NARRATOR: TODAY, THERE ARE 4 BILLION PEOPLE LIVING IN ASIA-- ALL OF THEM MEMBERS OF THE SAME SPECIES: HOMO SAPIENS--US.

BUT IT WASN'T ALWAYS THIS WAY.

WHEN WE LEFT AFRICA AND ENTERED ASIA, IT WAS ALREADY INHABITED BY DIFFERENT SPECIES OF HUMAN.

THEY WERE VETERANS IN THIS LAND.

WE WERE THE ROOKIES.

AAH!

BUT WHAT HAPPENED NEXT?

IS IT POSSIBLE WE MATED WITH THESE ANCIENT HUMANS AND PICKED UP THEIR GENES?

WHEN WE ANALYZED THE DNA FROM THIS, THIS WAS A NEW FORM OF EXTINCT HUMAN.

MAN: EVEN THOUGH THEY WENT EXTINCT 30,000 YEARS AGO, THEIR DNA LIVES ON IN EACH OF US.

DIFFERENT MAN: WE HAVE INSIDE OF US THE GENES OF THESE ANCIENT PEOPLE INFLUENCING US TODAY.

NARRATOR: BUT HOW HAVE THEIR GENES HELPED US COLONIZE THE PLANET?

THIS IS THE STORY OF OUR ANCESTORS AS THEY SPREAD TO EVERY CONTINENT OF THE WORLD.

WHAT WAS THE SECRET TO THEIR SUCCESS?

THEIR STORY IS OUR STORY.

NARRATOR: SOUTHEAST ASIA, 63,000 YEARS AGO.

A BAND OF HUNTER-GATHERERS ON THE LOOKOUT FOR FOOD.

THEY'RE NEWCOMERS HERE-- MIGRANTS FROM AFRICA-- AT THE EDGE OF THE KNOWN WORLD.

[ANIMAL GRUNTING] [PEOPLE SPEAKING INDISTINCTLY] PEOPLE IN THE TROPICS TEND NOT TO LIVE IN CAVES.

THERE ARE TOO MANY POISONOUS PREDATORS.

THEY USE THEM AS A SHELTER TO ESCAPE THE RAIN.

[THUNDER] BUT FOR ONE YOUNG WOMAN, THIS CAVE WILL BE HER FINAL RESTING PLACE.

HER REMAINS HAVE RECENTLY BEEN DISCOVERED.

THEY MAY BE THE OLDEST MODERN HUMAN REMAINS FOUND ANYWHERE IN ASIA.

LAOS IS ONE OF THE MOST ISOLATED COUNTRIES IN SOUTHEAST ASIA-- UNTIL RECENTLY, CUT OFF FROM THE WESTERN WORLD.

BUT TWO WESTERN ARCHAEOLOGISTS HAVE BEEN WORKING HERE SINCE 2007.

LAURA SHACKELFORD FROM CHICAGO AND FABRICE DEMETER FROM FRANCE.

THEY CAME TO LAOS ON A HUNCH THAT PREHISTORIC PEOPLE MAY HAVE LIVED IN THESE HILLS.

THE NORTH OF LAOS IS RIDDLED WITH CAVES.

DURING THE VIETNAM WAR, THEY WERE USED AS SHELTER AGAINST THE BOMBS, BUT NO ONE HAS PROPERLY EXPLORED THEM UNTIL NOW.

WITH THOUSANDS OF CAVES TO CHOOSE FROM, DEMETER AND SHACKELFORD STARTED EXCAVATING HERE AT THIS CLEFT IN A MOUNTAINSIDE... TAM PA LING.

ASSISTED BY A SMALL ARMY OF LAO WORKERS, THEY BEGAN DIGGING THROUGH TONS OF CLAY.

WOMAN: DO YOU THINK THAT THEY CAN FINISH THIS LEVEL TODAY?

MAN: MAYBE NOT TODAY.

MORNING, I GUESS, SHOULD BE OK.

I THOUGHT THERE SHOULD BE SOMETHING-- THE WHITE, UM-- OH, THE WHITE...

THE WHITE LINE, YES.

YES.

AND THAT'S IT.

YES.

THAT IS THE SAME ONE.

NARRATOR: EVEN WITH A DIG THIS SIZE, THEY WEREN'T CONFIDENT OF FINDING ANYTHING.

WOMAN: THERE ARE VERY FEW FOSSILS HERE IN ASIA, IN LARGE PART BECAUSE THIS IS A WET TROPICAL ENVIRONMENT.

UM, THINGS DON'T FOSSILIZE WELL HERE FOR THAT REASON.

NARRATOR: BUT IN 2008, THEY DISCOVERED SOME FRAGMENTS OF CHARCOAL-- EVIDENCE, PERHAPS, OF A PREHISTORIC CAMPFIRE.

SPURRED ON BY WHAT THEY FOUND, THEY CARRIED ON DIGGING.

SHACKELFORD: WE OPENED UP THE SAME PIT, WE ENLARGED IT JUST A LITTLE BIT, AND IT WAS IN THAT FIRST-- THAT FIRST PIT THAT WE FOUND OUR FIRST FOSSILS.

NARRATOR: EMBEDDED IN THE SEDIMENT WERE FRAGMENTS OF A HUMAN SKULL.

[MAN SPEAKING FRENCH] TRANSLATOR: HERE ARE THE HUMAN REMAINS THAT WE DISCOVERED.

THE FRONT PART HERE CORRESPONDS TO THE TOP OF THE SKULL.

AFTERWARDS, WE FOUND A COMPLETE UPPER JAW.

WHEN I HAD TWO PIECES IN MY HANDS, I KNEW STRAIGHT AWAY THAT IT WAS A MODERN HUMAN.

I WAS VERY EXCITED.

NARRATOR: A CT SCAN REVEALS THE SIZE AND SHAPE OF THE SKULL IS CLEARLY THAT OF A YOUNG WOMAN, BUT WHAT'S REMARKABLE IS ITS AGE.

PREVIOUSLY, THE MOST ANCIENT MODERN HUMAN FOUND IN THIS PART OF THE WORLD WAS 40,000 YEARS OLD, BUT THIS SKULL WAS MUCH OLDER.

AT 63,000 YEARS OLD, THIS IS THE OLDEST FOSSIL IN-- IN EAST OR SOUTHEAST ASIA, AND IT REALLY PUTS THE AGE OF MODERN HUMANS IN THIS PART OF THE WORLD BACK BY AT LEAST 20,000 YEARS.

[THUNDER] [RAIN POURING] NARRATOR: THE WOMAN FROM TAM PA LING IS NOW THE OLDEST DEFINITIVELY MODERN HUMAN FOUND ANYWHERE IN ASIA.

HER PEOPLE LEFT AFRICA AND MIGRATED 4,000 MILES EASTWARD FOLLOWING GAME-- LOOKING FOR NEW SOURCES OF FOOD.

BUT THEIR PRESENCE IN SOUTHEAST ASIA SO EARLY RADICALLY ALTERS THE PREVAILING THEORY OF HUMAN MIGRATION.

CONVENTIONAL WISDOM SAYS MODERN HUMANS MIGRATED OUT OF AFRICA AND INTO ASIA ABOUT 60,000 YEARS AGO, BUT THE WOMAN FROM TAM PA LING WAS LIVING ON THE OTHER SIDE OF THE CONTINENT-- IN THE HILLS OF LAOS-- 63,000 YEARS AGO.

PEOPLE MUST HAVE LEFT AFRICA EARLIER THAN PREVIOUSLY THOUGHT.

ARCHAEOLOGIST JEFF ROSE IS TRYING TO PIECE TOGETHER THE ROUTE TAKEN BY THE FIRST PEOPLE OUT OF AFRICA.

HE'S LOOKING FOR EVIDENCE WHERE THE JOURNEY BEGAN-- ON THE ARABIAN PENINSULA.

MAN: UP UNTIL JUST A COUPLE YEARS AGO, MOST ARCHAEOLOGISTS THOUGHT THE GREAT HUMAN EXPANSION OUT OF AFRICA OCCURRED ALONG THIS COASTLINE.

PEOPLE CALLED IT "COASTING OUT OF AFRICA," "COASTAL MIGRATION," "COASTAL EXPRESS."

THE IDEA WAS THAT THEY HAD BECOME SEAFARERS.

THEY HAD SOMEHOW DEVELOPED AQUATIC SUBSISTENCE.

THEY COULD FISH, THEY COULD COLLECT SHELLFISH ALONG THE COAST, AND THAT THEN LED TO THIS COLOSSAL EXPANSION INTO ASIA AND THE REST OF THE WORLD.

NARRATOR: THAT'S THE THEORY.

BUT IS IT REALLY TRUE?

ROSE: IF AN AFRICAN CULTURE CAME TO THIS PLACE, THEY'RE GONNA LEAVE EVIDENCE OF THAT CULTURE BEHIND IN THEIR STONE TOOL TECHNOLOGY, AND WE SHOULD BE ABLE TO SEE THIS.

SO IT'S TESTABLE.

NARRATOR: IN 2010, JEFF ROSE GOT FUNDING TO EXCAVATE IN OMAN AND FIND THAT EVIDENCE.

ANYTHING ON THE SHORE ITSELF WOULD HAVE BEEN WASHED AWAY LONG AGO, BUT IN THE HILLS BEHIND THE SHORE, HE EXPECTED TO FIND SOME SIGN OF PREHISTORIC HUMAN LIFE.

ROSE: NOW, THIS MOUNTAIN RANGE HERE RISES UP PRETTY STEEPLY FROM THE COAST.

IT IS FULL OF CAVES.

IT'S LIKE SWISS CHEESE.

WE JUST THOUGHT, "IT'S A FOREGONE CONCLUSION.

"IT'S JUST A MATTER OF TIME NOT IF WE FIND ANYTHING, BUT WHEN WE FIND ANYTHING."

WE DUG A TEST PIT OVER THERE.

WE DUG ONE THERE.

WE DUG ONE THERE.

WE DUG ONE ON THE SLOPE LEADING UP TO THE CAVE.

IN EACH CASE, WE WENT DOWN REALLY DEEP-- AS FAR AS WE COULD GO-- AND IN ALL THAT TIME, WE FOUND NOTHING.

WE NEVER FOUND A SINGLE ARTIFACT.

I REMEMBER JUST SITTING THERE AND SAYING, "THAT'S IT.

FORGET IT.

FORGET THE COAST.

"WE NEED TO GO SOMEWHERE ELSE.

WE NEED A COMPLETELY DIFFERENT STRATEGY."

NARRATOR: THE PROJECT WAS FAILING, SO JEFF ROSE HEADED INLAND.

OVER 100 MILES FROM THE COAST, IN THE MIDDLE OF THE DESERT, HE CAME ACROSS A DRIED-UP RIVERBED.

ROSE: THIS IS THE WADI GHADUN, ONE OF THE BIGGEST ANCIENT RIVER VALLEYS IN ALL OF ARABIA, PERHAPS.

IT'S PROBABLY HALF A MILE TO THE OTHER SIDE.

IT'S DRY NOW, BUT 100,000 YEARS AGO, THIS THING WAS FULL OF WATER.

ALL OF ARABIA WAS GREEN, FOR THAT MATTER.

IT WAS COVERED IN SAVANNAS AND LAKES, AND YOU HAVE THIS VAST NETWORK OF RIVER VALLEYS THAT RUN THROUGH THIS PLATEAU.

NARRATOR: HE BEGAN TO EXPLORE THE HILLTOPS ABOVE THE ANCIENT RIVERBED, AND HERE HE FOUND A FIELD OF FLAKED ROCKS.

SO, LIKE, THIS IS THE STUFF I'VE BEEN LOOKING FOR ALL THOSE YEARS ALONG THE COAST.

THIS IS CALLED CHERT, OR FLINT, AND IT'S THE RAW MATERIAL THAT WAS ESSENTIAL TO MAKE THE STONE TOOLS THROUGHOUT PREHISTORY.

AND THE SITE LIKE THIS IS LITERALLY COVERED IN A CARPET OF THIS MATERIAL.

PREHISTORIC TOOL MAKERS WOULD COME TO A SITE LIKE THIS.

THEY WOULD BANG AWAY, MAKE THEIR STONE TOOLS, LEAVE THE REFUSE BEHIND, AND THEN GO OFF DOWN INTO THE VALLEYS WHERE THE WATER WAS.

SO THIS IS A PREHISTORIC WORKSHOP.

NARRATOR: IT WAS UNMISTAKABLE.

PEOPLE HAD PASSED THROUGH HERE AND HUNG AROUND LONG ENOUGH TO MAKE THOUSANDS OF TOOLS.

BUT WHO AND WHEN?

THE STONES TOLD HIM ALL HE NEEDED TO KNOW.

ROSE: THAT IS GORGEOUS!

THAT IS A CLASSIC NUBIAN POINT.

THIS IS THE TOOL THAT WAS MADE BY THE FIRST PEOPLE THAT LEFT AFRICA.

NARRATOR: NUBIAN POINTS CAME FROM A PART OF AFRICA NOW IN MODERN-DAY EGYPT.

THEY WERE MADE ABOUT 100,000 YEARS AGO.

IF THE SAME TOOLS CAN BE FOUND IN OMAN, IT SUGGESTS PEOPLE WERE ON THE ARABIAN PENINSULA AT THE SAME TIME-- 100,000 YEARS AGO.

ROSE: WE'RE NOT LOOKING AT SOMETHING THAT LOOKS LIKE IT MAY HAVE A SIMILARITY TO AN AFRICAN TECHNOLOGY.

WE'RE LOOKING AT THE IDENTICAL AFRICAN TECHNOLOGY.

NARRATOR: IF AFRICANS WERE IN ARABIA 100,000 YEARS AGO, THEN PEOPLE WERE MIGRATING INTO ASIA MUCH EARLIER THAN ARCHAEOLOGISTS THOUGHT-- 40,000 YEARS EARLIER.

BUT WHAT DROVE THEM ON INTO THE UNKNOWN?

[THUNDER] PERHAPS IT WAS THE CLIMATE.

TODAY, NORTHERN AFRICA AND ARABIA ARE HOT AND DRY, BUT 100,000 YEARS AGO, THIS REGION WAS VERY DIFFERENT.

THE DESERTS BLOOMED.

THE LANDSCAPE WAS GREEN.

THE WORLD WAS IN A WET PHASE.

ROSE: THE CLIMATE HERE IS CYCLICAL.

SO YOU HAVE THIS MONSOON SYSTEM THAT'S CIRCULATING THROUGH THE INDIAN OCEAN, AND THEN EVERY ONCE IN A WHILE, IT INTENSIFIES, IT STRENGTHENS, AND IT PUSHES INTO THE INTERIOR OF ARABIA.

RIVERS LIKE THIS THAT WERE ONCE DRY WERE FLOWING AGAIN, AND SO IT WAS ALMOST LIKE THE HIGHWAYS WERE OPEN, AND THEY JUST FILLED THEM UP.

NARRATOR: IT WASN'T THE BEACHES THAT LURED PEOPLE INTO ARABIA, BUT THE RIVERS FILLED BY MONSOON WATERS.

ROSE: WHAT WE REALIZED WAS THAT PEOPLE WEREN'T INTERESTED IN THE COASTLINE.

THEY WEREN'T THESE FISHERMEN WHO HAD ADAPTED A NEW TECHNOLOGY AND WERE MOVING ALONG THIS-- THIS NEW ECOSYSTEM.

THEY WERE THE SAME PEOPLE AS THEY WERE IN AFRICA, AND THEY WERE OPPORTUNISTIC.

THESE RIVER VALLEYS WOULD HAVE BEEN PARADISE, BECAUSE NOT ONLY IS IT THE EXACT LANDSCAPE THAT THEY ALREADY KNEW, IT WAS ALSO UNOCCUPIED, SO THERE WAS NOBODY TO COMPETE WITH.

SOME OF THE QUALITIES THAT WE CAN SEE IN THESE PEOPLE WHICH WE REALLY HAVEN'T SEEN BEFORE IN PREHISTORY IS, THEY'RE CURIOUS.

THERE WAS NO OVERPOPULATION, SO THEY WEREN'T FORCED OUT OF AFRICA, BUT THEY WOULD WAKE UP EVERY DAY ON ONE SIDE OF THE RED SEA, AND THEY'D LOOK ACROSS, AND AT SOME POINT, SOMEBODY COULDN'T STAND IT ANYMORE AND HAD TO GO SCRATCH THAT ITCH.

I THINK IT WAS THIS CURIOSITY THAT WAS DRIVING THEM, AND I THINK THAT'S-- THAT'S WHAT WE ARE.

THAT'S THE DEFINING ASPECT OF OUR SPECIES, IS WE WANT TO KNOW WHAT'S ON THE OTHER SIDE.

[THUNDER] NARRATOR: FOR THE FIRST PEOPLES, THE WAY INTO ASIA WAS OPEN.

FROM ARABIA, THEY MIGRATED EASTWARD INTO CENTRAL ASIA, NORTHERN INDIA, AND BEYOND.

THEY WOULD HAVE FOLLOWED A NETWORK OF RIVERS-- NATURAL CORRIDORS THROUGH THE CONTINENT.

SOME OF THE LONGEST RIVERS IN THE WORLD ARE IN ASIA-- A CONSTANT SOURCE OF FRESH WATER AND FOOD.

STILL TODAY, OVER 90% OF THE WORLD'S POPULATION LIVES WITHIN 10 MILES OF A RIVER.

THE MEKONG RIVER FLOWS 2,000 MILES THROUGH SOUTHEAST ASIA.

IT'S KNOWN LOCALLY AS THE MOTHER OF WATER.

IT COULD HAVE LED THE FIRST PEOPLES TO THE HILLS OF LAOS AND EVENTUALLY ON TO TAM PA LING.

IF PEOPLE ENTERED ASIA 100,000 YEARS AGO, THEY WOULD HAVE HAD MORE TIME TO MIGRATE ACROSS THE CONTINENT.

THEY HAD TO MOVE ONLY TWO MILES EVERY GENERATION TO REACH HERE 63,000 YEARS AGO.

ARCHAEOLOGISTS KNOW SO LITTLE ABOUT THESE PEOPLE BECAUSE THEY FIND SO FEW ARTIFACTS IN SOUTHEAST ASIA.

THE PROBLEM IS, THERE'S BARELY ANY FLINT TO MAKE TOOLS OR SPEAR POINTS.

BUT THERE IS SOMETHING ELSE-- BAMBOO.

IT'S DAWN IN BAN SA-- A VILLAGE IN NORTHERN LAOS.

THE WORK OF THE DAY BEGINS, AND BAMBOO IS EVERYWHERE.

IT'S STRONG ENOUGH TO BUILD HOUSES BUT SUPPLE ENOUGH TO BEND AT WILL AND TENDER ENOUGH TO EAT.

BAMBOO IS A SUPERGRASS THAT GROWS UP TO 3 FEET A DAY.

IT CAN WITHSTAND MORE WEIGHT THAN CONCRETE, AND WHEN DRY, IT'S STRONGER THAN STEEL.

IN A LAND WITHOUT FLINT, IT WOULD HAVE BEEN A MULTIPURPOSE SOLUTION FOR THE FIRST PEOPLES.

SHACKELFORD: IF YOU'RE TRAVELING, THEN STONE TOOLS AREN'T NECESSARILY SOMETHING THAT YOU WANT TO TRAVEL WITH.

ORGANIC MATERIALS LIKE BAMBOO ARE EASY TO-- TO USE, TO DISCARD, AND THEN TO-- TO MAKE AGAIN WHEREVER YOU MOVE TO.

IT'S CERTAINLY SOMETHING THAT PEOPLE 60,000 YEARS AGO COULD EASILY HAVE USED.

NARRATOR: BUT BAMBOO IS ALSO HIGHLY ABSORBENT.

ONCE IT GETS WET, IT SOFTENS AND ROTS AWAY, LEAVING NOTHING FOR ARCHAEOLOGISTS TO FIND.

THAT MAKES THE TRAIL OF THE FIRST ASIANS HARD TO FOLLOW.

SO MUCH OF THE EVIDENCE HAS DISAPPEARED.

THE NEXT HINT OF THAT TRAIL TURNS UP 300 MILES FURTHER EAST IN SOUTHERN CHINA, AMONG THE LIMESTONE OUTCROPS OF GUANGXI PROVINCE.

CHANG ZHU JIN IS ONE OF CHINA'S MOST EXPERIENCED GEOLOGISTS.

HE REALIZED THIS WAS A LANDSCAPE THAT WOULD HAVE ATTRACTED PREHISTORIC HUMANS.

[MAN SPEAKING CHINESE] TRANSLATOR: THAT OVER THERE IS MULAN MOUNTAIN.

IT'S A TYPICAL LIMESTONE FORMATION.

AND AS YOU CAN SEE, IT HAS LOTS OF PEAKS AND LOTS OF CAVES.

THEY'RE QUITE TREACHEROUS.

BUT THEY'RE NATURAL DWELLINGS, AND FOR THE ANCIENT PEOPLE, THEY PROVIDED A PERFECT HABITAT.

NARRATOR: IN 2007, PROFESSOR JIN WORKED HIS WAY UP MULAN MOUNTAIN TO THIS CAVE-- ZHIRENDONG.

LOCAL FARMERS HAD FOUND ANIMAL FOSSILS AT THE ENTRANCE-- ANIMALS THAT MAY HAVE BEEN BUTCHERED BY EARLY HUMANS-- SO PROFESSOR JIN DECIDED TO EXPLORE THE INTERIOR OF THE CAVE.

AFTER 18 MONTHS OF EXCAVATIONS, HIS TEAM MADE A DISCOVERY.

A HUMAN JAWBONE--A MANDIBLE.

[JIN SPEAKING CHINESE] TRANSLATOR: THIS IS WHERE WE FOUND THE BONE.

ABOVE IT, YOU CAN SEE WHERE THE FIRST LAYER IS, AND THEN DOWN HERE, THERE'S ANOTHER LAYER.

AND IT WAS RIGHT THERE IN THE MIDDLE OF THE TWO LAYERS THAT WE FOUND THE ZHIRENDONG JAWBONE.

NARRATOR: WHEN HE DATED THE SURROUNDING LAYERS, HE DISCOVERED THE MANDIBLE WAS 102,000 YEARS OLD.

IT WAS OLDER THAN THE SKULL AT TAM PA LING.

IT WAS OLDER EVEN THAN THE TOOLS IN OMAN.

IF THE DATES ARE RIGHT, PEOPLE MUST HAVE LEFT AFRICA AND ENTERED ASIA EVEN EARLIER THAN THE EVIDENCE SUGGESTS-- REACHING CHINA AT LEAST 102,000 YEARS AGO.

BUT THERE IS AN ALTERNATIVE EXPLANATION.

PERHAPS THE MANDIBLE IS NOT FROM A MODERN HUMAN.

PERHAPS IT'S FROM A DIFFERENT SPECIES.

EVERYONE ALIVE TODAY, ALL 7 BILLION OF US, ARE MEMBERS OF THE SAME SPECIES... HOMO SAPIENS.

WE ARE NOW THE ONLY HUMANS ON THE PLANET, BUT IT WASN'T ALWAYS THIS WAY.

THE HUMAN FAMILY TREE STARTS MORE THAN TWO MILLION YEARS AGO.

BEFORE THAT TIME, WE WERE MORE APE THAN HUMAN.

EVER SINCE, THERE HAVE BEEN INNUMERABLE BRANCHES, TWIGS, AND SHOOTS GIVING RISE TO DIFFERENT SPECIES.

ANTHROPOLOGISTS CLASSIFY ALL THOSE THAT CAME BEFORE US AS ARCHAIC HUMANS.

THE MOST SUCCESSFUL OF THESE SPECIES WAS HOMO ERECTUS.

DESPITE HAVING A BRAIN ONLY 2/3 THE SIZE OF OURS, HOMO ERECTUS SPREAD OUT FROM AFRICA 1 1/2 MILLION YEARS BEFORE MODERN HUMANS.

THEY, NOT US, WERE THE FIRST INHABITANTS OF ASIA.

MAYBE THAT'S WHY THE MANDIBLE FROM ZHIRENDONG IS SO ANCIENT.

IT MIGHT NOT BE HOMO SAPIENS.

IS IT HOMO ERECTUS?

WU LIU IS AN EXPERT ON THE DIFFERENCES BETWEEN THE TWO HUMAN SPECIES.

THE MANDIBLE IS A DEFINING FEATURE-- IN PARTICULAR, THE SIZE AND ANGLE OF THE CHIN.

IN HOMO SAPIENS, IT JUTS FORWARD AHEAD OF THE TEETH, BUT IN HOMO ERECTUS, IT SLOPES BACKWARD.

NARRATOR: WHAT ABOUT THE MANDIBLE FROM ZHIRENDONG?

IS IT ERECTUS OR SAPIENS?

BY ANALYZING IT IN A CT SCANNER, IT'S POSSIBLE TO MEASURE THE ANGLE OF THE CHIN-- ALSO THE THICKNESS OF THE BONES AND THE SIZE OF THE TEETH CAVITIES.

ON ALL THESE SCORES, THE MANDIBLE SEEMS TO BE BETWEEN SAPIENS AND ERECTUS-- A MIX OF BOTH SPECIES.

IT HAS LONG BEEN THOUGHT HOMO ERECTUS WENT EXTINCT BEFORE WE TURNED UP IN ASIA, BUT WHAT IF THAT'S NOT TRUE?

WHAT IF THEY WERE STILL AROUND, AND THE TWO SPECIES MET EACH OTHER?

[PEOPLE TALKING INDISTINCTLY] NARRATOR: WOULD WE HAVE HAD ENOUGH IN COMMON TO INTERACT AND INTERBREED?

PERHAPS THAT'S WHY THE MANDIBLE LOOKS LIKE A MIX OF TWO SPECIES.

IT'S THE RESULT OF INTERBREEDING.

SO FAR, NO OTHER EVIDENCE OF SUCH ENCOUNTERS HAS BEEN FOUND IN SOUTHERN CHINA, BUT IT'S AN INTRIGUING IDEA THAT MODERN HUMANS ARRIVED AND MATED WITH OTHER ARCHAIC TYPES OF HUMAN.

IF IT HAPPENED HERE, WHERE ELSE MIGHT IT HAVE HAPPENED?

WHO ELSE WAS LIVING IN ASIA WHEN MODERN HUMANS ARRIVED?

FOR THE FIRST ASIANS THAT HEADED NORTH, THIS WAS A NATURAL CORRIDOR ACROSS THE WILDERNESS OF SIBERIA.

AND HERE IN THE ALTAI MOUNTAINS, ARCHAEOLOGISTS HAVE FOUND A CAVE LIKE NO OTHER--DENISOVA.

MAN: YOU HAVE THIS LARGE OPENING, AND IT'S VISIBLE FROM QUITE A DISTANCE AS SOON AS YOU GET INTO THAT PART OF THE VALLEY.

IT WAS INHABITED IN, I THINK, THE 18th CENTURY BY A HERMIT CALLED DENIS-- THE HOLY DENIS-- AND THE CAVE IS NAMED AFTER HIM.

SO IT'S DENIS' CAVE-- DENISOVA CAVE.

NARRATOR: DENISOVA WAS ONCE A HOME TO THE ROCK STARS OF PREHISTORY-- THE NEANDERTHALS.

OUR CLOSEST COUSINS, THE NEANDERTHALS ARE A SEPARATE SPECIES THAT EVOLVED IN ICE-AGE EUROPE BUT EXPANDED NORTHEAST AS FAR AS SIBERIA.

SINCE THE 1970s, ARCHAEOLOGISTS HAVE UNEARTHED THOUSANDS OF ARTIFACTS MADE BY NEANDERTHALS, BUT THEY ALSO FIND ARTIFACTS MADE BY MODERN HUMANS.

THERE ARE FEW CAVES IN THE WORLD WHERE THERE'S EVIDENCE THAT TWO TYPES OF HUMAN LIVED IN THE SAME PLACE, ALBEIT AT DIFFERENT TIMES.

BUT VERY FEW BONES HAVE EVER BEEN EXCAVATED HERE, SO IN 2008, ARCHAEOLOGISTS INTENSIFIED THEIR SEARCH.

VIOLA: I JOINED A RUSSIAN TEAM THAT WAS EXCAVATING THE CAVE SINCE A LONG TIME, UM, AND MY ROLE WAS MOSTLY STUDYING THE HUMAN REMAINS.

THE TEAM VERY CAREFULLY AND GRADUALLY REMOVES THE SEDIMENTS THAT ARE IN THE CAVE TO RECOVER EVEN THE SMALLEST REMAINS.

A LOT OF WORK IS ACTUALLY LOOKING AT ALL THE LITTLE BONE PIECES AND TRYING TO FIGURE OUT, "IS THERE SOMETHING HUMAN IN THERE?"

AND SOMETIMES YOU ACTUALLY DO FIND HUMAN REMAINS.

NARRATOR: WITHIN A LAYER OF SEDIMENT THAT WAS 41,000 YEARS OLD, RUSSIAN ARCHAEOLOGISTS UNCOVERED A TINY FRAGMENT OF BONE FROM A HUMAN FINGER.

VIOLA: WE HAVE AN ENLARGED 3D PRINT, UM, OF THE FINGER BONE.

IT'S 5 TIMES AS LARGE AS THE ORIGINAL.

IT'S PART OF THE-- PROBABLY OF THE LITTLE FINGER.

THIS FINGER WOULD STILL BE GROWING.

IT'S A CHILD THAT'S STILL DEVELOPING.

MAYBE A 5-- 5-, 7-YEAR-OLD KID, SOMETHING--SOMETHING AROUND THAT AGE.

THE ORIGINAL SIZE IS THIS HERE.

IT'S A TINY LITTLE PIECE, AS YOU CAN SEE.

IT'S PROBABLY THE LEAST SPECTACULAR FOSSIL THAT EXISTS ANYWHERE-- A TINY FRAGMENT OF A FINGER BONE OF A CHILD.

NARRATOR: WHO WAS THIS CHILD IN THE SIBERIAN HILLS 41,000 YEARS AGO-- MODERN HUMAN OR NEANDERTHAL?

THERE'S ONLY ONE WAY TO TELL... DNA.

THE FINGER BONE WAS SENT TO THE MAX PLANCK INSTITUTE-- TO SWEDISH GENETICIST SVANTE PAABO, A PIONEER IN THE FIELD OF ANCIENT DNA.

WHEN HIS TEAM FOUND SOME DNA STILL PRESERVED IN THE BONE, THEY SET ABOUT ANALYZING IT.

AS DNA AGES, IT BREAKS DOWN INTO BILLIONS OF TINY FRAGMENTS THAT ARE MIXED UP AND CONTAMINATED WITH THE DNA OF INSECTS, BACTERIA, AND FUNGI.

THE CHALLENGE FOR A GENETICIST IS TO IDENTIFY THE HUMAN FRAGMENTS, ISOLATE THEM, AND REASSEMBLE THEM ALL IN THE CORRECT ORDER.

A DECADE AGO, IT WOULD HAVE BEEN IMPOSSIBLE, BUT SUPERFAST SEQUENCING MACHINES HAVE MADE THE IMPOSSIBLE MERELY IMPROBABLE.

MAN: THE WHOLE LAST 30 YEARS IN MOLECULAR GENETICS IS A HISTORY OF REVOLUTIONS DRIVEN BY TECHNOLOGY.

THE BIG STEP WAS REALLY THE ABILITY TO SEQUENCE MILLIONS OF DNA MOLECULES QUITE RAPIDLY AND INEXPENSIVELY, AND THAT CHANGED THE WHOLE BALL GAME.

NARRATOR: IN 2010, PAABO'S TEAM CREATED AN INTERNATIONAL STORM.

THEY SEQUENCED THE GENOME OF A NEANDERTHAL.

IT WAS THE FIRST TIME ANYONE HAD MANAGED TO READ THE GENETIC CODE OF AN EXTINCT HUMAN.

THEY USED THE SAME TECHNIQUES ON THE SIBERIAN FINGER BONE, BUT ITS SEQUENCE WAS NOT WHAT THEY IMAGINED.

WHEN WE ANALYZED THE DNA FROM THIS, WE EXPECTED IT TO BE EITHER A NEANDERTHAL OR A MODERN HUMAN THAT WOULD HAVE BEEN THERE.

WE WERE VERY SURPRISED TO FIND THAT IT WAS ACTUALLY SOMETHING ELSE.

THIS WAS A NEW FORM OF EXTINCT HUMAN.

NARRATOR: FROM THE SINGLE FINGER BONE OF A CHILD, THEY HAD UNCOVERED A NEW BRANCH OF THE HUMAN FAMILY TREE.

PAABO: WE HAD A BIG DEBATE ABOUT WHAT WE WOULD CALL THESE INDIVIDUALS, AND WE CAME UP WITH THE NAME "DENISOVANS" AFTER DENISOVA CAVE, WHERE THEY HAD BEEN FIRST FOUND.

THIS WAS THE FIRST TIME THAT WE IDENTIFIED A NEW FORM OF EXTINCT HUMAN JUST FROM A GENOME SEQUENCE AND HARDLY ANY INFORMATION AT ALL FROM BONES OR STONE TOOLS-- JUST FROM A GENOME SEQUENCE.

NARRATOR: IT SOUNDS LIKE SCIENCE FICTION.

FROM NOWHERE, A NEW POPULATION-- THE DENISOVANS-- ENTER THE HUMAN STORY.

AS MODERN HUMANS PUSHED THROUGH ASIA, THEY MUST HAVE MOVED INTO DENISOVAN TERRITORY.

[SHOUTS INDISTINCTLY] NARRATOR: WHAT HAPPENED NEXT?

[MEN SPEAKING INDISTINCTLY] NARRATOR: ACCORDING TO THE GENETICS, THE TWO PEOPLES INTERBRED.

MAN: WHEN GENETICISTS LOOKED AT THAT DENISOVAN GENOME AND THEY LINED IT UP NEXT TO MODERN HUMAN GENOMES, THEY FOUND THAT THERE WERE MODERN PEOPLE WHO HAVE PIECES OF THIS DENISOVAN GENOME INSIDE OF THEM.

THIS IS SO INCREDIBLE THAT THIS ANCIENT POPULATION IS STILL THERE INSIDE OF US.

NARRATOR: THIS IS NOT THE FIRST EVIDENCE OF INTERBREEDING BETWEEN MODERN AND ARCHAIC HUMANS.

WHEN THE TEAM AT THE MAX PLANCK INSTITUTE SEQUENCED THE NEANDERTHAL GENOME, THEY FOUND THE SAME SORT OF OVERLAP.

THEY CONCLUDED THAT MODERN HUMANS INTERBRED WITH NEANDERTHALS 55,000 YEARS AGO AND CARRIED SOME OF THEIR GENES WITH THEM INTO ASIA.

WE USED TO THINK OF MODERN HUMANS ENTERING ASIA AND ENTERING SOMEWHERE THAT WAS RELATIVELY, YOU KNOW, SPARSELY INHABITED, IF AT ALL.

WE NOW KNOW THAT THERE WERE MULTIPLE POPULATIONS OF THESE ARCHAIC HUMANS-- DENISOVANS, NEANDERTHALS, MAYBE SOME LATE-SURVIVING HOMO ERECTUS POPULATIONS-- AND THE MODERN HUMANS SHOW UP.

THEY'RE THE NEW KIDS ON THE BLOCK IN A PLACE WHERE THERE'S ALREADY LOTS OF DIFFERENT PEOPLE THAT ARE PRETTY SUCCESSFUL THERE.

THERE MUST HAVE BEEN INTERBREEDING, AND THE PEOPLE WHO WERE THE PRODUCTS OF THAT INTERBREEDING WENT ON TO COLONIZE MUCH OF THE EAST AND SOUTHEAST ASIA.

NARRATOR: THERE ARE 4 BILLION PEOPLE IN ASIA TODAY.

ALL OF THEM HAVE GENES INHERITED FROM ARCHAIC HUMANS.

BETWEEN 1% AND 3% OF THEIR DNA COMES FROM NEANDERTHALS, AND IN SOME PEOPLE, UP TO 5% MORE FROM DENISOVANS.

ASIA--THE GREAT MELTING POT OF HUMAN DIVERSITY.

WHAT SORT OF GENES MIGHT HAVE PASSED FROM ARCHAIC TO MODERN HUMANS?

WHAT EFFECT DID THEY HAVE ON US?

THE CLUES ARE TO BE FOUND ON THE TIBETAN PLATEAU.

[METAL CLANKING] IT'S EARLY MORNING IN THE HIMALAYAS, AND FARMERS ARE LETTING OUT THEIR YAKS FOR THE DAY.

IT'S A FAMILIAR ROUTINE.

THEY LEAVE THE VILLAGE AND LOOK FOR PASTURE UP THE MOUNTAINSIDE.

THIS IS 13,000 FEET ABOVE SEA LEVEL.

THERE'S 40% LESS OXYGEN THAN AT THE COAST, BUT TIBETANS SEEM UNTROUBLED WORKING AT THIS ALTITUDE.

THEIR BODIES HAVE ADAPTED TO LIFE ON TOP OF THE WORLD.

RASMUS NIELSEN BELIEVES THE KEY TO THEIR SURVIVAL IS A LACK OF HEMOGLOBIN-- THE PROTEIN THAT CARRIES OXYGEN IN RED BLOOD CELLS.

MAN: YOU MIGHT EXPECT THAT IF YOU'RE AT HIGH ALTITUDE, YOU NEED MORE HEMOGLOBIN, BUT IN FACT, TIBETANS ACTUALLY PRODUCE LESS HEMOGLOBIN.

THAT MIGHT SOUND BACKWARDS.

WHY WOULD THEY PRODUCE LESS HEMOGLOBIN?

BUT THAT'S BECAUSE THAT HAVING TOO MUCH HEMOGLOBIN IN YOUR BLOOD CAN ACTUALLY BE DETRIMENTAL, BECAUSE IF YOU HAVE TOO MANY RED BLOOD CELLS, THEN YOUR BLOOD BECOMES TOO THICK.

WE DON'T REALLY KNOW EXACTLY HOW THE TIBETANS ARE FUNCTIONING WITH LESS RED BLOOD CELLS IN THEIR BLOOD.

WE THINK IT'S BECAUSE THEY'RE BETTER AT PRODUCING ENERGY IN THE MUSCLES WITHOUT USING OXYGEN.

NARRATOR: ONE OF THE GENES THAT CONTROLS THE LEVEL OF HEMOGLOBIN IS EPAS1.

EVERYONE HAS A COPY OF THIS GENE LOCATED ON CHROMOSOME 2, BUT TIBETANS HAVE A SPECIAL VARIANT THAT RESTRICTS THEIR LEVEL OF HEMOGLOBIN AND ALLOWS THEM TO WORK IN THE HIGH PASTURES WITHOUT ILL EFFECTS.

BUT WHERE DOES THIS VARIANT COME FROM?

HOW DID IT ENTER THE TIBETAN GENE POOL?

COULD IT BE THE RESULT OF INTERBREEDING?

WHAT WE SAW WAS THAT THE DNA SEQUENCE THAT THE TIBETANS HAVE REALLY DIDN'T MATCH ANY OTHER SEQUENCES IN ANY OTHER HUMANS.

BUT THEN WE LOOKED AT THIS OTHER SPECIES--THE DENISOVANS.

TO OUR SURPRISE, WHAT WE FOUND WAS THAT THERE WAS AN ALMOST COMPLETE MATCH BETWEEN THE DNA IN THE TIBETANS AND THE DENISOVAN SEQUENCE.

WE COULD SHOW THAT THE TIBETAN SEQUENCE HAD COME FROM THIS ARCHAIC SPECIES--THE DENISOVANS-- INTO THE TIBETANS.

WE DON'T KNOW VERY MUCH ABOUT THE DENISOVANS, BUT IT'S POSSIBLE THAT THEY WERE ALSO HIGH-ALTITUDE ADAPTED.

BY INTERBREEDING WITH THEM, MODERN HUMANS PICKED UP THIS GENE THAT THEN ALSO ALLOWED THEM TO BE ADAPTED TO THE HIGH-ALTITUDE ENVIRONMENT.

NARRATOR: THE SAME GENE IS STILL REGULATING THE HEMOGLOBIN OF TIBETANS TODAY.

IT'S AS IF THEY'VE HITCHED A GENETIC RIDE ON THE BACK OF THE DENISOVANS.

HAWKS: MIXTURE WITH THESE ARCHAIC HUMANS WAS A TREMENDOUS SHORTCUT FOR NATURAL SELECTION.

THE THING IS THAT IF YOU HAVE A TRAIT THAT YOU'RE TRYING TO GET, THAT'S USEFUL FOR YOU, YOU'RE WAITING FOR THE RIGHT MUTATIONS TO HAPPEN.

YOU COULD BE WAITING FOR THOUSANDS OF GENERATIONS.

BUT ARCHAIC HUMANS HAVE HUNDREDS OF THOUSANDS OF YEARS OF ADAPTATION ALREADY.

EVERYTHING THAT'S USEFUL FROM THIS POPULATION HAS A HUGE POTENTIAL OF BEING PICKED UP INTO MODERN HUMANS WITH A LITTLE BIT OF INTERBREEDING.

NARRATOR: EPAS1 IS THE FIRST GENE SCIENTISTS KNOW OF THAT'S CLEARLY PASSED FROM ONE TYPE OF HUMAN INTO ANOTHER THROUGH INTERBREEDING, BUT IT'S LIKELY TO BE JUST THE TIP OF THE ICEBERG.

MANY OTHER GENES HAVE PROBABLY TAKEN THE SAME ROUTE AND HAD A PROFOUND IMPACT ON US--MODERN HUMANS.

THE SKIN IS THE LARGEST ORGAN IN THE HUMAN BODY.

IT WEIGHS UP TO 8 POUNDS.

IT'S PACKED WITH SWEAT GLANDS, BLOOD VESSELS, AND NERVE ENDINGS, BUT IT'S ONLY 1/10 OF AN INCH THICK.

IT'S ALL THAT EXISTS BETWEEN US AND THE OUTSIDE WORLD.

MAN: THESE GUYS ARE PROVIDING A GOOD EXAMPLE OF SHOWING WHY THE HUMAN SKIN IS SO IMPORTANT.

THEY'RE ABLE TO WITHSTAND THE IMPACTS BECAUSE OF THE SPECIAL PROPERTIES AND TOUGHNESS OF SKIN.

AND MUCH OF THAT TOUGHNESS IS GOVERNED BY A PROTEIN CALLED KERATIN, WHICH IS A VERY TOUGH, FIBROUS STRUCTURAL PROTEIN THAT PROVIDES THE SKIN WITH THINGS LIKE ELASTICITY AND STRENGTH.

NARRATOR: ACCORDING TO THE LATEST RESEARCH, GENES THAT CONTROL THE PRODUCTION OF KERATIN WERE INHERITED FROM ARCHAIC HUMANS-- NOT DENISOVANS THIS TIME, BUT NEANDERTHALS.

THERE WAS SOMETHING BENEFICIAL ABOUT NEANDERTHAL SKIN BIOLOGY AND THE WAY THEY PROCESSED KERATIN THAT WAS PASSED ON BY INTERBREEDING AND HAS STAYED WITH US EVER SINCE.

AKEY: IT COULD BE RELATED TO SKIN PIGMENTATION.

IT COULD BE RELATED TO WOUND HEALING.

BUT WE KNOW THAT INHERITING THE NEANDERTHAL VERSION OF THESE GENES PROVIDED AN ADVANTAGE TO OUR ANCESTORS.

WE'RE ONLY JUST BEGINNING TO UNDERSTAND ABOUT HOW HUMANS AND NEANDERTHALS AND OTHER ARCHAIC HUMANS INTERACTED AND WHAT THOSE CONSEQUENCES WERE.

NARRATOR: GENETICS IS REWRITING THE HUMAN STORY.

EVIDENCE OF ABOUT WHO MET WHOM, WHERE, AND WHEN SHOWS UP IN TODAY'S DNA MORE CLEARLY THAN IN ANY FOSSIL, AND THAT EVIDENCE SUGGESTS OUR COUSINS WERE MORE IMPORTANT TO US THAN WE EVER REALIZED.

AKEY: IN ANY INDIVIDUAL, ABOUT 1.5% OF OUR GENOME CAME FROM NEANDERTHAL ANCESTORS, BUT MY 1.5% MIGHT BE DIFFERENT THAN YOUR 1.5%.

IF YOU TAKE ALL OF THE DIFFERENT 1.5% AND YOU ADDED THEM ALL TOGETHER, YOU'D COME UP WITH ABOUT 20% OR 30% OF THE NEANDERTHAL GENOME, WHICH IS QUITE REMARKABLE, BECAUSE EVEN THOUGH NEANDERTHALS WENT EXTINCT 30,000 YEARS AGO, THEIR DNA LIVES ON IN EACH OF US.

NARRATOR: JUST HOW INFLUENTIAL WERE THE GENES WE INHERITED FROM OUR EVOLUTIONARY COUSINS?

THEY MAY MAKE THE DIFFERENCE BETWEEN LIFE AND DEATH.

EVERY DAY, OUR BODIES ARE UNDER ATTACK FROM MICROBES SEEKING NEW HOSTS TO INVADE AND INFECT.

BUT WHEN MICROBES ATTACK, OUR BODY COUNTERATTACKS...

THANKS TO OUR IMMUNE SYSTEM, WHICH ENLISTS AN ARMY OF WHITE BLOOD CELLS KNOWN AS NATURAL KILLERS.

JOHN TROWSDALE IS A PATHOLOGIST AND AN EXPERT IN THE MECHANICS OF HUMAN IMMUNITY.

MAN: THESE ARE KILLER CELLS.

NOW, KILLER CELLS WORK BY KILLING ANY CELLS INFECTED WITH A VIRUS OR A MICROBE, SO WE NEED THESE CELLS IN ORDER TO TAKE OUT ANY INFECTED CELLS IN OUR BODIES.

IN ORDER TO DO THAT, THOUGH, THESE KILLER CELLS NEED A SPECIAL NETWORK OF GENES, AND THESE GENES ARE CALLED HLA GENES.

THEY'RE KIND OF AN EARLY WARNING SYSTEM.

NARRATOR: HLA GENES-- OR HUMAN LEUKOCYTE ANTIGENS-- HAVE LONG BEEN REGARDED AS KEY IN THE BATTLE AGAINST DISEASE, BUT SCIENTISTS HAVE RECENTLY NOTICED A GEOGRAPHICAL PATTERNING TO THESE GENES.

MODERN-DAY ASIANS HAVE CLUSTERS OF THEM THAT DON'T EXIST AMONG AFRICANS.

IT'S AS IF THEIR ANCESTORS PICKED THEM UP FROM A SOURCE ALREADY LIVING IN ASIA.

MAN: SOME OF THOSE GENES LOOK VERY DIFFERENT TO MODERN HUMAN GENES.

IT LOOKS AS THOUGH THEY'VE BEEN ACQUIRED FROM A DIFFERENT SPECIES.

NARRATOR: IT SEEMS INCREASINGLY LIKELY THAT SOME TYPES OF HLA WERE INHERITED FROM NEANDERTHALS OR DENISOVANS.

MAN: LET'S IMAGINE THESE DICE REPRESENT DIFFERENT HLAs.

AND IN A POPULATION LIKE THE AFRICAN POPULATION, EACH NUMBER REPRESENTS A DIFFERENT HLA AND ACTUALLY PROVIDES A DIFFERENT KIND OF LEVEL OF IMMUNITY.

AND THE MORE VARIETY IN THE POPULATION, THE HARDER IT IS FOR A VIRUS TO GET A HOLD ON THAT POPULATION.

BUT AS SMALL NUMBERS OF PEOPLE MOVE OUT OF AFRICA, THEY'VE GOT MUCH LESS VARIETY.

SO IN OTHER WORDS, THEY'RE MORE VULNERABLE TO INFECTION.

SO HOW DOES THIS POPULATION NOW ACQUIRE SOME MORE VARIETY?

WELL, IT CAN WAIT FOR A MUTATION TO COME ALONG, BUT, OF COURSE, MUTATION IN THE NORMAL COURSE OF EVOLUTION IS VERY SLOW.

HOWEVER, IF THEY ACTUALLY MATE WITH ANOTHER POPULATION-- LET'S SAY NEANDERTHALS OR DENISOVANS-- THEY CAN GAIN MORE VARIETY.

SO THEY MIGHT GET A 5 HERE OR A 6 HERE-- AN ADDITIONAL VARIETY THAT WERE MISSING.

IN ADDITION TO THAT, THEY CAN ACTUALLY ALSO ACQUIRE SOME VERY NOVEL HLA GENES THAT MAYBE WEREN'T EVEN PRESENT IN THE-- IN THE AFRICAN POPULATION, AND THEY PROVIDED RESISTANCE TO PARTICULAR INFECTIONS THAT WERE PREVALENT IN THOSE ENVIRONMENTS.

SO IT'S A KIND OF EVOLUTIONARY SHORTCUT, IN OTHER WORDS.

THESE GENES HAVE BEEN BORROWED FROM OTHER POPULATIONS AND INCORPORATED INTO THE POPULATION TO PROVIDE THAT VARIETY THAT-- THAT THEY WERE MISSING.

NARRATOR: BILLIONS OF US TODAY ARE BETTER AT FIGHTING OFF DISEASE BECAUSE OF ARCHAIC HUMANS.

WE ACQUIRED THEIR IMMUNITY.

GENES THAT WERE IN THEIR CELLS ARE NOW INTEGRAL TO OUR BIOLOGY.

MAN: SOME PEOPLE SAY THAT MAYBE 1% TO 3% OF OUR GENES HAVE COME FROM NEANDERTHALS.

SO WHAT'S THE BIG DEAL?

I MEAN, 1% TO 3% ISN'T VERY MUCH.

HOWEVER, IF YOU LOOK AT HLA GENES, IT APPEARS THAT THESE HAVE ACTUALLY PROVIDED A MAJOR ADVANTAGE, AND IN SOME POPULATIONS, IT'S ESTIMATED UP TO 90% OF THE GENES HAVE COME FROM NEANDERTHALS OR DENISOVANS.

SO IT MAY BE THAT THE IMMUNITY THROUGH HLA IS ONE OF THE MOST IMPORTANT GIFTS WE'VE HAD FROM THESE ANCIENT POPULATIONS.

NARRATOR: IT'S THE END OF THE DAY ON THE TIBETAN PLATEAU.

FARMERS ARE BRINGING THEIR YAKS HOME FOR MILKING.

[FARMER SHOUTS INDISTINCTLY] NARRATOR: THESE ARE HYBRID ANIMALS THAT HAVE BEEN CROSSED WITH DOMESTIC CATTLE TO BECOME MORE RESILIENT AND MORE PRODUCTIVE.

ALTHOUGH THE FARMERS DON'T REALIZE IT, THEY ARE ALSO HYBRIDS-- CROSSED WITH DENISOVANS, NEANDERTHALS, AND PERHAPS HOMO ERECTUS.

AS WITH THE YAKS, HYBRIDIZATION SEEMS TO HAVE MADE THEM STRONGER, TOUGHER, AND HEALTHIER.

PEOPLE TODAY ARE STILL LIVING WITH THE LEGACY OF WHAT HAPPENED IN THE ANCIENT FORESTS OF ASIA.

HAWKS: WHEN MODERN HUMANS MOVED INTO ASIA, THEY HAD SOME OF THE TOOLS THEY NEEDED TO MAKE IT, BUT NOT ALL OF THEM.

THEY MIXED WITH POPULATIONS THAT HAD BEEN THERE FOR HUNDREDS OF THOUSANDS OF YEARS, AND THEY BEGAN TO PICK UP SOME OF THE ADAPTATIONS THAT ENABLED THEM TO SURVIVE IN THAT NEW PLACE.

WE HAVE INSIDE OF US THE GENES OF THESE ANCIENT PEOPLE INFLUENCING US TODAY.

WE'RE ONLY BEGINNING TO LEARN HOW MUCH WE OWE TO THEM-- THESE ENCOUNTERS THAT HAPPENED TENS OF THOUSANDS OF YEARS AGO.

NARRATOR: THE IMPACT OF INTERBREEDING GOES WAY BEYOND ASIA.

THE SAME PEOPLE WHO EXPANDED INTO SIBERIA WOULD GO ON AND COLONIZE NORTH AMERICA.

OTHERS FURTHER SOUTH WOULD TAKE THE GREATEST JOURNEY OF ALL ACROSS THE SEA INTO AUSTRALIA-- THE WORLD'S MOST ISOLATED CONTINENT.

WHEREVER THEY WENT, THEY TOOK WITH THEM THE GENES OF ANCIENT SPECIES.

HYBRID HUMANS--FIRST PEOPLES.

"FIRST PEOPLES" IS AVAILABLE ON DVD.

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