NARRATOR: ACROSS THE PLANET 4 TIMES EVERY SECOND, A NEW HUMAN IS CONCEIVED AND AN EXTRAORDINARY JOURNEY BEGINS.

IN JUST 9 MONTHS, A SINGLE CELL NO BIGGER THAN A SPECK OF DUST TRANSFORMS INTO THE MOST COMPLEX ORGANISM ON EARTH.

THIS IS THE MOST EXTRAORDINARY JOURNEY WE'LL EVER TAKE.

AND NOW, WITH THE LATEST BIOLOGICAL SCIENCE, WE CAN FOLLOW--WEEK BY WEEK, MINUTE BY MINUTE-- THE EVENTS THAT SHAPE AND DEFINE US LONG BEFORE WE'RE BORN.

FROM EACH BEAT OF OUR HEART... TO EVERY FINGER ON OUR HANDS... FROM OUR SEX TO OUR PERSONALITY AND EVEN HOW LONG WE'LL LIVE... EACH MOMENT WILL BE DIFFERENT FOR US ALL BECAUSE THE STEPS AND MISSTEPS ALONG THE WAY CREATE VARIATIONS THAT MAKE EACH OF US UNIQUE AND ULTIMATELY HUMAN.

SO LET'S RELIVE THE JOURNEY AS WE ZERO IN ON THE WEEKS, DAYS, EVEN HOURS OF THE GREATEST STORY ABOUT HUMANS EVER TOLD.

[SEA GULLS CRYING] NARRATOR: RIGHT FROM THE START, IT'S A JOURNEY PACKED FULL OF SURPRISES.

WOMAN: WHO WAS BORN FIRST?

WHO'S THE OLDEST?

ME.

THEN ME.

THEN ME.

THEN ME.

I'M THE YOUNGEST.

ONE 8 MINUTES BETWEEN EACH OF US.

NARRATOR: HOLLY, JESSICA, ELLIE, AND GEORGIE ARE GENETICALLY IDENTICAL.

FROM ONE EGG AND ONE SPERM, ONE SET OF GENETIC INSTRUCTIONS SOMEHOW ENDED UP BUILDING NOT JUST ONE HUMAN LIFE...

BUT 4 IDENTICAL ONES.

WHAT HAPPENED IS SO UNUSUAL THAT ON OUR PLANET OF OVER 7 BILLION, THERE ARE JUST 70 LIKE THEM.

WOMAN: 64 MILLION TO ONE, THE ODDS OF MY PREGNANCY.

MAN: YEAH, THAT'S THE ODDS OF THE PREGNANCY.

TO ACTUALLY GO THROUGH IT AND SURVIVE IT WAS ONE IN BILLIONS, HE SAID.

NARRATOR: THESE SISTERS' STORY IS EXCEPTIONAL, BUT IT STARTED WITH THE SAME WONDROUS EVENTS THAT HAPPEN TO ALL OF US.

SHORTLY AFTER SPERM AND EGG MEET, AN EXTRAORDINARY EVENT TAKES PLACE, 23 CHROMOSOMES IN OUR FATHER'S SPERM FUSE WITH 23 FROM OUR MOTHER'S EGG, WHICH TOGETHER CREATE THE FIRST-EVER EDITION OF THE UNIQUE SET OF GENETIC INSTRUCTIONS FOR A HUMAN BEING.

24 HOURS LATER, CELLS MIRACULOUSLY BEGIN TO DIVIDE.

ONE CELL BECOMES TWO... TWO BECOMES 4... UNTIL AROUND DAY 5, IT'S A BUNDLE OF A FEW HUNDRED INTERCONNECTED CELLS, TIGHTLY BOUND TOGETHER.

AND IT WAS AT THIS POINT THAT THIS STORY TOOK A DRAMATIC TURN.

MAN: IT WAS SORT OF ARMS AND LEGS AND IT WAS ALL A BIT... INCOMPREHENSIBLE, I SUPPOSE, WASN'T IT?

YOU COULDN'T FATHOM OUT WHAT WAS WHAT.

AND THEN...

I DON'T THINK THE SONOGRAPHER COULD FIND OUT, EITHER, COULD HE?

NO, HE WENT, HE WENT A BIT... ASHEN.

YEAH, I SUPPOSE THAT'S THE RIGHT WORD.

AND HE SAID, "OH, RIGHT."

HE SAID IT'S A MULTIPLE PREGNANCY.

WE SORT OF SMILED, THINKING, OH, TWINS, BOY, GIRL, HAPPY EVER AFTER SORT OF THING.

AND HE SAID, "NO, THIS IS REALLY SERIOUS.

THERE'S 4 BABIES IN HERE."

NARRATOR: EXACTLY WHAT HAPPENS TO PRODUCE TWINS, TRIPLETS, OR QUADS IS STILL A COMPLETE MYSTERY.

BUT ONE IDEA IS THAT AROUND 5 DAYS INTO THE JOURNEY, THE VITAL CONNECTIONS THAT USUALLY HOLD CELLS TOGETHER COULD BE BROKEN... AND SEPARATE CLUMPS OF CELLS BEHAVE INDEPENDENTLY.

THIS INCREDIBLY RARE CHANCE EVENT SPLIT ONE INTO 4.

WITH EACH CLUMP GOING ON TO BUILD AN ENTIRELY SEPARATE YET IDENTICAL HUMAN.

WOMAN: SO WHAT'S THE BEST THING ABOUT BEING A QUAD?

HAVING THE BEST SISTERS IN THE WORLD.

HAVING FUN.

NOT BEING ALONE ALL THE TIME.

YEAH, HAVING--HAVING PEOPLE AROUND.

IF I DIDN'T HAVE ANY SISTERS, I'D BE REALLY SAD AND LONELY.

THEY CAN HELP YOU.

QUITE HONORED...

MOST OF THE TIME.

APART FROM WHEN THEY'RE PLAYING UP.

[LAUGHS] IT'S USUALLY YOUR FAULT ANYWAY.

YEAH, I GUESS SO.

[LAUGHTER] NARRATOR: HOLLY, JESSICA, GEORGIE, AND ELLIE ARE A MARVEL OF NATURE WHO DEFY ALL THE ODDS-- A LIVING EXAMPLE OF THE MANY TWISTS AND TURNS THAT CAN HAPPEN ALONG THE EXTRAORDINARY JOURNEY THAT SHAPES AND DEFINES US LONG BEFORE WE ARE BORN.

WOMAN: GOING FROM NO CHILDREN TO 4.

YEAH.

MASSIVE JUMP.

WOMAN: HI, GIRLS!

COME IN AND SAY HI.

GIRLS: HI!

HI, FILMERS!

HELLO.

HELLO.

JUST WANTED TO SEE YOU.

NARRATOR: IT'S THE END OF THE FIRST WEEK, AND THIS MICROSCOPIC BUNDLE OF CELLS IS BARELY VISIBLE-- JUST A HUNDREDTH OF AN INCH ACROSS.

IT NOW STARTS TO IMPLANT IN THE WALLS OF THE MOTHER'S UTERUS-- AND AT THIS VITAL TIME, SOME MOMENTOUS CHANGES ARE HAPPENING, DEEP INSIDE EVERY SINGLE CELL... CHANGES THAT ARE DRAMATICALLY INFLUENCED BY THE WORLD OUTSIDE AND BY WHAT A MOTHER HAPPENS TO BE EATING...

WHICH, WE'RE LEARNING, COULD DETERMINE THE LENGTH OF OUR LIVES.

[BABY CRYING] BABY NUHA IS THE NEWEST LIFE TO ARRIVE IN THIS REMOTE GAMBIAN VILLAGE...

BUT THERE'S EVERY REASON TO BELIEVE THAT WHAT HAPPENED TO HIM JUST DAYS AFTER BEING CONCEIVED MAY MEAN HE WILL LIVE AS LONG AS THE OLDEST MEMBER OF THE VILLAGE-- BORN OVER 9 DECADES BEFORE HIM.

MR. KARMO TOURAY AND BABY NUHA WERE BOTH CONCEIVED IN THE WET SEASON, AND THIS ONE THING MAY HAVE A DRAMATIC IMPACT ON THEIR LIFE EXPECTANCY.

WE KNOW THIS BECAUSE OF AN EXTRAORDINARY 70-YEAR-LONG STUDY THAT'S BEEN MONITORING BABIES THROUGH CHILDHOOD INTO OLD AGE.

DR. MATT SILVER IS WORKING ON THIS LONG-RUNNING STUDY THAT'S REVEALED SOME SHOCKING RESULTS.

SILVER: THIS DATA COLLECTION HAS LED US TO THE RATHER EXTRAORDINARY DISCOVERY THAT THE SEASON IN WHICH YOU ARE BORN HERE IN RURAL GAMBIA PREDICTS YOUR LIFE EXPECTANCY.

BY THE TIME YOU REACH THE AGE OF 65, YOU'RE UP TO 6 TIMES MORE LIKELY TO HAVE DIED IF YOU WERE BORN IN THE RAINY SEASON THAN IF YOU WERE BORN IN THE DRY SEASON.

NARRATOR: SOME EXCITING NEW EVIDENCE IS SUGGESTING THIS STARK DIFFERENCE IN LIFE EXPECTANCY COULD ALL COME DOWN TO WHAT A MOTHER IS EATING VERY EARLY ON IN HER PREGNANCY.

THE WEATHER IN THIS PART OF THE GAMBIA FLUCTUATES DRAMATICALLY BETWEEN THE WET AND DRY MONTHS OF THE YEAR.

AND THIS VARIATION IN DIET COULD MAKE ALL THE DIFFERENCE TO HOW LONG A PERSON LIVES.

SILVER: THESE SEASONAL DIFFERENCES ARE REFLECTED IN IN BIG DIFFERENCES IN THE KINDS OF DIETS THAT MOTHERS ARE GOING TO BE EATING AROUND THE TIME THEY'RE CONCEIVING THEIR BABIES AND CARRYING THEM.

THE SORT OF COCKTAIL OF NUTRIENTS THAT THE MOTHER IS EATING REALLY WILL VARY QUITE STARKLY BETWEEN THE SEASONS.

THERE WILL BE MORE GREEN, LEAFY VEGETABLES IN THE RAINY SEASON AND OTHER NUTRIENTS WILL ALSO VARY ACCORDING TO SEASON.

[CHILDREN SINGING IN NATIVE LANGUAGE] ♪ GOOD MORNING, MR. [INDISTINCT], GOOD MORNING, FRIENDS... ♪ NARRATOR: THOSE CONCEIVED IN THE DRY MONTHS APPEAR PERFECTLY NORMAL AS BABIES, BUT INSIDE THEM, THERE'S A TICKING TIME-BOMB FOR LATER IN LIFE.

TEACHER: HOW LONG DO YOU THINK YOU STAY IN THE MOTHER'S WOMB?

CHILDREN: 9 MONTHS.

TEACHER: 9 MONTHS.

OK. OK, SO THE SPERM WILL TRY TO PENETRATE IN, TRANSFERRING... NARRATOR: AND IT COULD BE DETERMINED BY WHAT'S HAPPENING DURING THE FIRST DAYS OF DEVELOPMENT IN THE WOMB.

WE USED TO THINK THAT THE GENES WE INHERIT FROM OUR PARENTS ARE FIXED...

BUT THE LATEST SCIENCE SHOWS US THAT THIS ISN'T ENTIRELY TRUE.

IN THE FIRST DAYS AFTER CONCEPTION, SOME GENES ARE SWITCHED ON WHILE OTHERS ARE SWITCHED OFF.

AND A MOTHER'S DIET OF GREEN, LEAFY VEGETABLES PROVIDES A COCKTAIL OF NUTRIENTS... THAT BIND TO STRANDS OF DNA...

WHICH FOREVER CHANGES HOW SOME GENES WORK.

AND THIS PERMANENTLY AFFECTS HOW THE BODY IS BUILT.

SILVER: OUR LATEST DISCOVERY IS A GENE WHICH HAS BEEN SHOWN TO REGULATE THE IMMUNE SYSTEM, WHICH COULD AFFECT THE BODY'S ABILITY TO DEAL WITH A VIRAL INFECTION.

NARRATOR: HERE IN THE GAMBIA, IT SEEMS THAT FOR BABIES CONCEIVED IN THE DRY SEASON, A KEY GENE THAT SHOULD HAVE BEEN SWITCHED OFF HAS BEEN LEFT ON, WHICH MAKES THEM LESS ABLE TO FIGHT INFECTIONS THAN THOSE WITH ACCESS TO THE RIGHT NUTRITION IN THE WOMB.

THESE EFFECTS ARE VERY EVIDENT HERE, BUT THEY HAPPEN TO EVERY ONE OF US.

SILVER: THESE TYPES OF EFFECTS ARE REVEALED MOST STARKLY IN AN ENVIRONMENT LIKE WE HAVE HERE IN THE GAMBIA, WHERE WE HAVE VERY MARKED VARIATIONS BETWEEN THE SEASONS...

BUT WE CAN EXPECT TO SEE SIMILAR EFFECTS IN OTHER PARTS OF THE WORLD.

SO THE KINDS OF DISCOVERIES WE ARE MAKING HERE, I THINK, REALLY HAVE GLOBAL SIGNIFICANCE FOR GLOBAL HEALTH.

NARRATOR: IT'S DAY 15, AND THE EMBRYO IS NOW LESS THAN 1/50th OF AN INCH ACROSS.

IT'S TIME FOR A MOMENTOUS TRANSFORMATION TO TAKE PLACE.

UNTIL THIS POINT, EACH CELL HAS BEEN IDENTICAL.

BUT NOW, AS IF BY MAGIC, EVERY SINGLE ONE BEGINS A PROFOUND CHANGE...

TURNING FROM ONE CELL TYPE INTO THE 350 DIFFERENT TYPES OF CELLS THAT MAKE UP EVERY PART OF THE BODY.

THIS HIGHLY COMPLEX CELLULAR METAMORPHOSIS IS THE FOUNDATION OF EVERY PART OF US.

INCREDIBLY, THIS PROCESS ALMOST ALWAYS RUNS ACCORDING TO PLAN...

BUT OCCASIONALLY, A SMALL CHANGE OCCURS, AND THAT CAN HAVE PROFOUND RESULTS.

[CAMERA SHUTTER CLICKING] WOMAN: ONE OF MY EARLIEST MEMORIES IS ACTUALLY A STRANGER STARING AT ME AND ASKING MY FAMILY QUESTIONS ABOUT ME.

I DIDN'T UNDERSTAND WHY THEY WERE LOOKING AT ME IN A CERTAIN WAY, AND GROWING UP, I DIDN'T UNDERSTAND WHY PEOPLE TREATED ME DIFFERENTLY.

IN SCHOOL, KIDS WOULD BE REALLY AFRAID OF ME.

IF I WAS AROUND THEM, UH, CHILDREN WOULD RUN AWAY.

I DIDN'T REALLY HAVE A LOT OF FRIENDS GROWING UP.

PHOTOGRAPHER: SO, MELANIE, LET'S JUST TRY A COUPLE MORE OF THESE.

NARRATOR: MELANIE GAYDOS IS NOW AN INTERNATIONAL MODEL.

I LOOK LIKE THE FIRST LADY.

YOU DO.

[LAUGHS] NARRATOR: IN SPITE OF HER TOUGH EARLY YEARS, SHE'S BROKEN DOWN MANY BARRIERS TO MAKE IT IN THE WORLD OF FASHION.

THAT LOOKS GREAT.

IT LOOKS BEAUTIFUL.

YAY!

GAYDOS: I WAS BORN WITH A GENETIC DISORDER CALLED ECTODERMAL DYSPLASIA.

IT AFFECTS MY SKIN, HAIR, TEETH, NAILS.

[CAMERA SHUTTER CLICKS] NARRATOR: MELANIE'S CONDITION WAS DETERMINED BY A CRUCIAL MOMENT IN TIME... [CAMERA SHUTTER CLICKING] 26 YEARS AND 9 MONTHS AGO, WHEN SHE WAS JUST A BUNDLE OF CELLS.

IT HAPPENED WHEN MELANIE'S CELLS FIRST STARTED THE MIRACULOUS TRANSFORMATION PROCESS OF CHANGING INTO THE SPECIALIZED CELLS THAT FORM ALL THE PARTS OF HER BODY.

AT MAGNIFICENT SPEED, CELLS BEGIN TO MIGRATE UNTIL 3 LAYERS ARE FORMED.

AT THE BOTTOM, CELLS TO BUILD THE GUTS AND LIVER.

IN THE MIDDLE, THOSE DESIGNED FOR KIDNEYS, MUSCLES, BONES, AND BLOOD.

AND AT THE TOP, CELLS TO MAKE SKIN, HAIR, TEETH, AND BRAIN.

BUT AT THIS MOMENT, MELANIE'S DESTINY TOOK A DRAMATIC TURN.

GAYDOS: I DON'T GROW A LOT OF HAIR.

MY NICKNAME WHEN I WAS YOUNGER WAS ACTUALLY TWEETY BIRD, BECAUSE I HAD NO HAIR EXCEPT FOR 3 LONG HAIRS THAT SPROUTED FROM THE TOP.

NARRATOR: WHEN THE CELLS IN MELANIE'S SURFACE LAYER SHOULD HAVE BECOME SPECIALIZED, SOMETHING WENT WRONG, SO HER SKIN BECAME EXTREMELY SENSITIVE, HER TEETH CRUMBLED, AND HER HAIR GREW INWARDS.

AND THIS HAS HAD A POWERFUL IMPACT ON HER LIFE.

GAYDOS: I REALLY DIDN'T THINK I HAD A FUTURE.

I REALLY THOUGHT THAT SOMETHING REALLY BAD WOULD HAPPEN TO ME.

WOMAN: ♪ LET, LET THEM DO WHAT THEY DO ♪ ♪ JUST WORRY 'BOUT YOU, GOTTA KEEP YOUR COOL ♪ ♪ NOBODY'S GONNA HAND IT TO YOU ♪ ♪ GOTTA GET UP AND SHOW HOW ♪ ♪ UH, UH, N-N-NEVER SAY NEVER, NO, KEEP IT TOGETHER ♪ GAYDOS: I NEVER--I NEVER THOUGHT ABOUT HOW I WOULD KILL MYSELF.

I--I REALLY THINK I NEVER LET MYSELF THINK TOO CLEARLY ABOUT IT BECAUSE IF I THOUGHT ABOUT IT CLEARLY, THEN I PROBABLY WOULD HAVE DONE IT.

OBVIOUSLY, I'M STILL ALIVE, AND I STILL KEPT GOING.

I THINK MY OWN PERSONAL STRENGTH IS... [VOICE BREAKING] REALLY WHAT HELPED ME CONTINUE IN LIFE BECAUSE...

I DIDN'T HAVE ANYTHING ELSE TO...LOOK FORWARD TO.

I KNEW THAT...

THROUGH BEING AN ARTIST, THAT WAS SOMETHING THAT I WANTED TO DO AND SOMETHING I WANTED TO BE, AND IT REALLY GAVE ME HOPE THAT, UM, MY LIFE COULD CONTINUE AND MY LIFE COULD CHANGE.

NARRATOR: MELANIE HAS NEVER KNOWN THE EXACT CAUSE OF HER CONDITION...

BUT TODAY SHE'S FINDING OUT FOR THE FIRST TIME.

DOCTOR: SO WE GOT YOUR RESULTS BACK, AND WE FOUND A CHANGE IN YOUR GENE.

GAYDOS: AGAIN?

WE FOUND A CHANGE IN THE TP63 GENE.

THIS CHANGE WE FOUND IN YOU ISN'T JUST A NORMAL VARIATION...

RIGHT.

IN THE GENE, BUT SOMETHING THAT MAY BE CAUSING YOUR CONDITION.

GAYDOS: SO IT'S A NEW FORM.

YES.

IT MAY BE A NEW FORM OF ECTODERMAL DYSPLASIA?

YOU'RE VERY UNIQUE-- RIGHT.

THAT THIS CHANGE IS YOUR CHANGE.

THAT SOUNDS VERY GREAT.

IT'S AMAZING NEWS.

IT'S QUITE COOL TO FIND THAT I MAY BE THE FIRST-- THE REALLY TRULY THE FIRST EVER OF MY KIND.

HA HA HA!

NARRATOR: MELANIE'S GENETIC TEST HAS REVEALED A CHANGE IN A GENE IN HER SURFACE LAYER NEVER BEFORE SEEN IN MEDICINE.

AND SO MOST OF THE CELLS THAT BUILD TEETH, HAIR, AND SKIN DIDN'T TAKE ON ALL THE VITAL PROPERTIES THAT SHOULD MAKE THESE PARTS OF HER BODY WORK.

SO THAT IS WHERE MY GENETIC MAKEUP REALLY COMES INTO PLAY-- WHEN EVERYTHING IS FORMED.

YES, AND THAT HAPPENS VERY EARLY ON.

WOMAN: ♪ THE CAMERAS AND THE FAME ♪ ♪ NO, YOU WON'T BE THE SAME ♪ ♪ THIS LIFE WON'T REALLY CHANGE... ♪ NARRATOR: MELANIE'S ENTIRE BODY STILL REFLECTS A SNAPSHOT OF THAT KEY MOMENT OF HER TIME IN THE WOMB.

AND THE STORY OF HER FORMATION, ALONG WITH OTHERS, IS HELPING US BETTER UNDERSTAND HOW THIS FUNDAMENTAL PROCESS OF CELL DIFFERENTIATION UNFOLDS IN US ALL.

I DO FEEL BEAUTIFUL.

IT'S NOT SOMETHING THAT IS EASY FOR ME.

BUT AS TIME GOES ON, IT'S SOMETHING THAT GROWS STRONGER AND STRONGER EVERY DAY.

[CAMERA SHUTTER CLICKING] WOMAN: ♪ UH, UH ♪ GAYDOS: IF I COULD TURN BACK THE CLOCK TO WHEN I WAS IN THE WOMB, TO THE GENE THAT CREATED ME, I DON'T THINK I WOULD.

IT IS THE WAY THAT I AM, AND...I LOVE MY LIFE REGARDLESS.

[WOMAN SINGING INDISTINCTLY] NARRATOR: IT'S TWO WEEKS INTO THE 9 MONTHS THAT MADE YOU...

BUT FOR THIS JOURNEY TO CONTINUE, WE ALL HAVE TO PASS THE NEXT STEP.

WE HAVE TO GROW, DOUBLING IN SIZE EVERY 14 DAYS FOR THE NEXT 6 WEEKS.

AND HOW THIS HAPPENS IS THE RESULT OF THOUSANDS OF YEARS OF EVOLUTION...

WHICH HAVE CREATED A WONDROUS BALANCE BETWEEN THE GENES WE INHERIT FROM OUR PARENTS.

THE GENES FROM OUR FATHER ARE PROGRAMMED FOR GROWTH TO MAKE US BIG AND STRONG.

BUT OUR MOTHER'S GENES ARE DESIGNED TO RESTRICT HOW BIG WE BECOME, BECAUSE SHE NEEDS A BABY SMALL ENOUGH TO DELIVER.

THIS REMARKABLE DESIGN HAS EVOLVED SO WHEN WE ARE BORN, MOST OF US ARE NEITHER TOO BIG...OR TOO SMALL.

BUT SOMETIMES, THE BALANCING ACT CAN BE TIPPED AND WE ARRIVE RATHER LARGER THAN EXPECTED.

WOMAN: I THINK IT WAS PROBABLY AFTER ABOUT THE 20-WEEK SCAN AND THEY SAID THAT THE BABY WAS BIG... AND THEN, WHEN WE WENT BACK FOR THE NEXT SCAN, SHE WAS STILL BIG AND IT SORT OF WENT FROM THERE, REALLY, AND SHE JUST GOT BIGGER AND BIGGER.

NARRATOR: NELL IS JUST 7, BUT SHE'S HEAD AND SHOULDERS ABOVE HER PEERS.

[CAMERA SHUTTER CLICKS] THERE'S ACTUALLY LITTLE IN THE WAY OF HEIGHT BETWEEN HER AND HER 9-YEAR-OLD BROTHER SETH.

WOMAN: SHE WAS NEARLY 5 WEEKS EARLY AND SHE WAS 8 POUND 6, SO, YEAH, SHE WAS PRETTY BIG AND IN SPECIAL CARE WITH ALL THE SORT OF TWO-POUND BABIES.

YEAH, SHE WAS... SETH USED TO GO RUNNING IN, DIDN'T HE, TO VISIT HER, AND YOU WOULDN'T HAVE TO TELL HIM WHICH COT BECAUSE SHE'D BE THERE IN THE BIG ONE.

"IT'S THE BIG ONE, ISN'T IT, MUMMY?"

"IT'S THE BIG ONE."

SO HE'D RUN TO THE BIG ONE, AND THAT WOULD BE NELL.

NARRATOR: EARLY IN THE WOMB, NELL'S DEVELOPMENT TOOK A DIFFERENT COURSE THAN THE REST OF US.

INSTEAD OF GETTING ONE COPY OF HER FATHER'S MOST POWERFUL GROWTH GENE, SHE INHERITED TWO, AND THE EXTRA SET OF INSTRUCTIONS TELLING HER BODY TO GROW... MEANT SHE JUST GREW AND GREW AND GREW.

WOMAN: WITHIN SORT OF A FEW HOURS, YOU COULD TELL SHE HAD A VERY BIG TONGUE THAT WAS SORT OF PROTRUDING OUTSIDE HER MOUTH, AND HER STOMACH WAS HUGE.

IT WAS JUST REALLY MASSIVE, SO THAT'S PROBABLY WHEN WE FIRST NOTICED, AND THEN PEOPLE STARTED MENTIONING OVER THE NEXT DAY OR TWO, YOU KNOW, THAT GENETICALLY THERE MAY BE SOMETHING WRONG.

NARRATOR: THE EXTRA GENE NELL INHERITED FROM HER DAD MEANS SHE OVERPRODUCES A HORMONE THAT TELLS HER BODY TO GROW.

BUT AS WELL AS ACCELERATING HER NORMAL GROWTH, IT ALSO ACCELERATES ABNORMAL GROWTH, PUTTING HER AT RISK OF CANCER BECAUSE HER CELLS DIVIDE FASTER THAN THEY SHOULD, WHICH CAN CAUSE TUMORS TO GROW.

THANKS TO RECENT MEDICAL ADVANCES, NELL HAS ALWAYS BEEN CAREFULLY MONITORED... AND TODAY SHE'S COME TO HAVE A CHECKUP ON THE PART OF HER BODY MOST AT RISK.

HIYA.

I'M DR. DAVIS.

OK, CAN I HAVE A LOOK AT YOUR TUMMY TODAY, YEAH?

OK. NOW WE TUCK THAT UP UNDER THERE LIKE THAT.

RIGHT.

WELL, THERE'S THE KIDNEY... AND WHAT WE'RE LOOKING FOR IS TO MAKE SURE THAT THE KIDNEY IS DEVELOPING NORMALLY.

WOMAN: THEY CONFIRMED THAT, UM, YOU KNOW, THERE WAS ABNORMALITIES IN HER KIDNEYS WHEN SHE WAS PROBABLY-- OR QUITE POSSIBLY GOING TO DEVELOP A TUMOR AT SOME STAGE, SO--[SNIFFLES]--IT WAS QUITE SCARY, YEAH.

YEAH, VER--YEAH.

AHEM.

YEAH, VERY SCARY, YEAH.

AND SO WE'VE LIVED WITH THAT, REALLY.

YEAH.

I SUPPOSE YOU JUST WONDERED WHEN SHE WAS GOING TO GET-- WE USED TO THINK, OH, YOU KNOW, IT WOULD BE BETTER, ACTUALLY, IF SHE GOT THE TUMORS WHEN SHE WAS 3 AND THEN THE TREATMENT AND THEN SHE'D START SCHOOL AS-- IT SOUNDS MAD, BUT THAT'S-- BUT THAT'S WHAT WE THOUGHT.

BECAUSE YOU JUST ASSUMED AT SOME POINT IT WAS GOING TO HAPPEN, YEAH.

[LAUGHTER] DAVIS: THAT LOOKS A LOVELY NORMAL KIDNEY.

WE'LL JUST MEASURE THE SIZE ON IT NOW.

WOMAN: WHAT DOES THAT MEAN NOW FOR THE FUTURE?

WELL, AT HER AGE NOW, HER RISK IS NO DIFFERENT FROM ANY OTHER CHILD OF HER AGE.

LOVELY.

THAT'S GREAT.

HA HA!

DAVIS: YES.

YEAH.

NARRATOR: NOW THAT SHE'S 7, THE RISK OF DEVELOPING TUMORS WILL SIGNIFICANTLY REDUCE, BECAUSE AT THIS AGE, NELL'S EXCEPTIONAL GROWTH STARTS TO LEVEL OFF.

[LAUGHS] WOMAN: YEAH, PLEASED.

REALLY PLEASED, YEAH.

EVERYTHING WAS FINE, SO YOU SORT OF GET A BIT MORE HAPPY THAT, YOU KNOW, EVERYTHING'S FINE, EVERYTHING'S GOING TO BE OK. GO ON, NELL!

NARRATOR: EMBRACING LIFE AT FULL SPEED, NELL IS NOW RACING AGAINST KIDS TWO YEARS OLDER THAN HER.

MAN: KEEP GOING, NELL!

KEEP GOING!

KEEP GOING, NELL!

KEEP GOING!

WELL DONE.

EXCELLENT.

I STARTED OFF SLOW, BUT THEN I STARTED TO GET A BIT FASTER, AND I WAS COMING SECOND, BUT THEN I WAS TRYING MY HARDEST SO I CAME FIRST AGAINST THE OTHER GIRL, AND SHE WAS TRYING HER HARDEST, BUT I BEAT HER.

SMILE.

MAN: SHE'S JUST A SUPERSTAR.

YOU KNOW, SHE BEATS THE BOYS, THE GIRLS, AND SHE'S QUICKER THAN YOU.

[CHEERING AND APPLAUSE] NARRATOR: 450 HOURS, AND BY NOW, SOME MAJOR MILESTONES HAVE BEEN OVERCOME.

CELLS HAVE RAPIDLY DIVIDED AND BECOME SPECIALIZED.

AND INSIDE EVERY ONE, PROFOUND GENETIC CHANGES HAVE TAKEN PLACE THAT WILL REVERBERATE THROUGHOUT OUR LIVES.

NOW ABOUT HALF AN INCH LONG AND INSIDE THE EMBRYO, HUGE TRANSFORMATIONS ARE UNDERWAY AS CLUSTERS OF CELLS ARE INTRICATELY ASSEMBLING INTO THE BODY'S ORGANS.

AND EXACTLY WHERE THEY END UP COMES DOWN TO A FEW REMARKABLE HAIR-LIKE STRUCTURES CALLED CILIA.

AROUND DAY 19, THESE CILIA SPRING INTO LIFE, SPINNING CLOCKWISE AND CREATING A LEFTWARD CURRENT IN THE FLUID AROUND THEM.

NO ONE REALLY KNOWS HOW, BUT THIS CURRENT MIRACULOUSLY SWITCHES ON GENES ON THE LEFT, WHICH IN TIME TELLS EVERY ORGAN OF THE BODY WHERE TO GO, AND OUR PRE-DETERMINED LAYOUT FALLS INTO PLACE.

BUT ON VERY RARE OCCASIONS, OUR ORGANS DON'T END UP WHERE THEY SHOULD.

MAN: ♪ DO YOU WANT TO BE A CHAMPION?

♪ ♪ GOT WHAT IT TAKES TO BE A CHAMPION ♪ ♪ I'M A REAL STRONG BABY AND I CALL MY OWN DIGITS ♪ ♪ GOT MY OWN ENTOURAGE AND THEY BE COMIN' WITH IT ♪ ♪ I THINK I'M BUILT TO BE A CHAMPION ♪ ♪ YEAH, I RISE AGAINST THE ODDS ♪ ♪ AND I BLAZE MY OWN PATH ♪ ♪ HARD WORK, DEDICATION, Y'ALL DO THE MATH ♪ MAN: I FIRST STARTED PLAYING BASKETBALL WHEN I WAS, UH, 7, 8 YEARS OLD.

WE'D GO TO THE BASKETBALL COURTS EVERY DAY.

WE HAD A LOT OF FUN, A LOT OF BATTLES GOING BACK AND FORTH FROM EARLY ON.

MALE COMMENTATOR: FOYE ON THE DRIVE...COUNT IT.

THERE'S RANDY FOYE.

HITS IT TWO IN A ROW.

THIS ONE FROM DOWNTOWN.

FOYE AGAIN.

NARRATOR: RANDY HAS ALWAYS BEEN A TOP ATHLETE, WINNING A SCHOLARSHIP TO VILLANOVA UNIVERSITY FOLLOWED BY BECOMING BIG EAST CONFERENCE PLAYER OF THE YEAR.

MAN: ♪ IT'S BEEN A LONG JOURNEY, MAN, I'M FEELING GOOD ♪ ♪ FINALLY REACH MY PRO LIKE A REAL PLAYER SHOULD ♪ ♪ NO MORE CLIMBING LADDERS, NO MORE STING OF DEFEAT... ♪ NARRATOR: HE WAS SEVENTH IN THE 2006 NBA DRAFT AND HAS NOW COMPLETED 9 SEASONS, WHEN MOST CAREERS LAST JUST 4 OR 5.

BUT THERE'S SOMETHING ELSE SPECIAL ABOUT RANDY, WHICH HAPPENED JUST 19 DAYS INTO HIS DEVELOPMENT.

FOYE: I WENT TO THE HOSPITAL.

THE NURSE WAS DOING SOME TYPE OF TEST ON ME, AND I THINK SHE COULDN'T FIND A PULSE WHERE IT WAS SUPPOSED TO BE.

HEH.

USUALLY, EVERYONE'S HEART IS ON THE LEFT SIDE.

MY HEART IS ON THE RIGHT SIDE.

MY LIVER IS ON THE LEFT SIDE, SO EVERYTHING IS BASICALLY A MIRROR IMAGE IN MY BODY.

NARRATOR: RANDY HAS A RARE CONDITION CALLED SITUS INVERSUS, WHICH ONLY HAPPENS TO ONE IN EVERY 10,000 OF US.

IN RANDY'S CASE...

THE MOMENT WHEN HIS CILIA WERE SUPPOSED TO SPIN... SENDING HIS ORGANS TO THE RIGHT PLACE...

THEY DIDN'T SPIN AT ALL.

WITHOUT HIS CILIA TO DIRECT THINGS, HIS LEFT GENE SOMEHOW BECAME A RIGHT GENE INSTEAD-- AND EVERYTHING THAT SHOULD HAVE GONE LEFT WENT RIGHT.

RANDY FOYE FROM VILLANOVA UNIVERSITY.

[CROWD CHEERING] NARRATOR: RANDY FEARED HIS RARE CONDITION COULD AFFECT HIS CAREER, SO INITIALLY, HE TRIED TO KEEP IT SECRET.

MAN: ...IS HE A TWO GUARD?

AS GREAT AS HE IS, YOU PUT THE BALL IN HIS HANDS... FOYE: I WAS WORRIED LATER ON WHEN I WENT FOR THE DRAFT BECAUSE I DIDN'T WANT MY STOCK TO FALL.

I THOUGHT IF MY-- IF I LET THEM KNOW THAT MY HEART WAS ON THE SIDE, IF I HAD, UM, SITUS INVERSUS, YOU KNOW, MY STOCK WOULD DROP AND, YOU KNOW, IF MY STOCK DROPPED, IT'D FEEL LIKE ALL THE WORK THAT I DID LEADING UP INTO THE DRAFT, UM, THAT IT WOULD JUST BE, UM...

IT JUST WOULD BE DEVASTATING TO, YOU KNOW, LET THIS SMALL THING THAT REALLY NEVER AFFECTED ME, YOU KNOW, HURT ME.

MY TEAMMATES LAUGH AT ME WHEN WE DO THE NATIONAL ANTHEM AND I PUT MY HAND, UM, OVER MY--RIGHT HERE, OVER MY HEART, BUT MY HEART IS OVER HERE, AND MY TEAMMATES WOULD LIKE, "MAN, WHY YOU PUT YOUR HAND OVER, UM, OVER YOUR CHEST RIGHT THERE?

YOU'RE SUPPOSED TO PUT IT THIS WAY."

I'M EXTREMELY BLESSED AND LUCKY TO BE A PERFECT MIRROR IMAGE.

NARRATOR: RANDY IS LUCKIER THAN HE KNOWS.

IF ONLY A FEW OF HIS ORGANS HAD SWITCHED SIDES, HIS HEART WOULD HAVE BEEN SEVERELY AFFECTED.

AND THIS IS FATAL FOR 30% OF EMBRYOS WITH THIS CONDITION, AND THE REMAINDER SUFFER HEART PROBLEMS FOR THE REST OF THEIR LIVES.

YOU DON'T HEAR IT?

YOU DON'T HEAR IT-- BOOM-BOOM, BOOM-BOOM?

THAT'S YOUR HEART.

YEAH, THAT'S YOUR HEART BEATING.

YOU HEAR IT?

NARRATOR: SITUS INVERSUS CAN BE INHERITED, SO RANDY AND HIS WIFE WERE CONCERNED FOR THEIR CHILDREN.

FOYE: WHEN SHE WAS PREGNANT, LIKE, WE LET THE DOCTORS KNOW THAT I HAD SITUS INVERSUS, SO THAT WAS THE FIRST THING THAT THEY WOULD CHECK FOR.

LIKE, THEY--THEY KNEW RIGHT AWAY THAT WITH THE TECHNOLOGY NOW THAT NONE OF MY DAUGHTERS HAD SITUS INVERSUS.

NARRATOR: RANDY'S LIFE COULD HAVE TURNED OUT SO DIFFERENTLY.

BUT WITH A LITTLE LUCK AND A LOT OF DETERMINATION, HE'S BECOME THE MOST SUCCESSFUL MIRROR MAN ON THE PLANET.

FOYE: I DONE BEEN TO CHINA, I'VE BEEN TO LONDON.

I DONE BEEN TO TURKEY, AMSTERDAM, GERMANY, MALAYSIA, AND THE LIST GOES ON AND ON AND ON, AND I JUST THINK THAT WITHOUT THIS-- THIS BEAUTIFUL GAME OF BASKETBALL, I WOULDN'T BE ABLE TO EXPERIENCE THOSE THINGS THAT I HAVE EXPERIENCED.

BEING ABLE TO PLAY BASKETBALL HAS SAVED MY LIFE.

[GAME BUZZER, CROWD CHEERING] MALE COMMENTATOR: NUGGETS WIN!

NARRATOR: 21 DAYS, AND THIS MASS OF CELLS IS STILL ONLY A FLAT DISK.

BUT NOW, AN AWE-INSPIRING TRANSFORMATION BEGINS.

THE ENTIRE DISK STARTS TO FOLD, CREATING A TUBE, AND ALONG THE EDGE, A SHEATH OF CELLS THAT WILL BECOME THE CONTROL CENTER OF THE ENTIRE BODY-- THE CENTRAL NERVOUS SYSTEM.

THIS IS THE REMARKABLE MOMENT THE BODY BEGINS TO TAKE SHAPE.

AND RIGHT IN THE CENTER, A COLLECTION OF TINY MUSCLE CELLS IS ASSEMBLING INTO A NEW ORGAN.

IT'S NO BIGGER THAN A GRAIN OF SAND, BUT REMARKABLY, BARELY 4 WEEKS OLD, IT BEGINS.

THE HEART WILL MAKE OVER 3 BILLION BEATS THROUGHOUT THE COURSE OF A LIFETIME.

BUT IT'S A PRECARIOUS MOMENT BECAUSE THE ASTONISHING DESIGN OF THIS VITAL ORGAN IS SO INTRICATE THAT EVERY STEP IN ITS DEVELOPMENT MUST BE COMPLETED ACCURATELY, OR CORRECTED WITH PRECISION ONCE BORN.

SO IF WE DECIDED THAT WE DIDN'T WANT TO GO AHEAD WITH THE OPERATION, WHAT WOULD SORT OF BE THE OUTCOME WITH THE BABY, THEN-- WOULD IT SURVIVE?

THE HEART PUMPING CHAMBERS WILL DILATE AND THEN EVENTUALLY, UM, DON'T WORK AND NO BLOOD WOULD GO DOWN TO THE REST OF THE BODY, TO THE KIDNEY, TO THE BRAIN, SO THE BABY ACTUALLY WOULDN'T SURVIVE.

SO THE OPERATION IS LIFE-SAVING AND IS NECESSARY.

SO THE RIGHT SIDE LOOKS BIG, BUT YOU CAN SEE THE LEFT SIDE... NARRATOR: HANNAH IS NOW 36 WEEKS PREGNANT, BUT SEVERAL MONTHS AGO, SHORTLY AFTER THE EMBRYO'S HEART MADE ITS FIRST FEW BEATS, SOMETHING WAS NOT RIGHT.

WITHIN A WEEK OF BEING BORN, IT WILL NEED A LIFE-SAVING OPERATION.

SO THIS IS YOUR BABY'S HEART NOW AT 36 WEEKS.

AS YOU CAN SEE, THE RIGHT PUMPING CHAMBER IS RATHER LARGE IN COMPARISON TO THE LEFT.

BABY'S JUST MOVED...MOVING.

MAN: OUR LIVES ALMOST SEEMED AS IF THEY FLIPPED UPSIDE DOWN.

WE WENT THERE ALL HAPPY, SORT OF LOOKING FORWARD TO SEEING A BIT MORE IN-DEPTH PICTURES OF THE BABY AND, UM... THEN ALL OF A SUDDEN WE'RE BEING TOLD THAT ACTUALLY, "YOUR BABY'S GOT, UM, AN ISSUE WITH THE HEART."

WELL, AT FIRST, WE WERE JUST BOTH TOO DISTRAUGHT, REALLY, TO UNDERSTAND, REALLY, WHAT WAS GOING ON.

NO, I DON'T REALLY SUPPOSE YOU CAN SUM IT UP ANY BETTER THAN JUST SAYING, YEAH, YOUR BABY'S BROKEN.

NARRATOR: AT THEIR 20-WEEK SCAN, HANNAH AND ADAM DISCOVERED THAT DURING EARLY DEVELOPMENT, THE AORTA, THE MAIN ARTERY THAT TAKES BLOOD TO THE BODY, WAS TOO NARROW, SO BLOOD COULDN'T GET THROUGH.

THEY KNOW THAT IN THE WOMB, THE FETUS IS COPING BECAUSE A BYPASS VESSEL KEEPS BLOOD MOVING.

BUT SHORTLY AFTER BIRTH, DOCTORS WILL NEED TO OPERATE.

HANNAH: WE SORT OF DIDN'T REALLY KNOW WHAT TO DO WITH OURSELVES MORE THAN ANYTHING.

IT WAS...YEAH, QUITE-- IT HAS BEEN A--A MASSIVE ROLLER COASTER, REALLY, BUT YEAH, WE'RE-- WE'RE CONFIDENT THAT THE TEAM AT THE END CAN FIX IT-- ALMOST PUT A PLASTER ON IT, UM, AND THEN WE CAN CARRY ON WITH OUR HAPPY LITTLE FAMILY THAT WE FIRST SORT OF PLANNED.

YEAH.

NARRATOR: UNEXPECTEDLY, THEIR BABY ARRIVED 3 WEEKS EARLY... AND IS RUSHED INTO INTENSIVE CARE.

AND WE'VE GOT 4 NEEDLE HOLDERS.

1, 2, 3, 4.

NARRATOR: AFTER 5 DAYS, DOCTORS DECIDE TO OPERATE IN ORDER TO FIX THE MISTAKES THAT HAPPENED IN THE WOMB.

WOMAN: 2, 4, 6, 8.

NARRATOR: DR. PANDEY IS GOING TO HONE IN ON THE MOMENT WHEN THINGS WENT WRONG...

REMOVING THE OBSTRUCTION THAT'S CAUSING ALL THE TROUBLE.

AMELIA'S HEART IS JUST A COUPLE OF CENTIMETERS BIG, WHICH MAKES SURGERY HIGHLY RISKY, BUT WITH INCREDIBLE SKILL AND PRECISION, DOCTORS ARE ABLE TO SUCCESSFULLY REMOVE THE MICROSCOPIC BLOCKAGE.

WOMAN: HOW DID IT GO?

IT WENT WELL.

WE'RE PLEASED.

SMALL, BUT-- WELL, YEAH, I MEAN, IT'S SORT OF A TINY BABY, SO, AS EXPECTED, ALL THE STRUCTURES WERE SMALL, BUT WE MANAGED TO REPAIR IT.

YOU JOINED THE BITS UP.

YEAH, WE JOINED THE BITS UP, SO YES, AS FAR AS I'M CONCERNED, THE REPAIR LOOKS GOOD, AND LET'S SEE HOW SHE GROWS.

NARRATOR: BABY AMELIA WILL ALWAYS NEED TO BE MONITORED.

BUT THANKS TO TODAY'S CUTTING-EDGE MEDICINE, IT'S POSSIBLE TO REPAIR HER TINY HEART...

SO LIFE AT HOME WITH HER PARENTS CAN BEGIN.

HANNAH: SHE SINGS TO HERSELF LATE AT NIGHT, AND IT MAKES YOU FEEL LIKE THE LUCKIEST PERSON IN THE WORLD, GIVEN EVERYTHING THAT'S HAPPENED.

YEAH.

WHAT IS THAT?

AND EVEN AT 3:00 IN THE MORNING WHEN SHE'S SCREAMING HER HEAD OFF AND YOU LOOK IN THERE AND SHE JUST GIVES YOU A LITTLE SMILE, IT JUST MAKES IT ALL WORTHWHILE.

UM, YOU ACTUALLY JUST FORGET ABOUT THE TIME AND THAT YOU ACTUALLY GOT TO BE UP IN TWO HOURS FOR WORK, AND YOU JUST SPEND 20 MINUTES, HALF AN HOUR HAVING A CUDDLE.

HANNAH: IT'S NOT UNTIL WE GIVE HER A BATH AND YOU REMEMBER, WHEN YOU LOOK AT HER SCAR, HOW MUCH SHE'S BEEN THROUGH IN JUST 3 MONTHS OF HER LIFE THAT MAKES YOU FEEL LIKE A SPECIAL PARENT AND MAKES YOU...VALUE JUST THAT LITTLE BIT EXTRA HOW MUCH SHE'S BEEN THROUGH AND THE FACT THAT SHE'S ACTUALLY STILL HERE AFTER EVERYTHING.

[AMELIA FUSSING] NARRATOR: IN ONLY 6 WEEKS, SO MUCH HAS HAPPENED.

MILLIONS OF INTRICATELY ORGANIZED GROUPS OF CELLS FORM THE BASIS OF ALL THE BODY'S MAJOR SYSTEMS.

IT'S 130 TIMES LARGER, BUT STILL NO BIGGER THAN A BEAN, AND YET THE FOUNDATIONS OF THE BRAIN AND SPINAL CORD ARE IN PLACE.

AND THE MICROSCOPIC HEART IS ALREADY BEATING.

THE EMBRYO IS GROWING MORE AND MORE DISTINCT EVERY DAY...

BUT SOMETHING VITAL STILL HAS TO EMERGE THAT STARTS AROUND 38 DAYS.

CELLS WITHIN TWO FLATTENED PADDLES AT THE END OF EACH LIMB SLOWLY DIE AWAY...

REVEALING 5 PERFECTLY FORMED DIGITS.

OR SOMETIMES, JUST SOMETIMES, 6.

[PLAYING MID-TEMPO TUNE] MEET THE DA SILVA FAMILY.

WHEN IT CAME TO THE MOMENT AROUND 7 WEEKS WHEN THEIR HANDS WERE FIRST SCULPTED...

THEY GOT MORE THAN MOST OF US.

INSTEAD OF 5 FINGERS ON EACH HAND, THEY HAVE 6.

IN FACT, OUT OF 22 MEMBERS OF THE FAMILY... 14 OF THEM HAVE AN EXTRA FINGER ON EACH HAND.

BOY: I THINK IT'S GOOD BECAUSE MY HAND'S BIGGER THAN OTHER HANDS, AND I HAVE A BETTER GRIP--I CAN HOLD THE BALL MUCH BETTER THAN PEOPLE WITH 5 FINGERS.

WELL, IT'S ONLY DIFFICULT FOR ME TO GET GLOVES, SO, I HAVE TO--TO BUY TWO GLOVES, CUT ONE FINGER FROM A GLOVE, PUT IN ANOTHER SO I CAN HAVE A 6-FINGER GLOVE.

[SPEAKING NATIVE LANGUAGE, LAUGHS] TRANSLATOR: WE CAN WEAR MORE RINGS.

MEN WHO MARRY US SPEND A LOT OF MONEY.

[LAUGHS] WHEN WE GET PREGNANT AND GO FOR A SCAN, WE JUST WANT TO KNOW HOW MANY FINGERS.

WE DON'T CARE IF IT'S A BOY OR A GIRL.

[SPEAKS NATIVE LANGUAGE] WE ARE DELIGHTED TO FIND IT WAS 6.

[SPEAKS NATIVE LANGUAGE] [PLAYING NOTES] TRANSLATOR: MY MUSIC TEACHER SAYS HE WISHES HE HAD 6 FINGERS SO YOU CAN PLAY EASILY AND YOU CAN REACH MORE NOTES.

[PLAYING NOTES] [ALL CHEERING] NARRATOR: ALTHOUGH THE DA SILVAS' PHYSIOLOGY IS EXTRAORDINARY, IT CAME ABOUT THROUGH THE SAME PROCESS THAT BUILDS ALL OUR HANDS.

BY THE SEVENTH WEEK, A KEY PROTEIN KNOWN AS SONIC HEDGEHOG IS ACTIVATED.

THIS IS THE VITAL INSTRUCTION THAT TELLS THE FINGERS WHAT TO BECOME AND WHERE TO GROW.

BUT UNLIKE THE REST OF US, THE DA SILVAS HAVE A RANDOM GENETIC MUTATION, WHICH MEANS THEY PRODUCE MORE SONIC HEDGEHOG THAN NORMAL, AND SO, WHEN THEY WERE IN THE WOMB, THEY GOT AN EXTRA SET OF INSTRUCTIONS TO GROW ANOTHER FINGER.

[GUITAR PLAYING] BOY: AAH!

BOY: IT'S A THING ALL OF MY FAMILY HAVE.

UH, IT'S SOMETHING THAT MAKES ME ME, SO I WOULDN'T CHANGE IT.

UH, I WOULDN'T CHANGE IT.

YEAH, THAT'S WHO I AM.

THIS MAKES ME WHO I AM.

NARRATOR: FROM JUST A SINGLE CELL, A SERIES OF MOMENTS OF DEVELOPMENT HAVE BEEN ADDING UP.

BUT THERE'S ONE MORE PIECE OF THE JIGSAW THAT HAS TO FALL INTO PLACE-- THE PIECE THAT ALLOWS US TO WALK UPRIGHT THROUGH LIFE.

MAN: BREATHE DEEP.

I THINK IT'S THE ONLY WAY THAT I HAVE TO CONTROL MY--MY FEAR.

HEH HEH.

NARRATOR: BRUNO CARRERA IS ABOUT TO TAKE A DRAMATIC LEAP OF FAITH...

CROSSING A RAVINE 844 METERS ABOVE SEA LEVEL.

AND HE'S GOING TO DO IT ON A SINGLE ROPE JUST TWO CENTIMETERS THICK.

CARRERA: IT'S PRETTY MUCH LIKE FLYING.

YOU CAN SEE THE BIRDS PASSING NEXT TO YOU OR YOU CAN HEAR THE BIRDS SOMETIMES.

YEAH, I LOVE THIS, REALLY.

HA HA!

NARRATOR: BRUNO'S INCREDIBLE FEAT IS ONLY POSSIBLE BECAUSE OF THE EXTRAORDINARY SEQUENCE OF EVENTS THAT CAME TOGETHER IN HIS EIGHTH WEEK IN HIS MOTHER'S WOMB.

AROUND THE TIME THE HANDS AND FEET ARE BEING SCULPTED, A SERIES OF DEVELOPMENTS ARE HAPPENING THAT WORK IN PERFECT HARMONY TO GIVE US OUR REMARKABLE ABILITY TO BALANCE, EVEN IN THE MOST PRECARIOUS SITUATION.

DEEP INSIDE THE HEAD, A MAZE OF LOOPING TUNNELS IS BEING CARVED OUT AND FILLED WITH FLUID TO FORM THE INNER EAR.

THIS IS WHAT GIVES US THE ABILITY TO TELL UP AND DOWN AND KNOW WHERE WE ARE IN THE WORLD.

AT THE SAME TIME, A SERIES OF BLOOD VESSELS ARE BEING CONNECTED SO BLOOD CAN REACH OUR UPRIGHT BRAINS.

AND THE HIPS ARE PRECISELY ALIGNED TO PRODUCE OUR UNIQUELY HUMAN UPRIGHT STANCE.

IT IS THE MOST AMAZING FEELING THAT I TRY IN MY LIFE, SO IT'S WHY IT'S GOT ME SO BAD.

HA HA!

NARRATOR: AND WITH THESE LAST FEW DEVELOPMENTS, ALL THE FURIOUS SHIFTING OF CELLS STOPS AND ALL THE CRITICAL STRUCTURES OF THE BODY ARE IN PLACE.

TINY AND FRAGILE AND NOT YET CONSCIOUS, ONE SINGLE CELL HAS NOW BECOME A HUMAN FETUS.

WHILE WE ALL FOLLOWED THE SAME PATH THROUGH OUR FIRST 8 WEEKS IN THE WOMB, WE ALL DID IT IN OUR OWN UNIQUE WAY.

AND EACH OF US IS DEFINED BY THIS EXTRAORDINARY TIME IN OUR DEVELOPMENT, A TIME THAT CONTINUES TO AFFECTS US ALL, FROM OUR FIRST BREATH TO OUR VERY LAST.

NEXT TIME, THE MIDDLE 16 WEEKS, WHEN WE BECOME A UNIQUE INDIVIDUAL, AS EVERYTHING FROM OUR FACE TO OUR SEX...

YOUR MOM AND DAD THINK YOU'RE A BOY, THEN YOU'RE ACTUALLY A GIRL.

NARRATOR: AND THE COLOR OF OUR SKIN IS SET.

IT'S EVEN THE TIME WHEN PERSONALITY STARTS TO TAKE SHAPE.