NARRATOR: FOR ANY CIVILIZATION TO SUCCEED, IT MUST MEET THE NEEDS OF ITS PEOPLE, TO PROVIDE THEM WITH THE STUFF OF LIFE.

THIS REQUIRES A SYSTEM OF EXCHANGE WHERE DEMAND FOR GOODS IS MET BY SUPPLY.

IN OTHER WORDS, CIVILIZATION HAS ALWAYS REQUIRED TRADE.

BUT HOW DID TRADE SHAPE THE COURSE OF CIVILIZATION?

MAN: FROM THE VERY EARLY PERIOD, TRADE AND EXCHANGE WAS REALLY IMPORTANT.

IT'S PART OF THE ECONOMIC AND SOCIAL GLUE THAT STITCHES PEOPLE TOGETHER.

WOMAN: TRADE IS THE ELIXIR.

IT'S THE THING THAT CREATES COLLABORATION AND CONNECTION.

IT'S WHAT ENABLES THE SOCIETY TO PROGRESS.

MAN 2: THAT'S, IN THE END, WHAT SETS US APART AND WHICH HAS EVENTUALLY ENABLED US TO BUILD A CIVILIZATION THAT IS SO COMPLEX.

NARRATOR: WE DIDN'T ALWAYS LIVE THIS WAY.

FOR 99% OF OUR TIME ON EARTH, HUMANS HAD NO MERCHANTS OR TRADERS.

BUT THEN... WE SETTLED DOWN, GREW FOOD, WORSHIPED GODS, FOUGHT BATTLES, WROTE STORIES, BUILT CITIES, AND CREATED MARKETS.

[PEOPLE SHOUT] NARRATOR: THIS IS THE STORY OF THAT TRANSITION, STEPPINGSTONES ON THE ROAD TO CIVILIZATION.

[SHOUTS] NARRATOR: IT'S A STORY SET ACROSS THE GLOBE IN THE MIDDLE EAST, CENTRAL AMERICA, SOUTHERN ASIA... ALL OF THEM SEABEDS OF CIVILIZATION.

HERE, OUR ANCESTORS SHAPED THE IDEAS BY WHICH WE STILL LIVE OUR LIVES.

THIS IS WHERE THE MODERN WORLD BEGAN.

NARRATOR: IN THE MOUNTAINS OF OMAN, ARCHAEOLOGIST JEFF ROSE HAS MADE A REMARKABLE DISCOVERY... AN ANCIENT ROCK PAINTING, A PETROGLYPH, OVER 4,000 YEARS OLD... 50 FEET UP A CANYON WALL.

ROSE: SO HERE IT IS, THE TANUF MURAL.

ROSE: I'VE SEEN ROCK ART ALL UP AND DOWN THESE CANYONS, AND SO I DIDN'T REALLY THINK THAT MUCH OF IT, EXCEPT THAT WHEN I LOOKED AT IT UP CLOSE, I SAW THAT THERE'S ACTUALLY THIS MOTIF WHICH I'D NEVER SEEN BEFORE IN ANY OTHER ROCK ART IN OMAN.

SO YOU HAVE THIS FIGURE HERE, AND HE'S GRASPING THESE-- THESE TWO OPPOSED BULLS LIKE THIS, AND HE'S SURROUNDED BY ANIMALS.

BUT THEN, WHEN YOU LOOK AT THE BULLS THEMSELVES, THEY'VE GOT HUMPS, SO THESE ARE--THESE ARE ZEBU COWS FROM THOUSANDS OF MILES TO THE EAST, AND IT MAKES YOU WONDER WHAT THE HECK IS GOING ON HERE.

NARRATOR: THE ZEBU IS A TYPE OF COW FOUND NOT ON THE ARABIAN PENINSULA, BUT IN SOUTHERN ASIA ON THE INDIAN SUBCONTINENT.

WHY WOULD THEIR IMAGE BE ON A ROCK WALL IN OMAN, A THOUSAND MILES AWAY?

WAS IT CARVED BY PEOPLE FROM INDIA?

NARRATOR: CLOSE TO THE CANYON ARE THE REMAINS OF AN ANCIENT BURIAL SITE WITH MORE EVIDENCE OF FOREIGN CONTACT.

ROSE: THIS IS CALLED A BEEHIVE TOMB.

I'M STANDING HERE RIGHT AT THE MOUTH OF THE CANYON, SO FOR ALL WE KNOW, THOSE PETROGLYPHS COULD BE TELLING US THE STORY OF THE GUY OR GIRL BURIED HERE IN THIS TOMB.

NARRATOR: BEEHIVE TOMBS ARE COMMON IN OMAN, BUT ARCHAEOLOGISTS HAVE FOUND POTTERY HERE THAT'S NOT LOCAL.

AGAIN, THE ORIGIN SEEMS TO BE FROM ACROSS THE INDIAN OCEAN.

WHEN WE ADD IT ALL UP TOGETHER, IT LOOKS LIKE WE'VE GOT THE FINGERPRINT OF A FOREIGN CULTURE FROM THE INDIAN SUBCONTINENT.

NARRATOR: HOW DID THESE OBJECTS FROM SOUTHERN ASIA END UP IN THIS REMOTE, MOUNTAINOUS REGION?

IN A WORD, TRADE.

WELL, THE FIRST METAL THAT'S USED ON ANY INDUSTRIAL SCALE IN THE ANCIENT WORLD IS COPPER... AND THESE MOUNTAINS HERE ARE LOADED WITH COPPER.

NARRATOR: COPPER WAS ONE OF THE FIRST COMMODITIES TO BE EXCHANGED IN LARGE QUANTITIES, TRADED OVER HUNDREDS OF MILES.

IT FUELED THE RISE OF CIVILIZATION.

TODAY, COPPER IS STILL ONE OF THE MOST IMPORTANT COMMODITIES IN THE WORLD.

EACH YEAR, OVER $90 BILLION WORTH IS BOUGHT AND SOLD THROUGH A GLOBAL TRADE NETWORK CENTERED ON THE LONDON METAL EXCHANGE.

[PEOPLE SHOUTING] NARRATOR: BUSINESS STRATEGIST AND AUTHOR RACHEL BOTSMAN BELIEVES TRADE WAS VITAL TO THE BIRTH OF CIVILIZATION.

BOTSMAN: SINCE HUMANS EXISTED, WE HAVE TRADED.

IT'S A NECESSITY, BUT IT WAS PRETTY BASIC.

IT WAS LOCAL, IT WAS WITH PEOPLE THAT THEY KNEW, SO I MIGHT EXCHANGE A METAL POT FOR MY NEIGHBOR'S ANIMAL SKINS; YOU MIGHT EXCHANGE SOME FOOD FOR A WEAPON.

AND THEN WHAT HAPPENED OVER TIME IS THAT TRADE BECAME MORE AND MORE SOPHISTICATED IN TERMS OF THE GOODS THAT PEOPLE WERE TRADING, BUT ALSO THE DISTANCES THAT THEY WERE TRADING OVER.

WE TRANSITIONED FROM LOCAL TRADE-- TRADE WITH PEOPLE THAT WE KNEW, TRADE WITH PEOPLE THAT WE TRUSTED-- TO TRUSTING STRANGERS.

BOTSMAN: IT WAS A REVOLUTION WHEN WE STARTED TO TRADE LONG DISTANCES.

NARRATOR: THE ANCIENT COPPER FROM OMAN ENDED UP BESIDE ONE OF ASIA'S GREAT RIVERS...

THE INDUS.

IT'S THE RIVER AFTER WHICH INDIA IS NAMED.

BUT TODAY, MOST OF ITS WATER FLOWS THROUGH PAKISTAN.

PEOPLE HAVE BEEN LIVING ALONGSIDE THE INDUS FOR THOUSANDS OF YEARS...

ALTHOUGH IT'S ONLY NOW ARCHAEOLOGISTS ARE UNDERSTANDING THE SOPHISTICATION OF THE ANCIENT CIVILIZATION ONCE HERE.

BUILT 4,500 YEARS AGO, THIS WAS A GREAT CITY... MOHENJO-DARO.

WOMAN: THE SITE, IN A VERY ROUGH ESTIMATE, IS 250 HECTARES, WHICH IS OVER 600 ACRES.

IT WOULD BE ABOUT 40,000 PEOPLE.

SOME ESTIMATES ALSO GO AS HIGH AS 60,000.

NARRATOR: MOHENJO-DARO WAS AT THE CENTER OF A CIVILIZATION IN THE INDUS VALLEY THAT WAS AS OLD AS ANY IN EGYPT OR MESOPOTAMIA.

IT WAS THE FIRST CIVILIZATION IN ASIA.

ARCHAEOLOGIST UZMA RIZVI BELIEVES THE INDUS CIVILIZATION WAS BUILT ON TRADE, MORE SO THAN ANY OTHER FIRST CIVILIZATION.

RIZVI: WHAT THE INDUS DID REALLY, REALLY WELL WAS REALLY GET IN ON WHAT IT WAS THAT PEOPLE WANTED, RIGHT?

SO WHAT DOES AN ELITE WANT?

EVERYONE WANTS THE BEAUTIFUL CARNELIAN BEADS THAT ARE ETCHED AND ARE JUST GORGEOUS TO LOOK AT.

THEY DID A FANTASTIC JOB CORNERING THAT MARKET, AND THEY ALSO DID A FANTASTIC JOB KNOWING EXACTLY WHERE THE RESOURCES WERE, THE BEST RESOURCES.

THEY'RE NOT USING MEDIOCRE MATERIAL.

IT'S ALMOST LIKE HAVING A BRAND, YOU KNOW, LIKE IN THE CONTEMPORARY SENSE.

ONE OF THE KEY FINDS WE HAVE IS THE FIGURINE.

IT'S A SMALL FIGURINE, BUT IT IS QUITE BEAUTIFUL WHEN YOU LOOK AT IT CLOSE UP.

IT SEEMS LIKE A SEATED FIGURE WITH SORT OF ALMOND-SHAPED EYES.

YOU HAVE A BAND ACROSS THE TOP, BEAUTIFULLY MANICURED HAIR AND BEARD.

THE LINES ARE QUITE PRISTINE.

THE CARVING IS ACTUALLY QUITE EXQUISITE.

EXEMPLARY CRAFT, EXEMPLARY RAW MATERIAL, AND EXEMPLARY TRADE NETWORKS THAT REALLY ALLOW FOR THESE CITIES TO BLOSSOM.

NARRATOR: THE CITY IS SO VAST THAT MUCH STILL LIES UNEXCAVATED...

BUT ARCHAEOLOGISTS HAVE UNCOVERED LARGE AREAS DEDICATED TO THE MANUFACTURE OF SPECIALIST CRAFTS, SUCH AS THE SMELTING OF COPPER.

SO EVERY ONE OF THESE BLACK, SORT OF GRAY-BLACK STONES THAT YOU'RE SEEING HERE, IS ACTUALLY THE WASTE MATERIAL FROM A SECONDARY SMELTING PROCESS.

SO YOU CAN TELL JUST BY THE DENSITY OF WHAT WE'RE SEEING HERE THAT THIS WAS A VERY ACTIVE PLACE FOR PRODUCTION OF METALS, OF COPPER, OF BRONZE.

RIZVI: WE CAN ACTUALLY ISOLATE THE VARIOUS ISOTOPES, AND SO WE KNOW, THEN, THE LEAD ISOTOPE SIGNATURE FROM DIFFERENT REGIONS, AND WE CAN SAY, "OH, A CERTAIN AMOUNT OF COPPER IS COMING FROM RAJASTHAN, A CERTAIN AMOUNT OF COPPER IS COMING FROM OMAN."

THERE IS A VERY CLEAR SENSE THAT EXCHANGE IS HAPPENING, AND THINGS THAT ARE BEING MADE IN THESE KINDS OF PRODUCTION CENTERS ARE BEING USED IN THAT WIDER NETWORK.

NARRATOR: ONE OF THE THINGS MADE AT MOHENJO-DARO IS THIS STATUETTE CRAFTED FROM BRONZE, PRODUCED BY COMBINING COPPER AND TIN.

IT'S 4,500 YEARS OLD.

RIZVI: A WOMAN WITH IMMENSE BEAUTY AND POISE AND POSTURE, AND SHE'S STANDING THERE AND SHE'S JUST LIKE, "HERE I AM."

SHE HAS HER BANGLES, AND THERE IS A CERTAIN KIND OF PRIDE TO HER.

AND YOU CAN SEE IT'S BEEN CRAFTED WITH THAT SAME KIND OF PERFECTION.

THAT CRAFTSMANSHIP IS SOMETHING THAT IS UNIQUE TO THE INDUS.

NARRATOR: HOW DID THE PEOPLE OF THE INDUS VALLEY ACHIEVE SUCH EXPERTISE SO LONG AGO?

PA. NARRATOR: THE DISCOVERY OF A SMALL, CORRODED AMULET MAY PROVIDE AN ANSWER.

NARRATOR: IN FRANCE, PHYSICIST MATHIEU THOURY IS EXAMINING THE AMULET AT THE SYNCHROTRON PARTICLE ACCELERATOR NEAR PARIS.

USING A PROCESS KNOWN AS PHOTOLUMINESCENCE SPECTROSCOPY, HE CAN WORK OUT HOW IT WAS MADE.

THOURY: THE AMULET IS 25 MILLIMETERS WIDE, SO WE ARE WORKING WITH A MICROSCOPE, WHICH RESOLUTION, THE SMALLEST OBJECT THAT YOU CAN SEE, IS A FRACTION OF MICRON, AND A MICRON IS A FRACTION OF AN HAIR.

NARRATOR: A POWERFUL BEAM IS PROJECTED AT THE AMULET.

ITS ELECTRONS EMIT THEIR OWN LIGHT IN RESPONSE, ALLOWING THOURY TO ANALYZE THOSE PARTS OF THE AMULET HIDDEN BY CORROSION.

THOURY: I KNEW THAT CONSIDERING THIS PATTERN WAS INVISIBLE AND EXTREMELY ORGANIZED, IT WAS VERY INTERESTING INFORMATION.

NARRATOR: THE MICROSCOPIC ANALYSIS REVEALS THE AMULET IS THE OLDEST EVIDENCE IN THE WORLD OF A TECHNIQUE KNOWN AS LOST-WAX CASTING.

SPECIALIST METAL WORKERS USE THE SAME TECHNIQUE TODAY FOR DOING PRECISION WORK.

ARCHAEOLOGIST BENOIT MILLE IS AN EXPERT IN ITS ANCIENT HISTORY.

[MILLE SPEAKING FRENCH] TRANSLATOR: LOST-WAX CASTING ALLOWS US TO MAKE VERY COMPLEX OBJECTS.

NARRATOR: THE FIRST PART OF THE PROCESS IS TO CREATE THE OBJECT YOU WANT TO CAST OUT OF MALLEABLE WAX.

TRANSLATOR: THIS MAKES IT POSSIBLE TO PRODUCE A NEW TYPE OF METAL OBJECT, MADE BY SMELTING AND CASTING, WITH GREAT PRECISION; LITTLE OBJECTS WHICH HAVE DELICATE DETAILS, AS WELL AS VERY COMPLEX AND VARIED FORMS.

NARRATOR: ONCE THE WAX MODEL IS FINISHED, IT'S ENCASED IN CLAY TO MAKE A MOLD.

THIS IS THEN HEATED TO MELT THE WAX AWAY, LEAVING AN EMPTY MOLD, WHICH IS FILLED WITH MOLTEN METAL.

THIS IS WHY IT'S CALLED LOST-WAX CASTING.

[MILLE SPEAKING FRENCH] TRANSLATOR: DURING THE INDUS CIVILIZATION, THESE CRAFT SKILLS BECAME HIGHLY DEVELOPED.

YOU NEED TO MASTER A NUMBER OF DIFFERENT TECHNIQUES.

YOU NEED TO KNOW HOW TO MAKE AN OVEN, AND THEN HOW TO WORK WITH THE CAST.

TO HANDLE METALS AT 1,000 DEGREES IS SOMETHING QUITE DANGEROUS, WHICH YOU WOULDN'T ENTRUST TO A NOVICE.

[SIZZLES] TRANSLATOR: THIS RESEARCH SHOWS THERE WAS AN EXTRAORDINARY INGENUITY BACK THEN, THAT THESE SOCIETIES WERE AMAZINGLY INNOVATIVE IN TERMS OF CRAFTSMANSHIP.

NARRATOR: THIS TECHNICAL SKILL WAS BEHIND THE GROWTH OF TRADE IN THE INDUS VALLEY.

NARRATOR: ARTISANS MADE ORNAMENTS, MERCHANTS TRADED THEM, AND EVERYONE PROSPERED.

BOTSMAN: THE BENEFITS OF TRADE, THEY START WITH US.

THEY START WITH THE INDIVIDUAL, SO THE FIRST THING THAT THEY DO IS THEY EXPAND OUR NEEDS AND WANTS.

THEY EXPAND THE CHOICES AVAILABLE TO US, BUT THEN MAGIC THINGS START TO HAPPEN BECAUSE FOR TRADE TO HAPPEN, MARKETS HAVE TO GROW, NEW ALLIANCES HAVE TO FORM, IDEAS HAVE TO-- AND PEOPLE HAVE TO BUMP INTO ONE ANOTHER.

SO TRADE IS THE ELIXIR.

IT'S THE THING THAT CREATES COLLABORATION AND CONNECTIONS.

IT'S WHAT ENABLES IDEAS TO MOVE FORWARD.

I THINK IT'S WHAT ENABLES THE SOCIETY TO PROGRESS.

NARRATOR: BUT WHAT WAS SO SPECIAL ABOUT THE INDUS VALLEY THAT ALLOWED A TRADE NETWORK, AND THEN A CIVILIZATION, TO START HERE, RATHER THAN ELSEWHERE?

ULTIMATELY, ANY CIVILIZATION DEPENDS ON ITS FARMERS TO PRODUCE ENOUGH FOOD TO FEED ITS PEOPLE.

ONE OF THE MOST RECENT SITES TO BE EXCAVATED IS ON THE INDIAN SIDE OF THE VALLEY.

IT'S A SMALL, RURAL SETTLEMENT CALLED LOHARI RAGHO.

ARCHAEOLOGIST CAMERON PETRIE BELIEVES EARLY FARMERS HERE GOT A HELPING HAND FROM THE CLIMATE.

THE INDUS CIVILIZATION IS AT THE VERY EASTERN EDGE OF THE RAINS THAT AFFECT THE MEDITERRANEAN AND THE MIDDLE EAST.

BUT IT'S ALSO AT THE WESTERN EDGE OF THE INDIAN SUMMER MONSOON, SO WHERE THE INDUS POPULATIONS LIVE IS DIRECTLY WHERE THESE TWO WEATHER SYSTEMS OVERLAP.

[THUNDER] NARRATOR: THIS MADE THE INDUS VALLEY UNIQUE.

IT BENEFITED FROM TWO RAINY SEASONS-- WINTER AND SUMMER-- WHICH ENABLED ITS FARMERS TO GROW MORE THAN ONE SET OF CROPS EACH YEAR.

ITS BENEFICIAL LOCATION GAVE THE INDUS VALLEY AN ADVANTAGE OVER ALL OTHER FIRST CIVILIZATIONS.

IN MESOPOTAMIA AND EGYPT, THEY HARVESTED WHEAT, BARLEY, SORGHUM, AND MILLET.

IN MESOAMERICA, IT WAS SQUASH AND CORN.

BUT NOWHERE WAS FARMING AS PRODUCTIVE AS THE INDUS VALLEY.

PETRIE: SO WHAT THAT MEANS IS THAT THEY COULD BE MUCH MORE ADAPTABLE TO THEIR SITUATIONS.

THEY'RE ABLE TO MAXIMIZE THE OUTPUT FROM THE HINTERLANDS AROUND THEIR SITES.

IT DOES CREATE, IN SOME WAYS, A SORT OF ECONOMIC DRIVE FOR THE LOCAL ECONOMIES TO HAVE A SURPLUS AND BE RELATIVELY WELL-OFF, AND THEREFORE TO ENGAGE IN THINGS LIKE TRADE.

AND IN SOME WAYS, IT'S PART OF THE ECONOMIC AND SOCIAL GLUE THAT STITCHES PEOPLE TOGETHER.

WE'VE GOT A NICE, INTERESTING DYNAMIC WITH THE INDUS, SO IT'S SIMULTANEOUSLY DIFFERENT--PEOPLE ARE VARIED AND THEY'RE PROBABLY SPEAKING DIFFERENT SORTS OF LANGUAGES-- BUT THERE'S THINGS THAT STICK THEM TOGETHER, AND IT'S THE ACCESS TO THIS TRADE NETWORK AND THE ECONOMIC PRODUCTS THAT THEY CAN OBTAIN.

PETRIE: TODAY WE'RE IN LUCK.

SO THE TEAM'S BEEN EXCAVATING CAREFULLY, AND WE'VE FOUND A NICE CARNELIAN BEAD.

WHAT'S INTERESTING IS THAT IT'S PROBABLY COMING FROM SOMEWHERE DOWN IN GUJARAT, 800 KILOMETERS AS THE CROW FLIES.

BUT IT DOES GIVE US A NICE, NEAT EXAMPLE OF-- OR A DEMONSTRATION OF HOW THIS TRADING SYSTEM OPERATES AND THE SORTS OF THINGS THAT ARE MOVING AROUND.

NARRATOR: THE INDUS VALLEY WAS PREVIOUSLY THOUGHT TO BE A PREDOMINANTLY URBAN CIVILIZATION.

BUT NOW, ARCHAEOLOGISTS ARE DISCOVERING THAT ALONGSIDE THE CITIES, THERE WERE THOUSANDS OF SMALLER SITES, JOINED UP IN A NEXUS OF TRADE.

[MEN SINGING INDISTINCTLY] NARRATOR: HERE, THE AGENTS OF CIVILIZATION WERE NOT SOLDIERS, BUREAUCRATS, OR PRIESTS, BUT TRAVELING MERCHANTS WHO WOULD ESTABLISH NEW TRADE ROUTES TO BUY AND SELL THEIR WARES.

COTTON PROVIDED A LUCRATIVE RAW MATERIAL FOR MERCHANTS, WHO WOULD BARTER FOR IT BY OFFERING OTHER GOODS IN EXCHANGE.

AS A RESULT, SETTLEMENTS EMERGED TO SERVICE THIS NEW TRADE.

ARCHAEOLOGISTS ADAM AND LILY GREEN ARE USING ANCIENT MAPS AND LOCAL KNOWLEDGE TO ESTABLISH THE FULL EXTENT OF THE INDUS VALLEY CIVILIZATION.

SO WHAT I THINK WE HAVE HERE AT THIS STAGE IS A SMALL-SCALE SETTLEMENT.

WE'RE PICKING UP LOTS OF EVIDENCE OF ACTIVITY, EVIDENCE OF BUILDING UP THE SITE, LIVING DAY-TO-DAY-- EVERYDAY-LIFE KIND OF THINGS.

SO IF YOU KIND OF WALK ALONG HERE, YOU'LL SEE AN OCCASIONAL PIECE OF BROKEN POTTERY THAT'S BEEN PLOWED OVER AND OVER AND OVER AGAIN BY TRACTORS AS THEY'RE PREPARING THE SOIL FOR THESE COTTON CROPS.

YOU CAN SEE THIS.

THIS IS REALLY GREAT.

HAVE THIS BLACK-PAINTED BODY SHARD.

THIS IS A PIECE OF A VESSEL WITH BLACK PAINT ON IT.

ADAM: ALL THESE GROUPS WERE ENGAGING IN DIFFERENT FORMS OF SPECIALIZATION, CREATING A RANGE OF GOODS THAT WERE BEING FED INTO THE SYSTEM AS A WHOLE.

SO THERE HAD TO HAVE BEEN LARGE-SCALE, LONG-DISTANCE SYSTEMS OF CONTACT AND INTERACTION THAT TIED THE WHOLE CIVILIZATION TOGETHER.

NARRATOR: FOR TRADE TO FLOURISH, THERE NEEDS TO BE A SET OF RULES, A PROTOCOL FOR EXCHANGE BETWEEN STRANGERS HUNDREDS OR THOUSANDS OF MILES APART.

THIS WAS TRUE IN ANCIENT TIMES JUST AS MUCH AS TODAY.

BOTSMAN: YOU'D ASK MANY PEOPLE HOW DOES TRADE WORK, THEY WOULD SAY, "WELL, IT'S MONEY."

MONEY IS A CURRENCY OF TRANSACTIONS, BUT FOR HUMAN BEINGS TO INTERACT, FOR HUMAN BEINGS TO TRADE, THE SOCIAL GLUE, THE LUBRICANT THAT MAKES THIS WORK, IS TRUST.

THAT'S THE MOST FRAGILE AND PRECIOUS ASSET THAT EXISTS IN ANY FORM OF TRADE NETWORK.

BOTSMAN: WHEN YOU TRADE WITH A STRANGER, YOU DON'T KNOW THE OUTCOME.

THIS IS WHAT I DESCRIBE AS A TRUST LEAP... [OVERLAPPING CHATTER] AND THE EASIEST WAY TO THINK OF A TRUST LEAP IS IT'S WHEN WE TAKE A RISK TO DO SOMETHING NEW OR TO DO IT DIFFERENTLY FROM THE WAY WE'VE DONE IT BEFORE... AND THEREFORE, THE CURRENCY THAT MAKES IT WORK IS TRUST.

IF THERE IS NO TRUST, YOU CANNOT TRADE.

NARRATOR: BUT HOW DO YOU DECIDE IF YOU CAN TRUST SOMEONE?

IS IT BASED ON THEIR APPEARANCE...

THE WAY THEY LOOK YOU IN THE EYE... OR THEIR BODY LANGUAGE?

PSYCHOLOGIST TIM HAHN BELIEVES TRUST IS SO FUNDAMENTAL TO HUMAN INTERACTION THAT OUR BRAINS HAVE EVOLVED TO BE INNATELY TRUSTING.

TO TEST THIS, HE IS RUNNING AN EXPERIMENTAL TRUST GAME IN WHICH TWO STRANGERS ARE PAIRED THROUGH A COMPUTER LINK.

EACH IS GIVEN A POT OF MONEY TO TRADE WITH.

HAHN RECORDS THEIR BRAIN ACTIVITY DURING EVERY STAGE OF THE GAME.

HAHN: YOU'RE ACTUALLY MEASURING HOW YOUR BRAIN RESPONDS TO THE STIMULI IN THE TRUST GAME BY LOOKING AT THE BRAIN WAVES.

FEMALE COMPUTER VOICE: HOW MUCH MONEY DO YOU WANT TO GIVE TO PLAYER TWO?

NARRATOR: WHATEVER AMOUNT PLAYER ONE DECIDES TO GIVE TO PLAYER TWO WILL BE TRIPLED.

HAHN: TRUST, IN THE CASE OF OUR EXPERIMENT, IS ALWAYS ENTRUSTING MONEY TO ANOTHER PERSON IN THE HOPE THAT THIS PERSON WILL RETURN SOME OF YOUR MONEY BACK TO YOU... AND EVEN MORE, HOPEFULLY, THAN YOU'VE INITIALLY ENTRUSTED TO THAT PERSON.

COMPUTER: PLAYER ONE HAS GIVEN YOU 45.

HOW MUCH MONEY DO YOU WANT TO GIVE BACK?

NARRATOR: WHATEVER AMOUNT PLAYER TWO DECIDES TO RETURN WILL ALSO BE TRIPLED.

THE GAME IS DESIGNED SO THAT THE MORE YOU TRUST A STRANGER, THE MORE YOU ARE REWARDED.

IT'S A WIN-WIN SCENARIO, MIRRORING THE PROCESS OF TRADE.

HAHN: IF THAT WORKS WELL OVER THE COURSE OF SEVERAL ROUNDS OF THE TRUST GAME, BOTH PLAYERS CAN LEARN THAT THEY CAN TRUST EACH OTHER, AND THEN THEY START TO INVEST MORE AND MORE MONEY INTO THE OTHER PERSON.

MORE AND MORE MONEY IS FLOWING BACK, WHICH BENEFITS BOTH PARTIES.

NARRATOR: HAHN'S RESEARCH REVEALS THAT THE MOST CRUCIAL MOMENT OCCURS AT THE VERY START OF THE GAME, WHEN THE TWO STRANGERS MEET FOR THE FIRST TIME AND THEY HAVE TO OVERCOME THEIR INITIAL CAUTION.

[SPEAKS NATIVE LANGUAGE] HAHN: THE THING THAT WE'VE SHOWN IN OUR EXPERIMENT IS THAT EVEN BEFORE YOU START TO KNOW A PARTNER, YOU WILL DISPLAY SOME LEVEL OF TRUST, WHICH MEANS THAT OVER TIME, PRETTY MUCH ANY RELATIONSHIP WHERE TRUST IS ANSWERED WITH TRUST WILL EVOLVE INTO A MORE TRUSTING RELATIONSHIP.

WE AS HUMANS ARE PROBABLY VERY WELL-EQUIPPED TO TRADE BECAUSE WE ALL SHOW AT LEAST THE BASIC LEVEL OF TRUST.

AND THAT SEEMS TO BE, IF NOT HARD-WIRED INTO OUR BRAINS, THEN STILL IMPLEMENT IN OUR BRAINS IN SUCH A SMART WAY THAT WE WILL ALWAYS TRUST, BUT THAT WE'RE FLEXIBLE IN RESPONDING TO DIFFERENT CIRCUMSTANCES AND DIFFERENT PARTNERS IN TRADE INTERACTIONS.

HAHN: THAT'S, IN THE END, WHAT SETS US APART AND WHICH HAS EVENTUALLY ENABLED US TO BUILD A CIVILIZATION THAT IS SO COMPLEX.

COMPUTER: CONGRATULATIONS, PLAYER ONE.

YOU HAVE WON 240.

CONGRATULATIONS, PLAYER TWO.

YOU HAVE WON 180.

NARRATOR: TO ENSHRINE TRUST BETWEEN BUYER AND SELLER, THE MERCHANTS OF THE INDUS VALLEY PIONEERED THEIR OWN METHOD OF DOING BUSINESS.

MAN: THE PEOPLE OF THE INDUS VALLEY WERE AMONG THE FIRST TO REALIZE THE FULL POTENTIAL OF TRADE.

THEY FOUND MANY INNOVATIVE WAYS TO FACILITATE THE EXCHANGE OF GOODS BETWEEN MERCHANTS, AND PERHAPS THE MOST INVENTIVE WAY WAS USING SEALS.

MANY OF THESE SEALS HAVE TALLY MARKS-- YOU KNOW, 1, 2, 3, 4, UP TO 7 TALLY MARKS-- FOLLOWED BY SYMBOLS, SUCH AS A STALK OF WHEAT OR A STALK OF BARLEY, WHICH SEEMS TO INDICATE THAT SOMETHING IS BEING COUNTED, PERHAPS A QUANTITY OF GRAIN.

AND THERE'S OTHER SYMBOLS THAT COULD POTENTIALLY STAND FOR WEIGHTS AND VOLUME, AND PERHAPS SYMBOLS THAT STAND FOR THE NAMES OF MERCHANTS' LOCATIONS... OR EVEN STATE INSTITUTIONS SUCH AS, YOU KNOW, THE TAX DEPARTMENT OR THE CUSTOMS DEPARTMENT.

THESE INDUS SEALS COULD ALSO HAVE BEEN USED AS AN ANCIENT BARCODE, SO NUMEROUS CLAY TAGS HAVE BEEN FOUND IN DIFFERENT SITES OF THE CIVILIZATION.

THE INFORMATION THAT THESE CLAY TAGS MIGHT CONTAIN INCLUDE THE SOURCE AND THE DESTINATION OF THE GOODS.

SOME OF THESE SEALS AND THE TAGS ASSOCIATED WITH THEM HAVE BEEN FOUND SNAPPED, ALMOST DELIBERATELY SNAPPED.

THESE SNAPPED SEALS OR TAGS COULD DENOTE THE FACT THAT A TRANSACTION, A BUSINESS TRANSACTION, HAS ENDED OR CONCLUDED, AND THAT'S BEEN DONE TO PREVENT FUTURE ABUSE OF THE LEGITIMATE AUTHORITY OF THE SEALS.

NARRATOR: EACH SEAL FEATURED A DIFFERENT ANIMAL, WHICH MAY HAVE BEEN USED TO IDENTIFY INDIVIDUAL TRADERS, LIKE A MODERN-DAY LOGO.

THOUSANDS OF YEARS BEFORE COCA-COLA AND NIKE, THE PEOPLE OF THE INDUS VALLEY DISCOVERED THE POWER OF BRANDING.

RAO: IT'S SOMETHING THAT EVEN AN ILLITERATE PERSON CAN JUST GLANCE AT AND UNDERSTAND THE POWER OF THAT PARTICULAR IMAGE...

ANCIENT BRANDS OF THE QUALITY OF THE GOODS OR THE AFFILIATION OF THE MERCHANT TO A PARTICULAR CLAN OR COMMUNITY.

NARRATOR: THE SAME ATTENTION TO DETAIL WENT INTO THE PLANNING OF THEIR CITIES.

EVERYTHING WAS DESIGNED TO PROMOTE THE FREE FLOW OF TRADE.

IN ANY CIVILIZATION, HAPPY, WELL-ORDERED PEOPLE ARE MORE EFFICIENT PEOPLE WITH MORE TIME FOR BUSINESS.

RIZVI: WHAT IS DISTINCT ABOUT THE INDUS, AS COMPARED TO MANY OTHER SITES FROM 5,000 YEARS AGO AROUND THE WORLD IS THAT WE DON'T HAVE THE ZIGGURAT.

WE DON'T HAVE THE MONUMENTAL PALACES, RIGHT?

WE DON'T HAVE THE LARGE TEMPLES WITH THE COLUMNS.

BUT WHAT WE DO HAVE IS AN INCREDIBLE MONUMENTALITY OF ORGANIZATION AND PLANNING, OF STANDARDIZATION.

THERE IS A MONUMENTALITY OF THOUGHT HERE THAT GOES BEYOND WHAT WE GENERALLY SEE AT THIS TIME PERIOD.

THE INDUS, IN MY MIND, WAS ACTUALLY FAR AHEAD OF THE CURVE.

IT'S NOT KIND OF AN ORGANIC FORMATION RESPONDING TO EACH OTHER, SEEING HOW THE POPULATION GROWS.

THIS CITY IN PARTICULAR IS JUST PLANNED FROM TOP TO BOTTOM.

YOU CAN WALK THROUGH THE CITY AND YOU CAN FEEL IT.

IT FEELS LIKE A MODERN CITY.

ALL THE ARCHITECTS AND THOSE WHO WERE PLANNING THIS PLACE WERE VERY CLEAR IN THEIR MIND.

THEY KNEW WHAT THE CITY NEEDED IN ORDER FOR IT TO BE A SUCCESSFUL CITY... LIKE THINKING ENVIRONMENTALLY, LIKE THINKING ABOUT WHERE IS THE WIND FLOWING FROM, SO YOU MAY NOT HAVE AN AIR CONDITIONER, BUT YOU HAVE REALLY FANTASTIC WIND TUNNELS.

SO YOU CAN STAND HERE IN A LITTLE BIT OF SHADE, AND YOU ACTUALLY GET A GREAT BREEZE COMING BY.

NARRATOR: FRESH WATER AND A SEWAGE SYSTEM ARE OFTEN THOUGHT TO HAVE BEEN INVENTED DURING THE HEYDAY OF ROME, BUT THE PEOPLE OF THE INDUS VALLEY GOT THERE 2,000 YEARS BEFORE THE ROMANS.

HERE WE HAVE A GREAT EXAMPLE OF A SECOND-FLOOR BATHROOM, AND THERE WAS A TERRACOTTA PIPE THAT WAS PLACED INSIDE FOR DRAINAGE, COVERED ENTIRELY, RIGHT--WE CAN SEE IT BECAUSE IT'S BEEN EXCAVATED-- AND THEN COMING DOWN THE CHUTE HERE.

AND ALONG BOTH SIDES OF THE STREET, WE HAVE DRAINAGE.

SO IT'S COMING STRAIGHT IN AND MOVING OUT, OUT OF THE CITY, WHICH IS JUST REALLY FANTASTIC.

MY STUDENTS ALWAYS LAUGH ABOUT THIS.

THEY'RE LIKE, "YES, THE ONE THING WE LEARNED ABOUT MOHENJO-DARO IS THE DRAINAGE.

WE KNOW THERE'S DRAINAGE."

BUT IT IS REMARKABLE DRAINAGE.

IT IS NOT JUST DRAINAGE.

IT IS FANTASTIC DRAINAGE.

EVERY SINGLE HOUSE HAS BEEN THOUGHT THROUGH.

THIS IS PLANNED.

THIS IS ORCHESTRATED.

THIS IS A LOT OF CONTROL AND A LOT OF THOUGHT.

RIZVI: THIS IS REALLY WHAT MAKES MOHENJO-DARO, AND THE INDUS IN GENERAL, REMARKABLE.

THESE INDIVIDUALS KNEW WHAT THEY WERE DOING.

THIS IS BETTER-ORGANIZED THAN MANY OTHER CITIES I'VE LIVED IN TODAY.

NARRATOR: AT THE SOUTHERN END OF THE INDUS VALLEY REGION, THERE WAS ANOTHER IMPORTANT TRADING CITY, DHOLAVIRA.

[BIRD CALLS] THE COASTAL LOWLANDS OF NORTHERN GUJARAT WERE A WILDERNESS COMPARED TO THE RICH FARMLAND FURTHER NORTH.

TODAY, THE AREA HAS BEEN ABANDONED TO NATURE...

BUT THE PEOPLE OF THE INDUS WERE ABLE TO MAKE THIS A HOME.

[VEHICLE APPROACHING] NARRATOR: ARCHAEOLOGIST MICHEL DANINO HAS MADE A LIFELONG STUDY OF DHOLAVIRA.

DANINO: IT'S A VERY ARID REGION, NOT VERY HOSPITABLE.

FOR THE CITY TO BE SUSTAINABLE, THEY HAD TO STORE RAINWATER.

NARRATOR: DHOLAVIRA IS RINGED BY A SERIES OF RESERVOIRS THAT STORED THE MONSOON RAINWATER EACH YEAR.

IT IS THE OLDEST SYSTEM OF URBAN WATER MANAGEMENT ANYWHERE IN THE WORLD.

DANINO: WE ARE HERE AT THE BOTTOM OF THE EASTERN RESERVOIR.

ITS LENGTH WAS ABOUT 73 METERS.

THAT'S NEARLY ONE AND A HALF OLYMPIC SWIMMING POOLS, SO THEREFORE, THIS WOULD HAVE BEEN A VERY IMPRESSIVE SIGHT, ESPECIALLY WHENEVER IT WAS FULL.

NARRATOR: IT HAS BEEN ESTIMATED THE CITY'S RESERVOIRS COULD HOLD 79 MILLION GALLONS OF WATER, ENOUGH FOR 14 GALLONS PER PERSON PER DAY.

THIS PART OF GUJARAT IS AN EARTHQUAKE ZONE, BUT THE RESERVOIRS SEEM TO HAVE BEEN DESIGNED TO WITHSTAND SEISMIC ACTIVITY.

DANINO: YOU CAN SEE THOSE-- WHAT LOOKS LIKE VERTICAL LINES.

ACTUALLY, THEY'RE SIMPLY LINES WHERE THE--I MEAN, THE STONES ARE NOT JOINED, THEY ARE NOT OVERLAPPING.

IN CIVIL ENGINEERING, THIS IS CALLED A FUSE; IN OTHER WORDS, THIS IS THE WEAKEST PART OF THE WALL, AND IF THERE IS, FOR EXAMPLE, AN EARTHQUAKE, IF THE WHOLE WALL IS INTERLOCKED, PERHAPS A BIG PORTION OF IT MIGHT JUST FALL OUT, WHEREAS, IF YOU KEEP A WEAKER PORTION HERE, PERHAPS ONLY THIS-- OR A LITTLE BIT ON THE SIDES, OR MAYBE ONLY ONE OF THE SIDES--WILL FALL OUT.

SO IF THIS IS AN EARTHQUAKE SAFETY MEASURE, THEN IT WAS REALLY WELL AHEAD OF ITS TIME.

NARRATOR: WHEREVER DANINO LOOKS, HE FINDS THE SAME EVIDENCE OF AN ENGINEERING MINDSET AMONG THE RUINS; THE STREETS, THE BUILDINGS, EVEN THE BRICKS, WERE ALL MADE TO MEASURE.

DANINO: THE BRICKS WERE STANDARDIZED IN THE SENSE THAT THE WIDTH IS TWICE AS MUCH AS THE HEIGHT, AND THE LENGTH IS TWICE AS MUCH AS THE WIDTH, SO IT'S 1:2:4.

THIS WAS ACTUALLY A STROKE OF GENIUS BECAUSE IT GIVES YOU THE MAXIMUM STRUCTURAL STRENGTH WITH A MINIMUM AMOUNT OF BUILDING MATERIAL.

NARRATOR: MATHEMATICAL PRINCIPLES WERE APPLIED TO ALL THE CITIES OF THE INDUS VALLEY.

CONSTRUCTION WAS ALWAYS BASED ON PRECISE, RISING RATIOS, WHERE THE LENGTH OF A BUILDING WAS LARGER THAN ITS WIDTH.

THIS SERVED NO PRACTICAL PURPOSE; IT MADE THE BUILDING WORK MORE COMPLICATED, BUT IT SUGGESTS A FAITH IN A CORE IDEA-- THE POWER OF PROGRESS.

THIS FAITH UNDERPINNED THE PRINCIPLES OF A TRADING CIVILIZATION.

DANINO: THERE'S A VERY CLEAR CONCEPT THAT IT IS SOMETHING WHICH HAS VALUE.

IN LATER INDIAN TRADITIONS, THIS CONCEPT OF GROWTH IS GOING TO BE VERY, VERY CLEARLY EXPRESSED IN THE BUILDING OF ALTARS, IN THE BUILDING OF TEMPLES AND SO ON, INVOKING A DESIRE FOR PROSPERITY, FOR AUSPICIOUSNESS.

AND THEY WOULD HAVE SEEN THIS RATIO AS-- CERTAINLY AS AUSPICIOUS, IF NOT, PERHAPS, AS SACRED.

NARRATOR: WHEN TRYING TO UNDERSTAND THE FIRST CIVILIZATIONS, ARCHAEOLOGISTS LOOK FOR EVIDENCE OF THE FORCES THAT MIGHT BIND PEOPLE TOGETHER... PERHAPS A KING OR AN EMPEROR WHO LAYS DOWN THE LAW AND IMPOSES ORDER.

BUT IN THE INDUS VALLEY, NO SUCH EVIDENCE HAS BEEN FOUND.

ARCHAEOLOGISTS ALSO LOOK FOR TEMPLES, THE EXISTENCE OF ORGANIZED RELIGION WITH A UNIFYING SYSTEM OF BELIEF.

AGAIN, HERE IN THE INDUS VALLEY, NONE HAS BEEN FOUND.

[WOMAN SCREAMS, MEN SHOUTING] NARRATOR: ANOTHER RECURRING FEATURE IS WAR... AS THE WEAK ARE WEEDED OUT AND THE STRONG PREVAIL.

BUT FOR THE PEOPLE OF THE INDUS, IT WAS A DIFFERENT STORY.

WITH TRADE CAME PEACE, GIVING CIVILIZATION HERE A DIFFERENT FLAVOR.

IT FUNCTIONED LIKE A MODERN BUSINESS CORPORATION, DESIGNED TO MAXIMIZE ITS OWN WEALTH, NOT BY RULING WITH AN IRON FIRST, BUT BY CREATING A LOOSE WEB OF LIKE-MINDED INTERESTS-- FRANCHISE HOLDERS UP AND DOWN THE VALLEY WHO SHARE THE SAME DESIRE FOR TRADE AND PROSPERITY.

IN DAYS WHEN THE FASTEST COMMUNICATION IS A BOAT, RIVER BOAT, OR A BULL AND CART, AND YOU HAVE CITIES 2,000 KILOMETERS APART, IT DOESN'T MAKE SENSE TO RUN THIS AS A CENTRALIZED EMPIRE.

THERE WERE ORIGINAL CHIEFTAINS WHICH WERE CONTROLLING THEIR REGIONS, BUT THEY WERE WORKING TOGETHER.

IT'S VERY CLEAR THAT YOU HAVE ONE MIND AT WORK.

[INDISTINCT CHATTER] BOTSMAN: OFTEN TRADE IS LOOKED THROUGH IN TERMS OF ECONOMIC BENEFITS, BUT IF YOU LOOK BACK IN HISTORY AND YOU LOOK AT TRADE PATTERNS, TYPICALLY, COUNTRIES THAT TRADE WITH ONE ANOTHER DO NOT GO TO WAR WITH ONE ANOTHER.

THERE IS A HUMAN UNDERSTANDING THERE.

IT CREATES CIVILIZATION.

IT'S WHAT ENABLES A CIVILIZED SOCIETY.

NARRATOR: THE IDEAS SEEDED IN THE INDUS VALLEY ARE THE VERY ESSENCE OF OUR OWN ECONOMIC SYSTEM-- THE LINK BETWEEN TRADE, WEALTH, CITIES, PRODUCTION, CONSUMPTION, CIVILIZATION-- IDEAS WE MAY THINK ARE MODERN, BUT WERE ACTUALLY ROAD-TESTED 4,500 YEARS AGO.

INSTEAD OF PRIESTS, TODAY THERE ARE TRADERS; INSTEAD OF PYRAMIDS AND TEMPLES, THERE ARE THE HIGH-RISES OF THE CENTRAL BUSINESS DISTRICT-- MONUMENTS TO A TRUST IN PROSPERITY.

BOTSMAN: TRUST AND TRADE, THEY WORK IN THIS BEAUTIFUL FEEDBACK LOOP.

FOR TRADE TO START IN ANY CIVILIZATION, YOU NEED TRUST, AND THEN THE MORE TRADE YOU HAVE, THE MORE TRUST YOU HAVE, AND SO THE LOOP CONTINUES, AND THE BENEFITS ARE REALLY EXPONENTIAL.

IN HIGH-TRUST SOCIETIES, THEY DON'T JUST THRIVE ECONOMICALLY; YOU ACTUALLY SEE INDIVIDUALS IN SOCIETY THRIVE BECAUSE THEY HAVE MORE FREEDOMS, THEY HAVE MORE EMPOWERMENT.

YOU SEE MORE ENTREPRENEURSHIP, YOU SEE MORE HUMAN EMPATHY.

NARRATOR: AT THE HEIGHT OF ITS EXPANSION, THE INDUS VALLEY CIVILIZATION COVERED AN AREA OVER HALF A MILLION SQUARE MILES.

BUT ITS TRADE LINKS REACHED EVEN FURTHER: IN THE NORTH AND EAST, TO MODERN-DAY CHINA AND AFGHANISTAN; IN THE WEST, AS FAR AS THE PERSIAN GULF.

THIS WAS A CIVILIZATION THAT PRODUCED BEAUTIFUL ARTIFACTS.

ITS PEOPLE WERE WELL-FED.

ITS CITIES WERE CLEAN.

INEQUALITY WAS LOW, AND IT WAS PEACEFUL.

WHAT COULD POSSIBLY GO WRONG?

RAO: AROUND 1900 BC, THE SEALS--SO THE ALL-IMPORTANT SEALS THAT WERE CRUCIAL TO THE SUCCESS OF THE INDUS CIVILIZATION AND THE INDUS MERCHANTS-- DISAPPEAR FROM THE ARCHAEOLOGICAL RECORD.

IT'S ALMOST AS IF SOMETHING CATASTROPHIC HAS HAPPENED TO DISRUPT TRADE AND, INDEED, THE INDUS CIVILIZATION.

PETRIE: ALL OF A SUDDEN, IF YOU DON'T GET A SURPLUS ONE YEAR, YOU CAN PROBABLY COMPENSATE FOR IT.

IF YOU DON'T GET A SURPLUS FOR A SECOND YEAR, YOU MIGHT BE ABLE TO COMPENSATE FOR IT, BUT IF IT KEEPS GOING ON AND ON AND ON, THEN THE ECONOMY SORT OF HAS TO CHANGE.

AND I SUSPECT THAT WOULD HAVE RESULTED IN A SOCIAL INSTABILITY.

MAYBE THE CITY STARTS TO COME UNDER STRAIN IN THIS SOCIAL STRIFE, AND THE URBAN FABRIC STARTS TO BREAK DOWN.

MORE AND MORE, THE THINKING TOWARDS CLIMATIC, ENVIRONMENTAL CHANGES.

A BIG DROUGHT THAT STRUCK 2200 BC, AND THE DRYING UP OF THE SARASWATI RIVER IN THE EASTERN REGION OF THIS CIVILIZATION.

RIZVI: ONE OF THE MOST STRIKING THINGS TO ME IS THAT SOMEHOW THAT ABILITY TO CONTROL AND ORGANIZE LARGE LANDSCAPES HAS CRUMBLED.

THERE'S A SHIFT IN BELIEF PATTERNS, AND THAT SHIFT ALTERS THE WAYS IN WHICH PEOPLE LIVE, AND THAT'S WHEN YOU BEGIN TO SEE ALL OF THIS BREAK DOWN.

NARRATOR: THE COLLAPSE OF ANY CIVILIZATION IS NEVER A SIMPLE STORY.

HOW DID THE WORLD OF THE INDUS PEOPLE IMPLODE?

WHEN ITS CITIES WERE FIRST EXCAVATED, SKELETONS WERE FOUND AMONG THE RUINS.

THEY WERE DATED TO THE FINAL PHASE OF THE CIVILIZATION.

SOME WERE BURIED IN CEMETERIES.

OTHERS APPEARED TO HAVE DIED WHERE THEY FELL.

BIOARCHAEOLOGIST GWEN SCHUG HAS FOUND A CLUE TO WHAT HAPPENED BY STUDYING ONE PARTICULAR SPECIMEN.

SCHUG: THIS SKELETON WAS THE FIRST INDIVIDUAL THAT WE DISCOVERED TO HAVE LEPROSY FROM THE INDUS CIVILIZATION, AND THE PRESENCE OF LEPROSY IN SOUTH ASIA AT THAT TIME WAS PREVIOUSLY UNKNOWN.

IT WAS THE OLDEST EVIDENCE OF LEPROSY IN THE WORLD BY ABOUT 1,400 YEARS.

WE FIND EVIDENCE FOR MYCOBACTERIAL INFECTION, LEPROSY, AND POSSIBLY ALSO TUBERCULOSIS.

NARRATOR: THIS WAS THE DOWNSIDE OF LONG-DISTANCE TRADE.

IT OPENED A DOOR FOR NEW PATHOGENS TO ENTER THE HUMAN POPULATION-- INFECTIOUS DISEASES, SPREAD BY CLOSE CONTACT-- THE PRICE OF CIVILIZATION.

THE BONES ALSO REVEAL THE DIET OF THE INDUS PEOPLE COLLAPSED AT THIS TIME.

SCHUG: WE START TO SEE EVIDENCE FOR VITAMIN-C DEFICIENCY.

THEIR BASIC NUTRITIONAL REQUIREMENTS COULDN'T BE MET.

THE FACT THAT THIS IS ALSO PRESENT IN VERY YOUNG INFANTS, RIGHT AROUND THE TIME OF BIRTH, DEMONSTRATES THAT PREGNANT WOMEN WERE NOT ABLE TO GET THEIR BASIC NEEDS MET FOR FOOD.

ALL OF THE FOODSTUFFS AND DIFFERENT PRODUCTS THAT WERE COMING IN AND OUT OF THE CITY, IT'S NOT HAPPENING ANYMORE.

NARRATOR: TRADE BROUGHT DISEASE.

DISEASE DISRUPTED THE SUPPLY OF FOOD.

THAT LED TO A BREAKDOWN IN SOCIAL ORDER.

THIS IS HOW CIVILIZATION FALLS.

SCHUG: WE FIND THAT THE PREVALENCE OF INTERPERSONAL VIOLENCE CLIMBS WITHIN THE SKELETAL MATERIAL THAT'S AVAILABLE, TO ABOUT 50% OF THE INDIVIDUALS.

A LARGE NUMBER OF THE CRANIA ARE IMPACTED BY TRAUMATIC INJURIES.

IT SORT OF PAINTS A PICTURE OF THE EXPERIENCE OF THAT LOSS OF SOCIAL CONTROL.

WHAT'S FASCINATING IS YOU CAN LOOK BACK OVER HISTORY AND LOOK AT THE COLLAPSE OF CIVILIZATIONS, AND THEY FOLLOW THIS SIMILAR PATTERN.

BOTSMAN: MOST RECENTLY, WE'VE SEEN THIS IN THE FINANCIAL CRASH, IN THAT YOU HAVE A SYSTEM THAT PEOPLE HAVE CONFIDENCE IN, AND THEN SOMETHING GOES WRONG.

SOMEONE BEHAVES BADLY, SOMEONE BECOMES GREEDY, AND THE FIRST THING TO GO IS THE CONFIDENCE.

[PEOPLE SHOUTING] BOTSMAN: AND THEN QUICKLY, IT'S LIKE A HOUSE OF CARDS.

THE WEAKEST LINK IN ANY SOCIETY IS US.

NARRATOR: OVER A 200-YEAR PERIOD, THE INDUS VALLEY'S VAST INTERNATIONAL TRADE NETWORK FELL APART.

ROSE: AS THE INDUS VALLEY CIVILIZATION IS COLLAPSING, THERE ARE REVERBERATIONS ACROSS THE ENTIRE REGION.

SO HERE IN THE ANCIENT KINGDOM OF NORTHERN OMAN, WE SEE A PROFOUND SOCIAL CHANGE, AND IT'S EXEMPLIFIED HERE AT THIS SITE.

DURING THE HEYDAY OF THE INDUS VALLEY, DOWN THERE ON THE COAST, THERE WAS A THRIVING VILLAGE OPENLY TRADING WITH THEIR NEIGHBORS ACROSS THE SEA.

AND AS THE INDUS VALLEY DECLINES, THAT SETTLEMENT MOVES UP TO THE TOP OF THIS MESA, THIS NATURAL CITADEL.

THEY'RE HUNKERING DOWN, AND THEY'RE BUILDING WALLS HERE TO PROTECT THEMSELVES, SO THE TRUST THAT WAS TYING THE WHOLE NETWORK TOGETHER IS BEGINNING TO UNRAVEL.

NARRATOR: WITHIN A SHORT TIME, THE WORLD OF THE INDUS PEOPLE TURNED TO DUST; TRADING CENTERS ABANDONED, CITIES RUINED, ITS LEGACY FORGOTTEN.

ROSE: CIVILIZATIONS ARE, ESSENTIALLY, SOCIAL EXPERIMENTS AND LARGE GROUPS OF PEOPLE LIVING TOGETHER, BEING CIVIL WITH ONE ANOTHER, BUT THERE'S A NATURAL EBB AND FLOW TO THIS PROCESS.

INEVITABLY, AT SOME POINT, ALL CIVILIZATIONS RISE AND FALL.

SO IT'S OUR JOB TO RIDE OUT THESE SOCIAL TRANSFORMATIONS AND BUILD ON THE BEST IDEAS OF OUR ANCESTORS.

NARRATOR: TRADE HAS ALWAYS BEEN A TRIGGER OF CHANGE.

[MAN SHOUTS] NARRATOR: IT ENCOURAGES US TO COME TOGETHER, TO EXCHANGE THINGS... TO SHARE IDEAS... TO CREATE SOCIETIES BUILT ON COOPERATION, TRUST, PEACE.

THIS WAS TRUE FOR THE FIRST CIVILIZATIONS... AND IT'S STILL TRUE TODAY.

TRADE-- THE DRIVING FORCE OF CIVILIZATION.

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