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Humans...
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Without a doubt, the smartest animal on earth.
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Yet we're unmistakably tied to our ape origins.
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Millions of years ago,
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we were apes, living ape lives in Africa.
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So how did we get from that...
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to this?
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What happened?
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What set us on the path to humanity?
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The questions are huge.
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But at last, there are answers.
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More than six million years ago we took that first step
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to separate from the apes.
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DON JOHANSON: We see the launching of the career
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that ultimately led to Homo sapiens.
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And three million years ago, we see the roots of our big brain
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begin to take hold in a tiny creature
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more like a chimp than a human.
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PETER deMENOCAL: The frontier of human evolution
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is really being brought to this razor sharp edge.
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And we now know that for millions of years,
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many different humanlike species lived together
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on the planet until one day there was only us,
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Homo sapiens,
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the most complex, adaptable animal on earth.
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So how did we get this way, and why?
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A radical new theory reveals how episodes of cataclysmic change
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forced our ancestors to adapt or die.
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MARK MASLIN: I think we
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should actually look to our proud ancestry
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and how we evolved in East Africa and say,
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That's how we survived that.
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We can survive the future.
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So get ready for a ride through millions of years
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of our history.
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It's the story of becoming human-- our story--
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right now, on NOVA.
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Millions of years ago, on the plains of Africa,
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a momentous event took place.
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Apes that had walked on four legs stood up and walked on two.
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Eventually, this change in posture
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would be followed by a change in their brains.
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Somehow, over time, they would become us.
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We know it happened, but we've never known when or why.
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Until now.
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In the Sahara desert, a six-million-year-old fossil
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called Toumai may hold the secret
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of how we first walked upright.
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: We are writing the first chapter of human evolution.
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We are very close to the beginning.
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Very close.
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And the fossilized bones of a child
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from three-and-a-half million years ago,
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hint to us about the beginnings of human thought.
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We're discovering how many different human species
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lived on earth at the same time and why all but one died out.
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We, Homo sapiens, are the first ever to be alone.
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So what powered our evolution?
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Why did we become human?
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Scientists are scouring the most remote parts of Africa
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for clues.
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The search for answers begins here, in the Afar--
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northeastern Ethiopia.
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It's part of the Great Rift Valley, a deep cut in the earth
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where geologic forces are ripping Africa apart.
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Millions of years of history are brought to the surface
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in layers of exposed rock.
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It's hot and desolate.
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Dangerous, too.
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Ancient rivalries and modern weapons have turned the Afar
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into a no-man's-land of simmering conflict.
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But Zeray Alemseged has made this forbidding place
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his life's work.
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He's searching for the fossilized traces
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of our earliest human ancestors.
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The fossil bones of animals like antelopes, elephants
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and pigs are abundant.
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But the fossils of our ancestors are extremely rare.
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Then, in a stroke of luck, Zeray makes the find of a lifetime,
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a find that illuminates our origins in a unique way.
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On that afternoon, we decided to survey this hillside
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and the first thing that was spotted was a cheekbone
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of the face.
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It was a face so tiny it had to be a baby.
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But not a baby chimp; he could tell that from its shape.
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The skull was embedded in sandstone,
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but as Zeray turned it over, he could see more bones inside.
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Everything was squashed
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against the base of the skull
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and completely covered by the sandstone block.
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Clues to the age of the fossil
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came from a distinctive feature in the landscape--
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white bands of volcanic ash.
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That is volcanic ash, dated to 3.4 million years ago.
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If the volcanic ash is 3.4 million years old,
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Zeray's fossil, which was lying just above it,
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must be younger.
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It was a child from the dawn of human evolution,
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about 3.3 million years ago.
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Zeray called the baby Selam, the Ethiopian word for peace.
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Then he set off on a quest to unravel her many mysteries.
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Her journey began a very, very long time ago.
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Imagine the entire span of recorded human history
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taking us back to the Egyptian pyramids,
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about 5,000 years.
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Double it-- 10,000 years ago--
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when plants were domesticated and agriculture begins.
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Double it again, to the time when Ice Age hunters
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paint stunning images on cave walls.
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And keep doubling six more times,
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taking us back 1.3 million years,
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when the first creature who really looked like us
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hunted on the plains of Africa and then keep traveling back
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another two million years
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and only then do we arrive in the time when Selam lived
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in Ethiopia nearly three- and-a-half million years ago.
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What were Selam and her family like?
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What kind of world did they live in?
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The answers are hidden in their fossil bones.
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Addis Ababa, Ethiopia.
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Zeray's home.
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He is one of a whole new generation of African scientists
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trying to unravel the mysteries of human origins.
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Zeray has brought his precious fossil here
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to the National Museum.
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His challenge is to release her from the tomb of sandstone
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in which her bones are encased.
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He quickly identifies her.
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She is from a species considered by most scientists
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to be an ancient ancestor.
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Australopithecus afarensis.
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A small, chimplike creature who walked on two legs.
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This is the same species as the famous Lucy
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discovered in the 1970s by Don Johanson.
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Lucy was terribly important because she was really
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an amalgam of different characteristics
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of ape and human.
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I think specimens like Selam and Lucy are
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extraordinary simply because you can look at them and see
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evolution in the making.
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But seeing Evolution in the making Will take some work.
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Selam's fossilized bones are solid rock
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held together by a mesh of soft sandstone.
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It has to be painstakingly removed.
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We spent hours, hours and hours and days and years and years
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and I removed the sand grains, grain by grain,
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working every day.
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He's been at it for eight long years,
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but the payoff has been amazing.
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As the work progressed,
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Zeray revealed an almost complete skull...
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and tucked beneath it was nearly her entire spine,
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along with both shoulder blades.
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Other bones were found nearby.
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An almost complete foot.
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This is the kneecap.
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The tibia here.
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Never before had a child's skeleton been found
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so ancient and so complete.
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Her bones would fit in a shoebox,
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but they speak volumes about her life.
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For example, to find out how old she was when she died,
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Zeray looked at her teeth.
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But not the baby teeth visible in her jaw...
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the adult teeth growing inside the bone as seen in a CT scan.
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From that, we know Selam died at age three.
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Like Lucy, she testifies to a crucial step in our evolution.
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Unlike apes, these creatures walked upright,
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as the first fossil Don Johanson found clearly revealed.
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It was sticking out of the ground like that.
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And I gently tapped it with my sneaker
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and this is what fell out of the ground.
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And it is the... this is your... the top end of your shinbone.
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So the kneecap would sit right in here.
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And very close by in two pieces I found this bone.
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And when you put them together
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and you see how they move and articulate,
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it has all the hallmarks of an upright person.
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Other bones confirm that Lucy walked on two legs like us.
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This is Lucy's pelvis.
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And you can see how different a chimpanzee is.
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And the reorientation of these hip bones--
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in a chimp they're facing straight forward.
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So here is... this is what everybody is sitting on
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in their living room right now.
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So they're not identical,
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but clearly these two resemble each other
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much more closely, right, than either one of these
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resembles the pelvis of an ape.
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From the waist down, Lucy was like us.
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From the waist up, she and her kind were all ape.
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Selam's skeleton is the same, with chimplike shoulder blades
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giving her the range of motion
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needed for climbing and swinging.
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These ancient creatures must have spent time in the trees,
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possibly sleeping there at night to keep away from predators,
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but walking upright on the ground during the day.
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They were at home in two worlds.
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What was their environment like?
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It must have been very different
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from the Great Rift Valley of today.
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Across the border in Kenya
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is one of the hottest and most barren places on earth,
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a vast expanse of volcanic rock and burning desert.
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That's how it is now.
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But there's good evidence
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that for most of its history it was very different.
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Researchers braving temperatures over 100 degrees
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are seeing signs of a dramatic transformation
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here in the Suguta Valley.
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YANNICK GARDIN: The Suguta Valley was entirely covered in water,
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up to an elevation of about 580 meters.
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So you can imagine that all this valley
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was filled by a huge lake.
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A huge lake that's deeper than any of the Great Lakes.
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In fact, the entire African continent used to be
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a lot wetter than it is today.
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Many millions of years ago, long before Selam and Lucy,
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Africa was a wet, tropical environment
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covered with rainforest.
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This is where the ancestors of Selam and Lucy lived.
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They probably looked a lot like chimps.
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But then, rica started to gradually dry out.
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The rainforest began to shrink.
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By Selam's time, three to four million years ago,
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the Great Rift Valley was a mosaic
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of different environments.
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We know that from the fossils of the animals that lived here.
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Their bones litter the ground.
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This is a canine of a hippopotamus,
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so this is probably the skeleton of a hippopotamus.
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How can one find a hippo in this type of environment?
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Nice antelope here.
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The fossils tell the story of a vanished landscape.
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This is the lower jaw of an antelope.
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Three million years ago, the Rift Valley was a patchwork
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of grassy plains, scattered woodlands, lakes and rivers.
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Definitely very different from what we see here today.
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Wow, a nice pig here.
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As their environment changed,
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scientists believe our ancestors changed, too.
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They had been creatures
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who spent most of their time in trees,
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like chimps and orangutans today.
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But as their forests shrank, some of them developed the trait
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that we take for granted:
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bipedalism-- walking on two legs.
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This is one of the defining characteristics of humans.
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But how did bipedalism develop, and why?
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BRIAN RICHMOND: Bipedalism is such an unusual trait.
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There's no other mammal
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that habitually walks on two legs like we do.
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Because it's unique, it's hard to figure out why it happened.
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There are a lot of theories.
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One of them is that they stood up
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to be able to see over tall grass.
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Another theory-- they stood up to be able to pick fruits
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off of the low branches of trees the way chimpanzees do today.
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Another theory states
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that they stood up to cool more efficiently,
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so that we don't have as much sun beating on
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so much of our body.
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DANIEL LIEBERMAN: I think the most compelling idea,
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the most compelling hypothesis, is that it saved us energy.
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And energy is crucial to survival.
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Let's go back to the dense forest,
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home to our ancient ape ancestors ten million years ago.
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Like many apes today, they were perfectly suited
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to a life in the trees.
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They're very good at climbing in trees.
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They're phenomenal at climbing in trees.
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On the ground,
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these ancestral apes could probably walk on two legs
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for short distances if they had to carry something.
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Fantastic climbers, but also able to walk and run
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rapidly and effectively but not economically.
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Walking was tiring, but they didn't have to walk far.
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But if you're a chimp and you only walk
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two to three kilometers a day,
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it doesn't really mean much,
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it's not going to have that much of an effect
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on your energy budget.
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But energy demands would change
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as the forest started to disappear.
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Our ape ancestors had to walk more.
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They have to go farther
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to get from one fruit patch
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to another fruit patch, for example.
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Dan Lieberman is an expert on bipedalism.
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He believes that walking on two legs evolved
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because it saved energy.
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When you compare the energy consumption of humans to chimps,
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there's no contest.
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A chimp is an energy glutton.
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It spends an enormous amount of energy--
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about four times as much energy as a human walking.
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Whether it walks on four legs...
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or two...
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a chimp can't compete with the human gait.
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It's poorly designed to withstand the forces of gravity.
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It has to spend a lot of muscular effort to keep itself
302
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from collapsing into a little pile of Chimpness Or whatever
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with each step.
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According to Lieberman, small anatomical differences
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created large energy savings,
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setting our ancestors on the path to bipedalism,
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a path that would eventually lead to us.
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But how long did it take?
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When Lucy's kind were first discovered,
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many people thought they were the so-called Missing link
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between apes and humans.
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But the science of genetics has transformed our understanding
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with a technique called The molecular clock.
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Today, scientists can compare DNA from closely related species
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to find out how long ago they split from a common ancestor.
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MARK STONEKING: It's just a very simple idea
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that the rate of change in DNA sequences
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is more or less constant over time.
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And that's an extraordinarily powerful concept
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00:21:52,570 --> 00:21:55,839
because it means that you have a way of determining
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00:21:55,907 --> 00:21:59,141
when two species last shared a common ancestor.
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Living forms evolve because DNA sometimes spontaneously changes
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as it copies itself.
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These changes happen at a surprisingly regular rate.
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By counting the differences between the genetic code
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00:22:20,594 --> 00:22:23,863
of chimps and humans, we can calculate
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how long they've been evolving away from each other.
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The dates that one almost always gets are around
329
00:22:31,202 --> 00:22:32,770
five to seven million years ago
330
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for when humans and chimpanzees last shared a common ancestor.
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Here was proof that humans diverged from the apes
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00:22:40,377 --> 00:22:47,416
much earlier than we thought, about six million years ago.
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00:22:47,484 --> 00:22:51,519
It shows Lucy and Selam weren't one step removed from chimps
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00:22:51,587 --> 00:22:54,055
but many.
335
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They may even be closer to us
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than to the first human ancestor.
337
00:22:59,726 --> 00:23:04,430
So what came before Lucy and Selam?
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Who was our earliest ancestor?
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Until the 1990s, the fossil record was blank.
340
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Fossil hunters combed East Africa's Great Rift Valley
341
00:23:24,115 --> 00:23:26,148
but could only find small fragments
342
00:23:26,216 --> 00:23:28,951
older than four million years.
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00:23:31,354 --> 00:23:35,356
Then, in 1997, a French anthropologist
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called Michel Brunet decided to look somewhere else.
345
00:23:45,032 --> 00:23:49,369
: We decided to go to Africa, but to the west,
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to the west of the Great Rift.
347
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1,600 miles to the west at the edge of the Sahara desert
348
00:23:58,512 --> 00:24:02,046
in northern Chad.
349
00:24:02,114 --> 00:24:05,349
BRUNET : Obviously, if you only go to the field in East Africa,
350
00:24:05,417 --> 00:24:11,121
then you are going to find fossils only in East Africa.
351
00:24:11,188 --> 00:24:13,023
This was the situation.
352
00:24:15,758 --> 00:24:20,429
Michel was looking in a place where the few animal fossils
353
00:24:20,497 --> 00:24:24,132
he turned up were all around six million years old.
354
00:24:28,837 --> 00:24:31,939
No one expected any humanlike fossils to be found
355
00:24:32,007 --> 00:24:34,606
in a layer that ancient.
356
00:24:34,674 --> 00:24:37,143
: And everyone said,
357
00:24:37,210 --> 00:24:41,179
No. There just aren't any fossils there.
358
00:24:41,247 --> 00:24:44,549
Michel was not to be deterred.
359
00:24:44,616 --> 00:24:48,219
He was stubborn, many thought to the point of madness.
360
00:24:48,287 --> 00:24:52,122
He and his team spent years searching the desert
361
00:24:52,190 --> 00:24:54,624
for signs of our ancestors.
362
00:24:54,692 --> 00:24:58,062
And year after year, they came up empty.
363
00:24:58,128 --> 00:25:03,466
Then, on their 26th expedition, in 2001,
364
00:25:03,534 --> 00:25:06,667
they found a smashed, misshapen skull...
365
00:25:09,171 --> 00:25:11,505
around six million years old.
366
00:25:14,543 --> 00:25:17,743
They called it Sahelanthropus tchadensis.
367
00:25:17,811 --> 00:25:20,246
There were no bones apart from the skull.
368
00:25:23,117 --> 00:25:27,619
Could it be a human ancestor? Or just another ape?
369
00:25:27,687 --> 00:25:30,889
The skull was so deformed it was difficult to tell.
370
00:25:32,991 --> 00:25:35,692
Michel would have to reconstruct it.
371
00:25:40,465 --> 00:25:42,565
His first step was to take the skull,
372
00:25:42,632 --> 00:25:46,034
now nicknamed Toumai, to a particle accelerator
373
00:25:46,103 --> 00:25:51,272
in Grenoble, France, to use its powerful X-ray scanner.
374
00:25:53,909 --> 00:25:57,245
Over a thousand pictures of the fossil were taken
375
00:25:57,313 --> 00:26:01,081
to build a 3-D image of the crushed skull.
376
00:26:01,149 --> 00:26:05,285
Using the virtual image, the skull could be restored
377
00:26:05,353 --> 00:26:08,587
to its original shape.
378
00:26:08,654 --> 00:26:12,390
It was then reproduced by a type of 3-D printer
379
00:26:12,458 --> 00:26:16,893
equipped with lasers which harden plastic.
380
00:26:16,961 --> 00:26:19,763
When it finally rose from its bath,
381
00:26:19,831 --> 00:26:24,400
the cast of Toumai's skull was ready for detailed study.
382
00:26:28,939 --> 00:26:33,475
The cast allows Michel to answer an important question.
383
00:26:33,543 --> 00:26:36,978
Did this ancient creature walk on two legs
384
00:26:37,046 --> 00:26:42,315
millions of years before Lucy or Selam?
385
00:26:42,383 --> 00:26:45,385
It's how the skull connects to the spine
386
00:26:45,453 --> 00:26:48,621
that provides the vital clue.
387
00:26:50,190 --> 00:26:54,460
And Michel could infer that from the shape of Toumai's skull.
388
00:26:54,528 --> 00:26:57,595
If Toumai's skull is set on the neck of an ape
389
00:26:57,663 --> 00:27:02,033
that walks on all fours, his eyes point downward.
390
00:27:02,100 --> 00:27:04,302
That can't be right.
391
00:27:04,370 --> 00:27:06,637
Set on the upright spine of a biped,
392
00:27:06,704 --> 00:27:09,305
his eyes point straight ahead.
393
00:27:09,373 --> 00:27:15,109
For Michel, this proved Toumai walked upright.
394
00:27:15,178 --> 00:27:18,880
: Anatomically speaking,
395
00:27:18,947 --> 00:27:22,683
he had the receding back skull of a biped.
396
00:27:22,750 --> 00:27:26,353
The back of his skull is not that of a gorilla,
397
00:27:26,421 --> 00:27:29,356
like some people are trying to say.
398
00:27:29,424 --> 00:27:31,791
No, not at all.
399
00:27:31,858 --> 00:27:33,726
All you have to do is look.
400
00:27:35,796 --> 00:27:37,730
Some scientists still question
401
00:27:37,798 --> 00:27:41,632
whether Toumai was really a biped.
402
00:27:41,700 --> 00:27:44,968
But if Michel is right, his six-million-year-old fossil
403
00:27:45,036 --> 00:27:48,471
is a good candidate for the first human ancestor.
404
00:27:51,609 --> 00:27:54,277
Discoveries like this are changing the way
405
00:27:54,345 --> 00:27:56,446
we see human evolution.
406
00:27:59,282 --> 00:28:03,352
Scientists used to have a simple idea:
407
00:28:03,420 --> 00:28:06,120
the growth of open grasslands forced our ancestors
408
00:28:06,188 --> 00:28:08,756
out of the trees.
409
00:28:08,824 --> 00:28:13,060
They became bipeds, and in short order, brain size increased.
410
00:28:14,261 --> 00:28:17,330
Human evolution took off.
411
00:28:17,397 --> 00:28:20,299
We were on our way to becoming human.
412
00:28:20,367 --> 00:28:25,871
That simple idea prevailed for more than a century.
413
00:28:25,939 --> 00:28:29,108
Darwin thought that we left the trees,
414
00:28:29,175 --> 00:28:31,075
walked on the ground upright,
415
00:28:31,143 --> 00:28:33,711
freed our hands, made tools, got big brains,
416
00:28:33,779 --> 00:28:35,046
reduced our canines and so on
417
00:28:35,114 --> 00:28:37,414
all at the same time.
418
00:28:37,482 --> 00:28:40,783
But walking upright may not have automatically led
419
00:28:40,851 --> 00:28:43,552
to big brains at all.
420
00:28:43,620 --> 00:28:48,990
From Toumai to Selam, both bipeds, brains stayed small.
421
00:28:49,058 --> 00:28:52,794
And they weren't the only ones.
422
00:28:52,862 --> 00:28:57,397
Over millions of years there was a profusion of upright walkers
423
00:28:57,465 --> 00:29:01,201
with complicated names and chimp-sized brains...
424
00:29:02,670 --> 00:29:04,737
Like Orrorin tugenensis.
425
00:29:04,805 --> 00:29:06,538
What we're seeing is a florescence of species,
426
00:29:06,606 --> 00:29:08,141
multiple species.
427
00:29:08,208 --> 00:29:09,575
They're probably subtly different from each other.
428
00:29:09,643 --> 00:29:12,111
NARRATOR: Ardipithecus ramidus.
429
00:29:12,179 --> 00:29:13,911
But it's important to recognize
430
00:29:13,979 --> 00:29:16,847
that there are not major differences among these species.
431
00:29:16,915 --> 00:29:20,184
NARRATOR: Australopithecus africanus.
432
00:29:20,252 --> 00:29:22,618
They were all bipeds, big snouts,
433
00:29:22,686 --> 00:29:24,788
more or less chimp-sized brains.
434
00:29:24,855 --> 00:29:29,058
NARRATOR: Kenyanthropus platyops.
435
00:29:29,126 --> 00:29:32,961
This way of life, this suite of adaptations,
436
00:29:33,029 --> 00:29:35,197
lasted for millions of years.
437
00:29:35,265 --> 00:29:40,134
Small-brained bipedal apes were extremely successful.
438
00:29:40,202 --> 00:29:43,136
Debates rage among scientists
439
00:29:43,204 --> 00:29:47,640
about which one eventually led to us.
440
00:29:47,708 --> 00:29:52,712
But as a group, they flourished for about 25 times longer
441
00:29:52,780 --> 00:29:55,080
than we've been around.
442
00:29:56,816 --> 00:29:59,117
They survived and thrived
443
00:29:59,185 --> 00:30:03,955
as brain size flat-lined for almost four million years.
444
00:30:11,363 --> 00:30:14,964
But that doesn't mean nothing changed.
445
00:30:16,866 --> 00:30:20,235
There's evidence that the seeds of our humanity were growing
446
00:30:20,303 --> 00:30:23,938
in these apelike creatures.
447
00:30:24,005 --> 00:30:26,708
One key difference between humans and apes
448
00:30:26,775 --> 00:30:31,645
is the length of childhood.
449
00:30:31,713 --> 00:30:34,113
But what do we know about the childhood
450
00:30:34,181 --> 00:30:36,750
of our early ancestors?
451
00:30:36,818 --> 00:30:38,084
We knew all about
452
00:30:38,151 --> 00:30:39,251
the adult individuals,
453
00:30:39,319 --> 00:30:41,387
but we didn't know much about the children.
454
00:30:43,690 --> 00:30:47,791
The brains of baby chimps have an early growth spurt.
455
00:30:47,859 --> 00:30:51,762
They're almost fully formed by age three.
456
00:30:51,830 --> 00:30:54,998
In humans that growth spurt is slower,
457
00:30:55,066 --> 00:31:01,103
and it takes nearly two decades for our brains to fully mature.
458
00:31:01,171 --> 00:31:08,443
But what about Selam's brain, 3.3 million years ago?
459
00:31:08,511 --> 00:31:12,380
Her skull tells us all we need to know.
460
00:31:12,447 --> 00:31:18,317
We have her milk teeth and her adult teeth,
461
00:31:18,386 --> 00:31:23,054
which give us her age-- three years old.
462
00:31:23,122 --> 00:31:26,324
And we have a cast of the inside of her skull,
463
00:31:26,393 --> 00:31:29,960
which tells us about her brain.
464
00:31:30,028 --> 00:31:34,064
When you have this you can directly measure
465
00:31:34,132 --> 00:31:40,904
how much of the brain was formed at age three.
466
00:31:40,971 --> 00:31:43,640
From other fossils, we know how large
467
00:31:43,708 --> 00:31:48,210
Selam's brain would have been as an adult.
468
00:31:48,278 --> 00:31:51,345
So Zeray could calculate how much of her brain
469
00:31:51,414 --> 00:31:55,549
was already formed by age three, when she died.
470
00:31:57,652 --> 00:32:01,154
He knows what the answer would be for a chimp.
471
00:32:01,222 --> 00:32:03,823
By age three, a chimpanzee would have
472
00:32:03,891 --> 00:32:05,991
over 90% of the brain formed.
473
00:32:06,059 --> 00:32:12,831
But Selam's brain was only around 75% of its adult size,
474
00:32:12,899 --> 00:32:15,867
suggesting it was growing up slower.
475
00:32:21,004 --> 00:32:23,940
Childhood would have been her time to learn,
476
00:32:24,007 --> 00:32:28,010
to learn the survival strategies her family group needed
477
00:32:28,078 --> 00:32:30,278
to live in a dangerous world.
478
00:32:33,917 --> 00:32:39,019
Perhaps this set the stage for our longer human childhood,
479
00:32:39,087 --> 00:32:41,456
when culture is handed down.
480
00:32:45,327 --> 00:32:49,261
But is there any other evidence Selam's brain was becoming
481
00:32:49,329 --> 00:32:51,931
more human and less ape?
482
00:32:53,934 --> 00:32:58,870
To find out, compare a human brain to a chimp's.
483
00:32:58,938 --> 00:33:01,038
TODD PREUSS: This is the brain of our closest relative,
484
00:33:01,106 --> 00:33:02,474
the chimpanzee brain.
485
00:33:02,541 --> 00:33:05,275
It's slightly larger than you would expect
486
00:33:05,343 --> 00:33:07,744
of a typical primate for their body size.
487
00:33:07,812 --> 00:33:11,447
Not greatly so.
488
00:33:11,515 --> 00:33:14,617
Scientists look for clues to the evolution of the brain
489
00:33:14,685 --> 00:33:18,187
in the folds and furrows on its surface.
490
00:33:18,254 --> 00:33:22,457
One important structure is called the lunate sulcus.
491
00:33:22,524 --> 00:33:24,692
In chimpanzees as in many primates,
492
00:33:24,759 --> 00:33:28,895
there is this big, deep sulcus here, the lunate sulcus.
493
00:33:30,731 --> 00:33:35,102
The lunate sulcus is a deep furrow in a primate's brain.
494
00:33:35,169 --> 00:33:38,604
It divides parts of the brain related to vision
495
00:33:38,671 --> 00:33:40,939
from the rest of the neocortex,
496
00:33:41,008 --> 00:33:44,276
which is where more complex thought happens.
497
00:33:44,344 --> 00:33:48,446
The human brain doesn't have this deep furrow,
498
00:33:48,514 --> 00:33:52,317
and the neocortex is bigger than the vision structures,
499
00:33:52,385 --> 00:33:54,917
which have moved far to the back.
500
00:33:56,320 --> 00:34:00,456
So did Selam have the deep furrow and small neocortex
501
00:34:00,524 --> 00:34:02,391
of a chimp?
502
00:34:02,458 --> 00:34:04,926
Or had something changed?
503
00:34:04,994 --> 00:34:09,364
Brains don't fossilize, but her remarkably complete skull
504
00:34:09,432 --> 00:34:12,199
provides a way to see some of the different structures
505
00:34:12,267 --> 00:34:13,967
of her brain.
506
00:34:15,803 --> 00:34:19,672
A cast of the brain case, called an endocast,
507
00:34:19,740 --> 00:34:23,809
preserves the impression of the brain's surface.
508
00:34:25,178 --> 00:34:30,048
Ralph Holloway has a collection of 300 brain endocasts
509
00:34:30,115 --> 00:34:32,050
from many of our ancestors.
510
00:34:32,117 --> 00:34:37,487
What a paleoneurologist like myself will be looking for
511
00:34:37,555 --> 00:34:41,224
are those indications on the endocast that might suggest
512
00:34:41,292 --> 00:34:43,493
reorganization taking place.
513
00:34:43,561 --> 00:34:44,960
And that's why things
514
00:34:45,028 --> 00:34:49,398
like the so-called infamous lunate sulcus becomes important.
515
00:34:51,435 --> 00:34:54,969
Ralph claims that as chimplike ancestors evolved
516
00:34:55,037 --> 00:34:57,037
into creatures like Selam and Lucy,
517
00:34:57,105 --> 00:35:01,375
the lunate sulcus, the furrow marking the vision structures,
518
00:35:01,443 --> 00:35:06,679
moved back, making room for a larger neocortex,
519
00:35:06,746 --> 00:35:09,848
the thinking part of the brain.
520
00:35:09,916 --> 00:35:12,384
If you look carefully,
521
00:35:12,452 --> 00:35:15,387
what you've got here is a depression
522
00:35:15,455 --> 00:35:19,056
that could very likely be the lunate sulcus.
523
00:35:19,124 --> 00:35:24,395
And so that suggests, then, by australopithecine times,
524
00:35:24,463 --> 00:35:27,162
that, you know, you're having a beast that is simply smarter
525
00:35:27,230 --> 00:35:30,031
than present-day chimpanzees.
526
00:35:32,668 --> 00:35:34,169
If that's the case,
527
00:35:34,237 --> 00:35:36,537
although still the size of a chimp's,
528
00:35:36,605 --> 00:35:40,341
Selam's brain had been rewired.
529
00:35:40,409 --> 00:35:43,377
But there was a long way to go.
530
00:35:49,049 --> 00:35:52,618
She and her kind were still very apelike.
531
00:35:56,689 --> 00:35:59,524
It would take another million years
532
00:35:59,592 --> 00:36:03,027
for the seeds of humanity contained in Selam's tiny frame
533
00:36:03,095 --> 00:36:04,662
to bear fruit.
534
00:36:08,133 --> 00:36:11,801
It's a time still shrouded in mystery.
535
00:36:11,869 --> 00:36:14,170
For almost half a million years,
536
00:36:14,238 --> 00:36:18,306
the fossil record is virtually silent.
537
00:36:18,374 --> 00:36:23,646
But in this blank period, something is happening.
538
00:36:23,713 --> 00:36:27,048
In 2.5 million-year-old layers,
539
00:36:27,116 --> 00:36:31,952
scientists begin to find something new.
540
00:36:32,020 --> 00:36:34,621
We might be tempted to call them rocks,
541
00:36:34,689 --> 00:36:37,790
but someone was shaping them.
542
00:36:37,858 --> 00:36:41,961
They are the first stone tools.
543
00:36:42,029 --> 00:36:45,997
The way we know this is a tool instead of just a broken rock
544
00:36:46,065 --> 00:36:49,100
is that it's broken in a very particular way,
545
00:36:49,168 --> 00:36:52,003
breaking a flake off this way, that way, this way,
546
00:36:52,071 --> 00:36:53,336
back and forth.
547
00:36:53,404 --> 00:36:58,508
So there is a method behind how this rock was broken
548
00:36:58,576 --> 00:37:00,608
in order to make it into a tool,
549
00:37:00,677 --> 00:37:04,079
and it's not a random method.
550
00:37:04,147 --> 00:37:05,681
It's considered unlikely
551
00:37:05,748 --> 00:37:10,751
they were made by Australopithecus, Lucy's kind.
552
00:37:10,819 --> 00:37:13,020
RICHMOND: Australopithecus was around
553
00:37:13,088 --> 00:37:16,290
for a couple of million years and did not make stone tools.
554
00:37:16,357 --> 00:37:21,527
But if not Lucy's kind, then who?
555
00:37:21,595 --> 00:37:26,531
The gap in the fossil record makes it difficult to say,
556
00:37:26,599 --> 00:37:28,500
but that's not surprising.
557
00:37:28,568 --> 00:37:33,738
Tools preserve easily, bones much less so.
558
00:37:35,173 --> 00:37:39,475
Finally, the skulls of a new creature begin to turn up.
559
00:37:39,543 --> 00:37:42,112
Is this the toolmaker?
560
00:37:42,180 --> 00:37:45,114
The skulls are different from what came before.
561
00:37:45,182 --> 00:37:48,250
They represent the dawn of a new era
562
00:37:48,318 --> 00:37:51,753
beginning around two million years ago.
563
00:37:53,288 --> 00:38:00,127
This is our era, the era of the genus Homo, humans.
564
00:38:00,195 --> 00:38:04,063
The mysterious toolmaker, Homo habilis,
565
00:38:04,131 --> 00:38:07,366
is the first of these new creatures.
566
00:38:07,434 --> 00:38:08,800
We definitely have evidence
567
00:38:08,868 --> 00:38:13,905
that the stone tools were being used to break the long bones
568
00:38:13,973 --> 00:38:16,574
in order to get to the marrow inside the long bones.
569
00:38:16,642 --> 00:38:19,977
There were clear cut marks on the bones of turtles,
570
00:38:20,045 --> 00:38:23,680
crocodiles, big antelopes, little antelopes, even hippos--
571
00:38:23,749 --> 00:38:25,615
really big animals like hippos.
572
00:38:25,682 --> 00:38:29,285
So we know that meat had become a new important part
573
00:38:29,353 --> 00:38:32,655
of the diet of Homo habilis.
574
00:38:32,723 --> 00:38:36,858
NARRATOR: The first fossil to be called Homo habilis included
575
00:38:36,925 --> 00:38:41,929
21 bones of the hand and was nicknamed Handy Man.
576
00:38:41,997 --> 00:38:44,731
This little bone is the bone at the end of the thumb,
577
00:38:44,799 --> 00:38:48,201
and that little bone in Homo habilis, like in humans,
578
00:38:48,269 --> 00:38:49,636
is very broad.
579
00:38:49,703 --> 00:38:54,907
And the broad bone reflects having a broad pad on the thumb,
580
00:38:54,975 --> 00:38:59,378
with a lot of surface area for fine, precision grip.
581
00:38:59,445 --> 00:39:01,179
With newly dexterous hands,
582
00:39:01,247 --> 00:39:03,081
this creature could make better tools.
583
00:39:06,450 --> 00:39:09,919
But what was it like?
584
00:39:09,987 --> 00:39:12,288
The few skeletal bones that have been found
585
00:39:12,355 --> 00:39:15,391
indicate a creature much smaller than us,
586
00:39:15,458 --> 00:39:18,260
about the same size as Lucy and Selam's kind--
587
00:39:18,327 --> 00:39:23,965
Australopithecus-- three to four feet tall.
588
00:39:24,033 --> 00:39:27,902
Homo habilis was still apelike in many ways,
589
00:39:27,969 --> 00:39:30,237
but with a critical difference.
590
00:39:30,305 --> 00:39:32,372
RICHMOND: What we see in the evolution of Homo habilis
591
00:39:32,440 --> 00:39:35,674
is an expansion in the brain size
592
00:39:35,741 --> 00:39:37,509
compared to Australopithecus.
593
00:39:37,576 --> 00:39:40,179
So here is the skull of Australopithecus,
594
00:39:40,246 --> 00:39:41,980
and it has no forehead,
595
00:39:42,048 --> 00:39:43,881
it just has the straight slope behind the orbits.
596
00:39:43,949 --> 00:39:45,917
Whereas here in Homo habilis
597
00:39:45,984 --> 00:39:49,620
you see a sloping, elevated forehead.
598
00:39:49,688 --> 00:39:52,655
And in Australopithecus, the area behind the orbits
599
00:39:52,723 --> 00:39:56,492
is pinched in, also reflecting a small frontal region.
600
00:39:56,560 --> 00:39:57,760
In contrast, in Homo habilis,
601
00:39:57,829 --> 00:40:01,063
we see an expansion of that area behind the orbits
602
00:40:01,131 --> 00:40:04,766
that points to an expansion in the cognitive capabilities
603
00:40:04,835 --> 00:40:07,268
of higher functions,
604
00:40:07,336 --> 00:40:09,937
of the higher reasoning functions of the brain.
605
00:40:10,004 --> 00:40:14,975
It was an expansion equivalent to a doubling of brain volume.
606
00:40:15,043 --> 00:40:16,342
Once you go from something
607
00:40:16,409 --> 00:40:19,846
like 400 cc in australopithecines
608
00:40:19,913 --> 00:40:24,116
to, say, 700, 800 cc in Homo habilis, yes,
609
00:40:24,184 --> 00:40:29,120
you're getting a big increase in cognitive capacity.
610
00:40:29,188 --> 00:40:31,456
And along with his bigger brain,
611
00:40:31,523 --> 00:40:37,493
Homo habilis was starting to look a lot more human.
612
00:40:37,561 --> 00:40:39,795
The contours of fossil skulls
613
00:40:39,864 --> 00:40:42,264
allow reconstructionist Viktor Deak
614
00:40:42,332 --> 00:40:45,533
to reveal the faces of early human beings.
615
00:40:50,605 --> 00:40:54,275
Gone is the projecting snout of an ape.
616
00:40:54,343 --> 00:40:58,945
In Homo habilis, the face of humanity is emerging.
617
00:41:06,153 --> 00:41:11,155
This poses a great enigma.
618
00:41:11,223 --> 00:41:14,759
Why, after millions of years of flat-lining,
619
00:41:14,826 --> 00:41:18,828
did brain size and mental capacities suddenly take off?
620
00:41:24,667 --> 00:41:29,738
Two million years ago, what jump-started human evolution?
621
00:41:35,811 --> 00:41:39,380
Scientists all over Africa looked for clues.
622
00:41:41,716 --> 00:41:44,150
Here in Kenya they found some,
623
00:41:44,218 --> 00:41:48,054
at the southern end of the Great Rift Valley.
624
00:41:48,122 --> 00:41:51,023
It's a hotbed of tectonic activity
625
00:41:51,090 --> 00:41:56,295
where ancient layers are forced to the surface.
626
00:41:56,362 --> 00:42:02,400
Ten million years ago, Africa was a much wetter place,
627
00:42:02,468 --> 00:42:06,504
a tropical jungle which has been slowly drying out ever since.
628
00:42:10,074 --> 00:42:12,075
But these rocks in Kenya show
629
00:42:12,143 --> 00:42:15,610
that Africa's gradual drying trend was punctuated
630
00:42:15,678 --> 00:42:19,714
by bursts of wild climate fluctuation.
631
00:42:19,782 --> 00:42:24,319
Rick Potts is an expert in reading the rocks.
632
00:42:24,386 --> 00:42:25,753
This layer right here
633
00:42:25,821 --> 00:42:29,424
represents about 1,000 years of environmental stability,
634
00:42:29,491 --> 00:42:32,592
but then we had an abrupt volcanic eruption,
635
00:42:32,660 --> 00:42:35,762
and then the lake was around for perhaps 500 years
636
00:42:35,830 --> 00:42:38,365
before a drought, then the lake came back.
637
00:42:38,433 --> 00:42:42,633
So in some cases we saw this through layer after layer
638
00:42:42,701 --> 00:42:44,502
of environmental change.
639
00:42:47,373 --> 00:42:49,873
With his trained eye, Rick could see
640
00:42:49,941 --> 00:42:55,112
some layers were once lake beds, others desert sands.
641
00:42:55,180 --> 00:42:58,114
Still others came from volcanic eruptions--
642
00:42:58,182 --> 00:43:01,817
a snapshot of a million years of climate history.
643
00:43:04,455 --> 00:43:08,991
This observation led him to an amazing new idea--
644
00:43:09,058 --> 00:43:14,061
rapid change as a catalyst for our evolution.
645
00:43:14,129 --> 00:43:16,897
And I began to think that, well,
646
00:43:16,965 --> 00:43:19,967
maybe it's not the particular environment of a savanna
647
00:43:20,034 --> 00:43:21,334
that was important,
648
00:43:21,402 --> 00:43:23,669
but the tendency of the environment to change.
649
00:43:24,972 --> 00:43:26,506
Could it be that the need
650
00:43:26,573 --> 00:43:28,774
to survive violent swings of climate
651
00:43:28,842 --> 00:43:31,943
made our ancestors more adaptable?
652
00:43:35,681 --> 00:43:39,518
A group of scientists has come here from Germany to find out
653
00:43:39,585 --> 00:43:43,821
just how radical these swings of climate really were.
654
00:43:47,491 --> 00:43:50,859
It's hard to believe, but these huge rock formations
655
00:43:50,927 --> 00:43:54,129
are made of the shells of tiny one-celled organisms
656
00:43:54,196 --> 00:43:56,999
called diatoms.
657
00:43:59,902 --> 00:44:05,839
There are many different kinds, but they all live in water.
658
00:44:07,207 --> 00:44:10,844
Their shells collect in layers of rock that pile up
659
00:44:10,912 --> 00:44:13,013
over millions of years,
660
00:44:13,080 --> 00:44:16,981
proving that this whole valley was once a giant lake.
661
00:44:19,552 --> 00:44:22,619
Annett Junginger analyzes these rock samples
662
00:44:22,687 --> 00:44:24,988
under the microscope.
663
00:44:30,195 --> 00:44:32,295
What I've discovered was
664
00:44:32,363 --> 00:44:35,565
that those white layers consist
665
00:44:35,632 --> 00:44:39,469
of a special kind of diatoms which only live in deep lakes.
666
00:44:39,537 --> 00:44:42,404
But between the white layers
667
00:44:42,472 --> 00:44:45,674
she also finds other species of diatoms
668
00:44:45,741 --> 00:44:50,410
which only live in shallow water.
669
00:44:50,478 --> 00:44:54,214
It means that in this spot, a massive lake appeared
670
00:44:54,282 --> 00:44:59,851
and disappeared and reappeared many times.
671
00:44:59,919 --> 00:45:01,853
JOHN KINGSTON: These lakes are really significant;
672
00:45:01,921 --> 00:45:04,022
these are not small ponds.
673
00:45:04,091 --> 00:45:05,790
And what we've been able to document now
674
00:45:05,857 --> 00:45:08,726
is a series of lakes that are cycling.
675
00:45:10,696 --> 00:45:14,965
We're talking freshwater lakes the size of Lake Victoria,
676
00:45:15,032 --> 00:45:17,701
filling the whole Rift Valley, and then disappearing.
677
00:45:17,768 --> 00:45:20,570
Enormous amount of water rushing through this area.
678
00:45:20,637 --> 00:45:23,671
This constant flux of turnover, of change.
679
00:45:23,739 --> 00:45:25,173
An awful time to live here.
680
00:45:25,241 --> 00:45:27,075
It's not just a unidirectional change.
681
00:45:27,143 --> 00:45:28,376
It's going back and forth.
682
00:45:31,012 --> 00:45:34,047
Against the backdrop of a slow drying trend,
683
00:45:34,116 --> 00:45:37,618
Africa was periodically pulsing with climate change:
684
00:45:37,685 --> 00:45:41,254
wet, dry, then wet again,
685
00:45:41,322 --> 00:45:44,524
sometimes in the space of 1,000 years.
686
00:45:44,592 --> 00:45:49,193
Punishing drought alternated with storms and monsoons.
687
00:45:49,261 --> 00:45:53,964
Rivers and forests sprang up, then turned to dry grassland
688
00:45:54,032 --> 00:45:58,168
all in the evolutionary blink of an eye.
689
00:45:58,236 --> 00:46:00,537
So we have a complete change of our ideas,
690
00:46:00,605 --> 00:46:03,507
from this slow drying out, to this incredible change
691
00:46:03,574 --> 00:46:05,174
between wet and dry, wet and dry.
692
00:46:06,743 --> 00:46:10,112
What effect did that have on our ancestors?
693
00:46:10,180 --> 00:46:13,681
Could these periods of climate instability be the key
694
00:46:13,749 --> 00:46:16,050
to understanding the evolutionary leap
695
00:46:16,118 --> 00:46:19,420
from small bipedal apes
696
00:46:19,488 --> 00:46:23,055
to the larger brained toolmaker, Homo habilis?
697
00:46:25,859 --> 00:46:29,061
To know that, scientists needed a detailed record
698
00:46:29,129 --> 00:46:32,297
that went back further than the diatoms--
699
00:46:32,365 --> 00:46:36,234
way back to the time when Homo habilis was evolving
700
00:46:36,302 --> 00:46:38,303
two million years ago.
701
00:46:38,371 --> 00:46:43,240
That's only found in one place-- under the ocean.
702
00:46:43,308 --> 00:46:46,443
Layers of deep-sea sediment tell a story
703
00:46:46,511 --> 00:46:49,245
that goes back millions of years.
704
00:46:49,313 --> 00:46:52,214
They have to be drilled from the ocean floor.
705
00:46:55,785 --> 00:46:58,687
At his laboratory in Upstate New York,
706
00:46:58,754 --> 00:47:01,489
Peter deMenocal keeps thousands of columns
707
00:47:01,557 --> 00:47:06,326
of sand, silt and rock-- a library of ocean cores.
708
00:47:09,597 --> 00:47:13,365
One of the really attractive features about ocean sediments
709
00:47:13,433 --> 00:47:16,535
is that they accumulate very slowly but very gradually
710
00:47:16,603 --> 00:47:18,004
and continuously over time.
711
00:47:20,207 --> 00:47:24,175
Each three-foot long core holds a continuous record
712
00:47:24,243 --> 00:47:28,346
of dust carried on the wind from Africa into the ocean,
713
00:47:28,414 --> 00:47:30,947
where it now sits on the bottom.
714
00:47:31,015 --> 00:47:32,583
Oh, nice! Wow! There we go!
715
00:47:32,650 --> 00:47:34,017
Sweet-- okay.
716
00:47:34,085 --> 00:47:37,387
An expert eye can detect distinct layers--
717
00:47:37,455 --> 00:47:38,954
thick in dry years
718
00:47:39,022 --> 00:47:42,225
when the dust is easily picked up by the wind;
719
00:47:42,292 --> 00:47:45,127
thin in wet years.
720
00:47:45,195 --> 00:47:46,794
By measuring the layers,
721
00:47:46,862 --> 00:47:51,366
they can tell when the climate was wet or dry.
722
00:47:51,434 --> 00:47:54,134
So we can read these deep-sea sediments
723
00:47:54,202 --> 00:47:56,802
almost like an earth history book of past changes in climate.
724
00:47:56,870 --> 00:48:00,072
To make sense of all this dirt, they have to know
725
00:48:00,140 --> 00:48:03,343
when it blew into the ocean.
726
00:48:03,410 --> 00:48:07,346
They can do this by dating the shells of tiny sea creatures
727
00:48:07,413 --> 00:48:11,450
that sank to the bottom at the same time.
728
00:48:11,517 --> 00:48:12,983
So this gives us an age,
729
00:48:13,051 --> 00:48:14,852
the other analysis gives us the climate.
730
00:48:14,919 --> 00:48:17,588
Oh, nice!
731
00:48:17,656 --> 00:48:20,224
Peter took this finely detailed climate diary
732
00:48:20,292 --> 00:48:25,694
and compared it to the grand arc of our human evolution.
733
00:48:25,762 --> 00:48:29,097
For the three million years between Toumai and Selam,
734
00:48:29,164 --> 00:48:34,369
when brain size was flat-lining, African climate was stable--
735
00:48:34,436 --> 00:48:38,705
dry... getting a little drier.
736
00:48:38,773 --> 00:48:43,109
Then came 200,000 years of wildly varying climate,
737
00:48:43,177 --> 00:48:48,847
careening unpredictably between wet and dry.
738
00:48:48,915 --> 00:48:51,950
During that time, stone tools appeared
739
00:48:52,018 --> 00:48:55,852
along with the larger brained creatures that made them.
740
00:48:55,920 --> 00:49:00,223
Africa was also home to many other humanlike species.
741
00:49:00,292 --> 00:49:03,526
Climate instability put pressure on all of them.
742
00:49:03,594 --> 00:49:05,227
So there are these time periods
743
00:49:05,296 --> 00:49:07,297
when African climate was really unstable,
744
00:49:07,364 --> 00:49:09,198
so anything that was living there at the time
745
00:49:09,266 --> 00:49:10,600
would have had to adapt
746
00:49:10,668 --> 00:49:13,067
to really dramatically different climate changes.
747
00:49:13,135 --> 00:49:18,106
Those that couldn't adapt died out, like Selam and Lucy's kind.
748
00:49:18,174 --> 00:49:23,911
Better problem-solvers, like Homo habilis, survived.
749
00:49:27,881 --> 00:49:29,381
The new discoveries
750
00:49:29,449 --> 00:49:32,217
about ancient climate upheavals in Africa
751
00:49:32,285 --> 00:49:35,721
have led Rick Potts to formulate a bold theory
752
00:49:35,789 --> 00:49:37,722
of human evolution.
753
00:49:39,558 --> 00:49:42,960
The traditional idea we have had about human evolution
754
00:49:43,028 --> 00:49:46,162
is that it was the savanna-- the grassy plain
755
00:49:46,230 --> 00:49:49,366
with some trees on it-- that was the driving force.
756
00:49:49,434 --> 00:49:51,668
But instead, what we've discovered
757
00:49:51,736 --> 00:49:54,503
is that climate changed all the time.
758
00:49:54,571 --> 00:49:58,473
And so the idea that we've come up with is
759
00:49:58,541 --> 00:49:59,808
that variability itself
760
00:49:59,876 --> 00:50:02,010
was the driving force of human evolution
761
00:50:02,078 --> 00:50:05,713
and that our ancestors were adapted to change itself.
762
00:50:09,017 --> 00:50:12,352
It's a simple but revolutionary idea--
763
00:50:12,419 --> 00:50:17,290
human evolution is nature's experiment with versatility.
764
00:50:17,358 --> 00:50:21,227
We're not adapted to any one environment or climate,
765
00:50:21,294 --> 00:50:22,461
but to many.
766
00:50:22,528 --> 00:50:26,465
We are creatures of climate change.
767
00:50:26,532 --> 00:50:29,367
I think we should actually look to our proud ancestry
768
00:50:29,434 --> 00:50:31,369
and how we evolved in East Africa and say,
769
00:50:31,436 --> 00:50:33,337
That's how we survived that.
770
00:50:33,405 --> 00:50:35,606
We can survive the future.
771
00:50:35,674 --> 00:50:39,008
Because we are that creature, because we are that smart.
772
00:50:41,011 --> 00:50:46,782
Today, climate change seems to threaten our survival.
773
00:50:46,850 --> 00:50:50,652
But it may have held the keys to the astonishing story
774
00:50:50,720 --> 00:50:54,454
of how we became who we are.
775
00:50:54,522 --> 00:50:58,759
Because it didn't stop two million years ago.
776
00:50:58,826 --> 00:51:01,160
These dramatic upheavals would continue
777
00:51:01,228 --> 00:51:05,598
for another million and a half years,
778
00:51:05,665 --> 00:51:10,135
propelling our ancestors down a road leading ultimately
779
00:51:10,203 --> 00:51:14,305
to the smartest creature the world has ever known.
