Original subtitles

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: