Neil Shubin: FROM THE PLAINS OF SOUTH AFRICA... TO THE SHORES OF NOVA SCOTIA...

IN THE BONES OF ANCIENT CREATURES... YOU HAVE A SKULL RIGHT IN THIS BLOCK.

AND DEEP INSIDE YOUR DNA...

LIES AN INCREDIBLE STORY-- THE STORY OF YOUR BODY AND WHY YOU'RE BUILT THE WAY YOU ARE.

YOUR SKIN AND HAIR, YOUR COMPLEX TEETH AND REMARKABLE SENSE OF HEARING CAN ALL BE TRACED BACK TO ANCIENT REPTILES THAT ONCE RULED THE EARTH.

THEIR BODIES WERE SHAPED BY GREAT TRANSITIONS IN THE HISTORY OF LIFE... [ROARING] AND THAT LEGACY STILL SHAPES OUR BODIES TODAY.

MY NAME IS NEIL SHUBIN.

AS AN ANATOMIST I LOOK AT HUMAN BODIES DIFFERENTLY FROM MOST PEOPLE.

WITHIN US I SEE THE GHOSTS OF ANIMALS PAST... [JUNGLE SOUNDS] DISTANT ANCESTORS WHO'VE SHAPED OUR ANATOMY IN SURPRISING WAYS.

PREPARE YOURSELF FOR A TRIP BACK TO AN ANCIENT WORLD...

IF YOU REALLY WANT TO KNOW WHY YOU LOOK THE WAY YOU DO.

IT'S TIME TO MEET YOUR INNER REPTILE.

Neil: IT WAS HERE, IN THE SMALL TOWN OF PARRSBORO IN NOVA SCOTIA, OVER 25 YEARS AGO, THAT I FIRST DISCOVERED MY INNER REPTILE.

OH, MY GOSH!

THIS WAS MY SECOND HOME WHEN I WAS IN GRADUATE SCHOOL.

THIS LITTLE MAIN STREET, I CAN'T TELL YOU HOW MANY TIMES I RIPPED BACK AND FORTH.

A LOT OF MY PERSONAL HISTORY IS ON THIS STREET.

BACK THEN I WAS A YOUNG, EAGER HARVARD GRADUATE STUDENT OFF TO LEAD MY VERY FIRST FOSSIL EXPEDITION.

YEAH, THERE'S THE BAY.

ISN'T IT BEAUTIFUL?

WE'RE CATCHING IT AT A REALLY NICE TIDE.

WHAT MAKES THE BAY OF FUNDY A SPECIAL PLACE TO HUNT FOR FOSSILS IS ITS HUGE TIDES.

THEY'RE THE LARGEST IN THE WORLD.

IN JUST SIX HOURS, 100 BILLION TONS OF WATER DRAINS HALF A MILE OUT TO SEA.

THESE MASSIVE TIDES CONSTANTLY ERODE THE CLIFF FACE, MAKING IT A WONDERFUL PLACE FOR EXPOSING NEW FOSSILS.

WE CAME TO NOVA SCOTIA TO LOOK AT THESE ORANGE ROCKS.

AND THESE ORANGE ROCKS ARE KIND OF PERFECT.

YOU HAVE ANCIENT LAKE BEDS, YOU HAVE ANCIENT STREAMS, YOU HAVE EVEN THE MARGIN OF AN ANCIENT DESERT, ALL IN ONE SMALL PLACE.

AND SO THIS AREA WE HIT REALLY HARD.

WE HAD OUR EYES TO THE ROCKS, WE HAD OUR COLLECTING BAGS AND OUR ROCK HAMMERS, AND WE WERE READY TO FILL THEM WITH BONES.

THE CHALLENGE IS WE DIDN'T FILL THEM WITH BONES, WE DIDN'T FIND MUCH OF ANYTHING, AND REALLY OUR LUCK WAS TO CHANGE BECAUSE OF EVENTS IN TOWN.

SO THE BREAKTHROUGH CAME WHEN THE PRESIDENT OF THE LOCAL LIONS CLUB INVITED US TO BE JUDGES FOR THE TOWN'S BEAUTY CONTEST.

SO IT WAS UP TO US TO PERFORM THIS ONEROUS TASK.

THE PROBLEM WAS, IN JUDGING THE CONTEST, AND IN THE CELEBRATION AFTERWARDS, WE STAY STAYED UP WAY TOO LATE IN THE EVENING... AND THAT LATE NIGHT REVERIE HAD CONSEQUENCES THE NEXT DAY.

SO THE NEXT MORNING... WE GOT UP TOO LATE.

IT WAS HIGH TIDE.

AND THE TIDES ARE JUST SO HUGE HERE, WHAT HAPPENS IS THE TIDE WILL LAP UP AGAINST THE ROCKS.

AND WHAT WE FOUND IS WE WERE STUCK.

WE WERE SURROUNDED BY DARK VOLCANIC ROCKS, FORMED FROM HOT LAVA FLOWS.

NOBODY IN THEIR RIGHT MIND WOULD LOOK FOR FOSSILS HERE.

IT WAS A BEAUTIFUL DAY, SO WE SKIPPED ROCKS.

THE ROCKS WEREN'T THE BEST IN THE WORLD FOR SKIPPING.

MY BUDDY BILL HAD DISAPPEARED.

THEN I HEARD HIS VOICE.

HE WAS LIKE, "NEIL, YOU MAY WANT TO COME OVER HERE AND TAKE A LOOK AT THIS."

I'LL NEVER FORGET THOSE WORDS AS LONG AS I LIVE.

HOLY COW!

THIS IS THE SPOT.

I MEAN, IT JUTTED OUT MORE.

IT WAS 1985.

WELL, WHAT HAPPENED WAS BILL WAS AT THIS POINT, AND WHAT HE HAD DISCOVERED WAS THAT INSIDE THESE BASALT BOULDERS LIE SEAMS OF THIS BROWN SANDSTONE, AND WITHIN THIS BROWN SANDSTONE WERE BONES.

AS WE CHIPPED AWAY AT THE ROCK, WE UNCOVERED COUNTLESS FOSSILS.

THERE WERE LEG BONES... TEETH, EVEN WHOLE JAWS.

THIS SITE YIELDED THOUSANDS OF PIECES OF BONE FROM A MOMENTOUS PERIOD IN OUR HISTORY.

WE FOUND LOADS OF THINGS, BUT THE REAL GEM IS IN HERE.

WHAT YOU'RE SEEING HERE IS AN UPPER JAW.

THERE'S A CANINE TOOTH, AND A SERIES OF OTHER SMALLER TEETH BEHIND IT.

THESE 200-MILLION-YEAR-OLD TEETH BELONG TO A CREATURE CALLED A TRITHELEDONT.

IT MAY LOOK LIKE AN INSIGNIFICANT LITTLE ANIMAL, BUT IT'S UNLIKE ANYTHING ALIVE TODAY.

NOW, WHAT WE HAVE HERE IS REALLY A UNIQUE KIND OF ANIMAL WITH A UNIQUE MIX OF FEATURES.

PART REPTILE, PART MAMMAL.

WE SEE IT IN ITS SKULL, IN ITS JAWS, IN ITS LIMB BONES-- VIRTUALLY EVERY FEATURE OF ITS ANATOMY.

[SQUAWK] THIS IS A CREATURE RIGHT ON THE CUSP OF THE TRANSITION FROM REPTILE TO MAMMAL... A KEY MOMENT IN THE EVOLUTION OF THE HUMAN BODY.

TRACE YOUR ANCESTRY WAY BACK IN TIME AND YOU'LL FIND AN ANCIENT RELATIVE-- A FISH-LIKE CREATURE THAT CRAWLED FROM THE WATER ONTO LAND ABOUT 375 MILLION YEARS AGO.

ANIMALS LIKE THIS GAVE RISE TO AMPHIBIANS AND, MORE IMPORTANT TO OUR STORY, REPTILES.

OVER TIME ONE LINE OF REPTILES GAVE RISE TO EARLY MAMMALS... AND EVENTUALLY TO US.

BUT BEFORE YOU GET THE IDEA THIS WAS ONE SMOOTH RIDE...

YOUR REPTILIAN ANCESTORS FACED A ROCKY ROAD.

THEY WOULD COLONIZE A HOSTILE LAND...

COMPETE FOR FOOD AND TERRITORY... [HISSING] AND EVEN DEAL WITH THE WORST MASS EXTINCTION THE WORLD HAD EVER SEEN.

OUR LIVES TODAY SEEM WORLDS AWAY FROM THIS STORY.

BUT EVERY CHAPTER SHAPED OUR REPTILIAN ANCESTORS, CHANGING THEIR BODIES.

AND WE CAN STILL SEE THOSE FEATURES IN OUR BODIES TODAY.

ONE OF THE MOST SURPRISING IS VISIBLE IN THE FIRST FEW WEEKS AFTER CONCEPTION.

WE'VE COME TO AN IVF CLINIC TO SEE OUR INNER REPTILE, AND SAPNA AND DEV HAVE GRACIOUSLY ALLOWED US TO SEE THEIR 8-WEEK-OLD FETUS AND WHAT'S GOING ON INSIDE.

Clinician: LET'S SHRINK THE IMAGE A LITTLE BIT.

OH, WELL, LOOK AT HERE.

THERE'S TWO.

WE HAVE TWINS.

Sapna: DID I TELL YOU OUR LIFE'S ABOUT TO CHANGE?

[LAUGHTER] Neil: IN THE FIRST FEW WEEKS OF PREGNANCY, IT'S ALMOST AS IF THE EMBRYO IS A WINDOW BACK IN TIME THAT REVEALS OUR EVOLUTIONARY HISTORY.

Clinician: THIS IS THE HEAD, THIS IS THE HEART, AND THIS IS THE TAIL.

AND HERE IS THE YOLK SAC.

Neil: LET'S SEE.

OH, WOW, THERE YOU GO.

YOU DON'T THINK ABOUT YOLK IN A HUMAN, RIGHT?

YOU THINK OF YOLK FOR CHICKEN EGGS.

Clinician: CHICKEN EGGS!

[LAUGHTER] Dev: IS THAT EXTRA LARGE OR JUMBO?

Sapna: IT'S ORGANIC, FREE-RANGE.

Neil: FREE-RANGE, EXACTLY!

WHEN YOU THINK OF YOLK, YOU DON'T THINK OF HUMANS, BUT WE HAVE A LITTLE YOLK SAC IN EARLY DEVELOPMENT THAT GETS SMALLER AND SMALLER AS THE EMBRYO DEVELOPS.

[HEART BEATING] THERE'S NO YOLK INSIDE.

IT'S A REMNANT FROM A TIME WHEN OUR ANCESTORS LAID EGGS.

AND THAT'S NOT THE ONLY GIFT THEY LEFT US.

Clinician: THIS LITTLE BRIGHT WHITE LINE AROUND IS THE AMNION.

AND THEN THE BLACK AROUND THE BABY IS THE AMNIOTIC FLUID.

[HEART BEATING] Neil: THE AMNION FORMS A FLUID-FILLED SAC THAT CUSHIONS THE EMBRYO.

WHEN A MOTHER'S WATERS BREAK, THIS IS THE SAC THAT BURSTS.

THAT LITTLE TISSUE REPRESENTS SOMETHING THAT HAPPENED HUNDREDS OF MILLIONS OF YEARS AGO AND OUR CONNECTION TO ONE OF THE GREAT EVENTS IN THE HISTORY OF LIFE.

375 MILLION YEARS AGO, LIFE TOOK A CRITICAL TURN.

FOR MOST OF EVOLUTIONARY HISTORY OUR ANCESTORS NEEDED TO LIVE IN WATER.

IT'S WHERE THEY FED, MATED... AND MOST IMPORTANTLY, LAID THEIR EGGS.

BUT WHEN THE FIRST VERTEBRATES VENTURED OUT ONTO LAND, INTO A COMPLETELY ALIEN WORLD, THEY FACED A LIFE AND DEATH CHALLENGE... HOW TO PREVENT THEIR EGGS FROM DRYING OUT.

AMPHIBIANS GOT AROUND THE PROBLEM BY RETURNING TO WATER TO LAY THEIR EGGS.

REPTILES EVOLVED A FAR MORE RADICAL SOLUTION... A WHOLE NEW TYPE OF EGG.

THEY ENCLOSED THEIR EMBRYOS IN A SAC FILLED WITH FLUID-- THE AMNION--AND THEN COVERED IT WITH A PROTECTIVE SHELL.

THIS MEANT THEY COULD LAY THEIR EGGS ON SHORE... AND START TO INVADE THE LAND.

THIS INNOVATION OPENED THE FLOODGATES FOR TOTALLY NEW KINDS OF ANIMALS.

FROM SNAKES AND BIRDS TO CROCODILES AND TURTLES... AND EVENTUALLY, MAMMALS LIKE US.

[HEART BEATING] THAT "EGG LAYING" HISTORY SHOWS UP IN MORE THAN JUST OUR ANATOMY.

IT'S ALSO WRITTEN IN OUR GENES.

WHAT'S REALLY AMAZING ABOUT GENES IS YOU CAN LOOK AT THEIR CHANGES.

THAT IS, YOU CAN COMPARE THE GENOME OF A HUMAN TO THE GENOME OF A REPTILE, TO THE GENOME OF A CHICKEN, AND YOU CAN ASK THE QUESTION, "HOW ARE THEY SIMILAR AND HOW ARE THEY DIFFERENT?"

AND THAT'S EXACTLY WHAT RESEARCHERS DID.

THEY SEARCHED FOR A PARTICULAR KIND OF GENE THAT PRODUCES YOLK IN THE EGG.

REPTILES AND BIRDS HAVE SEVERAL OF THESE YOLK GENES THAT PRODUCE GOBS OF YOLK PROTEIN TO FEED THE EMBRYO.

GUESS WHAT?

WHEN RESEARCHERS LOOKED AT HUMANS, WHAT DID THEY FIND?

THEY FOUND WE, TOO, HAVE YOLK GENES.

BUT THEY'RE NO LONGER FUNCTIONAL, THEY'RE KNOCKED OUT.

THEY'RE DERELICTS.

THAT'S BECAUSE LONG AGO OUR ANCESTORS STOPPED RELYING ON YOLK TO NOURISH THEIR EMBRYOS.

THE YOLK GENES BEGAN TO DECAY... UNTIL EVENTUALLY THEY PRODUCED NO YOLK AT ALL.

BUT THOSE BROKEN GENES STILL LIE BURIED IN OUR GENOME, LIKE FOSSILS FROM THE PAST.

SO WHAT THE GENES TELL US, WHAT DNA TELLS US, IS THAT WE HAVE COME FROM AN EGG-LAYING ANIMAL.

WE'RE RELATED TO EGG-LAYERS.

WE'RE RELATED TO REPTILES, WE CARRY THE GENETIC SIGNATURE OF OUR EGGY PAST INSIDE OF US.

NOT ALL OUR REPTILIAN PAST IS BURIED SO DEEP INSIDE OF US.

IF YOU GO TO THE BEACH, THERE IS AMPLE EVIDENCE ALL AROUND YOU.

OUR SKIN CAN ALSO BE TRACED BACK TO WHEN OUR ANCESTORS LEFT THE WATER.

TO COMBAT THE DRY AIR ON LAND, THEY EVOLVED A NEW KIND OF SKIN.

THEY BUILT UP LAYER UPON LAYER OF DEAD CELLS TO FORM A WATERTIGHT BARRIER AROUND THEIR BODIES.

AND YOU CAN SEE THESE LAYERS MOST CLEARLY WHEN A SNAKE SHEDS ITS SKIN.

WE HAVE INHERITED THOSE SAME DEAD SKIN LAYERS TO KEEP OUR BODIES FROM DRYING OUT.

BUT UNLIKE REPTILES, OUR SKIN IS FILLED WITH ITS OWN MOISTURIZERS-- SPECIAL GLANDS THAT SECRETE OILS TO KEEP THE SURFACE LAYER HYDRATED.

THAT'S WHY OUR SKIN IS SOFT AND SUPPLE TO THE TOUCH, WHILE A REPTILE'S IS DRY.

SO OUR SKIN AROSE FROM AN ANCIENT BATTLE AGAINST THE ELEMENTS OVER 300 MILLION YEARS AGO.

BUT ONCE OUR REPTILE ANCESTORS HAD BROKEN THEIR TIES TO WATER, A NEW CHALLENGE LAY AHEAD.

THE COASTLINE OF SOUTH AFRICA IS ONE OF THE MOST DRAMATIC LANDSCAPES ON THE PLANET.

BUT TO FOSSIL HUNTERS, THE REAL DRAMA LIES INSIDE THE ROCKS.

THEY'RE HOME TO ONE OF THE RICHEST REPTILIAN BONE YARDS ON THE PLANET.

ON THE OTHER SIDE OF THESE MOUNTAINS, YOU'LL FIND A DESOLATE LANDSCAPE STRETCHING FOR THOUSANDS OF MILES.

IT'S CALLED THE KAROO-- THE LAND OF GREAT THIRST.

AND IT HOLDS CLUES TO THE EVOLUTION OF SOME IMPORTANT PARTS OF OUR BODIES.

ONE OF THE REGION'S BEST FOSSIL HUNTERS IS AN OLD FRIEND OF MINE-- ROGER SMITH.

DO YOU REMEMBER THE MID-90s WHEN WE RAN INTO YOU GUYS IN NAMIBIA?

Roger Smith: YEAH.

Neil: THAT PLACE HAD THORNS LIKE I'VE NEVER SEEN IN MY ENTIRE LIFE.

I GAVE SO MUCH BLOOD THAT SEASON!

Roger: ONCE HOOKED INTO YOUR FLESH THERE'S ONLY ONE WAY TO GO, AND THAT'S BACKWARDS.

[LAUGHTER] Neil: THE THING ABOUT ROGER IS HE LOVES THIS PLACE.

YOU CAN'T BE WITH HIM, YOU KNOW, FOR MORE THAN FIVE MINUTES TILL YOU REALIZE HE LIVES, EATS, SLEEPS, DRINKS THESE ROCKS, AND IT'S A PASSION THAT JUST IS SO INFECTIOUS.

FOR ROGER, THIS DUSTY BUSH HOLDS EVIDENCE OF AN ARMS RACE BETWEEN PREDATORS AND PREY-- PIECES OF BONE THAT REVEAL AN ESCALATING STRUGGLE AMONG OUR ANCESTORS AS THEY FOUGHT FOR FOOD AND TERRITORY ON DRY LAND.

I WAS REALLY EXCITED TO COME HERE, I MEAN, I'VE STUDIED ABOUT THIS PLACE, YOU KNOW, FOR MUCH OF MY CAREER, FOR DECADES.

MY EXPECTATIONS WERE THIS VAST DESERT, YOU KNOW, WITH BEAUTIFUL EXPOSURES OF ROCK.

YOU KNOW, SO WHEN I FIRST CAME ON THIS PLACE, I WAS KIND OF DISAPPOINTED.

I SEE THESE GRASSLANDS.

I DIDN'T SEE A WHOLE LOT OF ROCK.

IT WAS NOT WHAT I EXPECTED.

TO THE UNTRAINED EYE, WE COULD HAVE BEEN WALKING OVER A PILE OF RUBBLE.

BUT LOOK CLOSER, AND YOU BEGIN TO SEE EVIDENCE OF LIFE IN THESE ROCKS.

Roger: IT'S THE BACK END OF A SKULL, YEAH.

THAT'S WHERE THE NERVE CORD GOES THROUGH DOWN THE SPINE.

Neil: AND THEN THIS WOULD BE THE FRONT OF THE SKULL?

Roger: THAT'S RIGHT, YEAH.

Neil: SO, THE TOP OF THE HEAD AND THEN THE SNOUT WOULD JUST DIVE DOWN?

Roger: YEAH.

Neil: WOW!

YOU HAVE A SKULL RIGHT IN THIS BLOCK.

Roger: THAT'S RIGHT.

Neil: THE CREATURES THAT LIVED HERE WERE VERY DIFFERENT FROM ANYTHING ALIVE TODAY.

THEY LOOKED LIKE REPTILES, BUT THEY ALSO HAD SOME MAMMALIAN FEATURES...

SO WHEN PALEONTOLOGISTS FIRST FOUND THESE FOSSILS, THEY NAMED THEM THE MAMMAL-LIKE REPTILES.

AND THE KAROO IS COVERED WITH THEIR REMAINS.

I MEAN, OVER 50 FEET HERE, WE'RE STUMBLING OVER, YOU KNOW, SKELETON AFTER SKELETON OF, YOU KNOW, MAMMAL-LIKE REPTILE.

I'VE NEVER EXPERIENCED THAT IN MY ENTIRE CAREER.

IT WAS A STUNNING MORNING.

[MOTOR STARTS] THE FOSSILS FOUND HERE REVEAL KEY CHANGES HAPPENING IN THE BODIES OF MAMMAL-LIKE REPTILES.

Woman: IT ALMOST LOOKS LIKE A TIBIA OR FIBULA, MAYBE FROM THE OTHER SIDE.

Neil: AMONG THE MOST TELLING FOR OUR STORY WERE CHANGES HAPPENING INSIDE THEIR MOUTHS.

BEAUTIFUL CUSPS.

OH, YOU SEE THEM NICELY ON THIS SIDE.

Roger: LOOK AT THAT.

Neil: AND THAT TAKES US TO ONE OF MY FAVORITE TOPICS: TEETH.

FOR ANATOMISTS, TEETH CAN REVEAL A LOT ABOUT AN ANIMAL'S LIFE.

ONE OF THE GREAT ANATOMISTS OF ALL TIME WAS BARON GEORGE CUVIER, WHO LIVED IN PARIS IN THE 1800s... WHO WAS REMARKABLY IMPORTANT, REMARKABLY SMART AND REMARKABLY ARROGANT.

HE HAD A BOAST, AND HIS BOAST WAS, "SHOW ME THE TOOTH OF ANY ANIMAL AND I CAN RECONSTRUCT ITS ENTIRE SKELETON."

TURNS OUT CUVIER WAS PRETTY MUCH RIGHT.

HERE'S A TOOTH-- THIS IS A HUGE TOOTH.

CLEARLY CAME FROM A BIG ANIMAL.

FLAT SURFACE ON TOP, THAT ANIMAL CLEARLY CHEWS PLANT MATERIAL.

WHAT IS A BIG ANIMAL THAT EATS PLANTS?

AN ELEPHANT.

HERE'S ANOTHER ONE.

LARGE CANINES, BIG SCISSOR-LIKE TEETH IN THE BACK, THIS CREATURE CLEARLY, YOU KNOW, IS BUILT TO EAT MEAT.

THIS IS A LION.

SO TEETH CONTAIN SO MANY ANSWERS TO THE BASIC BIOLOGY OF CREATURES.

THEY CAN TELL US WHAT ANIMALS EAT, WHAT THEY LIKELY LOOKED LIKE.

BUT THEY CAN DO SO MUCH MORE.

THEY CAN ANSWER MYSTERIES THAT CUVIER COULD ONLY HAVE DREAMED OF KNOWING.

TEETH GIVE US A WINDOW INTO OUR DISTANT PAST BY TELLING US HOW OUR ANCESTORS LIVED AND EVOLVED.

ROGER WANTS TO SHOW ME A PARTICULARLY STRIKING SPECIMEN THAT'S OVER 250 MILLION YEARS OLD.

OH, WOW, LOOK AT THAT.

FEROCIOUS, LOOK AT THOSE TEETH.

Roger: NOW, THIS IS A GORGONOPSID-- THESE ARE THE CARNIVORES OF THAT TIME.

Neil: BOY, THE TEETH REALLY TELL THE STORY ON THAT ONE.

Roger: THEY CERTAINLY DO, YEAH.

Neil: GORGONOPSIDS MIGHT LOOK A BIT LIKE T-REX, BUT THEY LIVED MILLIONS OF YEARS BEFORE ANY DINOSAUR WAS AROUND.

THIS IS A FOSSIL THAT WHEN YOU SEE IT, YOU KNOW EXACTLY HOW IT LIVED, WHAT IT DID, YOU KNOW.

THIS WASN'T MESSING AROUND, THIS CREATURE.

Roger: ABSOLUTELY.

[LAUGHTER] Neil: GORGONOPSIDS WERE FEROCIOUS PREDATORS WITH MOUTHS FULL OF HIGHLY SPECIALIZED TEETH.

AND THAT REPRESENTS A DRAMATIC CHANGE FROM THE MUCH MORE BASIC TEETH OF REPTILES.

MOST REPTILES HAVE SHARP PEG-SHAPED TEETH THAT ALL LOOK REALLY SIMILAR.

THESE TEETH ARE ONLY GREAT FOR ONE THING--BITING!

THEY CAN'T STRIP OR CHEW THEIR FOOD-- INSTEAD THEY TEAR OFF CHUNKS... AND THEN SWALLOW THEM WHOLE.

THE GORGONOPSIDS' TEETH HAD COME A LONG WAY FROM THEIR REPTILIAN COUSINS.

Roger: COMPARED TO THE REPTILES BEFORE IT, THERE WAS NO DIFFERENTIATION OF INCISORS AND CANINES, BUT THIS ELONGATION OF THE CANINE IS PARTICULARLY A MAMMAL-LIKE FEATURE.

AND THEIR FRONT TEETH WERE LIKE LITTLE NEEDLES THAT CAME TOGETHER...

VERY WELL ADAPTED FOR FLESH TEARING.

Neil: SO HERE YOU START TO SEE DIFFERENTIATION ALONG THE TOOTH ROW.

THEY'RE ALL SORT OF THE SAME SHAPE, BUT THEY'RE DIFFERENT SIZES, SPECIALIZED IN DIFFERENT WAYS.

GORGONOPSIDS HAD VIRTUAL SWISS ARMY KNIVES OF TEETH.

[ROARING] GIANT CANINES FOR PIERCING TOUGH SKIN... AND INCISORS FOR STRIPPING FLESH OFF A CARCASS...

SO THEY COULD AVOID SWALLOWING WHOLE BONES.

NEVER BEFORE HAD SUCH MENACINGLY EFFICIENT EATING MACHINES RULED THE LAND.

Roger: THESE PARTICULAR GORGONOPSIANS WERE FAIRLY LONG LEGGED, AND THE FRONT LEGS WERE SLIGHTLY SPRAWLED, THE BACK LEGS WERE NICE AND UPRIGHT.

EVEN IN THEIR GAIT THEY'RE SOMEWHAT BETWEEN A REPTILE AND A MAMMAL.

Neil: OVER MILLIONS OF YEARS, BACK TEETH WOULD ALSO BECOME MORE SPECIALIZED.

THEY DEVELOPED EXTRA RIDGES-- CALLED CUSPS-- FOR CHEWING FOOD...

ALLOWING MORE EFFICIENT DIGESTION AND MORE ENERGY FOR CHASING PREY.

THESE ANCIENT INNOVATIONS WERE THE PRECURSORS TO THE MOLARS WE HAVE IN OUR MOUTHS TODAY.

WITHOUT A MOUTH OF MOLARS, COULDN'T EAT THE APPLE.

WITHOUT THE INCISORS I COULDN'T EAT THE WHATEVER YOU CALL THIS.

IT'S GOOD, BY THE WAY.

Roger: GUAVA ROLL.

Neil: GUAVA ROLL.

THANK YOU FOR HAVING MOLARS, BECAUSE I'M GLAD I CAN CHEW THIS ONE.

YOU KNOW, BUT THE HUMBLE TOOTH IS VERY IMPORTANT FOR OUR LIVES TODAY.

EVERY TIME YOU BITE INTO AN APPLE, THAT IS A VERY COMPLEX BEHAVIOR.

OUR FRONT OF OUR TEETH, THE INCISORS, BITE INTO THAT APPLE, AND THEY DO SO IN ALMOST A PERFECT WAY.

THE TEETH OCCLUDE IN A VERY FIXED PATTERN, A VERY PRECISE PATTERN.

WE TAKE FOR GRANTED HOW WE RELY ON PRECISE TOOTH-TO-TOOTH OCCLUSION, EXCEPT WHEN THINGS GO WRONG, EXCEPT WHEN WE CRACK A TOOTH OR SOMETHING LIKE THAT.

Dentist: A GOOD LOOK AT YOU, NEIL, AND MAKE SURE EVERYTHING IS WHERE IT'S SUPPOSED TO BE.

AND THERE'S NOTHING IN HERE THAT ISN'T SUPPOSED TO BE HERE.

Neil: WONDERFULLY COMPLEX TEETH MIGHT ALLOW US TO HAVE A GREAT BITE...

BUT IT'S COME AT A PRICE.

YOU KNOW, ONE OF THE THINGS THAT'S INTERESTING TO THINK ABOUT IS, YOU KNOW, THE CONSEQUENCES OF THE REPTILE-MAMMAL TRANSITION ARE THE FACT THAT WE NEED DENTISTS!

IT'S REALLY TRUE.

Dentist: THANK YOU!

Neil: YEAH, RIGHT!

THANK YOU, SOUTH AFRICAN FOSSILS!

Dentist: THANK YOU VERY MUCH!

Neil: I MEAN, REPTILES DON'T NEED DENTISTS, RIGHT, BECAUSE THEY'RE PRODUCING LOTS OF TEETH THROUGHOUT THEIR LIVES, YOU KNOW, BOOM, BOOM, BOOM, THEY PRODUCE THEM AND THEY'RE OUT OF THERE.

MOST REPTILES, LIKE THE SNAKE, GROW MULTIPLE SETS OF TEETH IN THEIR LIFETIME.

THERE'S A CONSTANT STREAM OF NEW TEETH ERUPTING IN THEIR MOUTHS.

BUT AFTER WE LOSE OUR BABY TEETH, WE'RE LEFT WITH ONE ADULT SET TO LAST US THE REST OF OUR LIVES.

THIS MAY NOT SEEM LIKE THE BEST OF STRATEGIES...

BUT THERE IS A PLUS SIDE.

IT ALLOWS US TO DEVELOP OUR VERY PRECISE BITE.

IF YOU'RE CONTINUALLY REPLACING YOUR TEETH, RIGHT, I MEAN, I THINK IT WOULD BE HARD TO GET THAT MICRON-SCALE OCCLUSION WHICH IS SO REALLY IMPORTANT TO US.

Dentist: YOU'RE RIGHT.

WHEN WE LOSE A TOOTH, THE CHANGES THAT OCCUR TO COMPENSATE FOR THE LOSS OF THAT TOOTH AND THE SPACE THAT IT HELD HAS A PROFOUND EFFECT ON OUR OCCLUSIONS.

Neil: THE REASON OUR ROWS OF TEETH MATCH UP SO PRECISELY IS THAT OUR ANCESTORS REDUCED THE NUMBER OF SETS THAT THEY PRODUCED.

WE'VE MADE UP FOR THAT BY STRENGTHENING OUR TEETH WITH EXTRA ROOTS AND THICK ENAMEL, TO HELP THEM LAST A LIFETIME...

ALONG WITH A LITTLE HELP FROM OUR DENTISTS!

Dentist: THAT'S NEAT.

I'LL HAVE TO TAKE A BETTER LOOK WHEN I GO TO THE ZOO WITH MY KIDS.

THE REPTILE HOUSE IS NOT MY FAVORITE PLACE.

I WOULD LIKE THEM IN THE FORM YOU FIND THEM.

Neil: DEAD?

Dentist: YEAH, DEAD 200 MILLION YEARS!

[LAUGHTER] Neil: FOR MY DENTIST, ROGER'S TALES FROM THE KAROO WOULD BE THE STUFF OF NIGHTMARES.

BUT FOR ROGER, WHAT HAPPENED HERE IS AN ENDLESSLY FASCINATING DRAMA.

TO HIM, THE ROCKS AND FOSSILS TELL THE STORY OF A VANISHED WORLD.

Roger: THESE ROCKS WERE LAID DOWN BY LARGE MISSISSIPPI-SIZED, MEANDERING RIVERS THAT FLOWED PRETTY MUCH THROUGHOUT THE YEAR.

Neil: LONG AGO, THIS LANDSCAPE LOOKED DRAMATICALLY DIFFERENT.

IT WAS WET, HUMID, WITH HUGE, LUMBERING ANIMALS CROWDING THE RIVER BANKS.

Roger: AND THESE LARGE ALSATIAN-SIZED CARNIVORES, THE GORGONOPSIDS, WHICH FED UPON THEM.

SO THEY WERE FLOURISHING.

Neil: FOR NEARLY 20 MILLION YEARS THE GORGONOPSIDS SLICED AND DICED THEIR WAY ACROSS THE PLANET.

[ROARING] BUT THEIR REIGN OF TERROR WAS ABOUT TO END.

[THUNDER] 252 MILLION YEARS AGO THE CLIMATE BEGAN TO CHANGE DRAMATICALLY.

THIS CATACLYSM WIPED OUT MOST ANIMAL SPECIES, INCLUDING THE GORGONOPSIDS.

Roger: AS WE GO UP HERE, WE START TO SEE THE CHANGE IN THE ROCKS FROM THESE WET FLOOD PLAINS TO THE DRYING OUT.

Neil: PLANTS DIED.

SOILS WASHED AWAY.

THE KAROO BECAME A DEAD ZONE.

Roger: THIS WAS THE GREAT END PERMIAN MASS EXTINCTION, THE GREAT DYING.

Neil: THE MOTHER OF ALL MASS EXTINCTIONS, AS THEY SAY?

Roger: THAT'S RIGHT, THE MOTHER OF ALL MASS EXTINCTIONS.

Neil: YOU KNOW, WHEN YOU THINK ABOUT THE HISTORY OF LIFE ON EARTH, IF YOU LAY OUT EVERY FOSSIL THAT WE'VE EVER DISCOVERED, THERE ARE CERTAIN PERIODS OF TIME WHERE MANY CREATURES JUST DISAPPEAR, NEVER TO BE SEEN AGAIN, AND WE CALL THESE, YOU KNOW, THE MASS EXTINCTIONS.

WELL, THESE MASS EXTINCTIONS ARE HUGELY IMPORTANT FOR UNDERSTANDING OUR WORLD TODAY, UNDERSTANDING OUR BODIES, HOW THEY CAME INTO BEING, BECAUSE THESE MASS EXTINCTIONS DON'T ONLY JUST REMOVE CREATURES, BUT THEY CREATE NEW OPPORTUNITIES FOR THE SURVIVORS.

NOT ALL OUR DISTANT RELATIVES WERE WIPED OUT BY THE PERMIAN EXTINCTION.

THE FOSSIL EVIDENCE TELLS US THAT SOME SURVIVED ON THE SURFACE... AND OTHERS BURROWED UNDERGROUND.

Roger: NEIL, HAVE A LOOK AT THIS.

WE'RE EXCAVATING AN UNDERGROUND BURROW.

THIS WAS ORIGINALLY A HOLE DOWN INTO THE ANCIENT FLOODPLAIN, DUG BY ONE OF THESE ANIMALS, AND HAS BEEN FILLED WITH SAND, AND THE SAND HAS SOLIDIFIED NOW, AND IT'S WHAT WE CALL A BURROW CAST.

Neil: THIS IS SO COOL.

WHEN YOU LOOK AT THIS THING, IT'S SORT OF AN S-SHAPED CURVE, BUT THERE IS A BRANCH GOING OFF OF IT.

WHAT'S GOING ON WITH THE SHAPE?

Roger: WELL, THAT'S THE ENTRANCE CHAMBER, AND WE KNOW FROM SOME OF OUR MODERN BURROWING ANIMALS THAT THESE SIDE CHAMBERS ARE USED AS LATRINES OR PLACES TO STORE THEIR WASTE.

Neil: VERY SANITARY OF THEM.

I'M IMPRESSED.

ROGER THINKS THAT LIVING UNDERGROUND MAY HAVE HELPED OUR ANCESTORS SURVIVE IN A WORLD OF EXTREME TEMPERATURES.

AND IT'S WITHIN BURROWS LIKE THIS THAT A NEW BODY FEATURE MAY HAVE EMERGED, ONE THAT IS IMPORTANT TO ALL MAMMALS TODAY.

HAIR.

HAIR RARELY FOSSILIZES.

BUT ROGER SEES TANTALIZING EVIDENCE IN THESE ANCIENT BURROWERS THAT MAY EXPLAIN HOW AND WHY IT FIRST EVOLVED.

THAT IS BEAUTIFUL.

TWO SPECIMENS?

Roger: THAT'S TWO THRINAXODON, TWO JUVENILE THRINAXODON.

Neil: WOW.

THAT IS JUST ABSOLUTELY BEAUTIFUL.

DO YOU KNOW HOW LUCKY YOU ARE TO FIND THINGS LIKE THIS?

THIS IS GREAT.

Roger: YEAH, IT WAS QUITE A SENSATION TO FIND THAT.

THE WAY THEY ARE LYING, THIS BODY BEAUTIFULLY CURLED AROUND, THIS ONE LYING STRAIGHT.

IT REALLY LOOKS LIKE THE BURROW HAS COLLAPSED ON THESE AND JUST PRESERVED THEM AS THEY WERE SLEEPING OR HIBERNATING.

Neil: SO WHAT DO YOU KNOW ABOUT THE BIOLOGY OF THESE CREATURES, JUST FROM WHAT WE'RE SEEING HERE, THE SKELETONS?

Roger: ITS TEETH CLEARLY SHOW THAT IT HAD AN INSECTIVOROUS DIET-- BEETLES, COCKROACHES, THAT SORT OF THING.

IT HAS LARGE EYES, SO IT APPEARS TO HAVE BEEN NOCTURNAL OR AT LEAST IN THE DAWN AND DUSK, WHERE THEY WOULD COME OUT OF THEIR BURROWS TO HUNT INSECTS.

Neil: HAIR, THAT'S THE MILLION DOLLAR QUESTION-- DOES THRINAXODON HAVE HAIR?

Roger: WE PREFER NOT TO RECONSTRUCT THRINAXODON WITH FULL BODY HAIR, BUT WE DO HAVE EVIDENCE AROUND THE CHEEKS, SO WHISKERS, AROUND THE LOWER JAW, AS WELL AS ON THE MAXILLA AROUND HERE, SO MORE AS A SENSORY ORGAN THAN THERMOREGULATION.

Neil: THESE PITS ON THE SKULL SUGGEST THAT THRINAXODON HAD WHISKERS, JUST LIKE A CAT OR A DOG TODAY.

AND, ALTHOUGH WE CAN'T BE COMPLETELY CERTAIN, THERE'S NO EVIDENCE THAT IT HAD HAIR ANYWHERE ELSE.

SO A HAIRLESS GUY WITH WHISKERS AT THE FRONT?

Roger: THAT'S RIGHT.

VERY MUCH A HALF-AND-HALF ANIMAL.

Neil: IF WE COULD GO BACK IN TIME AND VENTURE DEEP UNDERGROUND, WE'D SEE THAT HAIR MAY HAVE FIRST EVOLVED AS A SENSORY ORGAN.

WHISKERS WOULD HAVE HELPED THRINAXODON TO NAVIGATE INSIDE ITS BURROW... AND WHEN IT EMERGED AT DUSK, THEY WERE USED TO FIND ITS WAY IN THE DARK.

IT'S THOUGHT THAT OVER MILLIONS OF YEARS OUR DISTANT RELATIVES GOT HAIRIER AND HAIRIER.

AND BY 50 MILLION YEARS AFTER THRINAXODON, THEY HAD STARTED USING HAIR FOR A NEW PURPOSE-- TO KEEP WARM.

SO HAIR DATES BACK TO A WHISKERED CREATURE THAT LIVED IN THE AFTERMATH OF A GREAT EXTINCTION.

COMPLEX TEETH ARE LINKED TO PREDATORS THAT BATTLED IN LONG LOST LANDS.

[HISSING] AND A WATERPROOF SKIN TO A TIME WHEN OUR ANCESTORS MOVED ONTO LAND.

EACH OF THESE BODY FEATURES IS LINKED TO A NEW CHAPTER IN THE HISTORY OF LIFE.

BUT HAIR, TEETH AND SKIN SHARE AN EVEN DEEPER CONNECTION.

ONE THAT LIES AT THE HEART OF OUR INNER REPTILE.

THERE'S A SCIENTIST IN LONDON NAMED ABIGAIL TUCKER WHO'S DECIPHERING THIS ANCIENT HISTORY.

Abigail Tucker: THIS IS SPONGEBOB THE BEARDED DRAGON, AND EVEN THOUGH WE LOOK SO DIFFERENT FROM OUR REPTILE FRIENDS HERE, WE ACTUALLY SHARE LOTS OF COMMON FEATURES IN OUR SKIN.

Neil: SKIN IS NOT JUST AN OUTER COVERING.

OUT OF IT GROW BODY PARTS AS DIFFERENT AS HAIR, TEETH, CLAWS AND SCALES.

ABIGAIL'S PASSION LIES IN PROBING THE MYSTERY OF HOW THESE SO-CALLED "SKIN ORGANS" FORM.

AS AN EXAMPLE... TAKE THE TOOTH.

FROM A MOUSE EMBRYO SHE CUTS OUT AN AREA OF SKIN TISSUE FROM THE LOWER JAW, WHICH WILL EVENTUALLY TRANSFORM INTO A TOOTH.

YOU NEED REALLY STEADY HANDS FOR THIS KIND OF WORK!

Abigail: HERE I'M USING A TUNGSTEN NEEDLE, BUT FOR VERY SMALL TISSUE WE GLUE AN EYELASH TO A GLASS STICK AND USE THAT AS A REALLY FINE LITTLE KNIFE.

IT IS STARTING TO GO NOW.

THERE WE GO.

SO HERE WE'RE LOOKING AT A TOOTH AT THE BUD STAGE.

IT HASN'T REALLY SORT OF GOT A TOOTH SHAPE YET.

SO IT WAS A FLAT SHEET, AND NOW IT'S PUSHED ITS WAY IN, FORMING THIS LITTLE BUD SHAPE.

Neil: AFTER PLACING THE TOOTH BUD IN AN INCUBATOR, ABIGAIL CAN OBSERVE HOW IT GROWS.

AND WHAT SHE SEES IS REMARKABLE.

THE SKIN FOLDS IN UPON ITSELF, VERY MUCH LIKE WATCHING ORIGAMI.

Abigail: AND NOW YOU REALLY CAN SEE THAT THAT'S FORMED A TOOTH, AND IT IS NOT JUST ANY OLD TOOTH, IT'S A MOLAR TOOTH, WITH ALL THESE NICE CUSPS, DEVELOPING HERE.

Neil: AND THE TOOTH IS JUST THE BEGINNING.

ALL OUR SKIN ORGANS-- FROM HAIRS... TO GLANDS-- ARE FORMED IN A SIMILAR WAY... FROM SIMPLE FOLDS IN SKIN TISSUE.

Abigail: WE THINK THAT THIS IS A REALLY SORT OF ANCIENT PRINCIPLE.

IT'S THIS SORT OF FOLDING THAT WAS ONE OF THE FIRST STEPS TO BE ABLE TO ACTUALLY THEN GO ON FROM A VERY SIMPLE SKIN TO SOMETHING WITH LOTS OF DIFFERENT ORGANS DEVELOPING WITHIN IT.

Neil: ONCE THE PROCESS OF FOLDING SKIN WAS IN PLACE, IT COULD BE MODIFIED TO PRODUCE ALL KINDS OF SKIN ORGANS... FROM A REPTILE'S SCALE, TO A BIRD'S FEATHER, AND EVEN THAT DEFINING BODY FEATURE OF MAMMALS-- MAMMARY GLANDS.

AND WHAT'S SURPRISING IS THE FORMATION OF ALL SKIN ORGANS IS CONTROLLED BY A SINGLE "MASTER" GENE CALLED E.D.A.

Abigail: E.D.A.

WE KNOW IS VERY IMPORTANT FOR DOING THINGS LIKE CONTROLLING HOW HAIRY YOU ARE, THEREFORE HOW MANY HAIRS YOU FORM IN A PARTICULAR AREA, OR HOW MANY TEETH YOU HAVE, AND WHAT ARE THOSE TEETH GOING TO LOOK LIKE.

IT SEEMS TO BE DOING SOMETHING SLIGHTLY DIFFERENT IN ALL THOSE DIFFERENT ORGANS, BUT IT'S ABSOLUTELY ESSENTIAL IN ALL OF THEM.

Neil: THIS GENE PLAYS A CRUCIAL ROLE IN BUILDING ALL THE VITAL STRUCTURES THAT ORIGINATE WITHIN SKIN.

AND IF YOU REALLY WANT TO SEE JUST HOW POWERFUL E.D.A.

IS, MEET ACTOR MICHAEL BERRYMAN.

SO MY BIG QUESTION IS, WHAT WAS IT LIKE TO BE ON STAR TREK?

Michael Berryman: IT WAS FANTASTIC!

I WAS THE FIRST BOLIAN.

Neil: BLUE SKINNED... Michael: YES.

I THINK WE WERE A WONDERFUL MUTATION.

Neil: MICHAEL OFTEN PLAYS MONSTERS AND ALIENS IN SCIENCE FICTION MOVIES.

HE SUFFERS FROM A RARE MUTATION IN HIS E.D.A.

GENE THAT STOPPED HIS SKIN ORGANS FROM DEVELOPING PROPERLY.

Michael: I HAD THE EXPERIENCES AT A VERY YOUNG AGE.

I STARTED REALIZING I HAD ISSUES WITH MY SKIN.

FOR INSTANCE, NO FINGERNAILS, I HAVE TWO NAILS HERE.

ONE HERE, ONE HERE, ONE HERE, AND ONE HERE.

THE REST ARE NOT FULLY DEVELOPED.

THERE WERE AREAS THAT WERE JUST RAW MEAT... WHERE THERE WAS NO DERMIS ON IT AT ALL.

BUT I ALSO REALIZED THAT I, WELL, I HAD NO TEETH.

THE TEETH THAT I HAD HAD TO BE REMOVED.

I'VE WORN DENTURES MY ENTIRE LIFE.

IT ALSO AFFECTED ME IN THE MANNER OF WHICH I CANNOT DISSIPATE BODY HEAT, WHICH IS ABSOLUTELY HUGE.

Neil: YEAH, YOU'RE AN ACTOR, YOU'RE UNDER HOT LIGHTS.

I MEAN, WHAT DO YOU DO?

Michael: WELL, UM...

THE ACTING CAME ABOUT SECONDARY TO MY CHOSEN PROFESSION, WHICH WAS FOOD AND BEVERAGE.

I WAS GONNA BE-- I HAD A RESTAURANT, I WAS ON MY WAY TO BE A CHEF.

I STARTED TO REALIZE THAT I WOULD OVERHEAT QUITE READILY, SO THE ONLY WAY IN WHICH I COULD DISSIPATE THE BODY HEAT WAS ACTUALLY PHYSICALLY, MEANING I WOULD GO INTO A FREEZER THAT WAS 20 BELOW ZERO.

Neil: WOW.

Michael: AND, AND, AND BREATHE SLOWLY AND DEEPLY TO COOL THE BLOOD IN MY LUNGS.

Neil: SO, SWEAT GLANDS, FINGERTIPS AND NAILS.

UH, HAIR?

Michael: YES.

Neil: AS WELL.

Michael: TEETH.

Neil: TEETH.

I MEAN, THESE ARE ALL SKIN ORGANS.

SO YOU, OF ALL PEOPLE, UNDERSTAND JUST HOW IMPORTANT SKIN IS.

Michael: YEAH.

I LIKE IT WHEN IT'S HEALTHY.

[LAUGHTER] Neil: MICHAEL'S SKIN CONDITION DEMONSTRATES THE CRUCIAL ROLE E.D.A.

HAS HAD IN SHAPING OUR BODIES.

WHEN WE LOOK AT EVOLUTION, YOU KNOW, WE DON'T INHERIT STRUCTURES FROM OUR ANCESTORS.

I DIDN'T INHERIT AN ELBOW, YOU KNOW, FROM THE FISH WE FOUND UP IN THE ARCTIC.

WHAT WE INHERIT ARE THE DEVELOPMENTAL PROCESSES, THE GENES, SORT OF THE MANUFACTURING PROCESSES, IF YOU WILL, TO BUILD THOSE ORGANS.

AND E.D.A.

WAS A VERY IMPORTANT PART OF OUR EVOLUTIONARY HISTORY.

FROM TEETH TO HAIR AND EVEN SKIN GLANDS, THE E.D.A.

GENE HELPED SHAPE THE BODIES OF OUR REPTILE ANCESTORS.

AND WITH EVERY CHANGE, THEY BECAME MORE AND MORE LIKE MAMMALS.

BUT THERE WAS ONE MORE CRITICAL CHANGE THAT HELPED COMPLETE THE TRANSITION TO MAMMALS... AND IT ENRICHES OUR LIVES TODAY.

A NEW KIND OF EAR.

THE HEARING ANATOMY OF MAMMALS IS UNIQUE.

YOU WON'T FIND EAR FLAPS ON ANY REPTILE.

OUR EARS ARE EXTREMELY SENSITIVE, ALLOWING US TO PICK UP A WIDE RANGE OF SOUNDS.

FROM HIGH-PITCHED SQUEAKS... [SQUEAKING] TO LOW DISTANCE CALLS.

Roger: OK!

Neil: AND THE SECRET OF THEIR INCREDIBLE SENSITIVITY LIES IN THE TINIEST BONES IN OUR BODY.

THESE ARE THE THREE BONES OF OUR MIDDLE EAR.

ALL MAMMALS INCLUDING PEOPLE HAVE THESE THREE BONES THAT SIT INSIDE THE SKULL.

AND WHAT THEY DO IS THEY FORM A LITTLE LEVER SYSTEM THAT TURNS THE VIBRATION OF AIR INTO SOUND THAT OUR NERVOUS SYSTEM CAN PERCEIVE.

WE, LIKE ALL MAMMALS, HAVE THREE OF THESE BONES IN OUR MIDDLE EAR, BUT REPTILES ONLY HAVE ONE.

THESE TWO EXTRA BONES, THE MALLEUS AND INCUS, ENABLE US TO AMPLIFY SOUNDS THAT REACH OUR EARDRUM.

WHICH IS ONE REASON WHY OUR HEARING IS SO MUCH BETTER THAN REPTILES'.

THIS PRESENTS US WITH A SIMPLE, YET EXTRAORDINARY PUZZLE.

HOW DID OUR ACUTE SENSE OF HEARING USING THREE BONES EMERGE FROM OUR REPTILIAN ANCESTORS, WHO USED ONLY ONE?

FOR NEARLY TWO CENTURIES, SCIENTISTS HAVE BEEN FASCINATED BY THIS MYSTERY.

AND ONE OF THEM IS MY FORMER STUDENT, KAREN SEARS.

Karen Sears: SO THESE ARE THE GRAY SHORT-TAILED OPOSSUM, AND THEY HAVE BEEN USED IN LABS FOR A FEW DECADES AND ARE NOW BECOMING A MUCH MORE POPULAR LAB ORGANISM.

THEY ARE VERY CUTE.

[LAUGHS] SO THE OPOSSUMS, LIKE OTHER MARSUPIAL MAMMALS, THE MOTHER GIVES BIRTH VERY EARLY COMPARED TO WHAT WE WOULD CALL PLACENTAL MAMMALS, THINGS LIKE US, LIKE MICE, LIKE DOGS.

SO THESE GUYS, YOU KNOW, THEY'RE A MONTH AND A HALF OLD, THEY'RE PRETTY BIG.

WHEN THEY'RE BORN...

THEY CRAWL UP HERE AND ATTACH TO THESE NIPPLES.

AND THEY ARE ONLY A FEW MILLIMETERS IN LENGTH AT THAT STAGE.

THEY ARE ALMOST AT AN EMBRYONIC STAGE OF DEVELOPMENT, ABOUT THE EQUIVALENT OF A HUMAN EMBRYO AT ABOUT 40 DAYS OF DEVELOPMENT.

SO THEY DON'T REALLY HAVE HIND LIMBS, THEY HAVE NO HAIR, THEY CAN'T HEAR AT ALL, AT THAT POINT.

Neil: THESE OPOSSUMS ARE BORN JUST TWO WEEKS AFTER MATING, AND KAREN CAN SEE HOW THEIR EAR BONES FORM AT EACH STAGE OF DEVELOPMENT FROM EARLY EMBRYO TO ADULT.

IN ORDER TO FIND THE BONES, SHE EUTHANIZES THE OPOSSUMS... AND USES FLESH-EATING BEETLE LARVAE TO STRIP AWAY THE TISSUE AND LEAVE BARE BONES.

THESE DERMESTID BEETLES ARE USED BY LABS AND MUSEUMS THE WORLD OVER.

IT TAKES ABOUT 24 HOURS FOR THEM TO COMPLETE THEIR WORK.

KAREN IS THEN LEFT WITH A SERIES OF SKULLS, THROUGH WHICH SHE CAN TRACE THE DEVELOPMENT OF THE MIDDLE EAR BONES FROM EMBRYO TO ADULT.

AND WHAT THE SKULLS REVEAL IS ASTONISHING.

Karen: SO WHAT'S REALLY SURPRISING IS IF YOU LOOK AT THESE EARLY OPOSSUM EMBRYOS, WHAT YOU SEE IS ESSENTIALLY A REPTILIAN-STYLE EAR WITH ONE EAR BONE.

THOSE TWO EXTRA MIDDLE EAR BONES THAT ARE GONNA BE IN THE EAR IN THE ADULT, THEY ARE PART OF THE JAW.

AND SO THE JAW JOINT BETWEEN THE SKULL AND THE JAW REALLY LOOKS LIKE THAT OF A REPTILE.

THEN AFTER THEY'RE BORN, AND WHILE THEY'RE GROWING UP, THOSE LITTLE BONES THAT ARE GOING TO BE THE MAMMALIAN MIDDLE EAR DISCONNECT FROM THE JAW, THEY GET RELATIVELY SMALLER, AND MOVE UP TO THEIR FINAL POSITION TO BECOME A MAMMAL-LIKE EAR.

SO IF YOU HAVE MODERN REPTILES WITH ONE EAR BONE, YOU HAVE MODERN MAMMALS WITH THREE EAR BONES, I THINK WHAT THE EMBRYOS DO IS THEY ALMOST PROVIDE A LINK BETWEEN THOSE TWO.

SO WHAT WE SEE IN THE OPOSSUMS IS FROM THAT TIME THEY ARE BORN THROUGH THEY'RE ADULT, THEY ALMOST GO THROUGH 300 MILLION YEARS OF EVOLUTION IN TERMS OF THEIR EARS.

Neil: HOW DO WE KNOW THAT THE BONES WE USE TO HEAR WITH CAME FROM THE BONES REPTILES USED TO EAT WITH?

WELL, THIS AMAZING TRANSITION WE SEE IN DEVELOPMENT, FROM JAW BONES TO EAR BONES, WE ALSO SEE IN THE FOSSIL RECORD.

AND WE SEE IT IN FOSSILS FROM HERE IN SOUTH AFRICA.

AND IF WE LOOK AT SOME OF THESE PRIMITIVE MAMMAL-LIKE REPTILES, WE SEE SOMETHING IN THE LOWER JAW THAT'S VERY DIFFERENT FROM US.

BECAUSE WHAT IT HAS IS ONE BONE HERE, WHICH CARRIES THE TEETH, AND THEN THERE'S A BUNCH OF OTHER BONES THAT ARE SORT OF MODERATELY SIZED, THAT SIT AT THE BACK OF THE JAW, THEY FORM THE JAW JOINT ITSELF.

AS YOU GO FORWARD IN TIME WHAT YOU START TO SEE ARE CREATURES LIKE THIS, WHICH HAVE A LARGER BONE THAT HOLDS ALL THE TEETH, AND EVER SMALLER BONES AT THE BACK OF THE JAW THAT FORM THE JAW JOINT.

THEN WE HIT A POINT.

WHEN YOU GET TO ANIMALS THAT LIVED AROUND 200 MILLION YEARS AGO-- MORE RECENT IN THE FOSSIL RECORD-- YOU FIND THAT THEY HAVE FORMED A COMPLETELY NEW JAW JOINT.

THAT'S BECAUSE THE BONE THAT HOLDS THE TEETH HAD GROWN SO LARGE IT MADE CONTACT WITH THE SKULL.

SO OVER MILLIONS OF YEARS, BONES THAT FORMED THE OLD JAW JOINT BEGAN TO SHRINK... AND EVENTUALLY BECAME REDUNDANT.

BUT INSTEAD OF BEING LOST ALTOGETHER, THESE OLD JAW BONES WERE REPURPOSED TO TAKE UP A NEW ROLE...

IN THE EAR.

FINDING OUT WHEN THAT TRANSITION TOOK PLACE IS DIFFICULT BECAUSE THE TINY EAR BONES SIT LOOSE IN THE SKULL, AND THEY ARE RARELY FOUND WITH THE REST OF THE BODY.

BUT RECENTLY, A FASCINATING DISCOVERY WAS MADE IN ANOTHER PART OF THE WORLD.

CHINA.

ONE OF THE LEADING SCIENTISTS INVOLVED IN THE STUDY IS MY COLLEAGUE ZHE-XI LUO, WHO WORKS IN A LAB NEXT DOOR TO MINE.

SO, LUO IS A FABULOUS ANATOMIST.

HERE'S A GUY WHO KNOWS THE ANATOMY OF THE TEETH, THE ANATOMY OF THE JAWS, THE ANATOMY OF THE EARS OF REPTILES AND MAMMALS AS WELL AS ANYBODY ON THE PLANET.

FURTHERMORE, HE'S ABLE TO WORK WITH REALLY TINY SPECIMENS.

Zhe-Xi Luo: WHEN THE FOSSIL WAS FIRST SPOTTED IN THE FIELD, MY FRIEND THOUGHT IT'S JUST A PEBBLE.

HE PICKED IT UP AND HE REALIZED THERE WERE BONES IN THAT PEBBLE.

IT'S A FOSSIL!

AFTER MORE DETAILED WORK, WE KNOW THAT IT'S ACTUALLY A MAMMAL SKULL, AND IT'S RELATED TO THE ORIGIN OF ALL MAMMALS.

Neil: THEY NAMED IT "HADROCODIUM."

USING A 3-D PRINTER, LUO BUILT AN ENLARGED REPLICA OF ITS TINY SKULL.

THE TEETH HAD CUSPS LIKE OTHER MAMMALS, BUT IT'S THE LOWER JAW THAT REALLY STOOD OUT.

THE JAW JOINT LOOKED REMARKABLY LIKE THAT OF MODERN MAMMALS, WHICH SUGGESTS THAT THE OLD REPTILIAN JAW BONES WERE NO LONGER BEING USED TO EAT WITH-- THEY HAD BECOME PART OF THE EAR.

Zhe-Xi: ON THE INSIDE OF THE JAW THERE ARE STRUCTURES TO HOLD THE EAR BONE.

THIS SHOWS US HADROCODIUM'S EAR BONES ARE STILL ATTACHED TO THE JAW.

Neil: BUT THE ATTACHMENT WAS MINIMAL-- HADROCODIUM'S EAR BONES WERE ON THEIR WAY TO BEING COMPLETELY SEPARATE.

SO THIS 195-MILLION-YEAR-OLD CREATURE, NO BIGGER THAN A PAPER CLIP, HAD TAKEN A BIG STEP TOWARDS THE KIND OF HEARING WE HAVE TODAY.

ALONG WITH ITS SPECIALIZED TEETH AND FUR COAT, HADROCODIUM IS REGARDED AS ONE OF THE EARLIEST MAMMALS.

AND THERE WAS A FINAL SURPRISE-- ANOTHER IMPORTANT MAMMALIAN FEATURE INSIDE ITS SKULL.

Zhe-Xi: WE ORIGINALLY THOUGHT THE EARLY MAMMALS' BRAIN WERE A LOT SMALLER, BUT HADROCODIUM REALLY SURPRISED US.

IT HAS A HUGE BRAIN.

IN FACT, ITS BRAIN IS 50% BIGGER RELATIVE TO ITS BODY MASS THAN ANYTHING BEFORE IT.

THAT'S WHY WE CALLED THE FOSSIL HADROCODIUM.

IT MEANS IT'S GOT A GREAT BIG BRAIN.

Neil: THE QUESTION IS, WHY WOULD OUR ANCESTORS HAVE NEEDED SUCH A BIG BRAIN IN THE FIRST PLACE?

AFTER ALL, THEY WEREN'T BUILDING CITIES OR WRITING SHAKESPEARE.

195 MILLION YEARS AGO, A BIG BRAIN HELPED THEM TO SURVIVE IN A DANGEROUS NIGHTTIME WORLD.

THE RISE OF A NEW KIND OF REPTILE--THE DINOSAURS-- FORCED THE EARLY MAMMALS TO BECOME TINY CREATURES OF THE NIGHT.

FROM SMELLING TO HEARING TO TOUCHING WITH THEIR WHISKERS, PROCESSING SO MUCH SENSORY INFORMATION CAUSED THEIR BRAINS TO GROW.

THEY HUNTED IN THE DARK AND HID IN THEIR BURROWS DURING THE DAY.

MANY OF THEM MAINTAINED THAT LIFESTYLE FOR OVER 130 MILLION YEARS.

ONLY WHEN THE DINOSAURS WERE WIPED OUT IN YET ANOTHER MASS EXTINCTION 65 MILLION YEARS AGO COULD THE MAMMALS TRULY EMERGE AND FLOURISH.

SO WHEN THE DINOSAURS DISAPPEARED, THAT'S WHEN MAMMALS REALLY TOOK OFF IN A BIG WAY?

Roger: THAT'S RIGHT, THEY TOOK THE OPPORTUNITY WITH ALL THE VACANT NICHES, THE EMPTY SPACES ON THE LAND SURFACE, AND THEY STEPPED OUT FROM UNDER THE DINOSAURS' FOOTPRINTS AND RADIATED AND DIVERSIFIED.

[CALL] Neil: SO IN A VERY REAL SENSE, WE WOULDN'T BE HERE IF IT WASN'T FOR THE REMOVAL OR THE DEATH OF THE DINOSAURS.

Roger: CERTAINLY.

Neil: OUR EARLIEST MAMMALIAN ANCESTORS WERE TINY, BUT DEEP WITHIN THEIR BODIES-- JUST AS IN OURS-- IS AN INCREDIBLE HISTORY.

THE MORE YOU LOOK AT FOSSILS, THE MORE YOU LOOK AT EMBRYOS, THE MORE YOU SEE THAT WE'RE REPURPOSED REPTILES.

THE JAW I'M USING TO TALK WITH, THE TEETH I USE TO EAT WITH, THE EAR BONES I USE TO HEAR WITH, THE SKIN AND THE STRUCTURES THAT APPEAR WITHIN IT, THAT AROSE IN OUR REPTILE PAST.

SO IN A VERY REAL SENSE, EVERY DAY WE FEEL THE CONSEQUENCES OF OUR INNER REPTILE.

Announcer: NEXT TIME ON YOUR INNER FISH: HOW DID OUR HANDS BECOME SUCH REMARKABLE TOOLS?

AND HOW DID WE COME TO SEE THE WORLD IN VIVID COLOR?

JOIN NEIL SHUBIN AS HE TRACES THE HUMAN BODY BACK TO THE ANCIENT PAST.

Neil: WOW!

Announcer: SEE HOW OUR PRIMATE ANCESTORS SHAPED OUR ANATOMY, GAVE US BAD BACKS, AND LEFT THEIR MARK ON THE HUMAN BRAIN.

IT'S TIME TO MEET YOUR INNER MONKEY.

YOUR INN TO LEARN MORE ABOUT THIS PROGRAM, VISIT pbs.org/innerfish.

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