BECOMING HUMAN PART PBS NOVA Two million years ago the by liaoqinmei


									                               BECOMING HUMAN PART 2
                                                     PBS NOVA
          Two million years ago, the Great Rift Valley of East Africa. witnessed a mysterious event: the birth of the
first ancestor we can really call human. New discoveries are revealing a creature surprisingly like us, a world trav- eler, a
toolmaker, a hunter, tamer of fire, creator of the first human societies. Amazingly, the qualities that make us human began
not with our own species, Homo sapiens; the true birth of humanity began much further back in time, millions of years
ago. Imagine the entire span of recorded human history, taking us back to the Egyptian pyramids,
about 5,000 years. Double it: 10,000 years ago, when plants were domesticated and agriculture begins. Double it
again, 20,000 years: Ice Age hunters are painting stunning images on cave walls. And keep doubling six more
times: only then do we encounter our ancestor, Homo erectus, in Africa's Great Rift Valley. For millions of years, this
massive geological fault line running the length of East Africa was a stage on which our human evolution was
played out. It all started with the first apes to walk upright on two legs, about 6,000,000 years ago. There were many
different types, all variations on the same theme: ape-like creatures with small brains.
Turkana Boy
          The fossil known as Lucy is the most famous ex-                                     ample. Here she is: just three-
foot, eight-inches tall, with a brain the size of a chimp's. For                              millions of years, creatures
like her roamed the forests and grasslands of Africa, but                                     then something changed.
about 2,000,000 years ago, new creatures appeared with                                        abilities never seen before in
the animal kingdom. Meet Homo erectus, a toolmaker and                                        hunter, one of the first mem- to
bers of our genus, the genus Homo, humans. The transition the                                 Homo was probably one of
most important transformations that occurred in human legs                                    evolution. Arms got thinner,
got longer, brains got bigger. It was a huge evolution-                                       ary step from ape bodies to
bodies like ours. But what about the things that make us                                      distinctly human? Creativity,
intelligence, caring for each other; how can we know when                                     these got started? With only
skulls and bone fragments to go on, how could we ever                                         know what those first humans
were really like? It would take a momentous find to shed light on their lives. Lake Turkana, in Northern Kenya is
surrounded by volcanoes and vast expanses of baking desert. In 1984, famed anthropologists Richard and Meave
Leakey were working at this remote inland sea. As the first family of paleoanthropology, the Leakeys were used to
fossil finds. But this was very special. One of Leakey's team had found a skull fragment of one of those early hu- mans.
He could tell from its size and shape it was Homo erectus, and there was more than just a fragment. A com-
plete skull was rare enough, but it was just the beginning. Soon parts of the Homo erectus skeleton which had never
been found before, started to emerge.
          The Leakeys had found the actual bones of a human ancestor who lived over one and a half million years
ago. The Leakeys called him "Turkana Boy." His bones have revolutionized our understanding of the transition
from ape to human. The really important thing about Turkana Boy is how complete he is. There are arms and legs,
and bits of his spine and his ribs. What did he look like? His skeleton tells us he was five-feet, three-inches tall with a
build closer to a man's than an ape's. But how close? As Turkana Boy's forensically reconstructed head
nears completion, a face emerges that looks a lot like us. Now, for the first time in a million and half years, here he is, our
ancestor, the Homo erectus called Turkana Boy. But what he looked like is only the beginning of his story. To reconstruct
his life, we need to find out how old he was. If we look at his skeleton, we can see that the growth
plates on his limbs that would fuse when he's fully adult are all unfused, so even though he's very tall, we know
that he's still growing. The fact that Turkana Boy was not fully grown has turned out to be a boon to researchers. When
scientists compared his bones and teeth to ours, he seemed to be about 14 years old. But when dental spe-
cialist Chris Dean began to study his teeth he was in for a shock. It turns out that all teeth, fossil or not, preserve a
remarkably precise record of childhood. Turkana Boy wasn't fourteen years old. He was eight. What that implies is that
the growth of the Turkana Boy resembled more closely that of chimpanzees today. To be five-foot-three at age
eight, Turkana Boy must have grown up very fast, at a rate closer to chimps than us. A chimpanzee's childhood is
short. It is sexually mature at about seven. Human childhood is longer. We reach puberty at about 12. So as hu-
mans evolved from apes, childhood was extended. But what advantage could be gained by having helpless children
around to feed and care for, who take so long to grow up? The mystery of prolonged childhood is at the heart of
human evolution. It may be related to brain size. We humans have the biggest brains in the animal kingdom in rela-
tion to our body size. They are so big that most of our brain growth has to happen outside the womb or our heads would
never get through the birth canal. A long, slow childhood gives our brains time to grow after birth and time to learn
everything we need to function in our complex human societies. That's the advantage of prolonged child-
hood, for us at least. But what about Turkana Boy? His brain was 900 cubic centimeters, smaller than ours but
more than twice as large a chimp's. So was he on the way to thinking and talking like us? Some scientists believe he
was. An endocast is a mold taken from the inside of the skull which reveals the shape of the brain. In the brain of
Turkana Boy, Ralph believes he sees evidence for something remarkable, a change in the area tied to communi- cation.
The Broca's caps regions on the Turkana Boy are fully modern in terms of their appearance. It is good solid evidence for
having the ability of symbolic communication, in other words, language. It's a controversial idea, and we'll never know
for sure if Turkana Boy could speak. But there are other clues to his intelligence: the stone tools he left behind.
Tool Making
          Homo erectus made tools like the hand ax. It's been chipped extensively on both sides. The point enables one to
do piercing tasks; the heavy bit can be used for cracking bone or chopping wood. It's a very, very versatile tool, and a
sharp one. It may not look like much, but the stone hand ax marks the birth of technology. Homo erec-
tus has left us many signs of his inventiveness. Here in Central Kenya, Rick Potts has been studying a treasure trove
of Homo erectus stone tools. Stone tools represented a momentous change, because once you had tools in your hands,
all the foods in the world could open up to you. That represented a tremendous survival advantage.
Here is a cache of over 500 stone hand axes made by Homo erectus. Just a mile away, Rick visits the quarry where,
for thousands of years, these ancestors came to shape stone into tools, leaving behind unused fragments. A good
toolmaker has to understand the properties of stone. A skilled craftsman, Homo erectus had evolved a new type of
intelligence, but his bigger brain came with hidden costs.
          Modern brains consume 25 percent of our body's energy. Our brain happens to be the hungriest organ in the
body. And in order to support a brain our size, we need lots of calories. With his big brain and body, a Homo
erectus like Turkana Boy needed more nutrition from his savanna environment. Turkana Boy, had he grown to
adulthood, would probably have stood about six feet tall. This is a big, strong creature that would have had a huge energy
budget. One can satisfy an energy budget like that by eating plants, but you have to eat a lot, a lot of plants.
But there's one food that can supply the nutrients a growing brain and body need. And Africa was filled with it!
The one high quality resource that's probably most important for the evolution of the genus Homo is meat and meat
byproducts, such as brain and marrow and fat. They're high in protein, they're high in calories, and they're easy to
digest. But the one problem with getting meat is that it's hard to get. Most predators rely on strength or speed to kill
their prey. Our ancestors had neither. Today we are on top of the food chain, so it's hard to imagine the predica- ment
of those early humans. Here was a slow moving creature with no claws or fangs, easy prey for the hungry
predators around him. So how did Turkana Boy—a weakling with a big brain which needed calories—get his
meat? Homo erectus faced a problem: how do you kill a big, dangerous animal that has lots of meat and fat in it,
without that animal also killing you? Some scientists think the answer to that was a very clever set of innovations,
and that is endurance running and high activity in the middle of the day. The ancestors of Homo erectus, small,
hairy apes like Lucy, were bipedal but probably didn't do much running. But Turkana Boy's kind were built to run, like
us. It is believed that they could run long distances because, like us, they had lost their thick coat of body hair, and could
keep cool by sweating. This was the key to their success. But how do we know if these crucial changes
go back all the way to Turkana Boy's time, over a million years ago?
          Through the study of our varying types of body lice we know that long before Turkana Boy, maybe even
around Lucy's time, our ancestors had slowly begun to lose their body hair. Turkana Boy was mostly hairless, just
like us, and that may be what gave him an edge over other predators. Most animals are at a disadvantage in the
midday sun because they overheat. They can only cool down by panting. And when they run fast they can't pant.
That means they can only run in short sprints. Quadrupeds can gallop for about ten to fifteen minutes and then they
overheat, but hominids can cool down by sweating. They use their entire body like a dog's tongue. Our hairless
bodies allow air to circulate freely on our skin and cool us down as sweat evaporates. This makes us one of the best long-
distance runners in the animal kingdom. Dan Lieberman believes this gave our ancestors the ability to hunt in
a very unusual way. It's called persistence hunting, and he believes the modern ethnographic record can show us
how it was done. The Bushmen of the Kalahari offer us an insight into how Homo erectus might have hunted
2,000,000 years ago. The Bushmen know that at midday animals rest in the shade, which is why it's the perfect
time to be hunting. Once they locate their prey—in this case a kudu—the marathon begins. Their strategy is sim-
ple: run it to exhaustion. Every time the animal tries to rest, the hunters track it down and get it moving again.
They never give it a chance to cool down. And the reason they can keep going is that they can sweat. So if the the- ory is
right, the Bushmen hunt may help explain how Turkana Boy got his meat. Homo erectus had come up with an innovative
way of feeding his hungry brain. In a modern hunt, the Bushmen ran in the fierce heat for over four hours. The kudu was
finally immobilized by heat stroke. Turkana Boy wouldn't have had steel-tipped spears like
the Bushmen, but he wouldn't have needed them. Homo erectus probably hunted with close-quarters weapons, with
spears that were thrown at animals from a short distance, clubs, thrown rocks, weapons like that. They weren't us- ing
long distance projectile weapons that we know of. The Homo erectus hunt was simple but effective. It fed not just their
larger brains but the growing complexity of that early human society.
          There are other social animals, but none quite like us. Society is in every corner of our lives, our relation-
ships, communication, rules, symbolism, all the things that bind us together. What's behind it? Why did we become
so social? Could it have something to do with another innovation,
something unprecedented in our evolution: building fires and cooking?
The earliest evidence that our ancestors deliberately
used fire for cooking dates to long after Turkana Boy's time. But
Richard Wrangham is sure Homo erectus was building fires much
earlier. Now, for the first time, we had a species that was
committed to living on the ground because they lose their climbing adaptations. So how were they sleeping? They
had to be able to protect themselves from wild animals. On the African savanna, full of predators who hunt by
night, Richard believes Turkana Boy and his people couldn't have survived without fire. And he thinks only cook- ing,
which makes food more soft and digestible can explain why Homo erectus evolved smaller teeth and a much
smaller gut. These things are compatible with the reduced cost of digestion, produced by cooking food. Nothing
else is. As our ancestors reaped the benefits of cooking, something else happened too, at least according to
Wrangham: we became more social. Wrangham believes we learned to share and communicate sitting around fires,
waiting for food to cook. It's speculative, but one thing is for sure: in the Homo erectus world, new social relation- ships
had to be evolving. The bonds between mothers and children must've been very different from the apes. For example, a
mother orangutan will not allow any other individual to take her infant. She maintains constant skin-to-
skin contact with that baby for at least the first six months of life, not a moment out of contact. Secure in this un-
breakable mother-infant bond, ape babies need less capacity to read the intentions of others than human babies,
whose bond with their mothers is surprisingly less secure. The shocking fact is that human mothers abandon their
infants much more often than ape mothers. Infanticide by a mother is more common among humans than any other higher
ape. Maternal commitment is a lot more contingent in humans than it seems to be in apes. Unlike most pri-
mates, human mothers share parenting with others. A child's survival can depend on making itself appealing to a
number of caregivers. Perhaps that's why human infants have evolved a uniquely acute sensitivity. We are born
hard-wired with an awareness of the intentions and emotions of others, which is unique in the animal world. When
did humans develop this gift for attributing mental states and feelings to others and for caring about what others
thought about them? Could these social instincts have developed with Homo erectus? Along with cooperative
hunting, bigger brains, longer childhoods and the use of fire? Perhaps Turkana Boy and his people already had
social skills that would be familiar to us. Here were intelligent social beings with an increasing capacity for coop-
eration. It may be this that made possible another great achievement, the exodus from Africa. For millions of years,
our earliest ancestors stayed on the African savannas, but at some point they started to leave. Ancient fossil skulls
and tools have been found as far away as China and Indonesia. The question is: when did they leave Africa and
why? When Turkana Boy was found, scientists thought they had the answer. Here was a strong, large-brained an- cestor,
capable of an arduous migration. He had the look of a world conqueror.
Surprise in Dmanisi
          In the mid-1980s scientists were thinking that a hominid like Turkana Boy had left Africa, but had done it
maybe about a million years ago. For decades, scientists believed big, strapping humans, like Turkana Boy, left
Africa a million years ago. But new discoveries are showing the migration may have started a lot earlier than that.
Dmanisi, Georgia: the mountains and plains of the Caucasus, thousands of miles from the Great Rift Valley, had
                               never produced any fossils of early human ancestors, but then an astonishing discovery was
                               made. It was a lower jaw, with teeth downward, in the ground. When cleaned and those front
                               teeth came to light, it became obvious that some kind of hominid had been found. But what
                               kind? The jaw seemed to be a primitive form of Homo erectus, but at first hardly anyone
                               believed it. Since then, Dmanisi has been put on the map of human evolution in a big way.
                               The site has turned up a treasure trove of Homo erectus fossils. They've transformed our
                               understanding of who left Africa and when. They showed that
                               the first humans to leave Africa were much more primitive than Turkana Boy. At four and
                               a half feet tall, they were smaller than Turkana Boy, with more ape-like shoulders
and a simple stone technology. They were much more primitive. They had small brains. At the same time they
were using very primitive stone tools. The next surprise came when they dated the site.
          The ancient Dmanisi landscape has been built up, layer by layer, over millions of years. One-point-eighty-
one million years ago, massive volcanic eruptions deposited a layer of ash. The fossils sat on top of this ash, so
must have been slightly younger, around 1.8 million years old. To the vast majority of scientists who believe that
all our ancestors evolved in Africa, this was a stunning surprise. How had a small, primitive Homo erectus mi-
grated to the Caucasus almost 2,000,000 years ago—long before Turkana Boy? Scientists now accept that as soon as
Homo erectus appeared on the savannas of Africa, they started to leave. The Georgia fossils proved that Homo erectus
left Africa much earlier than previously thought. An even more provocative find shows the migration may have started
even earlier.
Hobbit Human
          Five thousand miles from Africa: the island of Flores, Indonesia. In 2003, researchers made a discovery
so strange nobody knew what to make of it. They found the bones of a tiny human ancestor, just over three feet
tall, even smaller than the Dmanisi fossils. They called this baffling new ancestor Homo floresiensis, and, because of its
tiny size, nicknamed it "the Hobbit." According to Ralph Holloway, "This has created a tremendous amount of grief,
because we're not really sure of what we're seeing here. The size of the Hobbit brain endocast is roughly 400 CCs." That's
barely bigger than the brain of Lucy, the famous bipedal ape from 3,000,000 years ago. It's not just a small brain and a
primitive looking face, but the foot's primitive, the hand's primitive, the leg is primitive.
The lower limb is very much like the Lucy skeleton. That was a big surprise. And in the cave where this primitive
creature was found, they also uncovered stone tools, something Lucy never had. People have, for a long time, said,
"You need a big brain to make stone tools." If Homo floresiensis is making stone tools—this creature has a brain the
size of an orange—clearly that equation's gone.
          Everything about these creatures is an enigma. Where did they come from and what were they? Some
researchers have argued that floresiensis is just a dwarfed population of modern people that suffered some kind of
disease that caused them to both dwarf and have relatively small brains. But when scientists took a closer look,
most saw no evidence of disease. The stone tools and the shape of the face moved the focus to our old friend,
Homo erectus. Some researchers also think that Homo floresiensis evolved from Homo erectus. But how did they get so
small? Something called "island dwarfism" may be the answer. Isolated on islands with limited food, large mammals
sometimes shrink, over time. On Flores, there were once pygmy elephants the size of cows. Could the
same evolutionary pressure have acted on Homo erectus to produce the Hobbit? Or was this
mysterious creature descended from an even more primitive ancestor? Per- haps we're
sampling a period which is at the very beginning of the Homo lineage. So whatever the
Hobbit was, perhaps its ancestors were the very first wave of migration
out of Africa, some unknown creature: part bipedal ape like Lucy and part Homo
erectus. If that's the case, then what we see in Indonesia makes sense, it's kind of a body
that existed before human bodies became more modern.
Exodus from Africa
          What would push such primitive creatures out of Africa? A key driving force behind the migration was
probably a climate shift which spread grasslands from Africa into Asia; and with the grasses went the game ani- mals.
Animals are going to be moving out of Africa, and the hominids will just be keeping pace with those ani-
mals. After all, that's their livelihood. Our ancestors didn't know they were leaving Africa. They just followed the
animals they depended on through the Sinai up into the Middle East and beyond. It's often been called an exodus,
but it really wasn't like that. When people think of exodus they think of the Bible, or if they think of migration,
they think of Europeans coming over here to the New World. It probably wasn't like any historical migration, this
dispersal of humans out of Africa. The process was probably very, very slow, much like the spread of any other
animal species into new territories. You could imagine a group of Homo erectus moving their range a kilometer a year in
one direction. And doing that continually over a long enough period of time, you can get the distance from Africa to
Indonesia covered in say, 15,000 years.
Caring for the elderly
         By a million years ago our ancestors had populated Asia from the Caucasus to Indonesia. And they were
in Europe too, as a recent discovery in Spain has shown. Homo erectus had conquered the Old World. The fact that they
made it so far with limited technology and relatively small brains makes them seem even more remarkable.
Their longevity was astonishing. A few pockets of Homo erectus may have been still clinging on in Asia just
50,000 years ago. That's a span of two million years. Our own species has only been around for 200,000. What was
the secret of Homo erectus' success? The amazing finds at Dmanisi have given us one last clue. One of the skulls
belonged to an old man. His jaw bone revealed he had lost all his teeth, well before he died. That was a real sur-
prise. It means that this individual survived two years without teeth. For an elder to have survived that long without
teeth must mean that others in the group were feeding him, perhaps even chewing his food for him. This was a
remarkable testimony from the past about the quality of emotional life that may have characterized Homo erectus.
         This is a tantalizing clue to what may be this ancestor's most important legacy: the instinct to look after each
other. And it helps us imagine Turkana Boy's final day on Earth. He starts the day out on a hunt, but he has
trouble keeping up with the hunting party. Why? The evidence from his skeleton is that he was sick and in pain at
the time he died. Under his teeth that he has a bit of an abscess and an infection. That kind of an infection could
have entered the rest of his body, could have killed him. An abscess that ate away that much of his jawbone, would
have been agonizing. Turkana Boy is in so much pain he's unable to continue the hunt. Knowing he would be
looked after, perhaps he returned to his campsite to find comfort among the females. However, as much as they may
have wanted to help him, there was nothing they could do about the infection that was probably spreading
through his body. His skeleton was so complete it is likely he died in water, which would have protected him. For
almost 2,000,000 years, his bones were preserved by the earth. The Leakey's discovery opened a window for us on
an unknown world, the world of the most successful human ancestor of all time: Homo erectus. They've revealed to us
that mysterious moment when almost everything human was born: our bodies, our minds, our emotions.
Think of all we've become. Trace the threads of our origins through the ancestors who went before. They all lead back to
Turkana Boy and his kind, the first humans.

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