It's the most fundamental act of human life.
And though we may have come a long way from our early ancestors,
one thing hasn't changed much.
Sex.
Yet, despite being widely practiced,
sex has always been a bit of a mystery.
But science is starting to unlock its secrets
literally getting under our skin to reveal a new, bizarre world.
A world where the truly unexpected can happen.
For the first time on television,
we follow a woman under the influence of hormones,
acquiring the body,
the mind and facial hair of a man.
Understanding sex is the key to understanding how to build a human.
All humans are built in bits,
far too small to be seen with the naked eye.
We're made up of cells.
And inside every cell is exactly the same mix of genes - our DNA.
Until recently, reproduction was regarded as pretty straightforward.
To build a new human you first take the genes that make up a woman,
then the genes that make up a man,
and then enthusiastically mix them up.
But things aren't quite that simple.
We might have been doing it since the dawn of time,
but scientists know very little about the basic processes.
Reproductive physiologist Doctor Roy Levin,
has struggled with the technical limitations of studying sex for decades.
We didn't really have the apparatus to allow us to do the measurements.
And there was a long period of time
when you only guessed what was happening
from the external appearances of men and women in coitus.
So that you couldn't really tell what was happening inside,
because you just can't see.
Cur understanding of sex hasn't moved on much
since Leonardo Da Vinci first started dissecting corpses
and studying them over 500 years ago.
The Queen holds this drawing by Leonardo in her very own private collection.
Doctor Levin's life-long quest to reveal
the secrets of sex is taking him to Germany.
What I've come for is a machine.
This is a machine that makes the body transparent.
You can look inside and see movements
and the secrets really of coitus and how people get sexually aroused.
There are very few machines like this in the world,
and one of them is here in a town called Krefeld,
quite a few miles outside of Dusseldorf.
And the machine Doctor Levin's come to see is this FMRI scanner.
It's basically a camera which uses
magnetic fields to penetrate human flesh.
Today, in the interests of science,
Michael DeGroot and his girlfriend Liz Leahy
are going to attempt to have sex in its cramped confines.
Well, this is the machine as you can see inside,
it's got, like, two doughnuts, those are the very magnets,
and in between is the space
that you'll lie down in and have coitus.
So it's been specially adapted,
that means just a single board has been put down,
and you'll lay in between the two magnets,
and hopefully that will capture
the images of what's going on during sexual intercourse.
The other thing that has to happen is,
Liz, is that you have to wear this girdle
it won't do much for your figure,
but it'll hopefully capture the images
from what's happening inside your body.
So you have to put this sort of around you,
in the pelvic area, and then you lay on top of that.
And so that's really basically all there is to it.
The other thing is though, Michael,
that we'll help you to perform with this pharmaceutical support for it.
So hopefully, it'll help you to do this.
So let's go and get changed. Okay?
Okay.
Doctor Levin is well aware of the problems
that need to be overcome
if this experiment is to be successful.
It's not the easiest thing in the world to maintain an erection
and have intercourse in terms of this particular set-up.
They're brave people that go into these machines.
We've seen the machine,
and I'm interested to see how we're
going to manoeuvre ourselves in there,
because it looks like a pretty constricted space.
And I know they want us in one certain position.
So I hope - I hope that that we're able to situate ourselves
so that they get the images that they want.
I'm just -that's my main concern.
Michael has a personal interest in taking part in this study.
I have a small background in human sexuality,
in the psychology aspect,
because I'm a psychology under-graduate.
So with my background I just kind of got an interest,
and hearing about these images that would be one of the first times
that these would ever be recorded,
I just thought it would be really fun to be a part of this work.
Okay, we're ready to go.
You can start now.
The scanner takes a picture every three seconds,
and produces images of the body from top to bottom.
This is the first time that such images
have been seen on British television.
You can sort of see the penis here.
That's outside of the body from about here.
And this is the root of the penis inside the body.
And this is inside the female's body.
That's her pubic symphysis, the bone,
and here would be the pubic hair just around here.
That's, of course, her bottom.
And this is the vagina that the penis is in.
And at the top here is the glans.
And the thing that's obvious in this cross-section
is the unusual shape of the penis during intercourse.
Well, it's like a boomerang.
That's what we've found out by this machines actually,
that, in fact, the penis does look like a boomerang.
It isn't straight like they drew it in the early times.
In fact, it is bent, as you can see quite clearly.
It's actually incredible,
because as far as when you have an erection you think it's,
you know, hard and solid as a rock or wood or something,
but when you look at those pictures it's unbelievable,
like, you have the 90 degree angle,
and you can't even imagine that it would bend that way. So...
It's really fascinating to see what the body does.
Just incredible. Yeah.
It's not something you really think about
when you're actually in an act like that,
and to actually have the pictures to see what's going on,
it's just extremely interesting.
Nobody knows why the penis has to
go through extraordinary contortions.
One theory is that it's a relic from our past,
when sex was more commonly done on all fours and not face to face.
Doctor Levin is aware these experiments
might be seen as prurient or trivial.
But he believes that understanding the mechanics
has important practical applications.
One possibility is that when we get a bank
of normal images of functional
the normal reproduction and the movements that occur,
and we have a whole range of images,
because this is just one couple that we've done,
we can then possibly look at people who are abnormal,
that they can't have children perhaps,
and their -the dysfunctions that are there might be in the movements,
might be in the motility that we can pick up,
and then we'll be able to look at that
and possibly have treatments for it.
But that's really looking into the future.
We¡¯re just starting out really, the first step this is.
Understanding anatomy is important
if you really want to understand what makes men and women tick.
But there's also much that can be learnt by studying their cells.
Down at this level, men and women are all very similar.
Each cell holds a nucleus which, in turn, contains DNA,
the substance that builds us.
Each cell contains over two metres of DNA,
divided and packaged neatly into chromosomes.
There are 46 of these.
The real difference between men and women is
that women normally have two X sex chromosomes,
while men have an X and a Y.
But it's not always that straightforward.
Look at Jan Johnson.
She's an athletic, feminine 42 year old woman.
She's also six foot three inches tall,
a distinct advantage when you're playing volley-ball.
The other thing that's different about Jan is that she's genetically male.
There's a huge stigma attached to my condition.
I think a lot of people don't want to talk about their personal conditions,
and, you know, no one's perfect.
People - lots of people have problems,
and it just - it's a matter of facing your deepest,
darkest secrets and bringing them out in the open.
She had no idea that there was anything strange
about her when she was growing up.
The only thing that I remember is that I was just very active,
and I just had a lot of energy.
In terms of, like, feeling different no, I didn't.
I just felt like a normal
a normal little girl, and, you know,
I had girlfriends, and I had all my dolls and,
you know, everything was just just normal.
It wasn't until she went to college to study anatomy
that she realised something was wrong.
Quite simply, at the age of 19
she still hadn't gone through puberty.
I felt self-conscious because I wasn't developing like
all my girlfriends were developing.
You know, it's painful to stand out and be different.
And I wanted to find out why I wasn't starting my period.
So I ended up going to the gynaecologist
for the first time.
I came into class one day very upset,
and - because I had gone to the doctor a few days before,
and they had very simply just written XY on a piece of paper,
and gave that to me.
And that was supposed to be
the explanation of why I was the way I was.
So why did Jan become a woman
when her genes were those of a man?
The answer lies in understanding how the male sex chromosome
the Y chromosome -works.
The most obvious thing about the Y is that it isn't very big.
The Y has around 60 genes and the eggs around 3,000.
The Y chromosome is the smallest chromosome,
and when you look at it in detail
only a very small part of it actually codes for anything.
So most of its a desert.
Peter Good fellow knows the Y chromosome intimately.
He found the gene responsible for turning embryos into men.
It's on the Y chromosome, and it's called the SRY.
It's hugely influential in how men develop.
And this is it.
But being very important doesn't mean it's big.
Imagine that your DNA was a piece of string,
so you could see it.
So we've magnified the DNA
which is present in every cell in your body so it's visible.
And that piece of string now would stretch from here to Moscow.
Now, if we were to start a journey from here to Moscow,
on foot, the Y chromosome would represent
the first 20 miles of that journey.
And the SRY gene would be the first step on the journey to Moscow.
It takes 30,000 genes to build a human,
but just one gene -the SRY to build a man.
The SRY is a switch which clicks
when the foetus is around six weeks old.
Until then, the baby could become either male or female.
When the SRY is switched on,
it fires up the development of the male sex organs.
There's a tiny genetic difference,
that genetic difference leads to one key difference,
and that one key difference is men have testes and women have ovaries.
And it's what's produced by the testes
and what's produced by the ovaries
which is responsible for the differences
you see between males and females.
But Jan Johnson has a copy of the SRY gene,
so why isn't she a man?
Well, the SRY triggers development of the testes,
and they produce a very versatile hormone called testosterone.
Testosterone is a steroid hormone,
and one of the things that steroid hormones are excellent
at is going in lots of different places in the body
and instigating lots of different changes,
and testosterone's a beautiful example of that.
Testosterone rampages through the male embryo building muscle
and developing the cardio vascular system.
It's this hormone which makes men so very different from women.
To a remarkable extent, testosterone is responsible
for the masculinisation of the human body.
It's really, in some ways,
it's kind of surprising that this one agent
seems to start this cascade
that has so many different effects.
And it's testosterone that is the answer
to the riddle of Jan Johnson's unusual genetic make up.
Jan has a condition called AIS Androgen Insensitivity Syndrome.
Her genes make testosterone,
but her body can't respond to it.
If it can't react to testosterone
the foetus assumes the default position, which is female.
And that's exactly what happened to Jan Johnson.
The discovery that she's genetically male
was so traumatising
that she couldn't even tell her then boyfriend
and future husband, Peter.
I told him well, I'm unable to have children,
and, you know, here's my situation,
and I don't have my period.
But I never really got into the nuts and bolts of it with him.
Why?
Because it was embarrassing for me.
I didn't want to talk about it.
And I didn't think anyone wanted to hear about it.
The only person she felt would listen
and understand was her anatomy professor Kris Blodget.
And I said well,
the first thing I want you to know is you're a normal girl,
you're not a boy, you're definitely a girl,
you're a very feminine girl.
This should not change your life.
It was devastating to know that I had XY chromosomes,
and those are the same chromosomes a man would have.
Then I said, well, you can't respond to testosterone so,
actually, you're more feminine than an actual
than an XX female who can respond to testosterone, too.
So I said - I said the people that have this develop as normal female,
usually above-average in attractiveness,
they're tall, they're very feminine.
Professor Blodget then got Jan to check her own genes.
So she could see there was nothing to be frightened of.
She was probably the strongest female person
in my life back then.
My mum couldn't really offer any support,
and you know, my siblings certainly couldn't offer support.
So I really didn't have anyone
who could explain it to me the way she explained it to me.
And the pain and the hurt of the discovery
that she was genetically male came flooding back
when they met for the first time since Jan left college 25 years ago.
I was hoping that she's put it behind her,
and was leading a normal life and not considering it a deep,
dark secret, and realised that she was totally feminine and normal.
She really made me feel a lot better about myself.
So it was great to see her again.
Jan's unusual genetic make up means she can't have children,
the thing she wants most.
She doesn't have a womb or ovaries.
And that's put an inevitable strain on her marriage.
We've been together for 23 years.
We've been married for 13 years.
And we've certainly had our ups and downs,
and some of the - some of the deepest lows were over,
you know, adoption.
And you know, and that's unfortunate.
I would - I hoped back then that I would meet someone
that would want to adopt children,
and it just - it just didn't work out.
It was not the right time,
and we didn't have the money or, you know,
for some reason or another it just never it never worked out.
Ironically, Jan Johnson works
for a company which makes high-tech medical equipment.
But there's no technological solution to her inability to conceive,
She just doesn't have the right genes.
Instead of a Y, she'd need another X.
That's because the second X sends out instructions to make ovaries.
These produce the female hormone oestrogen
which builds a womb and fallopian tubes.
The discovery that men are created
from a female blueprint was certainly a big surprise
and contradicts previously held notions
about the relative importance of men and women.
Societies have decided the relative importance of men
and women for thousands of years.
By and large, the men are regarded as more important than the women.
So when biology started to catch up,
at first it looked to reinforce that;
the idea was that the sperm contained the little person
and the woman was just the sort of soil in which the man would grow,
the baby would grow.
Gradually, as we've learnt more about it,
we've realised that the mum is rather more important,
and the woman's rather more important.
And the big shock came when it was realised
that if you don't have a sex chromosome,
or one of the sex chromosomes you don't come out as a boy,
you come out as a girl.
So what that tells us is that the Bible
was fundamentally the wrong way round.
Woman was not born of man's rib,
it's the other way round.
Adam, if anything, came from Eve's rib.
So the male is an adapted form of the female.
One very obvious remnant of our female past
can be found on every male chest.
Nipples.
The reason men have nipples is
because they developed in the foetus
before testosterone kicked in.
But if a hit of testosterone is
all it takes to turn the female form into the male...
what would happen if a genetic female were to be exposed to it?
Max Toth, a software programmer,
is physically and genetically a woman.
She wants to see if testosterone can turn her into a man.
I remembered having this distinct moment
where I thought that God had made a mistake
and that I should have been a boy,
which I thought was interesting,
because I grew up in an atheist household.
I didn't really have a good conception of what God was,
or that there was an entity that could have made a mistake.
But I remembered having this very clear moment.
Science can't explain why she feels like this,
but she knows she's not right the way she is.
She wants the body and looks of a man.
The idea that I could do this,
that I could change something about the way that I was made up,
that I can change the way that my body
had been born was very exciting to me.
To masculinise her body,
Max is planning to take a course of testosterone.
I'm taking my life into my own hands in a lot of ways
because there aren't a lot of studies
about the long-term effects of testosterone.
But Max really wanted to do it and have it properly evaluated.
So we followed her testosterone treatment for six months to see
what effect it would have, not only on her body, but also on her mind.
The body changes will be the most obvious.
Testosterone should build her muscles
and make her more powerful,
which should mean that she'll run faster.
To test this effect, we set up a round course of just over a mile
in Pan Handle Park in the middle of San Francisco,
and recorded her time.
11 minutes, exactly.
We'll come back and check that time
as we monitor the progress of her testosterone treatment.
Apart from the rather obvious physical effects,
testosterone might also change the way her brain works.
So we had her brain scanned by Doctor Ruben Gur,
one of the world's leading scientists
on the effects of hormones on the brain.
Doctor Gur also put Max through a battery of practical
and psychological tests designed to see
just how feminine Max currently is.
Max shows a fairly typical female profile cognitively,
and I'd be curious to see whether there is a change in that.
At the moment, by any measure, Max is a woman.
But that's about to change.
I had a birthday party recently,
and I thought as part of the festivities
that I would allow people to express t