and throwing everything
into question.

Eventually the water returned
and so did the minnows

having slowly worked
their way back up the hill

from pool to pool.

When Vrijenhoek
checked the top pool

he made a disturbing discovery.

Now the parasites
were decimating the sexual fish.

The clones were doing
quite well, thank you.

VRIJENHOEK:
This is absolutely contrary

to anything we'd seen
for many years previous to this.

Now it was completely
upside down.

Something... something
mysterious was going on.

Now an opportunity, you see,
to do some science.

Nature had done
an experiment for us.

We had to find out
what happened, what changed.

We collected the fish.

We can then examine
them, uh, carefully

and what we found in this case

is that the sexual species in
the process of recolonization

had lost its genetic variation.

It had become inbred.

NARRATOR:
Testing revealed the sexual fish
to be clonelike.

They now resembled each other
genetically.

And since they outnumbered
the true clones

they were being targeted
by the parasites.

As a final test, Vrijenhoek
conducted a simple experiment.

VRIJENHOEK:
I went downstream with my bucket
into a lower pool

where the fish still
had genetic variability

and I took some fish
from there

put them in my bucket,
hiked back up the mountain

and threw them in the pool

where... where the fish
had lost variation

and came back a year later
just to see what happened.

And it was just
wonderful result.

The parasite load in the sexuals
had dropped right down

to the levels it used to be
in the past.

The asexuals now were taking the
brunt of the parasite attacks.

And when we examined
those sexual fish, we found

that we had in fact successfully
transplanted genetic variation

into that pool.

And, you see,
that's precisely the point.

That's what sex does.

Sex generates variability
among offspring

and when you take that away
from a sexual reproducer

by inbreeding them
or cloning them

you've lost the very benefit
of sex.

It's that generation of
an immense amount of diversity.

That diversity
of your offspring

provides challenges
to everything around it:

challenges to the parasites,
challenges to the viruses,

challenges to your competitors.

That's the beauty
of the sexual process

is the variation and... and
wonderful diversity it creates.

NARRATOR:
The lesson taught by
Vrijenhoek's little fish

is that passing only half
your genes to your kids

is a price worth paying.

Sex generates variation

which improves a species'
chances of survival

in a world dominated
by relentless competition.

(players yelling)

For all their downside,
males are worth the trouble.

Think of them as
a female's insurance policy

against losing her children

to rapidly evolving threats,
like measles and the flu.

If the reason for sex is
a bit less mysterious these days

its origins remain
much more speculative.

Some believe it all got started
billions of years ago

with two single-celled creatures

sharing a chance encounter
in the primordial night.

They meet,
and genes are exchanged,

that's what sex is all about.

The moment is brief

but it leaves them
a little bit stronger,

a little more likely
to survive and reproduce.

Males and females came later

when random change produced a
creature that was small and fast

which turned out to be
an evolutionary advantage.

Organisms with reproductive
cells like that

are called "males."

Their goal is to find organisms
with a different specialty,

providing the nutrients
life requires.

They're called "females."

These early pioneers evolved
in time into sperm and eggs.

Males produce sperm
by the millions.

With so many potential offspring

it doesn't pay
to be fussy about eggs.

A better strategy is to try
to fertilize every egg you can.

Eggs are more complex than sperm

and take a larger investment
of energy.

Females make only
a limited number of them.

Fewer eggs mean fewer chances
to pass on genes.

And that means females,
unlike males

do better if they're choosy.

At a deep biological level

males and females
want different things

regardless of how things appear
on the surface.

Small sperm versus large eggs;

quantity versus quality,

these are the evolutionary roots
of the "war between the sexes."

This war is a lot more

than fodder for poets,
philosophers

and soap opera writers.

It can explain a lot
about how species evolve

and why they look
and act the way they do.

(frog croaking)

Charles Darwin was the first

to recognize the evolutionary
significance of sex.

He came to it because
his theory of natural selection

had a major problem.

It beautifully explained why all
polar bears have heavy coats:

over time, any trait

that improves an individual's
chances of survival

should spread through
the entire population.

But it offered no help
in explaining

the wild extravagances
found throughout nature

like the peacock's tail.

WOMAN:
Darwin had a real problem
with peacocks.

In fact, he once said

"The sight of a peacock
makes me sick"

because he really didn't
understand how it could evolve.

NARRATOR:
An extreme reaction, perhaps

but it is hard to see
a peacock's tail

as something other than
an impediment to his survival.

PETRIE:
They're heavy.

(chuckling):
They're difficult
to carry around.

They take a lot of energy
to grow.

They're conspicuous

and basically they're going
to slow an animal down

if it's running away
from a predator.

(squawks)

(tiger growling)

(growling)

NARRATOR:
And it wasn't just
the peacock's tail

that Darwin's theory of natural
selection couldn't explain.

There were also the elaborately
ornamented carapaces of beetles

and the baroque extravagance
of butterflies.

(bird singing)

And even the delicate songs
of birds.

Theologians of his day

argued that God created
ornate flowers and feathers

to inspire man's wonder
and devotion.

Darwin was convinced

there had to be
an evolutionary explanation

just as there had to be
an evolutionary explanation

for why so many
of nature's ornaments

are seen only on males.

SMALL:
If natural selection

is operating on all organisms
the same

why is it in nature
that you can see differences

between males and females?

And these differences
are actually quite large,

things like antlers
or large body size in males

that are clearly connected
to maleness or femaleness

as if there were two paths.

And this really
doesn't make sense

if you accept evolution
by natural selection.

It should be operating
the same on everybody.

It took him several decades
to think of it

but eventually he happened upon
the idea of sexual selection

which is really Darwin's
most ingenious idea, I think.

These ornaments
are not for our good.

They're to advertise
each individual's fitness,

its goodness as a mate

to the opposite sex
of its own species.

In a sexually reproducing
species

survival is no good
if you don't find a mate.

If you don't convince somebody
that you're good enough

to copulate with,
to have offspring with

your genes will die with you;

you won't leave any descendants.

(growls)

NARRATOR:
Darwin saw two strategies
at work

in the courtship idiosyncrasies
of different species:

for males, it's competition;

for females, it's choice.

Males fight for access to,
or control over

the females themselves,
or a resource females need

like food or territory.

(screeching)

Sometimes this competition
gets downright nasty.

But it's just as likely
the males of a species

will follow the path epitomized

by John Travolta
in Saturday Night Fever

the path of the peacock,

seduction through
sexual display.

(Bee Gees performing
"Night Fever")

"Reach out for me..."

NARRATOR:
This is where female choice
comes in.

"No"

"No-o-o..."

SMALL:
Female choice is that part of
sexual selection that has to do

with females choosing
particular males over others.

(emphatically):
"No!"

(song ends)

You would expect that the female

who invests more per egg,
per offspring,

should be much more choosy about
who she has offspring with,

who she combines her genes with.

Whereas the male,
who is investing so little,

you would expect
that he wouldn't care so much.

NARRATOR:
Darwin's contemporaries had no
trouble with male competition.

But females actively
directing evolution

through their choice of mates?

That was too much.

MILLER:
This was the aspect
of sexual selection

that Victorians really
had trouble with.

They couldn't imagine
that mere female animal brains

could be shaping something
as... as grand and important

as evolution itself.

In those days,
females didn't have choices.

Males decided who they were
to marry, for example, you know

and females really didn't
actually have that much say

in the matter.

(peahens calling)

NARRATOR:
So radical was the idea
of female choice

that it was more than a century
before anyone tested it.

Marion Petrie's experiments with
peacocks were among the first.

According to sexual
selection theory

peacocks grow their tails

because peahens
pay attention to them

and peahens pay attention

because only a healthy, fit,
strong peacock

can afford to grow one.

Yeah, he's got
a lovely tail.

NARRATOR:
To test that,
Petrie measured the tail lengths

of a captive population
of peacocks.

PETRIE:
The longest feather is 145.

NARRATOR:
Then she charted exactly
which males the females chose

over an entire mating season.

Her data left little doubt:

to peahens, size matters.

Next, Petrie tried reducing
the number of eyespots

in some otherwise
well-endowed tails.

The result was a lonely mating
season for the trimmed birds.

Finally, Petrie started
playing matchmaker.

PETRIE:
We paired females
with males with big trains

and we paired females
with males with small trains

and then we looked at how

being paired to a male
with a big train...

what effect that had
on the performance

of the female's offspring.

And what we found was

is that if you were mated to
a male with an elaborate train

your offspring survived
a lot better.

Paternity does matter.

(peafowl squawking)

NARRATOR:
Peacocks are a classic case

of evolution operating
through sexual selection.

Males compete
for the opportunity to mate

and females hold out
for the best genes.

When females choose a trait

that is an honest indicator
of good genes

that trait spreads throughout
the population over generations.

It can also become
highly exaggerated.

It's all a logical consequence

of the differing
reproductive strategies

of males, who have lots of sperm

and females,
who have fewer eggs.

But the goal isn't
just to have offspring,

the young have to survive
long enough

to have their own offspring.

Sometimes, that requires
paying as much attention

to behavioral traits
as to physical ones.

Lizzie, it's lonely as dyin'
out there.

Will you come with me?

NARRATOR:
In the Hollywood classic
The Rainmaker

Katherine Hepburn's struggle

to choose between the sexy,
quick-witted Burt Lancaster

and the dependable Wendell Corey

mirrors a deep
biological dilemma.

For some species, the chances
of offspring surviving

increase if a female chooses
a mate who'll stick around

over the one
with the best genes.

Evolution has favored

in many of the species
Stephen Emlen studies

males and females who share
the job of parenting.

(birds twittering)

EMLEN:
In songbirds

if a male were to be
a deadbeat dad and leave

and not raise the kids

the kids would die,
and basically no genes

would be passed
to the next generation

because the female alone
can't do it.

NARRATOR:
She needs help.

But he's only going
to give up philandering

if he believes the chicks
he's staying home to help raise

are his own.

The result is monogamy,

a social solution
to a biological dilemma.

Human infants are also born

heavily dependent
on parental care.

You can't get it
quick enough, huh?

It's not coming out
fast enough for you?

Let's try this.

JOSEPH TREMITIEDI:
Being a parent

is about bringing up the child,
loving the child

sacrificing for the child

and Nyah...

I would give my life for her

without blinking an eye.

Everything we do
revolves around her.

Our needs are second.

What she needs comes first.

NARRATOR:
A shared investment
in the next generation

can reinforce a couple's
commitment to each other.

What she gave us
was completeness;

that it wasn't just
him and I anymore,

it's the three of us

and, you know, we like
the way that sounds, you know.

NARRATOR:
But monogamy isn't easy
to maintain.

While some evolutionary forces
encourage it

others threaten the family
values that are at its core.

Songbirds are unusually
monogamous.

But even as they pair off
and set up nests

inevitably, some of them are
lusting after their neighbors.

EMLEN:
Say here in an Ithaca woodlot

you migrate back
from South America

and of the 100 birds
in this woodlot of your species

you're the 65th female
to return from migration.

You find that most of the males
have already been taken.

You choose the best male
available but you end up

paired with a fairly
low-quality male

in comparison
with your neighbors.

You're in a situation where
you now have a social mate

who's going to help provide the
food and the care for your young

but the neighbors are in fact
higher genetic quality,

perhaps more experienced,
more healthy,

and if your young
could be sired by them

you in fact would have
healthier young

that carry, therefore, the genes

to promote and have
healthier grandchildren.

NARRATOR:
But for a female songbird,
cheating is a risky strategy;

if she's caught,
her partner will leave.

Alone after the chicks are born

she won't be able
to meet their needs.

(chicks cheeping)

Yet a surprising number
of songbirds take that risk.

DNA testing has revealed that
as many as 40% of all chicks

are not sired by the male
that helps feed them.

Cheating, at least
for certain female songbirds

gives their chicks better genes,
and therefore a better chance

of surviving until
they can reproduce.

For the wattled jacanas
of Panama

survival of chicks
is so uncertain

it's led to an amazing
gender role reversal.

Jacanas lose a lot of chicks
to crocodiles.

They might have
died out long ago

if they hadn't found a way
to produce more offspring.

Their solution:
the female lays the eggs

but it's the male
who keeps them warm

and raises the chicks.

EMLEN:
If a female is the one
who is freed of parental care

she can produce more eggs
more rapidly

and both she and the male
benefit from that.

NARRATOR:
As jacana females
cut back on nurturing

their reproductive strategy
began to change.

Now it's the females

who care more
about quantity than quality.

Now it's the females
who fight over mates.

Over time, they've taken on

traditionally
male characteristics.

It's the females
that are larger

females are highly aggressive

females compete
for access to males

and a highly "successful" female

is one who is able to accumulate
and defend, if you will

a harem of four
or even five mates.

NARRATOR:
When a female
conquers another's territory

she often breaks the eggs

and kills the chicks
of the vanquished mother.

EMLEN:
This makes sense,
despite its grisliness.

The male instantly becomes
available to take her eggs

and in fact that's what happens.

Within hours, the female is
sexually soliciting to the male.

He starts mounting,
and within a few days to a week

he has a clutch of eggs
that are her eggs.

NARRATOR:
So here is an evolutionary
revelation about gender:

male and female roles
are not set in stone.

They're largely determined by
which sex competes for mates,

and which invests in the young.

Solving the problem
of how to pass on your genes

can even trigger
the emergence of a new species.

On the tree of life

different branches
are often occupied by species

that look poles apart.

But sometimes,
what separates species

is more social than physical

as it is with
our closest relatives

chimpanzees and bonobos.

Chimpanzees and bonobos

live in similar jungles
in equatorial Africa.

They look alike, live in
the same size communities

and eat similar foods.

Yet...

(chimpanzees shrieking)

Violence is a fact of life
for chimpanzees.

Battles between neighboring
communities are common.

So is the physical abuse
of females by males.

Bonobos, on the other hand,
are essentially peaceful.

In all instances

bonobos are predisposed
to make love, not war.

So why are humankind's
closest relatives so different?

For 20 years

Richard Wrangham has searched
for an answer to that

among the chimpanzees
of Uganda's Kibale Forest.

WRANGHAM:
Chimpanzee society

is horridly patriarchal,
horridly brutal in many ways

from the female point of view.

I mean, the young males,
the late adolescents...

it's almost
a rite of passage for them.

In order to be a...
an adult male chimpanzee

you have to be able to dominate
all of the females.

So that's rough
from the female's point of view.

They regularly get beaten up
in horrid ways.

NARRATOR:
Wrangham frequently
finds himself

in the middle of what,
for want of another term,

must be called
a domestic dispute.

(chimpanzee shrieking)

WRANGHAM
Imoso's... chasing Barbara.

He wants to have some sex.

Barbara's a young female

and she's quite upset
about being approached

by the dominant male for sex.

She's only used, really,
to mating with young boys.

(shrieking continuing)

There's our alpha male, Imoso.

He's not used to being denied,
and now he's after Tongo

and with his erection,
his penile erection,

his hair erection,
he really wants to get at Tongo

but for the moment
she's escaped successfully.

(cacophony subsides)

NARRATOR:
Females chimps
aren't the only ones at risk.

Infanticide is thought
by primatologists

to be a major factor

in the evolution
of chimpanzee sexuality.

As a response to this danger

females try to copulate
with all the males in the troop.

The grisly logic
of infanticide is disrupted

if every male thinks
every infant might be his.

WRANGHAM:
Under this regime
in which the females

are trying to get matings
from lots of different males

then it's favored males
to have these tremendous testes

and large seminiferous tubules
for storing the sperm

so they can put in a tremendous
number of sperm,

about five times
as many as humans.

It's very high-quality sperm.

If you look at human sperm,
you know, the classic quote

is from the vet who picks up
a slide of human sperm and says

"If this was a bull,
I would shoot it."

But chimps,
by comparison to humans

have very high-quality sperm

and they can have five or more
copulations per day.

The whole thing only takes
seven seconds, though.

I mean, this is not fun sex
by human standards.

NARRATOR:
Bonobos, on the other hand

seem to find sex
thoroughly enjoyable.

For the past decade

Amy Parish has been observing
bonobo behavior

at the San Diego
Wild Animal Park.

She's seen them go at it
in every way imaginable.

PARISH:
You get standard
heterosexual interactions

which are often face-to-face,
the way they are in humans.

You also see what we call
ventral upright matings

where a male and a female
will hang together out of a tree

suspended, and have sex.

Males have sex with other males

in what we call
"rump-rump rubbing"

where they stand and rub
their scrotums together.

We also see something among
males called "penis fencing"

where males will suspend
off of branches by their arms

and rub their erect penises
back and forth.

And then a very remarkable
behavior in which two females

rub their genital
swellings together

in rapid sideways motions.

(bonobos panting)

NARRATOR:
So what's allowed bonobo females

to establish such peaceful
relations with males?

Parish believes the answer
is female solidarity.

(bonobos shrieking)

PARISH:
By cooperating with each other
and solidifying their bonds

and reducing any tension
that does exist

they're able to form alliances
with each other

and cooperatively dominate males

and this changes
the whole balance of power

and the whole social dynamic
in the group

and makes it radically different
from chimpanzees.

NARRATOR:
And why have bonobo females
evolved this strategy

and chimpanzee females haven't?

It looks as though
a relatively simple change

in the feeding ecology

is responsible
for this dramatic difference

in sexual behavior.

The bonobos live in an
environment where you have herbs

much more continuously
on the ground.

And there are chimpanzees
that live in similar forests

but wherever those forests
are occupied by chimpanzees

they're also occupied
by gorillas.

NARRATOR:
The gorillas
eat the food on the ground

leaving the chimpanzees heavily
dependent on fruit trees.

To get their share,
the female chimps forage alone.

WRANGHAM:
Mothers, with their babies
ranging in age

from one to about five

can't move as quickly
as the males.

I mean, one infant is up here
playing in the tree

and a couple are nibbling slowly

and the mothers have to sit
and wait for them.

So it's absolutely typical

that the males reach
the big feeding ground first

and the males
have finished all the food

by the time the mothers arrive.

So the mothers disperse,
away from each other

and away from the males

and that means they can't
have much opportunity

to form bonds with each other.

NARRATOR:
The simple fact that there
was food available on the ground

appears to have been the force

that drove
the evolution of bonobos.

Wrangham believes the catalyst
was a long-lasting drought

two million years ago,
in what is now Zaire.

The plants, and the gorillas
that depended on them, died.

It was tough on the chimpanzees;

but they could live on
the fruit in the trees.

When the rains and the plants
returned, the gorillas didn't.

Now the chimpanzees could
get to the food on the ground.

In time, they evolved
into bonobos.

It's been suggested that same
drought forced our ancestors

out of East Africa's forests
and onto the plains.

WRANGHAM:
And once you had drying
in a savanna area

then conditions
became quite harsh.

It was impossible
for early humans

to travel around
in groups together

in the way that bonobos do

and therefore for females
to form alliances

and dominate the males in
the way that happens in bonobos.

But a little bit different
climatic history,

a little bit different
in our food history

and we might have evolved
to be a totally different

more peaceful, less violent,
more sexual species.

NARRATOR:
Today, this theory

is little more
than interesting speculation.

But the idea behind it
is consistent

with a growing but controversial
body of scientific thought

that claims much
of present-day human behavior

is rooted in our distant past.

There's a new group
of scientists

called evolutionary
psychologists

and they're interested in
how human evolutionary history

affects the way we think today.

Now, keep in mind
that that means

four million years
of evolutionary history,

a time during which
we were almost always roaming

the plains and forests
of Africa.

How does that affect
the way we operate today?

They've been looking at things
like mate choice

different kinds of standards
of beauty, social exchange.

And they have
a very long way to go

before they can prove
some of their ideas.

But they're still
some of the most interesting

and provocative issues
around today.

NARRATOR:
Evolutionary psychologists
begin by pointing out

that regardless of the culture
in which we grow up

we all tend
to respond the same way

to a surprising variety
of things.

Most of us find spiders
unpleasant,

certain body types sexy

and particular smells
disgusting.

All, they say, are legacies
of our evolutionary past.

MAN:
If we ask, for example

do rotten eggs smell bad?

It's just a molecule.

Hydrogen sulfide gas,
it doesn't have a smell.

We have evolved a brain
to generate a negative feeling

for something that's detrimental
to our gene survival.

This indicates
biological contamination, right?

If you were a dung beetle

the smell of rotten eggs might
smell wonderful to you

because the smell
doesn't reside in the molecule;

it resides in the evolved brain.

NARRATOR:
To an evolutionary psychologist

it's no accident
Hollywood turns to snakes

when it wants
to put a hero in danger.

Snakes.

Why did it
have to be snakes?

(thunder cracks)

NARRATOR:
Dangerous snakes don't slither
into most of our lives.

Still, more than
a few of us share

Indiana Jones's
instinctual fear.

You go first.

NARRATOR:
Another deeply embedded instinct

we may have inherited
from our ancestors

is the ability to smell
a genetically compatible mate.

In one unusual experiment

scientists had young men
sleep in the same T-shirt

for a couple of nights

until the shirts were infused
with their unique smell.

It turns out,
when women were asked

to rate the sex appeal
of the different men

based only on the smell
of the shirts

they consistently rated higher
the shirts of men

whose immune genes
were unlike their own.

From an evolutionary
perspective, this makes sense.

Choosing a mate
with different immune genes

gives offspring a greater
protection against viruses

parasites and other pathogens.

The ability to smell good genes
is a remarkable talent

but like most instincts,
we don't even know it's at work.

We just like
the way someone smells

or the way they look,
or because they make us laugh.

People don't have sex

because they want
to perpetuate their genes.

They don't like someone because
they want to get better genes.

They do things
because they feel good to them.

They have sex
because it feels good.

The relationship between those
good feelings and gene survival

may not even be known to them.

They never think about that.

They do things
because it feels good

and they never think,
"Why do those things feel good?"

"Why have we evolved a brain

"that makes certain things
feel bad to us

or certain things
feel good to us?"

And that's the question
that we're addressing.

NARRATOR:
Like many men, Victor Johnston
spends as much time as he can

looking at beautiful women.

But for Johnston

it's an academic pursuit,
part of his effort to understand

the evolutionary roots
of beauty.

You have to remember
that beauty is a bit like sugar.

In a hunter-gatherer world, the
source of sugar was ripe fruit

so if you liked
the taste of sugar

you got a very nutritional diet.

And now we've separated
the sugar from the nutrient

the sugar's actually killing us,
it's bad for us

and yet we still have
this preference for it.

Well, beauty may be something
the same.

Beauty's just a configuration
of facial cues

that we find to be attractive.

These are cues
that are telling us

something about
that individual,

something
biologically important.

People like very full lips

and these are due to estrogens
at puberty.

And they also like females
with short lower jaws

and narrow lower jaws

and this indicates
a low testosterone level.

And that combination of high
estrogens and low testosterone

is correlated with fertility.

In a hunter-gatherer world

it was important
to detect these cues

but in today's world, you know,
we can manipulate fertility.

We have fertility drugs
and contraception.

It doesn't have
the same importance

but we still react to it;
it still affects our behavior.

The next face I want
you to find, Sarah,

is the face
that would be

the most
attractive male face.

Whoo-hoo!

"The most
attractive male"...

Yeah, the most
attractive male face

from this
range of faces.

NARRATOR:
To study facial preferences

Johnston constructed a computer
program that scrolls seamlessly

between a sexy,
fertile female face

and a macho, virile male face.

SARAH:
That guy looks like
he's right out of prison.

He's not
very popular.

Yeah, he's not very popular.

I'd like for you to find
the best possible mate

for a short-term
relationship.

Short-term, okay...

RESEARCHER:
A secret weekend.

Tends to be
more masculine and...

pretty far down there.

JOHNSTON:
For a short-term mate

they want someone who's got

a lot of testosterone markers
on their face

compared with a choice
for a long-term mate.

So whenever people
are looking for good genes

or a short-term relationship

they seem to be looking

for these more
masculine characteristics

but not so when they're looking
for a long-term mate.

They're looking for, maybe,
a softer, kinder person

for a long-term relationship.

WOMAN:
Maybe someone that's kind of

a little bit protective,
help me out, watch over us.

NARRATOR:
Johnston tested many of
his subjects more than once.

To his surprise

he sometimes got different
answers on different days.

When was the first day of
bleeding for your last period?

NARRATOR:
It turns out that when women
are ovulating

and the chances of getting
pregnant are high

there is a consistent shift
in what they find attractive

toward a more
masculine-looking male.

I want someone at least my age.

NARRATOR:
Just like with the songbirds,
that's when the unconscious lure

of those "good genes"
is strongest.

WOMAN:
There, that's the one.

NARRATOR:
Of all the claims of
evolutionary psychology

none are more sweeping
than Geoffrey Miller's.

He believes the human brain

like the peacock's
magnificent tail

is an extravagance that evolved,
at least in part

to help us attract a mate
and pass on genes.

The human brain
is the most complex system

in the known universe.

It's wildly in excess of what
it seems like we would need

to survive
on the plains of Africa.

In fact, the human brain
seems so excessive

that a lot of people

who believe in evolution
applied to plants and animals

have real trouble imagining

how natural selection produced
the human brain.

All the other species
on the planet seem to get by

with relatively small,
simple nervous systems

that seem tightly optimized

just to do what the species
needs to do to get by.

I think people are perfectly
sensible in being skeptical

about the ability
of selection for survival

to account for the human brain.

I think there was a sort
of guidance happening,

there was a sort
of decision-making process

that was selecting our brains.

But it wasn't God,
it was our ancestors.

They were choosing their sexual
partners for their brains

for their behavior,
during courtship

and I think
our semi-intelligent ancestors

were the guiding force.

They were the guiding hand
in human evolution.

NARRATOR:
When choosing a mate,
we still notice beauty

but what really counts is how
someone thinks, feels and acts.

All of these are products
of the brain.

SHARON TREMITIEDI:
He's a sweet, gentle man,
and he makes me laugh.

I don't think
I'd tell him that, but...

because there's times he
doesn't make me laugh, but...

He has really nice
qualities about him.

He was funny,
he's kind, he's generous.

He has all the good qualities
I think you want in a mate.

Her sense of humor, her laugh.

She always seemed to be upbeat,
always happy

and she has a great personality.

Plus she's
a very attractive woman.

I mean, I'm not going to lie.

That had a lot to do
with it as well.

You've never spoke to me
like this.

I tell you there comes
one moment once

and heaven help those
who pass that moment by

when beauty stands
looking into the soul...

NARRATOR:
In the classic tale
of Cyrano de Bergerac

his eloquent words
convince the beautiful Roxanne

there's more to him
than his extraordinary nose.

It's brains, not beauty,
that win her heart.

Your name is like
a golden bell hung in my heart

and when I think of you,
I tremble

and the bell swings and rings...

MILLER:
Everybody who falls in love

knows that
they're falling in love

not just with somebody's
physical features

but their personality,
their intelligence,

their creativity,
their wit and their charm.

Yes... that is love.

CYRANO:
Yes, that is love.

There are all sorts of things
that mess up brains

and paradoxically
for that reason

brains make
really good indicators

of how fit you are
during courtship.

In fact, they're probably
better indicators of that

even than a peacock's tail is
of how fit a peacock is.

Geoffrey Miller's idea

about sexual selection moving us
down the path of larger brains

is really interesting because
it's not the same old saw

of tool use, language, culture;

it's something
entirely different.

Now, I have some questions about
what's the female role in this,

how do women fit
into this process?

But still, it's
a very intriguing idea

and he might just be right.

(chorus singing Handel's
"Hallelujah Chorus")

NARRATOR:
Miller is just getting started

when he argues
that the size of our brains

can be attributed to our
ancestors' sexual choices.

He's also convinced
that artistic expression

no matter how sublime

has its roots in our desire
to impress the opposite sex.

("Hallelujah Chorus"
continuing)

And that includes music, art,

the poetic and storytelling uses
of language...

even a good sense of humor.

(studio audience laughing)

NARRATOR:
According to Miller

they all stem from our instincts
for sexual display.

MILLER:
I think when a lot of people
produce cultural displays

what they're doing, in a sense

is exercising
these sexual instincts

for impressing the opposite sex.

They're not doing it consciously

but what they're doing
is investing their product

with an awful lot of information
about themselves.

(classical ballet
music playing)

A lot of people are very upset
about this idea

that cultural displays are there
to attract sexual partners.

They find this somehow demeaning

as if sex is dirty
and culture is clean

and the two must
be kept separate.

I think this is a basic mistake.

I think the capacity
for artistic creativity is there

because our ancestors valued it

when they were making
their sexual choices.

NARRATOR:
Miller knows his ideas

about art, culture and the
human brain are controversial.

He's also convinced that, as
experimental techniques improve

it'll be possible to determine
whether he's right or wrong.

(birds squawking)

Sex is at the heart
of evolution.

The process of mixing
and passing on genes

produces variation
that helps species

meet the challenge of life
in a competitive world.

Sexually selected variations

are those that help
individuals find mates

and successfully raise young.

That's how, for humans

sex became fun
and parenting rewarding.

Those of our ancestors
who took pleasure from sex

and satisfaction from parenting

had more surviving offspring
than those who didn't;

that was true
generation after generation.

These traits
are now almost universal.

Even if we choose not to have
children, we still enjoy sex.

And even when we adopt a child
who doesn't carry our genes

we can still
find parenting rewarding.

SHARON TREMITIEDI:
The infertility specialist

said to try a different option,
and so that's what we did.

She's my baby.

The fact that Sharon
didn't bear her

or I didn't father her,
that's the physical thing.

Parenthood is so much more
than the physical.

The physical's the easy part.

Tell Mommy,
"I want a satellite dish."

Yeah... tell her
that's what you want

because that's the only way
I'm going to get it.

SMALL:
Humans are the only species

in which adults will care
for children

who are not biologically related
to them, over the long term.

With our
highly intellectualized brain

we have taken this compelling
biological urge

to mate and reproduce
and care for children

and translated that
onto children

with whom we share
no genes in common.

JOSEPH TREMITIEDI:
I tease Sharon; she's
the new woman in my life.

I never thought I'd fall in love
with another woman

but she's
the new woman in my life

and I wouldn't have it
any other way.

NARRATOR:
Humans are unique.

We're a product of evolution

but we've taken
the first tentative steps

towards controlling
our evolutionary destiny.

It's a brave new world
we're entering.

Only time will tell

if we'll be as successful
at guiding our future

as evolution has been.

BOTH (quietly):
"Out came the sunshine
and dried up all the rain"

"And the itsy-bitsy spider
went up the spout again."

Continue the journey

into where we're from
and where we're going

at the Evolution web site.

Visit www.pbs.org.

The seven-part
Evolution boxed set

and the companion book

are available
from WGBH Boston Video.

To place an order, please call: