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. [MUSIC PLAYING] 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. [VOLCANO EXPLODING] 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. [MUSIC PLAYING]
