Mapping Human History: Discovering the Past Through Our Genes - Hardcover

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.

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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 chr...