Mapping Human History: Genes, Race, and Our Common Origins

ISBN-13: 9780618352104

Media Type: Paperback

Publisher: HarperCollins Publishers

Publication Date: 04-01-2003

Pages: 304

Product Dimensions: 5.50(w) x 8.25(h) x 0.76(d)

Steve Olson’s Mapping Human History was a National Book Award finalist and won the Science-in-Society Award from the National Association of Science Writers. Olson has also written for the Atlantic Monthly, Scientific American, and Science. He lives in Bethesda, Maryland, where he coaches the math team at a public middle school.

Read an Excerpt

1
The End of Evolution
The African Origins of Modern Humans

I am an African. I owe my being to the hills and the valleys, the
mountains and the glades, the rivers, the deserts, the trees, the
flowers, the seas and the ever-changing seasons that define the face
of our native land.
— Thabo Mbeki, from a speech delivered upon the adoption of the
constitution of the Republic of South Africa, May 8, 1996

The past autumn has been the rainiest season in southern Africa in
more than a century, and the scrublands of northeastern Botswana are
bursting with life. Hornbills and shrikes glide among the acacia
trees. The bush is rich with flowers and seed. The leopard that lives
in this area, which no one has seen for months, left paw prints last
night a hundred yards from our camp.
About a dozen Bushmen are moving languidly through the
underbrush. They are following the tracks of a small antelope that
passed this way a couple of hours ago, but they are not really
serious about hunting. A young man named Xoma (that’s how he spells
his name in English, though in fact it begins with a complicated
click sound that’s very dif.cult to pronounce) spots a familiar vine.
With a few quick jabs of his digging stick, he unearths a plump tuber
the size of an orange. He hands the prize to a nearby woman, who
stashes it in the leather kaross slung over her shoulder, then
hurries off to join the other men for a smoke.
The lives of these people, who call themselves the Ju/’hoansi
and are also known as the !Kung San, have changed dramatically in
recent decades. Xoma and his family now live in a permanent house
made of wood and tin rather than the thatch huts that the Ju/’hoansi
used to construct when they established a new hunting camp. At school
the Ju/’hoansi children learn the national language of Botswana, not
the complex click-based language their ancestors spoke. They wear
shirts and slacks, not the traditional leather clothes made from the
animals they hunted. Young men of Xoma’s age often leave the bush to
work elsewhere in Botswana or in neighboring South Africa.
But for a few weeks each year, members of Xoma’s village move
back into the bush to live in the old ways. They forage for roots
with weighted digging sticks. They hunt with bows and arrows and cook
the spitted game over crackling fires. They talk and joke for hours
while carving ostrich shell beads or playing an impenetrable game in
which they move stones among indentations scooped from the ground.
Xoma is learning to be a healer. At night, when the Bushmen gather
around the fire to sing and clap the rhythms of ancient songs, he
dances with uncertain steps behind his mentor, learning to achieve
the trance state that will connect him with the spirit world.
Though they are fast becoming part of a cash economy, many of
the Bushmen who live in this part of Botswana still obtain some of
their food by hunting and gathering in the land surrounding their
villages. But disputes with neighboring ranchers and farmers are
common, and the allure of a more modern life is powerful. Whether the
tradition of hunting and gathering will survive for much longer
remains to be seen.
The Bushmen are the original people of southern Africa. (The
equivalent words “Bushmen” and “San” both have derogatory
connotations, but no other terms for this group of people are
available, and many of them prefer “Bushmen” because of its
association with the land.) Their ancestors have lived here for tens
of thousands of years, perhaps for more than 100,000 years. Over that
time the Bushmen developed a way of living in harmony with each other
and with the land. They took what they needed for the present while
ensuring that enough remained for the future. They built elaborate
social networks through marriages, alliances, and trade. They left
many thousands of paintings on rock walls scattered across southern
Africa.
But over the last few millennia, other groups have steadily
encroached on their homelands. Somewhat more than 1,000 years ago,
groups of farmers and herders who were taller and had darker skin
began to push into southern Africa from the north. Gradually the
Bushmen either mixed with the invaders or retreated into less
productive lands. Then, in the 1600s and 1700s, Dutch farmers began
to spread north from the Cape of Good Hope. Although the Bushmen and
their neighbors fought desperately to stop the settlers, gradually
the Europeans prevailed.
Throughout the history of their contact with others, the
Bushmen have been the objects of a virulent racism. Other Africans
have often treated them as vagrants and thieves. (One meaning
of “San” is “untrustworthy.”) Many European farmers, on the other
hand, simply decided that the Bushmen were not human. A late-
nineteenth-century tally from German South-West Africa lists the
animals shot by settlers and policemen over the previous year. At the
top of the list, under the heading “mammals,” is “female Bushmen:
400.”
Denying the humanity of other people has always been a way to
justify oppressing and exterminating them, and science has a long,
sad history of contributing to these atrocities. Well into the
twentieth century, anthropologists were speculating that Africans,
Asians, and Europeans had evolved from different kinds of primates.
The clear implication was that these groups belonged to different
species, one of which was more highly evolved than the others.
But one obvious problem has always plagued this idea. If two
animals belong to different species, they rarely are able to
interbreed. Yet whatever other limitations human beings have, the
inability to interbreed has never been one of them. Southern Africa
today is a genetic hodgepodge of groups descended from the Bushmen
and their pastoral cousins the Khoi Khoi, from neighboring farmers
and herders, and from European and Asian immigrants. The Xhosa, the
group to which Nelson Mandela, Thabo Mbeki, and many other South
African leaders belong, obviously has some Bushman ancestry.
The “Cape Coloureds” are the descendants of European pioneers, Asian
immigrants, and the indigenous people of southern Africa. Many
European South Africans have African ancestors from the early years
of European settlement, when different groups extensively interbred.
One of the great ironies of the apartheid era in South Africa, when
people were divided into the end of rigid racial categories, is that
few countries have such a rich legacy of genetic mixing.

Anyone who lives in Africa can immediately recognize a group of
Bushmen. They are small and wiry. Their skin color ranges from
reddish brown to almost yellow. Their hair grows in tightly wound
tufts and is so brittle that it naturally breaks off. With prominent
cheekbones and delicate features, they are a handsome people by
today’s standards.
Why are the Bushmen so distinctive in appearance? For that
matter, what makes any group of humans recognizable? What accounts
for the distinguishing features we use to categorize people?
Where the Bushmen live certainly has a big influence on their
appearance. The faces of elderly Bushmen are deeply lined from
constant exposure to the sun. Physical activity and diets rich in
vegetables have given most of them a lean, sinewy physique.
But the underlying reasons for the Bushmen’s similarities to
one another require a closer look. Under a microscope, the cells in
the top layer of their skin are indistinguishable from those of
people anywhere else in the world. But deeper in their skin, beneath
the transparent uppermost layers, are the cells known as melanocytes,
which give skin its color. In Bushmen these cells are darker than
those of Europeans and Asians because they contain larger amounts of
the pigment eumelanin. On the other hand, the melanocytes of the
Bushmen are lighter than the heavily pigmented cells of Africans
whose ancestors lived closer to the equator.
Beneath the melanocytes, the differences between the Bushmen
and other people again fade away. Every other type of cell in their
bodies looks no different from the corresponding cells in other
people. In that respect, the differences between the Bushmen and
anyone else on earth are truly skin deep.
But skin color is just one attribute. What about the
Bushmen’s small bodies, pointed chins, and hooded, almost Asian,
eyes? To find the origins of these differences, we have to look into
the nucleus, the small compartment that exists inside almost every
human cell. Floating in the nucleus, in a warm bath of nutrients and
enzymes, are forty-six structures called chromosomes. There are
twenty-three pairs of chromosomes in humans, numbered 1 to 22 in
order from longest to shortest. The twenty-third pair consists of an
X chromosome and a Y chromosome in males or two X’s in females. (The
vast majority of people have the usual complement of twenty-three
pairs, but a few have extra chromosomes — such as individuals with
Down syndrome, who have an extra chromosome 21.)
The pairwise organization of chromosomes reflects the
mysterious dualism of sex. One chromosome in each pair is descended
from a chromosome in the father’s sperm cell; the other is descended
from a chromosome in the mother’s egg. In that respect, each pair is
like a married couple, bound until death. The pairs even engage in
their own form of sex. When an adult organism begins to create new
sperm or egg cells, the chromosome pairs delicately intertwine and
exchange pieces in a process known as recombination. The result is
two hybrid chromosomes, as if a husband and wife had exchanged arms
and legs. These hybrids are separated, packaged in new egg or sperm
cells, and sent on their way to begin the process anew.
The odd couple are the X and Y chromosomes. Egg cells always
contain an X chromosome. Sperm cells contain either an X or a Y.
Fathers are therefore responsible for the sex of their offspring,
though it is largely a matter of chance whether a Y-bearing or an X-
bearing sperm swims up the fallopian tube, finds a fertile egg, and
is the first to breach the egg’s inner sanctum.
Except for the X and Y, the two members of each chromosome
pair are almost identical. (This is where the husband and wife
analogy breaks down.) They have to be, or the cells of the body would
not work properly. For example, when the members of a chromosome pair
exchange pieces during recombination, the chromosomes have to match
up, like partners on a dance floor. If the chromosomes are
incompatible, the dance cannot proceed, and the process of
reproduction grinds to a halt.
When people think about chromosomes, they often recall a
picture from a school biology textbook. At a certain point in the
life of a cell, the chromosomes scrunch up into stubby cigar-shaped
objects. If they are then exposed to a chemical called Giemsa stain,
bands appear around the chromosomes like the stripes on a croquet
mallet.
Except for people with rare chromosomal abnormalities, these
banding patterns are essentially the same for people anywhere in the
world. When male white settlers mated with female Bushmen in the
eighteenth century, their corresponding chromosomes lined up
perfectly. In the search for the origins of the Bushmen’s distinctive
attributes, the chromosomal banding patterns offer no clues.
But chromosomal banding patterns do differ from species to
species. We often hear, for example, that human beings and
chimpanzees are remarkably alike genetically. And, when stained and
compared, some human and chimp chromosomes in fact cannot be visually
distinguished from one another. A careful comparison turns up the
telltale differences, however. Chimps have twenty-four pairs of
chromosomes, not twenty-three, and some of the banding patterns are
subtly different. On nine of the chromosomes, certain segments are
flipped in humans compared with chimps. On other chromosomes, extra
material is tacked onto the ends, or some is missing. These
differences embody the evolutionary distance between our species. Our
lineages have been separated for so long that the structure of our
chromosomes has diverged.
If the banding patterns of the chromosomes tell us nothing
about the differences between the Bushmen and other people, then we
must look deeper. Each chromosome contains a single strand of
deoxyribonucleic acid, or DNA. DNA has achieved an almost iconic
status in our society. Biotech companies build double-helix
staircases for their headquarters. Glossy magazine illustrations show
the molecule twisting away into a dimly seen future. Shampoos trumpet
their DNA content, as if the inclusion of anything from a plant or
animal must be good for our hair. (Counterexamples are easy to find.
A major constituent of DNA, guanine, got its name from guano, from
which the molecule was first isolated.)
The problem with icons is that we tend not to think deeply
about them, which is unfortunate in the case of DNA, because it
really is one of nature’s most amazing creations. First of all,
molecules of DNA can be incredibly long. If the DNA in the forty-six
chromosomes of a single human cell were stretched out, it would
extend from one side of a kitchen table to the other — six feet
altogether. It seems impossible that so much material could be
packaged inside an object smaller than a dust mote. The secret is
DNA’s thinness. If the six feet of DNA on the kitchen table were
enlarged until it extended from New York to Los Angeles, the molecule
would still be no wider than a pencil.
Even more astonishing than the length of DNA is how much
information it can hold. The core of a DNA molecule consists of four
simple building blocks known as nucleotides — adenine, thymine,
cytosine, and guanine, abbreviated A, T, C, and G — strung together
in a chain. For example, a particular section of DNA on human
chromosome 2 consists of the following nucleotides: ATACTGGTGCTGAAT.
But that’s just 15 nucleotides. The twenty-three chromosomes in each
human sperm or egg cell contain about 3 billion nucleotides
altogether — 6,000 times as many nucleotides as there are letters in
this book. Electronics engineers often congratulate themselves on the
amount of data they can cram into a semiconductor chip. They have a
long way to go to catch up with the information density of DNA.
The string of nucleotides in DNA looks like gibberish to us.
But that’s because we don’t speak the language. To the cell, the
messages embodied in DNA are the wisdom of the ages. Each of us
inherited our DNA from our biological mother and father, who in turn
got their DNA from our grandmothers and grandfathers. The first
creatures who could be called human inherited their DNA from
creatures that could not be called human. The first mammals got their
DNA from their reptilian ancestors. And so it goes, back through
time, to the first single-celled organism that began using DNA to
transmit genetic information. DNA is our link to every other creature
that has ever lived on this planet.
If an earnest graduate student took copies of chromosome 2
from two people and began comparing the chromosomes’ nucleotide
sequences, the student would find the two sequences to be almost
identical. But about once in every 1,000 nucleotides, on average, the
two sequences would differ. One person might have an A at that point,
while the other has a G. Or a few nucleotides might be added,
deleted, or transposed in one person but not the other.
Here at last is the origin of the genetic differences between
individuals and groups. All humans everywhere in the world have
exactly the same set of genes. But many of the genes come in slightly
different versions. These differences in the DNA sequences of our
genes lie at the base of our physical uniqueness. They create the
color of our skin, eyes, and hair. They generate the shape of our
skull, the distribution of hair on our head, and the overall contours
of our bodies. They influence our likelihood of getting particular
diseases. They are the biological foundation on which we build our
lives.
Much of biomedical research in the twenty-first century will
revolve around the genetic differences among people. Biotechnology
companies are already studying how DNA varies from person to person
to figure out how these variations contribute to disease. Soon drugs
will be available that are designed to work in concert with each
person’s unique DNA. Eventually, biomedical researchers will figure
out how to change specific nucleotides in the cells of the body and
in the egg and sperm cells that create new people. At that point,
humans will be able to take evolution into their own hands and, for
better or worse, will be able to determine the genetic future of our
species.
But this book is not about biomedical research. It’s about
what DNA tells us about our past. Vast amounts of historical
information have emerged from genetics research in the past few
years, and much more is on the way. It’s as if biologists had
discovered a book written in code by observers from another planet
that recounts the convoluted history of a particularly unusual
species on the earth, the species we know as Homo sapiens.

Two thousand miles northeast of Botswana, the equator first makes
contact with the continent of Africa just north of the city of
Kismaayo, Somalia. From the rocky shoreline the equator sweeps up a
scrub-covered hill, through the banana plantations around the Jubba
River, and onto the barren flatlands that separate Somalia from
Kenya. About 400 miles from the ocean the equator climbs the northern
flank of Mount Kenya, passing just beneath the mountain’s glistening
snowfields, and then dives into the Great Rift Valley, with its
eroded streambeds and hot, flat plains. On the western edge of Kenya
the equator plunges into Lake Victoria, passes almost directly over
the airport at Entebbe, Uganda, and then parallels the Katonga River
to Lake George. On the lake’s west side, the equator rises again to
cross the Ruwenzori Range — the fabled Mountains of the Moon — and
then quickly drops into the basin of the Congo River, passing through
hundreds of miles of thick, sparsely populated rain forest. Finally,
more than 2,000 miles from the Indian Ocean, it cuts through the
tropical lowlands of Gabon and leaves the western side of the
continent 30 miles south of Libreville, a city founded by French
naval officers in 1849 as a refuge for freed slaves.
The four most important events in human evolution probably
all occurred in eastern Africa within some 500 miles of the equator’s
passage across the continent.

• About 6 million years ago, a population of African apes split into
two distinct species. One of those species would lead, through many
intermediary species, to human beings. The other would lead, through
a different set of intermediary species, to modern chimpanzees.

•More than 4 million years ago, one of the species on the
evolutionary path to humans began spending most of its time on two
feet. This upright stance seems to have set in motion a profound
evolutionary trend. This new species could use its hands in new ways —
to manipulate objects, for example, or throw stones to scare away
predators. It could peer over the underbrush and ponder what it was
seeing. No one knows exactly what led to dramatically larger brains
in the species leading toward humans, but bipedality may have been a
key factor. So important was this transition to verticality that
paleoanthropologists — scientists who study the fossil remains of
humans and their ancestors — place these apes in a distinct category.
In the system used for designating species — in which the first, or
genus, name denotes a group of similar species and the second name
indicates the exact species — the apes that stood upright are placed
in the genus Australopithecus.

• Around 2 million years ago, a species of large and particularly
brainy biped began to alter natural objects for use as tools. Members
of this species knocked pebbles together to create sharp-edged stones
that they could use to butcher animals killed by other carnivores.
They used bones as hammers and anvils to break apart other bones.
This transition to toolmaking marked another milestone in our
history. This species was the first to deserve the genus name Homo.

• Finally, sometime between 100,000 and 200,000 years ago, a new
group within the genus Homo appeared. It was different from any
previous group of humans: less heavily built, more mobile, with a
cognitive flexibility unknown before. This is the group of humans
from which we are all descended.

This chronology tends to imply that a fairly straight line led from
Australopithecus through early Homo to modern humans. This is the
view portrayed, for example, in the illustrated parade of ancestors
that accompanies so many books and articles on human evolution. A
modern human, almost always male, leads the parade, marching
resolutely toward the edge of the page. He is followed by something
resembling a caveman, then a bipedal ape, and finally a shambling,
foolish looking chimpanzee. The picture seems to suggest that we are
the end result of a preordained process, the inevitable goal of
evolution. It reinforces our belief that we are at the apex of a
great pyramid of life, with all other living and extinct organisms
arrayed below us.
But this picture of human evolution is wrong — or at least so
incomplete as to be seriously misleading. Human evolution has not
been a straightforward slog from lower to higher. It’s been a maze of
dead ends, unexpected detours, and sudden changes of direction. Many
of the fossils that we have assumed belong to our ancestors probably
represent failed evolutionary experiments, lineages of different
kinds of humans that did not survive. In the end, we are the product
of a relentless winnowing process, a trial by extinction.
Today, just a single human species lives on the earth. But
for portions of the history of both Homo and Australopithecus,
several anatomically distinct human species lived on the planet,
often in the same general region. About 1.8 million years ago, as
many as four distinct groups of humans and australopiths may have
shared the same homeland in eastern Africa. One was a heavily built
species known as Australopithecus boisei (many paleoanthropologists
place the more robust australopiths in a separate genus,
Paranthropus). The other three were distinctive populations of Homo.
Only one of these populations was ancestral to modern humans, and in
the absence of a detailed fossil record, we don’t know which one it
was.
Colin Groves was one of the first biologists to propose that
different species of humans lived in a single region at the same
time. A physical anthropologist at the Australian National University
in Canberra, Groves has led a life of high academic adventure. He has
traveled to more places and seen more kinds of animals than almost
anyone on earth. In a newspaper column he writes for the Canberra
Times, he regularly debates the views of creationists, psychics, and
other pseudoscientists. A special passion is the fight against the
poaching of rhinos, tigers, bears, musk deer, gibbons, and other
animals whose populations have been decimated by the demand for parts
of their bodies mistakenly believed to have medicinal properties.
Groves has put human evolution in context. He has studied the
evolution of tigers in southeastern Asia, wolves in the Canadian
Arctic, buffalo in North America, rhinoceroses and elephants in
Africa, and many other mammals. Why, he asks, should the evolution of
humans be different from that of any other species? Granted, humans
now have an advanced culture, but this is a relatively late
development in our history. And it is not clear that the attributes
of early humans altered underlying evolutionary processes. “That
human beings are animals like any other is perhaps the most important
message of the Darwinian revolution,” he says.
When Groves looks at the evolution of large mammals, he sees
a distinct pattern. Species do not change en masse from one form into
another. Rather, they tend to remain largely unchanged for long
periods of time. Then, quite suddenly, a small group of animals with
characteristics distinct enough to constitute a new species buds off
from the larger group. If the new species has an advantage over the
parent group, it may take over areas occupied by that group. In fact,
if the advantage is great enough, the previous species may go
extinct, setting the stage for the process to begin again.
This is exactly what Groves sees in the human fossil
record. “Speciation events have been the prime mechanism in human
evolution,” he says. “Gradual change within continuous lineages has
been a minor factor.” In other words, the australopiths did not
gradually change into the earliest Homo species, according to Groves.
Rather, the first Homo species budded off from one of the existing
Australopithecus species and gradually out-competed its predecessors.
By a million years ago, all the australopiths were gone.
Groves discovered something else in his study of mammalian
evolution. Biologists have long thought that speciation occurs most
often on the edge of a species’ range, where a small group of animals
could more easily become isolated from the larger group, perhaps by
moving beyond a mountain range or river. Once isolated, this small
group would evolve independently of the larger group. Over time the
small group could become so different that it would no longer
interbreed with its parent group, gaining evolutionary independence.
That’s what Groves expected to see when he began his
investigation of mammalian evolution — species spinning off new
species like drops of water flung from a twirled umbrella. But he
repeatedly found something quite different. New species were not
forming on the edges of a range; they were forming right in the
middle. Take lions, for example. Four subspecies of lions have
existed in historical times. The centrally located subspecies
occupies eastern and central Africa. The other three subspecies lived
in southern Africa, northern Africa, and western Asia (two of the
peripheral subspecies today survive only in zoos or game reserves,
and the third is extinct). According to the traditional view, the
newer species would be those on the periphery. But the situation is
reversed. The central subspecies, with its larger brain and more
complex social organization, is clearly the newest.
Exactly the same pattern can be seen in human evolution,
Groves says. “All of the major events in human evolution have
occurred in eastern Africa — there doesn’t seem to be any question
about it.” According to this view, it’s not an accident of geology or
history that most of the important human fossils have been found in
eastern Africa. For our species, that’s where the evolutionary action
has been.
The notion that new species can form in the center of an
existing species’ range is counterintuitive, but an accumulating body
of evidence supports the idea. The center of the range is where
resources tend to be greatest, and thus population densities are
high. Dense populations produce rich stores of genetic diversity on
which evolution can act. From these genetic hot spots, new variations
can emerge.
The savannas of eastern Africa certainly fit this
description. The highlands of Kenya and Tanzania are a biological
paradise. More large mammals live in these areas than anywhere else
on earth. It is a corner of the world overflowing with life.
But one major objection must be addressed: how can a new
species become biologically isolated right in the middle of an
existing species’ range? According to the usual understanding of
speciation, the new species would begin interbreeding with the
existing one and quickly lose its distinct identity.
This quandary has forced biologists to look more carefully at
what they call reproductive isolating mechanisms. These mechanisms
have a job much like that of the chaperones accompanying high school
seniors on an overnight graduation trip: to prevent successful
matings. Some reproductive isolating mechanisms depend on behavior.
For example, the mating rituals of two species may be so different
that sexual overtures end in a welter of mixed signals and
inappropriate responses. Others are more visual. Primates, in
particular, with their highly acute vision, often can identify
suitable mating partners at a glance.
Geography can act as a mechanism to prevent mating, even in
the very center of a species’ range. For example, our closest primate
relatives tend to live in clumps, even where their overall
populations are greatest. In the forests of equatorial Africa one