Practice English Speaking&Listening with: Great Transitions: The Origin of Humans — HHMI BioInteractive Video

SEAN CARROLL: Explaining the origins of key traits

that distinguish species has long

been one of biology's fundamental quests.

That's especially true for our own species.

If we look at humans, as a biologist would any animal,

certain features stand out--

our big brains, the way we get around on two legs,

instead of four, and the way we use our free hands

to make tools.

Each of those three traits marks an enormous difference

between us and our primate relatives.

But when did they evolve and in what order?

The quest to understand our past has revealed much

about the evolution of these features--

all of the milestones in the great transition

from apes to humans.

It was many years after Charles Darwin

had published his theory of evolution

that he finally addressed the question, "What about us?"

He speculated that we are descended

from a common ancestor we share with African apes.

The hope was that some geologists or paleontologists

would one day recover the fossils that

would settle the question.

Fossils are essential evidence when putting together

an evolutionary history.

But in Darwin's day, and for many decades after,

few early human fossils had been found anywhere.

Anthropologists Louis and Mary Leakey

thought Darwin was right about Africa,

so they searched for early human fossils

in places like Tanzania's Olduvai Gorge.

Here, they found abundant stone tools.

But for the longest time, the bones they sought eluded them.

For almost three decades, all the Leakey's is found

were tools--

tools, tools everywhere, but not their makers.

But all that finally changed the morning of July 17th, 1959.

On a hill Mary had walked by countless times,

something caught her eye.

Poking through the eroding sediment was a huge upper jaw.

Together, she and Louis carefully

extracted bones from the skull of an early hominid.

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Geochemists analyzed the sediment layer it was buried in

and determined this hominid had lived a stunning

1.76 million years ago.

Remarkably, the very next year, the Leakey's

made another discovery.

They designated it Olduvai Hominid Number 7.

It too was almost 1.8 million years old.

But the recovered skull pieces, and finger, and wrist bones

led them to conclude it was a separate species

of early hominid.

So there were at least two different evolving lineages

of humans alive at this time.

These discoveries helped swing the focus of human paleontology

to Africa.

Detailed casts of these, and many other fossil finds,

are kept at the Human Evolution Research Center

at the University of California at Berkeley.

Dr. Tim White, the center's director,

has been involved with many of the important hominid

discoveries of the past four decades.

TIM WHITE: Clearly, it was a hominid.

SEAN CARROLL: I asked him what the current view

is of the Leakey's first discoveries.

TIM WHITE: Well, I guess after chasing

the toolmaker for so many years, they initially thought,

oh, we've found the toolmaker.

But it turns out this large crest, the huge back teeth

of this show that it's on a side branch of human evolution--

probably not the toolmaker.

But fortunately, the next discovery

was Olduvai Hominid Number 7, with a cranium

much larger in size, and a face much smaller in size,

and probably the maker of these very primitive stone tools

from the very bottom of Olduvai Gorge.

SEAN CARROLL: The early humans found at Olduvai

were bipedal tool makers with brains not as big as ours,

but larger than those of modern chimps, our closest primate

relatives.

So all of these traits must have evolved

between 1.8 million years ago and whenever

the human and chimp line separated.

And when did that happen?

At that point, no one could say.

But then, Alan Wilson and colleagues, here at Berkeley,

developed a revolutionary new way

to use biomolecules, including DNA,

to estimate the time of that split.

Using this approach, researchers have

estimated that humans and chimps have

been evolving independently for almost 7 million years.

DNA tells us that our lineage goes back several million years

before the Olduvai fossils.

What DNA can't tell us is where and when

the traits that distinguish us, like bipedality, first emerged.

Only fossils and their ancient environments

can address those questions.

Eastern Africa is a fossil treasure trove

because of the geological forces that

have created the rift valleys that scar the region.

Over the eons, volcanoes associated with this rifting

regularly blanketed the region with ash

that included radioactive elements--

the steady decay of which allows geologists

to accurately date sediment layers and the fossils

within them.

Paleontologist Don Johanson remembers vividly

the first time he visited the Hadar region of Ethiopia.

1,000 miles north of Olduvai, it has

exposed sediments that are over a million years older.

DONALD JOHANSON: We drove up to the edge of this escarpment,

and it just unfolded.

And there it was, all of the sediments

getting deeper, and deeper, and deeper.

I could not wait to get down there.

The driving force was find something.

And then we walked out there.

SEAN CARROLL: Johanson recently shared

with fellow paleontologist, Neil Shubin, his memories

of the day he discovered the first small bone

fragment of one of the most famous fossil skeletons ever

found.

DONALD JOHANSON: My best recollection

is that it was right in this area.

And I looked at it, and almost instantaneously

said, that's a hominid.

Just a fragment of elbow that led to the skeleton.

SEAN CARROLL: An international team of scientists

helped Johanson recover almost half the bones

of an individual who had lived 3.2 million years ago.

They called her Lucy.

DONALD JOHANSON: Finding Lucy was really

the first step in this very long process of description,

investigation, evaluation, hypothesis testing,

trying to figure out where in the world she sat,

like we are on the human family tree.

SPEAKER 2: Something like this, and you put the male--

TIM WHITE: This is the Lucy skeleton, found

by Don Johanson in Ethiopia.

She's 3.2 million years old and very representative

of Australopithecus, the next, earlier phase

of human evolution.

And they are bipeds, relatively small brains, and no evidence,

so far, of any stone tool use.

SEAN CARROLL: So the stone tool use comes in much later

than Lucy and her brethren.

TIM WHITE: With early Homo.

SEAN CARROLL: What can we tell about this creature

from the fossils?

TIM WHITE: When we look down here into the pelvis,

we see evidence for bipedal walking--

a commitment to walking on two legs that

is very different from what we see among great apes.

So when we look at a chimpanzee, in the hip,

we see the hip bones behind.

They're long, they're tall, they're up the creature's back.

Whereas, in a human, our hip bone

is much broader, front to back, much shorter, and wrapping

around the side to put these muscles

that control pelvic tilt during walking

in an advantageous position.

Then we can ask the question, is Lucy more like a human or more

like a chimp?

She has a very short blade on the pelvis, much more

like a human.

She has muscle attachments much more like a human.

It's a, basically, biped's architecture.

And that's how we know that she walked on two legs.

But there was a little bit of controversy, even after that.

Some people said, well, how can we really be sure about that?

SEAN CARROLL: And how can we be sure?

TIM WHITE: Because we found these incredible things

in Northern Tanzania, older than Lucy,

sandwiched between layers of volcanic ash.

And it's not what you think.

It's not bones.

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There was a volcanic eruption 3.75 million years ago.

The volcanic ash came down on the Serengeti Plain.

And animals walked across it.

The ash hardened and was buried.

In the 1970s, I was lucky enough to be with Mary Leakey

out in this area.

And we found the trails of hominid individuals, left

as they walked across that volcanic ash millions of years

ago.

It was an amazing snapshot of time.

They went for meters, and meters, and meters.

There are no knuckle marks, no handprints, just

bipedal footprints.

It looked, more or less, like what you and I

would leave on a beach.

Human feet-- we're all used to them,

but they're really strange.

Our big toe is in line with our other toes.

We don't have a grasping big toe.

We have arches, transverse and longitude, in our feet.

All these features are present at 3.75 million years

ago in Australopithecus.

SEAN CARROLL: So Australopithecus

pushes us all the way back to 3.7 million years or older.

She's small brained, not using tools, to our knowledge,

but walking upright.

So that's telling us that walking upright is yet still

an earlier trait.

What do we know about that?

TIM WHITE: We didn't know very much about it,

because Lucy and her species only went back to 3.75.

So to take the next step back in time,

we had to find older fossils.

SEAN CARROLL: Just 50 miles south of where Lucy was found,

there are exposed rock layers reaching back 6 million years.

This is where Tim White and a large international team

of geologists, paleontologists, and archaeologists

have focused their combined efforts since the early 1980s.

TIM WHITE: What we wanted to do was

to plumb the unknown to figure out what

came before the Lucy species.

SEAN CARROLL: For a decade, what they'd

come for largely eluded them, until--

TIM WHITE: A graduate student at the time,

[? Johannes ?] [INAUDIBLE],, found two little pieces

from the palm of the hand-- just this bone here.

And these little pieces, he picked up and said,

this looks like a hominid.

SEAN CARROLL: The excitement of this and other early finds

quickly gave way to a disciplined search for more.

And there was, indeed, much more to find.

TIM WHITE: Hand, foot, arm, leg, teeth, skull--

head to toe, we had coverage of a creature nobody

had ever seen before.

We nicknamed her Ardi for the genus Ardipithecus.

The species is ramidus.

And it's really a skeleton that is

representative of the earliest known phase of human evolution.

SEAN CARROLL: And how old is she?

TIM WHITE: She's 4.4 million years old.

We know that because these bones were all

found sandwiched between volcanic horizons,

both dated to 4.4 million years ago.

SEAN CARROLL: So that's more than a million years older

than Lucy.

TIM WHITE: It was stepping into that black hole beyond Lucy

that nobody had been able to step into before.

SEAN CARROLL: Removing Ardi from her 4 million

year resting place was a real challenge.

Her bones were ready to turn to dust.

TIM WHITE: That little hill had to be excavated a millimeter

at a time.

We had to use chemical hardeners on her,

extract her in plaster jackets, and then work

on each bone under a binocular microscope with a needle

to clean the encasing sediment from the soft bone underneath.

But what we got as a result of that

is a really unrivaled look at the anatomy

of a very ancient hominid.

We could see the muscle attachments

on the finger bones.

We could see the scratches on the teeth.

It's beautiful anatomy.

SEAN CARROLL: With some real surprises,

especially below the neck.

TIM WHITE: It was an extension in the lower pelvis

that showed that she was a climber.

In the foot, a large toe that stuck out

to the side of the foot-- the first time

this was ever seen in a hominid, even though all other primates

have this.

She is this peculiar mosaic of traits,

capable of bipedality on the ground,

but also climbing abilities far superior to those

seen in later Australopithecus.

SEAN CARROLL: You couldn't possibly have expected this.

TIM WHITE: Nobody could have expected it

because you can't predict this from looking at chimps

and humans and triangulating.

Ardi is neither a chimp, nor is she a human.

She is a mosaic 4.4 million years old--

the step beyond Australopithecus,

a glimpse into that first phase of hominid evolution.

SEAN CARROLL: Buried along with Ardi

was fossil evidence of the habitat in which she lived

and where bipedality evolved.

It wasn't what anyone had been expecting.

For a long time, scientists predicted that bipedality

had evolved in a grassland.

TIM WHITE: The savanna has always

played a big role in people's speculations.

And what we had with Ardi was evidence

from her body and, indeed, her chemistry,

as well as evidence from her environment

that showed she was not adapted to an open grassland savanna

existence, even though she had already achieved bipedality.

SEAN CARROLL: That evidence included tens of thousands

of animal and plant fossils, indicating

that she was living in a woodland setting, not

an open African savanna.

So bipedality evolved while our ancient ancestors were still

spending time in the trees.

TIM WHITE: Ardipithecus took away any doubt

that bipedality was ancient.

And it was so ancient that it preceded

by over a million years, the expansion

of the brain, the incorporation of stone tool technology.

SEAN CARROLL: We now have thousands of hominid fossils

from the past 6 million years.

They reveal several phases in the biological evolution

of humans.

TIM WHITE: You have an early phase, Ardipithecus,

whose anatomy allows it to climb in the woodlands

and walk on two legs.

We see Australopithecus is the next phase--

Lucy, a representative of this.

It's a committed biped with a small brain,

but still big teeth for chewing, big robust faces.

Their niche has expanded beyond Ardipithecus.

They're in more open habitats.

They're found throughout the African continent.

And then the third phase of human evolution

is our own genus, the genus Homo.

And here, we have a creature that

really is a technological primate, depending

more and more on culture.

SEAN CARROLL: Stone tools allow early humans to compete, first

with scavengers, and then with predators.

They broaden their diets and, ultimately,

their geographic range, leaving Africa.

Recently, in the Republic of Georgia,

hominid fossils were discovered that are

as old as the Olduvai fossils.

They include the most complete early Homo skull ever found.

TIM WHITE: That is going to give us insight

into the biology of our ancestors,

the ancestors of Homo sapiens.

And it's a great illustration of how paleontology is not

a dead science.

Paleontology is the science by which we learn about our past--

how we became human.

SEAN CARROLL: And what that science shows

is that like all animals, we have

a long evolutionary history.

Just as four-legged animals evolved from fish ancestors,

and birds evolved from dinosaur ancestors

over a series of small steps over a long geological time

span, we evolved from small-brained, quadrupedal apes

over a long time span that is now well documented

in the fossil record.

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