Evolution (2002) s01e02 Episode Script
Great Transformations
MAN: It's a basic human need to ask, "Who are we?" "Where do we come from?" "How did we get here?" "Why do we look the way we do?" NARRATOR: The story of our evolution is just a small chapter in a much larger story: the evolution of all living things.
(trumpeting) MAN: Evolution shows us that we're much more connected to the rest of the world, the rest of animal life than we could ever have imagined.
NARRATOR: We can recognize the connection to our closest relatives but when we know how to look we can also find it in other mammals: birds reptiles fish even insects.
The deeper we dig, the farther back we go the more we see that everything alive has evolved from a single starting point.
The tree of life has been branching for four billion years.
And we can now follow its branches back to their roots.
MAN: When we look back over time we find certain signposts, certain key events the great transformations, the big evolutionary steps.
NARRATOR: In the history of our planet, a few great transformations have opened the door for new ways of life and new forms of life.
50 million years ago land mammals evolved into sea creatures.
Long before that, fish colonized land.
At the dawn of animal life itself the first bodies appeared.
These are some of the chapters in life's story our story.
MAN: And and part of the fun of studying this is understanding each different chapter because by understanding those chapters we can begin to see the unity of life, the common history of all life on Earth.
NARRATOR: Human civilization stretches back thousands of years.
But compared to the age of the earth we humans have only just arrived.
MAN: The earth is really old.
If you take the entire history of the earth from 4.
6 billion years ago to the present and sort of call that an hour (clock chime dings) SHUBIN: The first 50 minutes are largely spent in a world of microbes, single-celled organisms.
(clock ticking) Animal life appeared in the last ten minutes of that hour.
(clock ticking) All of human history, our civilization, our evolution, happened in the last hundredth of a second of that hour.
(clock chime dings) CHILDREN: ? Ring-around-a-rosy ? NARRATOR: We've come quite late to the party but we've been shaped by the same forces that have helped shape all life on Earth.
CHILDREN: ? We all fall down.
? NARRATOR: To understand how we fit in, we need to look back to long before our own origins to see how other living things evolved.
(whales singing) NARRATOR: Whales are the largest living animals.
Like us, whales and dolphins took on their present forms relatively recently.
For a long time, the origin of these marine mammals was a scientific mystery.
MAN: Whales are so different from every other kind of mammal that we can't easily relate them to anything else and so they're off by themself as a branch of mammal evolution.
NARRATOR: Mammals first appeared on Earth around 200 million years ago on land.
Mammals are warm-blooded they give birth to living young and they breathe air.
These are all adaptations to living on land.
(dolphin clicking) NARRATOR: But whales and dolphins are mammals, too.
SHUBIN: They're mammals that live in the water but we know that mammals evolved on land.
So it's a real puzzle how whales originally evolved.
By understanding how that happens we'll begin to understand how these big jumps these big transformations happen generally.
GINGERICH: People are interested in whales, and I can understand.
They're so beautiful.
Their origin is such a mystery.
Whales are one of the few groups of mammals that have really large, complicated brains like we do.
And so in a sense, they're our alter egos living in the sea while we live on land dominating the sea while we dominate land.
And I think for that reason we're very interested in what goes on there how they got there, as a reflection of our own history through geological time.
NARRATOR: When Phil Gingerich began his career 30 years ago he knew nothing about whales, and that was just fine with him.
He was drawn to geology mostly because he couldn't imagine a career spent behind a desk.
GINGERICH: I think I was interested in geology because it was a science outdoors.
And in geology, I became interested in paleontology because it was about life and the history of life.
NARRATOR: Gingerich's early interest in primitive land mammals eventually took him to Pakistan.
It was there that he made the kind of find most paleontologists only dream about: a fossil that would rewrite one of evolution's greatest stories.
GINGERICH: I found the back of a skull that I couldn't identify.
It had a very good, well-preserved ear region and that offered the clue to what it was.
NARRATOR: The shape was familiar but in other ways it was like nothing Gingerich had ever seen.
This is the original specimen.
It's the one we found in about 1978.
There's several things that strike you.
One is it's very similar in size and shape to the back of a skull of a wolf.
NARRATOR: But there was something odd about this skull.
On its underside was a walnut-sized bump.
GINGERICH: If this wasn't here I would think that this was an archaic carnivorous mammal or what we call a creodont, but it is here.
NARRATOR: It was part of the animal's inner ear and it had a distinctive shape, a shape found today in only one kind of animal: whales.
What was the ear of a whale doing on the skull of an animal that resembled a wolf? Gingerich was intrigued, so he constructed a model of what the creature's full skull might have looked like.
He wondered, was his find a crucial missing link the first fossil evidence ever found for one of Darwin's most daring claims, that whales had evolved from land mammals? To know for sure Gingerich would need to find more fossils ones that would show each stage of the whale transformation, what scientists call "transitional forms.
" I want to line them all up.
I want anyone to be able to see it and believe it because they've seen it.
NARRATOR: Gingerich tried to return to Pakistan to resume his search but war had broken out, and the borders were closed.
Frustrated, Gingerich decided to look elsewhere.
He had heard stories about whale skeleton sightings in a very unlikely place.
So he decided to check it out for himself.
The Sahara Desert is one of the driest places on Earth.
But 40 million years ago, things here were quite different.
GINGERICH: This used to be the sea.
Just think of this being the current Mediterranean coast of Egypt backed up about 40 million years but about 100 kilometers to the south of where it is today.
NARRATOR: Here, in what had once been the Southern Mediterranean Sea is a 100-square-mile stretch of layered sandstone with a surprising name Valley of the Whales.
The name is well suited.
Scattered everywhere across this arid landscape are what look like heaps of rose-colored stones.
Here's aBasilosaurus.
NARRATOR: But they're not stones You can see how big the vertebrae are.
Here's a lumbar partly weathered away.
NARRATOR: They're whale skeletons 40 million years old.
There's another one back here coming out of the mushroom.
There's one over here.
And back over there is one.
This whole place is full of whales.
NARRATOR: Why were there so many whales concentrated in this one spot? Gingerich believes that Whale Valley was once a protected bay, a lagoon hidden from the open sea by underwater sandbars.
Perhaps the whales birthed their young here and came here to die.
But even with hundreds of whale bones at his feet Gingerich was disappointed.
Nearly all of the skeletons belonged to a whale called"Basilosaurus--" a 40-million-year-old creature already known to science.
Basilosauruslived full time in the water.
This isBasilosaurus.
I got all excited NARRATOR: If whales had evolved from land mammals they had done so long beforeBasilosaurus.
So Gingerich didn't think the bones would be of much interest but he couldn't have been more wrong.
After only a few days of digging he made his second amazing find.
It turned out thatBasilosaurus had something modern whales have long since lost.
For the first time, we've got whales that have legs.
NARRATOR: The bones were small but unmistakable.
A pelvis.
A kneecap.
Even toes.
This whale had a complete set of leg bones.
Gingerich brought back as much of the skeleton as he could carry.
It was dramatic evidence that whales had once been four-legged animals.
Since Gingerich's finds he and others have filled in more of this fantastic story.
Scientists now think that the earliest ancestor of whales was similar to this 50 million-year-old wolflike mammal called sinonyx.
Sinonyx was a predatory scavenger that lived and hunted along the shores of an ancient sea.
Perhaps its descendants found the water a source of abundant food, and a haven from competition.
Over millions of years front legs became fins rear legs disappeared, bodies lost fur and took on their familiar streamlined shape.
Since Gingerich's first find, named Pakicetus the list of known transitional whales has grown.
It now includes Ambulocetus Rhodocetus Durodon as well as Basilosaurus.
They reveal another element of whale evolution the gradual migration of nostrils to the top of the head as whales adapted to breathing in the water.
(exhaling) GINGERICH: How did whales lose their legs? As the years went by, they evolved into newer types of NARRATOR: Gingerich's work demonstrates what Darwin himself insisted that the evidence for evolution is all around us if we choose to look for it.
And bones aren't the only evidence of whale evolution.
Their ancestry is also visible in the way they move.
Frank Fish studies how today's marine mammals swim.
He looks for their evolutionary heritage in the way they move through the water.
FISH: The big question is: How do you go from a terrestrial mammal that ran around on all four legs to something such as a dolphin which now doesn't have any legs at all and is well adapted to swimming in the oceans? NARRATOR: Even though whales look like fish, they don't swim like them.
Fish swim by flexing their spines from side to side like this shark.
But mammals swim differently.
This otter swims by undulating its spine up and down in exactly the same way that whales do.
And, as it turns out in the same way that land mammals use their spines when running.
Whales took with them into the water their ancestral way of moving and we can still see it 50 million years later.
SHUBIN: In one sense, evolution didn't invent anything new with whales; it was just tinkering with land mammals.
It's using the old to make the new and we call that tinkering.
And it does this in every animal group at every time during evolutionary history.
NARRATOR: The starting point for whales was a four-legged land animal that lived over 50 million years ago.
But land animals were also the product of a transformation, a much earlier one.
Hundreds of millions of years ago there were no animals on land.
SHUBIN: Before then all our distant ancestors lived in the water.
So at some point you had this shift from life in water to life on land.
That's a huge change.
NARRATOR: It was the moment when fish crawled out of the water and onto land.
WOMAN: If these early animals hadn't made the transition we wouldn't be here today.
And it's important to understand how and when and possibly, where that transition took place.
NARRATOR: The first creatures to leave the water really started something.
Their descendants eventually evolved into today's reptiles birds and mammals.
And these creatures' common origins are still visible in their bodies.
Just like us, they all have bodies with four limbs, they're all tetrapods.
SHUBIN: What that means is that all these different creatures are descended from a common ancestor whichhadsomething very similar or akin to limbs.
NARRATOR: Just what was that common ancestor? And how did it leave the water 370 million years ago? (men conversing) Those are the questions that paleontologists Neil Shubin and Ted Daeschler are trying to answer.
They think that the cliffs here in Central Pennsylvania may offer some clues.
DAESCHLER: All right, I think it's a good day for fossils.
What do you say? Great day; let's find some.
Hey, Doug.
Hey, Doug.
Good trip up? What you say we go over here? That's good.
Get some good digging in today.
NARRATOR: An unlikely spot to hunt for early tetrapod fossils.
But they're here because the rocks in these hills are just the right age, laid down during the period in Earth's history called the Devonian.
(men conversing) SHUBIN: Back in the Devonian, this place was very different.
It was south of the Equator, remember the continents are continually moving around, and back this time we're actually dealing with a much more tropical climate in Pennsylvania.
NARRATOR: Hundreds of millions of years ago the fossils and sediments in these layers were collecting on the bottom of a stream.
SHUBIN: What we have here is a snapshot of life in a stream about 370 million years ago.
These are fossilized broken fossils of scales, of teeth.
This little bone here, it's a spine of a creature known as a spiny shark.
NARRATOR: Most of the fossils are too fragmented to be of much value.
But in 1995, right at this spot Daeschler came across something he had never seen before.
It was a small shoulder bone, but not from a fish.
It was a tetrapod shoulder, 370 million years old.
Shubin and Daeschler had unearthed the remains of one of life's first four-legged creatures.
DAESCHLER: It was the first evidence of these early tetrapods from all of North America, and that made it very exciting.
NARRATOR: And there was another surprise.
Since it was found in the stream bed this tetrapod most likely livedin the water.
SHUBIN: And it's a very surprising discovery.
It's not something we necessarily would have predicted.
NARRATOR: Why would an animal with limbs live in the water? Limbs were thought to have evolved for getting around on land.
The old idea was that the fish came on shore first and then developed the legs.
And what we now think is that the tetrapods developed the fingers first and then left the water.
NARRATOR: Jenny Clack of Cambridge University suspected that the theory taught in many textbooks was wrong.
The story that you will find in many of the old textbooks and the pictures that you will see in children's books and museum galleries is a picture of a fish stranded in a drying pool trying to support itself out of water.
And it looks really odd if you look at it objectively.
NARRATOR: Clack thought there had to be a better explanation but where to look? Only a handful of early tetrapod fossils had ever been found, most of those in a remote part of Greenland at the turn of the century.
All she had to guide her was a note scribbled in a journal from a scouting trip to Greenland years earlier.
It referred to tetrapod fossils on an unnamed mountain.
Clack flew to Greenland to search for those bones.
CLACK: Eventually we found the spot, 800 meters up on a hillside.
NARRATOR: Clack returned with four tons of rock And spent the next four years drilling.
At the end she had the most complete early tetrapod skeleton ever found; and it forever changed the textbooks.
CLACK: One of the first things that we found was this forelimb.
NARRATOR: At the end of the animal's limb was an unmistakable array of bones.
This was a hand.
CLACK: This is a life reconstruction.
.
The artist is using imagination on the color scheme and on the eyes but we think this is as accurate as you can get.
NARRATOR: The creature, named Acanthostega was clearly a water-dweller: It had a fishlike tail and gills for breathing in the water.
But the ends of its arms were petal-shaped possibly the first hands on Earth.
CLACK: This was a swimming creature.
We don't know whether it could ever have come out on land but it certainly wouldn't havewalked in the conventional sense.
Basically, it's a fish with fingers.
NARRATOR: Clack's find was a scientific breakthrough.
It proved that some fish had arms and legs in the water.
So tetrapods didn't need to grow limbs after they got onto land.
The limbs had already evolved and helped them survive out of the water.
The basic pattern for limbs had been in place for millions of years.
SHUBIN: Here we have the fin of a 370-million-year-old fossil fish and an arm of a human.
In a human arm, what you have is one bone then two bones, the wrist and the digits.
In this fin what do you have? You have one bone, two bones even little bones that can be compared to a wrist and then rods that face away from the rest of the appendage itself just like our fingers or toes.
So you have, in a fish fin already set up about 370 million years ago many of the bones that are used in a tetrapod limb.
NARRATOR: With the basic pattern already there the fin-to-limb transition took place in a series of small changes occurring over millions of years.
SHUBIN: There's really no goal to evolution.
Evolution wasn'ttrying to make limbs it wasn'ttrying to push our distant ancestors out of the water.
What was happening was a series of experiments.
NARRATOR: In the crowded, freshwater streams where tetrapods first evolved the competition for survival was intense.
SHUBIN: These small streams were like an engine or a crucible of evolutionary change.
NARRATOR: Fish experimented with all sorts of survival strategies.
Some became predators.
The owner of this jaw was a 12-foot-long killer.
Its teeth were the size of railroad spikes.
Smaller fish developed elaborate defenses, like this heavy armor.
Others packed weaponry, like this sharp spike.
It protruded from behind its owner's neck.
These armaments were all tools for survival in a dangerous world.
Perhaps their new arms and legs gave the first tetrapods another way to survive.
SHUBIN: It was to get out of the way; it was to get onto land.
And what enabled those animals to get out of the way, that is, to get out of the water were these new features, like limbs.
NARRATOR: Those that did escape found a new world filled with opportunity.
The transformation from water to land was only the latest example of evolution experimenting with radically new forms.
An earlier transformation, perhaps the most significant of all, occurred just over half a billion years ago And it led to all animals as we know them.
Evolution tinkered with fish to make limbs.
But fish carry the baggage of their own past.
Think of a fish: It has a head, it has a tail and a bunch of fins in between.
That's a body plan, the way the body's put together.
But that's just one of many ways of putting animals together.
I mean, some animals are like disks, like jellyfish.
Other animals have lots of little legs.
The question is what sort of tinkering led to these body plans? I mean, really what we're dealing with here is the origin of animals.
NARRATOR: According to the fossil record animals appeared upon the earth in a short burst around 570 million years ago.
Scientists call this crucial transformation the Cambrian Explosion.
MAN: The Cambrian Explosion was effectively one of the greatest breakthroughs in the history of life.
About half a billion years ago, suddenly, things change and we have this extraordinary explosion of diversity.
And this sudden appearance of the fossils led to this term the Cambrian Explosion.
And Darwin, as ever, was extremely candid.
He said, "Look, this is a problem for my theory.
How is it that suddenly, animals seem to come out of nowhere?" And to a certain extent, that is still something of a mystery.
NARRATOR: Most of what we know of the Cambrian Explosion is a result of a single discovery probably the greatest fossil find in history.
In 1913, while climbing in the Canadian Rockies paleontologist Charles Walcott discovered a layer of shale containing thousands of exquisitely detailed fossils.
These animals, all sea dwellers were caught in a catastrophic underwater mudslide.
Over the next 500 million years the sea floor which entombed them rose to become the top of a mountain.
Walcott removed over 60,000 fossils from the site which he named the Burgess Shale.
Simon Conway Morris has studied the fossils for over 30 years.
It's almost as if you've gone to another planet.
You've been given a fishing boat and a net and you've been allowed to throw that net over into the deep ocean and you'd no idea what was going to come up.
NARRATOR: Some of the Burgess Shale creatures were familiar.
MORRIS: And here, we've got one of the trilobites.
We see the delicate soft parts, also preserved.
NARRATOR: Trilobites are extinct arthropods creatures with external skeletons.
Today's arthropods, like crabs, lobsters insects and spiders are all descendants of creatures like these.
Other Burgess Shale animals were bizarre, alien-seeming.
An animal with five eyes and a long retractable nozzle.
One with long, sharp spines protruding from its back.
Another with a circle of prongs around its mouth.
And yet, as alien as these creatures seem they are also surprisingly familiar.
Like living animals, they have bodies with heads, tails, appendages, specialized segments performing specialized functions.
All the basic body plans found in nature today are here.
Every animal that has lived for the last half billion years has come from tinkering with these initial designs.
We might even see our own ancestor here.
MORRIS: Maybe this is the crown of the Burgess Shale.
This isPikaia.
NARRATOR: A tiny creature,Pikaiais one of the rarest fossils from the Burgess Shale.
It's the only one with an internal nerve cord resembling a spine.
That might mean that creatures likePikaia were the earliest ancestors of all animals with skeletons.
MORRIS: The idea is that this might be the precursor of the fish and so, ultimately through a long evolutionary story, ourselves.
The Cambrian Explosion matters for lots of reasons.
Basically, it's part of our history.
It's where we came from and that matters very much.
This is the time when the animals first appear.
We look back and we can see part of our history unfolding.
So what do we learn by looking at 600 million years of animal history? Evolution's tinkering with mammalness to make whales.
In the same way, it's tinkering with fishiness to make tetrapods and it's tinkering with animalness to make all the different body plans that we see.
All these different creatures are variations of the same theme restated over and over again.
The question was, what was evolution tinkering with? One of the remarkable discoveries of the last 20 years is that evolution's not tinkering with the bodies.
It's tinkering with the recipe the machinery that builds bodies.
What is that recipe? What is that machinery? It's the genes.
NARRATOR: Fossils record the changes in animals' bodies over time but just how bodies change was unknown.
The search for the genetic mechanism of evolution took most of this century.
When scientists finally found it they were astonished by just how simple it was.
One of the key players was Mike Levine.
LEVINE: I was, um, I guess, kind of a weird kid.
I always liked bugs.
We had a nice, big backyard, and I could go back there.
It was kind of a sanctuary.
And, uh, I played with bugs dissected them, manipulated them.
That's really the most pleasant memory I have.
NARRATOR: Levine's affinity for bugs led to his study of biology.
One insect in particular became an object of fascination.
LEVINE: They have a quick generation time and they have lots of pattern.
I mean, you wouldn't know it if you look at a distance but when you look under a microscope at an adult fruit fly you'd be astounded by the number of bristles the intricacies of their wings, the patterns of their eyes.
But the embryos are something else.
I do love the embryos.
NARRATOR: Scientists had long suspected that embryos held clues to how animals evolve.
All embryos start out as clusters of nearly identical cells.
But soon, an embryo partitions itself into specialized segments which develop into the final form of the animal.
What controlled this process? How did the embryos know what shape to take? One of the first people to study these questions was a 19th-century naturalist named William Bateson.
Bateson wrote that animals' skeletons revealed an underlying structure of repeating segments.
He also observed that animals occasionally developed with some segments in the wrong places.
MAN: Insects with legs in the wrong place.
Crabs where a claw was transformed into a leg.
Pythons with extra ribs or frogs with extra cervical vertebrae and all these sorts of things.
NARRATOR: To Bateson, these developmental errors meant that the underlying blueprint for the animal was being disrupted.
He had no idea how it happened but he suspected that these random changes might provide the fuel for evolution.
By the 1940s, scientists working with fruit flies had learned how to cause disruptions in the developmental blueprint: by dousing growing embryos with radiation and poison.
MAN: And so when they did that they found flies with changed wing structures, changed legs and these very special flies that have one part of the body in the wrong place or a copy of a normal part of the body in another place.
NARRATOR: The scientists had triggered the changes by damaging the flies' DNA.
Within each cell of the developing embryo is a chainlike molecule called DNA.
The experiments showed that DNA was somehow causing the embryo to divide into segments.
But how? Scientists were just beginning to grasp that the DNA itself was made up of segments, called genes.
The question was: how did the genes shape the body? One researcher, Dr.
Ed Lewis of Caltech studied this question for 30 years by crossbreeding thousands of flies.
Lewis's work led him to a controversial idea.
He proposed that a surprisingly simple mechanism was shaping embryos.
He wrote that each segment of the fly was being directed to grow by a single gene.
A small set of genes, a kind of genetic toolkit appeared to be laying out the entire body.
And as he looked at these genes, he said "This one affects this part of the body.
"This affects the next part of the body.
And this affects the next part of the body.
" That was an astonishing observation.
NARRATOR: It was astonishing because it seemed too simple.
Nobody else thought single genes were powerful enough to control something as complex as the structure of the body.
Skeptics argued that Lewis's idea was guesswork.
Of course, he had never seen the genes because the techniques to do so didn't exist.
From the 1920s to the 1970s it was not possible to physically isolate any specific gene.
That opportunity first became available, fortunately for me at the time that I was a student.
And so, many of us thought, "Wow.
"We can finally dig in there and identify these really mysterious genes.
" NARRATOR: Levine enlisted his friend and fellow scientist Bill McGinnis.
The first gene they went after had an unusual name.
Antennapedia, which means "antenna leg.
" The gene was thought to control the growth of legs.
When the gene misfired flies grew legs in the wrong place: on their heads, in place of antennae.
In normal flies, legs grow from the midsection the area called the thorax.
So Levine and McGinnis decided to hunt for the gene in the thorax of a normal embryo.
LEVINE: The expectation is that antennapedia would be active expressed in the thorax the developing thorax, of the embryo.
But who knew? NARRATOR: Levine and McGinnis had to do something no one had ever done before.
They had to find a way to see a gene in action.
LEVINE: We wanted to light up the gene and it was very painstaking work.
NARRATOR: The project called for new and untested methods.
McGINNIS: At first, it didn't work very well and there were a number of technical problems to solve.
NARRATOR: The team had to find a delicate balance of radioactive probes and toxic enzymes.
Too much of either would destroy the embryos.
LEVINE: The process was not very gratifying on a day-by-day basis.
Unbelievably tedious.
NARRATOR: It took months of trial and error.
McGINNIS: People often said, "You know, you should try something else.
"You know, this is too long-shot.
You know, you're going to you're just wasting your time.
" But we kept going.
NARRATOR: Finally, late one night, all the work paid off.
LEVINE: And there was this moment when we saw that the gene was turned on in a band in the middle of a very early embryo.
This had never been seen before.
NARRATOR: The antennapedia gene was acting like a master switch turning on the segment of the embryo that would become the thorax.
The implications were mind-boggling: if single genes like antennapedia could define whole segments of an animal these genes were acting like architects of the body.
And if one of these genes turned on in the wrong place striking changes to the body could result.
It seemed that Levine and McGinnis had uncovered the genes responsible for the evolution of bodies.
But there were still doubts.
The work had all been done in fruit flies.
What about other animals? Did they use the same mechanism to build their bodies? An answer would come from Switzerland.
In 1994, Walter Gehring of the University of Basel isolated the gene that triggered the growth of eyes in fruit flies.
The gene was called Eyeless because flies without it developed with no eyes.
Gehring knew of a gene in mice that worked in the same way.
He wondered, were the two genes the same? GEHRING: And this question we tested by taking the mouse gene and putting it into fruit flies to see whether flies can understand the message of the mouse.
NARRATOR: Gehring replaced a fly's gene for eyes with the mouse gene.
GEHRING: And to everybody's surprise the mouse gene works perfectly well and can induce a compound eye in the fruit fly.
NARRATOR: The fruit fly grew normal fruit fly eyes using a gene from a mouse.
Not only did the two creatures use the same mechanism; they used the same gene.
This was the mechanism behind extra wings legs sprouting from heads, and Bateson's deformed animals.
The century-long search was complete.
The genetic engine of the body's evolution turned out to be a tiny handful of powerful genes.
CARROLL: So what this means is in some ways, some sense, evolution is a simpler process than we first thought when you think about all of the diversity of forms out there.
We first believed that this would involve all sorts of novel creations starting from scratch, again and again and again.
We now understand that no, that evolution works with packets of information, and uses them in new and different ways and new and different combinations without necessarily having to invent anything fundamentally new, but new combinations.
NARRATOR: Suddenly, the commonality of form among animals was understood: animals resembled each other because they all used the same set of genes to build their bodies a set of genes inherited from a common ancestor that lived long ago.
And what we see now among all the animals are just variations on a body plan that existed half a billion years ago.
And there's only one inescapable conclusion you can draw from that, which is if all of these branches have these genes then you have to go to the base of that which is the last common ancestor of all animals and you deduce, itmust have had these genes.
So the whole radiation of animals the whole spring of animal diversity has been fed by essentially the same set of genes.
NARRATOR: Ed Lewis shared the Nobel Prize in 1995 for the discovery of the universal set of genes that builds the bodies of animals.
And so, yes, it came as a huge surprise not only to people like my mother, who says "My God, an earthworm and a mouse? An earthworm and me, you know, share things in common?" But it came as a surprise to other scientists that there was this profound conservation of mechanism of building embryos among all these different kinds of animals.
NARRATOR: What about us? Our bodies are built from the same genes that build all other animals.
Yet we are different.
No other animal designs or creates like we do.
We seem so special it's hard not to think that we're somehow an exception to evolution but of course, we're not.
The transformation that led to us was no different from other transformations.
Our crucial turning point seems to have occurred when our ancestors left the trees and began to walk on two legs.
MAN: We don't know exactly when, or even where our ancestors became upright and bipedal.
We think it goes back well over four million years.
When these ancestors came out of the trees and began to exploit food sources on the ground in terrestrial habitats one of the key elements that would've been so useful to them would've been freeing their forelimbs, their hands to be able to gather and carry foodstuffs over long distances.
Once that happened, it opened up an extraordinary breadth of possibilities and opportunities.
NARRATOR: Most bipedal hominids went extinct but one branch went on to evolve larger brains.
That branch eventually led to modern humans.
So how did this crucial transition to two-legged walking begin? Liza Shapiro of the University of Texas looks for clues in living primates.
SHAPIRO: When you look at the fossil record all you have really is a pile of bones.
It's a nonmoving entity.
There's not much you can know about it unless you look for living analogs.
So if you look at living animals, you've got the bones but you can also look at how they're moving.
NARRATOR: In their movements, living lemurs resemble tree-dwelling primates that lived up to 50 million years ago.
We didn't evolve from lemurs but they may be the best living analog for those distant ancestors.
SHAPIRO: When we're trying to reconstruct the scenario about how humans evolved bipedally from this ancestor we have to know what it was we started from if we're going to come up with an explanation for not only how we made that transition, but why.
NARRATOR: Today, Shapiro is gathering data on the movement style of the lemur.
Small reflectors have been gently placed on the animal's back.
An array of infrared cameras will record the lemur as it walks across this makeshift bridge.
Of course, getting a lemur to do just about anything on cue takes a bit of doing.
There you go.
NARRATOR: Finally, the animal makes it across.
Here you go oh, good! All right.
Got that.
And he's down.
NARRATOR: The motion of the lemur's spine can now be analyzed in three dimensions.
The data reveal that lemurs' spines are extremely flexible capable of many kinds of movements.
SHAPIRO: Lemurs walk quadrupedally but they're also very good at leaping.
NARRATOR: Like these lemurs, the early primates probably moved in all sorts of ways: down on all fours, scampering up trees even leaping in an upright position.
They weren't limited to just one style of movement so they could serve as the starting point for a number of evolutionary experiments.
And most likely, that's just what happened.
We weren't the only ones to evolve from those early ancestors.
So did most of today's living primates.
Our closest living relative is the chimpanzee.
We didn't evolve from chimps but we do share a recent common ancestor.
Can you walk over here? NARRATOR: That's why our DNA is nearly identical to theirs and why our skeletons have the same number of bones arranged in nearly the same way.
But the few physical differences that set us apart seem to have made a great difference.
Chimps don't walk on two feet.
They've evolved a different style of getting around called knucklewalking.
JOHANSON: Knucklewalking is a very specialized adaptation that we see among chimps and gorillas today.
It's an adaptation to walking on the ground.
NARRATOR: Knucklewalking was as valid an evolutionary experiment as two-legged walking.
But the difference in our walking styles which may have affected our intellects is seen in the few slight differences in our skeletons.
Here are two skeletons of modern primates.
This skeleton I'm sure you'll all recognize because it's a skeleton like yours and mine.
This is a modern human.
But this smaller skeleton is one of our closest living relatives, the chimpanzee.
We began walking on two legs and that made a whole series of modifications in the skeleton.
In humans, the spinal chord comes out of the base of the skull and points straight downwards rather than coming out of the back of the skull.
The pelvis is shaped very differently.
A chimpanzee has a long, narrow pelvis.
Ours is short and squat.
We walk with our knees close together.
Chimpanzees walk with their knees wide apart.
These are minor differences.
These are the sorts of tinkering that evolution did to change us into a modern biped.
NARRATOR: What if our ancestors hadn't stood up? What if they had taken one different turn along the path to becoming human? JOHANSON: One of the great misconceptions that most people have is that that once our ancestors stood up, it was almost inevitable that we would be here today that the egocentric species, Homo sapiens would evolve in this manner.
But what we see is that evolution has worked the same way with us as it has with every, single organism on this planet.
We're here through a series of chance coincidences specific adaptations, chosen opportunities.
So I think that when we look at our own origins we see that it is extraordinary that humans are here to look back and ponder their past.
SHUBIN: Does that mean we are not unique in many ways? Of course not.
We're the ones telling this story.
And that's very important that evolution, that life has gotten to the point where it can tell this story.
Continue the journey into where we're from and where we're going at the Evolution web site.
The seven-part Evolution boxed set and the companion book are available from WGBH Boston Video.
To place an order, please call:
(trumpeting) MAN: Evolution shows us that we're much more connected to the rest of the world, the rest of animal life than we could ever have imagined.
NARRATOR: We can recognize the connection to our closest relatives but when we know how to look we can also find it in other mammals: birds reptiles fish even insects.
The deeper we dig, the farther back we go the more we see that everything alive has evolved from a single starting point.
The tree of life has been branching for four billion years.
And we can now follow its branches back to their roots.
MAN: When we look back over time we find certain signposts, certain key events the great transformations, the big evolutionary steps.
NARRATOR: In the history of our planet, a few great transformations have opened the door for new ways of life and new forms of life.
50 million years ago land mammals evolved into sea creatures.
Long before that, fish colonized land.
At the dawn of animal life itself the first bodies appeared.
These are some of the chapters in life's story our story.
MAN: And and part of the fun of studying this is understanding each different chapter because by understanding those chapters we can begin to see the unity of life, the common history of all life on Earth.
NARRATOR: Human civilization stretches back thousands of years.
But compared to the age of the earth we humans have only just arrived.
MAN: The earth is really old.
If you take the entire history of the earth from 4.
6 billion years ago to the present and sort of call that an hour (clock chime dings) SHUBIN: The first 50 minutes are largely spent in a world of microbes, single-celled organisms.
(clock ticking) Animal life appeared in the last ten minutes of that hour.
(clock ticking) All of human history, our civilization, our evolution, happened in the last hundredth of a second of that hour.
(clock chime dings) CHILDREN: ? Ring-around-a-rosy ? NARRATOR: We've come quite late to the party but we've been shaped by the same forces that have helped shape all life on Earth.
CHILDREN: ? We all fall down.
? NARRATOR: To understand how we fit in, we need to look back to long before our own origins to see how other living things evolved.
(whales singing) NARRATOR: Whales are the largest living animals.
Like us, whales and dolphins took on their present forms relatively recently.
For a long time, the origin of these marine mammals was a scientific mystery.
MAN: Whales are so different from every other kind of mammal that we can't easily relate them to anything else and so they're off by themself as a branch of mammal evolution.
NARRATOR: Mammals first appeared on Earth around 200 million years ago on land.
Mammals are warm-blooded they give birth to living young and they breathe air.
These are all adaptations to living on land.
(dolphin clicking) NARRATOR: But whales and dolphins are mammals, too.
SHUBIN: They're mammals that live in the water but we know that mammals evolved on land.
So it's a real puzzle how whales originally evolved.
By understanding how that happens we'll begin to understand how these big jumps these big transformations happen generally.
GINGERICH: People are interested in whales, and I can understand.
They're so beautiful.
Their origin is such a mystery.
Whales are one of the few groups of mammals that have really large, complicated brains like we do.
And so in a sense, they're our alter egos living in the sea while we live on land dominating the sea while we dominate land.
And I think for that reason we're very interested in what goes on there how they got there, as a reflection of our own history through geological time.
NARRATOR: When Phil Gingerich began his career 30 years ago he knew nothing about whales, and that was just fine with him.
He was drawn to geology mostly because he couldn't imagine a career spent behind a desk.
GINGERICH: I think I was interested in geology because it was a science outdoors.
And in geology, I became interested in paleontology because it was about life and the history of life.
NARRATOR: Gingerich's early interest in primitive land mammals eventually took him to Pakistan.
It was there that he made the kind of find most paleontologists only dream about: a fossil that would rewrite one of evolution's greatest stories.
GINGERICH: I found the back of a skull that I couldn't identify.
It had a very good, well-preserved ear region and that offered the clue to what it was.
NARRATOR: The shape was familiar but in other ways it was like nothing Gingerich had ever seen.
This is the original specimen.
It's the one we found in about 1978.
There's several things that strike you.
One is it's very similar in size and shape to the back of a skull of a wolf.
NARRATOR: But there was something odd about this skull.
On its underside was a walnut-sized bump.
GINGERICH: If this wasn't here I would think that this was an archaic carnivorous mammal or what we call a creodont, but it is here.
NARRATOR: It was part of the animal's inner ear and it had a distinctive shape, a shape found today in only one kind of animal: whales.
What was the ear of a whale doing on the skull of an animal that resembled a wolf? Gingerich was intrigued, so he constructed a model of what the creature's full skull might have looked like.
He wondered, was his find a crucial missing link the first fossil evidence ever found for one of Darwin's most daring claims, that whales had evolved from land mammals? To know for sure Gingerich would need to find more fossils ones that would show each stage of the whale transformation, what scientists call "transitional forms.
" I want to line them all up.
I want anyone to be able to see it and believe it because they've seen it.
NARRATOR: Gingerich tried to return to Pakistan to resume his search but war had broken out, and the borders were closed.
Frustrated, Gingerich decided to look elsewhere.
He had heard stories about whale skeleton sightings in a very unlikely place.
So he decided to check it out for himself.
The Sahara Desert is one of the driest places on Earth.
But 40 million years ago, things here were quite different.
GINGERICH: This used to be the sea.
Just think of this being the current Mediterranean coast of Egypt backed up about 40 million years but about 100 kilometers to the south of where it is today.
NARRATOR: Here, in what had once been the Southern Mediterranean Sea is a 100-square-mile stretch of layered sandstone with a surprising name Valley of the Whales.
The name is well suited.
Scattered everywhere across this arid landscape are what look like heaps of rose-colored stones.
Here's aBasilosaurus.
NARRATOR: But they're not stones You can see how big the vertebrae are.
Here's a lumbar partly weathered away.
NARRATOR: They're whale skeletons 40 million years old.
There's another one back here coming out of the mushroom.
There's one over here.
And back over there is one.
This whole place is full of whales.
NARRATOR: Why were there so many whales concentrated in this one spot? Gingerich believes that Whale Valley was once a protected bay, a lagoon hidden from the open sea by underwater sandbars.
Perhaps the whales birthed their young here and came here to die.
But even with hundreds of whale bones at his feet Gingerich was disappointed.
Nearly all of the skeletons belonged to a whale called"Basilosaurus--" a 40-million-year-old creature already known to science.
Basilosauruslived full time in the water.
This isBasilosaurus.
I got all excited NARRATOR: If whales had evolved from land mammals they had done so long beforeBasilosaurus.
So Gingerich didn't think the bones would be of much interest but he couldn't have been more wrong.
After only a few days of digging he made his second amazing find.
It turned out thatBasilosaurus had something modern whales have long since lost.
For the first time, we've got whales that have legs.
NARRATOR: The bones were small but unmistakable.
A pelvis.
A kneecap.
Even toes.
This whale had a complete set of leg bones.
Gingerich brought back as much of the skeleton as he could carry.
It was dramatic evidence that whales had once been four-legged animals.
Since Gingerich's finds he and others have filled in more of this fantastic story.
Scientists now think that the earliest ancestor of whales was similar to this 50 million-year-old wolflike mammal called sinonyx.
Sinonyx was a predatory scavenger that lived and hunted along the shores of an ancient sea.
Perhaps its descendants found the water a source of abundant food, and a haven from competition.
Over millions of years front legs became fins rear legs disappeared, bodies lost fur and took on their familiar streamlined shape.
Since Gingerich's first find, named Pakicetus the list of known transitional whales has grown.
It now includes Ambulocetus Rhodocetus Durodon as well as Basilosaurus.
They reveal another element of whale evolution the gradual migration of nostrils to the top of the head as whales adapted to breathing in the water.
(exhaling) GINGERICH: How did whales lose their legs? As the years went by, they evolved into newer types of NARRATOR: Gingerich's work demonstrates what Darwin himself insisted that the evidence for evolution is all around us if we choose to look for it.
And bones aren't the only evidence of whale evolution.
Their ancestry is also visible in the way they move.
Frank Fish studies how today's marine mammals swim.
He looks for their evolutionary heritage in the way they move through the water.
FISH: The big question is: How do you go from a terrestrial mammal that ran around on all four legs to something such as a dolphin which now doesn't have any legs at all and is well adapted to swimming in the oceans? NARRATOR: Even though whales look like fish, they don't swim like them.
Fish swim by flexing their spines from side to side like this shark.
But mammals swim differently.
This otter swims by undulating its spine up and down in exactly the same way that whales do.
And, as it turns out in the same way that land mammals use their spines when running.
Whales took with them into the water their ancestral way of moving and we can still see it 50 million years later.
SHUBIN: In one sense, evolution didn't invent anything new with whales; it was just tinkering with land mammals.
It's using the old to make the new and we call that tinkering.
And it does this in every animal group at every time during evolutionary history.
NARRATOR: The starting point for whales was a four-legged land animal that lived over 50 million years ago.
But land animals were also the product of a transformation, a much earlier one.
Hundreds of millions of years ago there were no animals on land.
SHUBIN: Before then all our distant ancestors lived in the water.
So at some point you had this shift from life in water to life on land.
That's a huge change.
NARRATOR: It was the moment when fish crawled out of the water and onto land.
WOMAN: If these early animals hadn't made the transition we wouldn't be here today.
And it's important to understand how and when and possibly, where that transition took place.
NARRATOR: The first creatures to leave the water really started something.
Their descendants eventually evolved into today's reptiles birds and mammals.
And these creatures' common origins are still visible in their bodies.
Just like us, they all have bodies with four limbs, they're all tetrapods.
SHUBIN: What that means is that all these different creatures are descended from a common ancestor whichhadsomething very similar or akin to limbs.
NARRATOR: Just what was that common ancestor? And how did it leave the water 370 million years ago? (men conversing) Those are the questions that paleontologists Neil Shubin and Ted Daeschler are trying to answer.
They think that the cliffs here in Central Pennsylvania may offer some clues.
DAESCHLER: All right, I think it's a good day for fossils.
What do you say? Great day; let's find some.
Hey, Doug.
Hey, Doug.
Good trip up? What you say we go over here? That's good.
Get some good digging in today.
NARRATOR: An unlikely spot to hunt for early tetrapod fossils.
But they're here because the rocks in these hills are just the right age, laid down during the period in Earth's history called the Devonian.
(men conversing) SHUBIN: Back in the Devonian, this place was very different.
It was south of the Equator, remember the continents are continually moving around, and back this time we're actually dealing with a much more tropical climate in Pennsylvania.
NARRATOR: Hundreds of millions of years ago the fossils and sediments in these layers were collecting on the bottom of a stream.
SHUBIN: What we have here is a snapshot of life in a stream about 370 million years ago.
These are fossilized broken fossils of scales, of teeth.
This little bone here, it's a spine of a creature known as a spiny shark.
NARRATOR: Most of the fossils are too fragmented to be of much value.
But in 1995, right at this spot Daeschler came across something he had never seen before.
It was a small shoulder bone, but not from a fish.
It was a tetrapod shoulder, 370 million years old.
Shubin and Daeschler had unearthed the remains of one of life's first four-legged creatures.
DAESCHLER: It was the first evidence of these early tetrapods from all of North America, and that made it very exciting.
NARRATOR: And there was another surprise.
Since it was found in the stream bed this tetrapod most likely livedin the water.
SHUBIN: And it's a very surprising discovery.
It's not something we necessarily would have predicted.
NARRATOR: Why would an animal with limbs live in the water? Limbs were thought to have evolved for getting around on land.
The old idea was that the fish came on shore first and then developed the legs.
And what we now think is that the tetrapods developed the fingers first and then left the water.
NARRATOR: Jenny Clack of Cambridge University suspected that the theory taught in many textbooks was wrong.
The story that you will find in many of the old textbooks and the pictures that you will see in children's books and museum galleries is a picture of a fish stranded in a drying pool trying to support itself out of water.
And it looks really odd if you look at it objectively.
NARRATOR: Clack thought there had to be a better explanation but where to look? Only a handful of early tetrapod fossils had ever been found, most of those in a remote part of Greenland at the turn of the century.
All she had to guide her was a note scribbled in a journal from a scouting trip to Greenland years earlier.
It referred to tetrapod fossils on an unnamed mountain.
Clack flew to Greenland to search for those bones.
CLACK: Eventually we found the spot, 800 meters up on a hillside.
NARRATOR: Clack returned with four tons of rock And spent the next four years drilling.
At the end she had the most complete early tetrapod skeleton ever found; and it forever changed the textbooks.
CLACK: One of the first things that we found was this forelimb.
NARRATOR: At the end of the animal's limb was an unmistakable array of bones.
This was a hand.
CLACK: This is a life reconstruction.
.
The artist is using imagination on the color scheme and on the eyes but we think this is as accurate as you can get.
NARRATOR: The creature, named Acanthostega was clearly a water-dweller: It had a fishlike tail and gills for breathing in the water.
But the ends of its arms were petal-shaped possibly the first hands on Earth.
CLACK: This was a swimming creature.
We don't know whether it could ever have come out on land but it certainly wouldn't havewalked in the conventional sense.
Basically, it's a fish with fingers.
NARRATOR: Clack's find was a scientific breakthrough.
It proved that some fish had arms and legs in the water.
So tetrapods didn't need to grow limbs after they got onto land.
The limbs had already evolved and helped them survive out of the water.
The basic pattern for limbs had been in place for millions of years.
SHUBIN: Here we have the fin of a 370-million-year-old fossil fish and an arm of a human.
In a human arm, what you have is one bone then two bones, the wrist and the digits.
In this fin what do you have? You have one bone, two bones even little bones that can be compared to a wrist and then rods that face away from the rest of the appendage itself just like our fingers or toes.
So you have, in a fish fin already set up about 370 million years ago many of the bones that are used in a tetrapod limb.
NARRATOR: With the basic pattern already there the fin-to-limb transition took place in a series of small changes occurring over millions of years.
SHUBIN: There's really no goal to evolution.
Evolution wasn'ttrying to make limbs it wasn'ttrying to push our distant ancestors out of the water.
What was happening was a series of experiments.
NARRATOR: In the crowded, freshwater streams where tetrapods first evolved the competition for survival was intense.
SHUBIN: These small streams were like an engine or a crucible of evolutionary change.
NARRATOR: Fish experimented with all sorts of survival strategies.
Some became predators.
The owner of this jaw was a 12-foot-long killer.
Its teeth were the size of railroad spikes.
Smaller fish developed elaborate defenses, like this heavy armor.
Others packed weaponry, like this sharp spike.
It protruded from behind its owner's neck.
These armaments were all tools for survival in a dangerous world.
Perhaps their new arms and legs gave the first tetrapods another way to survive.
SHUBIN: It was to get out of the way; it was to get onto land.
And what enabled those animals to get out of the way, that is, to get out of the water were these new features, like limbs.
NARRATOR: Those that did escape found a new world filled with opportunity.
The transformation from water to land was only the latest example of evolution experimenting with radically new forms.
An earlier transformation, perhaps the most significant of all, occurred just over half a billion years ago And it led to all animals as we know them.
Evolution tinkered with fish to make limbs.
But fish carry the baggage of their own past.
Think of a fish: It has a head, it has a tail and a bunch of fins in between.
That's a body plan, the way the body's put together.
But that's just one of many ways of putting animals together.
I mean, some animals are like disks, like jellyfish.
Other animals have lots of little legs.
The question is what sort of tinkering led to these body plans? I mean, really what we're dealing with here is the origin of animals.
NARRATOR: According to the fossil record animals appeared upon the earth in a short burst around 570 million years ago.
Scientists call this crucial transformation the Cambrian Explosion.
MAN: The Cambrian Explosion was effectively one of the greatest breakthroughs in the history of life.
About half a billion years ago, suddenly, things change and we have this extraordinary explosion of diversity.
And this sudden appearance of the fossils led to this term the Cambrian Explosion.
And Darwin, as ever, was extremely candid.
He said, "Look, this is a problem for my theory.
How is it that suddenly, animals seem to come out of nowhere?" And to a certain extent, that is still something of a mystery.
NARRATOR: Most of what we know of the Cambrian Explosion is a result of a single discovery probably the greatest fossil find in history.
In 1913, while climbing in the Canadian Rockies paleontologist Charles Walcott discovered a layer of shale containing thousands of exquisitely detailed fossils.
These animals, all sea dwellers were caught in a catastrophic underwater mudslide.
Over the next 500 million years the sea floor which entombed them rose to become the top of a mountain.
Walcott removed over 60,000 fossils from the site which he named the Burgess Shale.
Simon Conway Morris has studied the fossils for over 30 years.
It's almost as if you've gone to another planet.
You've been given a fishing boat and a net and you've been allowed to throw that net over into the deep ocean and you'd no idea what was going to come up.
NARRATOR: Some of the Burgess Shale creatures were familiar.
MORRIS: And here, we've got one of the trilobites.
We see the delicate soft parts, also preserved.
NARRATOR: Trilobites are extinct arthropods creatures with external skeletons.
Today's arthropods, like crabs, lobsters insects and spiders are all descendants of creatures like these.
Other Burgess Shale animals were bizarre, alien-seeming.
An animal with five eyes and a long retractable nozzle.
One with long, sharp spines protruding from its back.
Another with a circle of prongs around its mouth.
And yet, as alien as these creatures seem they are also surprisingly familiar.
Like living animals, they have bodies with heads, tails, appendages, specialized segments performing specialized functions.
All the basic body plans found in nature today are here.
Every animal that has lived for the last half billion years has come from tinkering with these initial designs.
We might even see our own ancestor here.
MORRIS: Maybe this is the crown of the Burgess Shale.
This isPikaia.
NARRATOR: A tiny creature,Pikaiais one of the rarest fossils from the Burgess Shale.
It's the only one with an internal nerve cord resembling a spine.
That might mean that creatures likePikaia were the earliest ancestors of all animals with skeletons.
MORRIS: The idea is that this might be the precursor of the fish and so, ultimately through a long evolutionary story, ourselves.
The Cambrian Explosion matters for lots of reasons.
Basically, it's part of our history.
It's where we came from and that matters very much.
This is the time when the animals first appear.
We look back and we can see part of our history unfolding.
So what do we learn by looking at 600 million years of animal history? Evolution's tinkering with mammalness to make whales.
In the same way, it's tinkering with fishiness to make tetrapods and it's tinkering with animalness to make all the different body plans that we see.
All these different creatures are variations of the same theme restated over and over again.
The question was, what was evolution tinkering with? One of the remarkable discoveries of the last 20 years is that evolution's not tinkering with the bodies.
It's tinkering with the recipe the machinery that builds bodies.
What is that recipe? What is that machinery? It's the genes.
NARRATOR: Fossils record the changes in animals' bodies over time but just how bodies change was unknown.
The search for the genetic mechanism of evolution took most of this century.
When scientists finally found it they were astonished by just how simple it was.
One of the key players was Mike Levine.
LEVINE: I was, um, I guess, kind of a weird kid.
I always liked bugs.
We had a nice, big backyard, and I could go back there.
It was kind of a sanctuary.
And, uh, I played with bugs dissected them, manipulated them.
That's really the most pleasant memory I have.
NARRATOR: Levine's affinity for bugs led to his study of biology.
One insect in particular became an object of fascination.
LEVINE: They have a quick generation time and they have lots of pattern.
I mean, you wouldn't know it if you look at a distance but when you look under a microscope at an adult fruit fly you'd be astounded by the number of bristles the intricacies of their wings, the patterns of their eyes.
But the embryos are something else.
I do love the embryos.
NARRATOR: Scientists had long suspected that embryos held clues to how animals evolve.
All embryos start out as clusters of nearly identical cells.
But soon, an embryo partitions itself into specialized segments which develop into the final form of the animal.
What controlled this process? How did the embryos know what shape to take? One of the first people to study these questions was a 19th-century naturalist named William Bateson.
Bateson wrote that animals' skeletons revealed an underlying structure of repeating segments.
He also observed that animals occasionally developed with some segments in the wrong places.
MAN: Insects with legs in the wrong place.
Crabs where a claw was transformed into a leg.
Pythons with extra ribs or frogs with extra cervical vertebrae and all these sorts of things.
NARRATOR: To Bateson, these developmental errors meant that the underlying blueprint for the animal was being disrupted.
He had no idea how it happened but he suspected that these random changes might provide the fuel for evolution.
By the 1940s, scientists working with fruit flies had learned how to cause disruptions in the developmental blueprint: by dousing growing embryos with radiation and poison.
MAN: And so when they did that they found flies with changed wing structures, changed legs and these very special flies that have one part of the body in the wrong place or a copy of a normal part of the body in another place.
NARRATOR: The scientists had triggered the changes by damaging the flies' DNA.
Within each cell of the developing embryo is a chainlike molecule called DNA.
The experiments showed that DNA was somehow causing the embryo to divide into segments.
But how? Scientists were just beginning to grasp that the DNA itself was made up of segments, called genes.
The question was: how did the genes shape the body? One researcher, Dr.
Ed Lewis of Caltech studied this question for 30 years by crossbreeding thousands of flies.
Lewis's work led him to a controversial idea.
He proposed that a surprisingly simple mechanism was shaping embryos.
He wrote that each segment of the fly was being directed to grow by a single gene.
A small set of genes, a kind of genetic toolkit appeared to be laying out the entire body.
And as he looked at these genes, he said "This one affects this part of the body.
"This affects the next part of the body.
And this affects the next part of the body.
" That was an astonishing observation.
NARRATOR: It was astonishing because it seemed too simple.
Nobody else thought single genes were powerful enough to control something as complex as the structure of the body.
Skeptics argued that Lewis's idea was guesswork.
Of course, he had never seen the genes because the techniques to do so didn't exist.
From the 1920s to the 1970s it was not possible to physically isolate any specific gene.
That opportunity first became available, fortunately for me at the time that I was a student.
And so, many of us thought, "Wow.
"We can finally dig in there and identify these really mysterious genes.
" NARRATOR: Levine enlisted his friend and fellow scientist Bill McGinnis.
The first gene they went after had an unusual name.
Antennapedia, which means "antenna leg.
" The gene was thought to control the growth of legs.
When the gene misfired flies grew legs in the wrong place: on their heads, in place of antennae.
In normal flies, legs grow from the midsection the area called the thorax.
So Levine and McGinnis decided to hunt for the gene in the thorax of a normal embryo.
LEVINE: The expectation is that antennapedia would be active expressed in the thorax the developing thorax, of the embryo.
But who knew? NARRATOR: Levine and McGinnis had to do something no one had ever done before.
They had to find a way to see a gene in action.
LEVINE: We wanted to light up the gene and it was very painstaking work.
NARRATOR: The project called for new and untested methods.
McGINNIS: At first, it didn't work very well and there were a number of technical problems to solve.
NARRATOR: The team had to find a delicate balance of radioactive probes and toxic enzymes.
Too much of either would destroy the embryos.
LEVINE: The process was not very gratifying on a day-by-day basis.
Unbelievably tedious.
NARRATOR: It took months of trial and error.
McGINNIS: People often said, "You know, you should try something else.
"You know, this is too long-shot.
You know, you're going to you're just wasting your time.
" But we kept going.
NARRATOR: Finally, late one night, all the work paid off.
LEVINE: And there was this moment when we saw that the gene was turned on in a band in the middle of a very early embryo.
This had never been seen before.
NARRATOR: The antennapedia gene was acting like a master switch turning on the segment of the embryo that would become the thorax.
The implications were mind-boggling: if single genes like antennapedia could define whole segments of an animal these genes were acting like architects of the body.
And if one of these genes turned on in the wrong place striking changes to the body could result.
It seemed that Levine and McGinnis had uncovered the genes responsible for the evolution of bodies.
But there were still doubts.
The work had all been done in fruit flies.
What about other animals? Did they use the same mechanism to build their bodies? An answer would come from Switzerland.
In 1994, Walter Gehring of the University of Basel isolated the gene that triggered the growth of eyes in fruit flies.
The gene was called Eyeless because flies without it developed with no eyes.
Gehring knew of a gene in mice that worked in the same way.
He wondered, were the two genes the same? GEHRING: And this question we tested by taking the mouse gene and putting it into fruit flies to see whether flies can understand the message of the mouse.
NARRATOR: Gehring replaced a fly's gene for eyes with the mouse gene.
GEHRING: And to everybody's surprise the mouse gene works perfectly well and can induce a compound eye in the fruit fly.
NARRATOR: The fruit fly grew normal fruit fly eyes using a gene from a mouse.
Not only did the two creatures use the same mechanism; they used the same gene.
This was the mechanism behind extra wings legs sprouting from heads, and Bateson's deformed animals.
The century-long search was complete.
The genetic engine of the body's evolution turned out to be a tiny handful of powerful genes.
CARROLL: So what this means is in some ways, some sense, evolution is a simpler process than we first thought when you think about all of the diversity of forms out there.
We first believed that this would involve all sorts of novel creations starting from scratch, again and again and again.
We now understand that no, that evolution works with packets of information, and uses them in new and different ways and new and different combinations without necessarily having to invent anything fundamentally new, but new combinations.
NARRATOR: Suddenly, the commonality of form among animals was understood: animals resembled each other because they all used the same set of genes to build their bodies a set of genes inherited from a common ancestor that lived long ago.
And what we see now among all the animals are just variations on a body plan that existed half a billion years ago.
And there's only one inescapable conclusion you can draw from that, which is if all of these branches have these genes then you have to go to the base of that which is the last common ancestor of all animals and you deduce, itmust have had these genes.
So the whole radiation of animals the whole spring of animal diversity has been fed by essentially the same set of genes.
NARRATOR: Ed Lewis shared the Nobel Prize in 1995 for the discovery of the universal set of genes that builds the bodies of animals.
And so, yes, it came as a huge surprise not only to people like my mother, who says "My God, an earthworm and a mouse? An earthworm and me, you know, share things in common?" But it came as a surprise to other scientists that there was this profound conservation of mechanism of building embryos among all these different kinds of animals.
NARRATOR: What about us? Our bodies are built from the same genes that build all other animals.
Yet we are different.
No other animal designs or creates like we do.
We seem so special it's hard not to think that we're somehow an exception to evolution but of course, we're not.
The transformation that led to us was no different from other transformations.
Our crucial turning point seems to have occurred when our ancestors left the trees and began to walk on two legs.
MAN: We don't know exactly when, or even where our ancestors became upright and bipedal.
We think it goes back well over four million years.
When these ancestors came out of the trees and began to exploit food sources on the ground in terrestrial habitats one of the key elements that would've been so useful to them would've been freeing their forelimbs, their hands to be able to gather and carry foodstuffs over long distances.
Once that happened, it opened up an extraordinary breadth of possibilities and opportunities.
NARRATOR: Most bipedal hominids went extinct but one branch went on to evolve larger brains.
That branch eventually led to modern humans.
So how did this crucial transition to two-legged walking begin? Liza Shapiro of the University of Texas looks for clues in living primates.
SHAPIRO: When you look at the fossil record all you have really is a pile of bones.
It's a nonmoving entity.
There's not much you can know about it unless you look for living analogs.
So if you look at living animals, you've got the bones but you can also look at how they're moving.
NARRATOR: In their movements, living lemurs resemble tree-dwelling primates that lived up to 50 million years ago.
We didn't evolve from lemurs but they may be the best living analog for those distant ancestors.
SHAPIRO: When we're trying to reconstruct the scenario about how humans evolved bipedally from this ancestor we have to know what it was we started from if we're going to come up with an explanation for not only how we made that transition, but why.
NARRATOR: Today, Shapiro is gathering data on the movement style of the lemur.
Small reflectors have been gently placed on the animal's back.
An array of infrared cameras will record the lemur as it walks across this makeshift bridge.
Of course, getting a lemur to do just about anything on cue takes a bit of doing.
There you go.
NARRATOR: Finally, the animal makes it across.
Here you go oh, good! All right.
Got that.
And he's down.
NARRATOR: The motion of the lemur's spine can now be analyzed in three dimensions.
The data reveal that lemurs' spines are extremely flexible capable of many kinds of movements.
SHAPIRO: Lemurs walk quadrupedally but they're also very good at leaping.
NARRATOR: Like these lemurs, the early primates probably moved in all sorts of ways: down on all fours, scampering up trees even leaping in an upright position.
They weren't limited to just one style of movement so they could serve as the starting point for a number of evolutionary experiments.
And most likely, that's just what happened.
We weren't the only ones to evolve from those early ancestors.
So did most of today's living primates.
Our closest living relative is the chimpanzee.
We didn't evolve from chimps but we do share a recent common ancestor.
Can you walk over here? NARRATOR: That's why our DNA is nearly identical to theirs and why our skeletons have the same number of bones arranged in nearly the same way.
But the few physical differences that set us apart seem to have made a great difference.
Chimps don't walk on two feet.
They've evolved a different style of getting around called knucklewalking.
JOHANSON: Knucklewalking is a very specialized adaptation that we see among chimps and gorillas today.
It's an adaptation to walking on the ground.
NARRATOR: Knucklewalking was as valid an evolutionary experiment as two-legged walking.
But the difference in our walking styles which may have affected our intellects is seen in the few slight differences in our skeletons.
Here are two skeletons of modern primates.
This skeleton I'm sure you'll all recognize because it's a skeleton like yours and mine.
This is a modern human.
But this smaller skeleton is one of our closest living relatives, the chimpanzee.
We began walking on two legs and that made a whole series of modifications in the skeleton.
In humans, the spinal chord comes out of the base of the skull and points straight downwards rather than coming out of the back of the skull.
The pelvis is shaped very differently.
A chimpanzee has a long, narrow pelvis.
Ours is short and squat.
We walk with our knees close together.
Chimpanzees walk with their knees wide apart.
These are minor differences.
These are the sorts of tinkering that evolution did to change us into a modern biped.
NARRATOR: What if our ancestors hadn't stood up? What if they had taken one different turn along the path to becoming human? JOHANSON: One of the great misconceptions that most people have is that that once our ancestors stood up, it was almost inevitable that we would be here today that the egocentric species, Homo sapiens would evolve in this manner.
But what we see is that evolution has worked the same way with us as it has with every, single organism on this planet.
We're here through a series of chance coincidences specific adaptations, chosen opportunities.
So I think that when we look at our own origins we see that it is extraordinary that humans are here to look back and ponder their past.
SHUBIN: Does that mean we are not unique in many ways? Of course not.
We're the ones telling this story.
And that's very important that evolution, that life has gotten to the point where it can tell this story.
Continue the journey into where we're from and where we're going at the Evolution web site.
The seven-part Evolution boxed set and the companion book are available from WGBH Boston Video.
To place an order, please call: