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 can
travel great distances without coming across any gorillas, then hit
an area where gorillas are common, then again travel for miles with
no gorillas present. Under such conditions, populations are largely
isolated and can begin to diverge.
Other reproductive isolating mechanisms work at the molecular
level. For example, if two individuals from distinct populations do
succeed in mating, compounds on the surface of an egg can detect
sperm from a different species and keep them from penetrating the
cell wall. Or, if a sperm does enter an egg, the chromosomes may be
incompatible and fail to produce an embryo. Finally, even if a hybrid
organism is born, it may be sterile, as in the case of mules, born
from the union of horses and donkeys.
The existence of many kinds of reproductive isolating
mechanisms supports the observation that the creation of new species
is far from unusual. On the contrary, the biological world seems to
have evolved to ensure the continual production of new species. This
is how nature experiments with life. If a new species is better
adapted to an environment than an existing species, it will prosper.
If a new or existing species is deficient, it often is thrown on the
bone heap of history.
Human evolution clearly displays this harsh reality. Once the
first species in the genus Homo appeared, it began to spin off new
varieties of Homo. At least one of these groups did something that
the australopiths had never done. Its members spread out of Africa
into Asia and Europe. More than a million years ago a species known
as Homo erectus was living in modern-day Indonesia. In Europe, a
population that we today call Neandertals survived until about 30,000
years ago. In Africa, species with names such as Homo heidelbergensis
and Homo ergaster strutted across the stage during various portions
of the past 2 million years.
Then, sometime less than 200,000 years ago, a distinct group
of humans appeared, almost certainly in eastern Africa. Initially
this group must have been very small, occupying an area that may have
been no larger than present-day Israel. Though its existence may at
times have been precarious, it survived.
The oldest known fossil skull thought to belong to a member
of this group was discovered by Richard Leakey in 1967 on the shore
of the Omo River in Ethiopia. Dated to about 130,000 years ago, the
reconstructed skull has a steeply rising forehead, a relatively flat
face, and much smaller brow ridges than previous members of Homo had.
Compared with earlier skulls, this one seems hauntingly familiar. An
observer peering into its empty eye sockets cannot escape the
impression that this face, despite its antiquity, is that of an
ancestor.

The human fossil record can be interpreted in other ways, and science
cannot yet say exactly which path led to anatomically modern humans.
Some paleoanthropologists, for example, adhere to a much more
gradualist view. They maintain that dividing past humans into
strictly separated species is a mistake. Instead, they say, humans
have remained a single species ever since the evolution of Homo
almost 2 million years ago. They demote the various kinds of humans
that spread throughout Asia and Europe to subspecies or races, which
continued to breed with each other and to evolve until all became
fully modern.
This hypothesis is known as multiregionalism, and it is the
culmination of an old tradition in paleoanthropology. It maintains
that the differences we see today between various human groups
predate the evolution of anatomically modern humans. In other words,
Africans descend in part from the archaic Homo sapiens who lived in
Africa, Asians descend in part from the Homo erectus who lived in
Asia, and Europeans descend in part from the Neandertals of Europe.
Some paleoanthropologists have always had doubts about this
view. But in 1987 it came under fire from an entirely new quarter. A
group of molecular biologists at the University of California,
Berkeley, decided to study human evolution by comparing the DNA
sequences of people around the world. Their results are as
astonishing today as when they were first published.
The Berkeley geneticists — Allan Wilson (who died of leukemia
in 1991), Rebecca Cann, who is now at the University of Hawaii at
Manoa, and Mark Stoneking, now at the Max Planck Institute for
Evolutionary Anthropology in Leipzig, Germany — did not study the DNA
in chromosomes. Rather, they focused on a tiny object called a
mitochondrion, hundreds of which are scattered through most cells.
Mitochondria have the important function of breaking down complex
compounds into a simple and highly energetic molecule — a sort of
cellular battery pack — that cells use to run many different chemical
reactions. Oddly enough, mitochondria are almost certainly the
descendants of bacteria that began to live inside other single-celled
organisms more than a billion years ago. They have hitched along for
the ride ever since, providing plant and animal cells with energy in
return for a comfortable place to live.
Because of their independent origin, mitochondria have their
own DNA — a circular loop about 16,500 nucleotides long. Each person
has trillions of mitochondria in his or her body, and the DNA
sequence in each of these mitochondria is, with rare exceptions,
identical. But the DNA sequence of my mitochondria probably differs
from the DNA sequence of yours, which is where the story becomes
interesting.
We all receive our mitochondria from our mothers. Sperm cells
have just one thing on their mind — delivering their package of
chromosomes into the egg. Sperm cells do have a few mitochondria, but
they are tossed away like worn-out flowers in the process of
fertilization. Therefore, only egg cells contribute mitochondria to
the next generation.
Because mitochondria are maternally transmitted, all the
human mitochondrial DNA sequences that exist in the world today are
descended from the mitochondrial DNA of a single woman. The first
time I heard this statement I thought it highly implausible. All 6
billion people on the planet descended from a single ancestor? Yet
this is one of those wonderful scientific conclusions that is not
only true but has to be true.
About 3 billion female humans are alive right now, all of
whom received their mitochondrial DNA from their mothers. (The 3
billion men on earth also received their mitochondrial DNA from their
mothers, but they can be ignored in this analysis.) Now think about
the generation of women one step removed from today’s women, the
generation that includes the mothers of every female alive today. Not
all of the women of that generation had daughters. Some had only
sons; some had no children at all. So the mitochondrial DNA of
today’s 3 billion females must be derived from a subset of the
mitochondria lineages that existed in the previous generation. (The
reason that more females are alive today than in the previous
generation is that some women had more than one daughter.)
The same conclusions can be drawn about the grandmothers of
all the women alive today. Fewer grandmothers than mothers
contributed mitochondrial DNA to today’s women, so the number of
mitochondrial lineages from two generations ago represented in people
today must be smaller than the number of such lineages from one
generation ago. This reduction in the number of extant mitochondrial
lineages continues with each previous generation — from the billions,
into the millions, down to the thousands, and finally into the
hundreds, tens, and single digits.
Eventually, the number of extant mitochondrial lineages must
shrink to two — two women from whom everyone in the world today is
descended. Perhaps those two female lineages extended for some time
back into the past, with each daughter getting her mitochondrial DNA
from a separate mother. But mathematically this process cannot
continue forever, because the number of extant lineages can only
decline with each previous generation — it cannot increase. When the
inevitable decline occurs, the women at the roots of the two separate
lineages would have to be sisters.
Their mother was the woman who produced all the mitochondrial
DNA on the planet today. She is sometimes called mitochondrial Eve,
but the name is profoundly misleading. It implies that she was the
only woman alive on the planet at the time. In fact, many other
humans lived at the same time as Eve. All those humans received their
mitochondrial DNA from a different (perhaps nonhuman) female who
lived in their past. But Eve’s is the only mitochondrial DNA from her
time that has survived. All the rest has gone extinct.
Geneticists call the process I’ve been describing
coalescence, in which a DNA sequence in many people can be traced
back to a sequence in just one person. The term is somewhat
confusing, because coalescence proceeds as one goes backward in time.
But it is a very powerful way of thinking about our DNA, because it
can be applied not just to our mitochondrial DNA but to the billions
of nucleotides in our chromosomes as well. Consider the Y chromosome,
for example. Men pass most of their Y chromosome on to their sons in
the same way that women pass their mitochondrial DNA to their
children, intact and unadulterated. If a man doesn’t have sons, his Y
chromosome dies with him. This means that the same winnowing process
that characterizes mitochondrial lineages applies to the Y. All 3
billion of the Y chromosomes that exist today coalesce in the Y of a
single man who lived sometime in the past. But this “Adam” for the Y
chromosome didn’t necessarily know Eve. He probably didn’t live at
the same time. The path taken by his Y was independent of the path
taken by Eve’s mitochondrial DNA.
The situation is somewhat more complicated for the DNA in the
other chromosomes. The recombination process that takes place between
pairs of chromosomes shuffles the genetic contributions of mothers
and fathers. Still, the coalescence process does apply to particular
chunks of the chromosomes. Each chromosomal segment in the 6 billion
people on earth today — for instance, the ten nucleotides at the end
of all the chromosome 6s in the world, to take a random example —
derives from the chromosome of a single person who lived sometime in
the past. In fact, geneticists have made a rough guess of the number
of people who contributed all of the DNA existing in the human
population today. Using various simplifying assumptions, it turns out
that something like 86,000 individuals, of whom mitochondrial Eve and
the Adam of the Y chromosome were two, are the sources of all the
human DNA in existence today.
As I’ve mentioned, the mitochondrial DNA sequences of today’s
people differ from person to person. No one now has exactly the same
mitochondrial sequence as did Eve, even though we all get our
mitochondrial DNA from her. The sequences of mitochondrial DNA
therefore must have changed over time as they were passed from mother
to children. The next chapter discusses the source of these changes.
What’s important to know now is that the number of differences in all
of the world’s existing mitochondrial DNA sequences reflects the
length of time that has passed since coalescence. In essence, each
new generation adds variation to DNA. As a result, the total amount
of variation in a DNA sequence measures the number of generations
that have passed since the coalescence of that sequence. In looking
at the differences in people’s mitochondrial DNA, the University of
California geneticists were trying to figure out when mitochondrial
Eve lived.
What they discovered is that Eve lived about 200,000 years
ago. More recent analyses of mitochondrial DNA, using additional
data, have produced a somewhat more recent coalescence: around
150,000 years ago. Similar calculations for the Y produce a
coalescence at about the same time (though other calculations point
toward a more recent Y coalescence).
Geneticists do not yet know for sure why human mitochondrial
DNA coalesces about 150,000 years ago. But they do know that a
coalescence is much more likely to occur when a population is small.
In a small population, a few people are the ancestors of a large
number of descendants, so their genes are passed through what is
called a genetic bottleneck.
This process may sound familiar by now. A genetic bottleneck
is what would be expected when the first anatomically modern humans
appeared. A small group of humans could have been isolated for an
extended period. Over time this group could have evolved a new set of
characteristics that set it apart from other humans. This view fits
extremely well with the genetic evidence, which indicates that our
ancestors went through a bottleneck between 100,000 and 200,000 years
ago. At that point, perhaps 20,000 people constituted the population
from whom we are all descended.
Not everyone agrees with this picture, and the evolution of
anatomically modern humans was undoubtedly more complex than my
description might imply. A group of archaic humans did not disappear
into a valley in Africa and emerge several thousand years later
thoroughly modernized. Early modern humans might have been subdivided
in some way. Modern humans and archaic humans might have interbred,
though clear genetic evidence of such mixing has not been found. Much
more genetic data and new fossil discoveries are needed to arrive at
airtight conclusions.
But no matter what the complexities, the genetic evidence
available today points to a straightforward conclusion. According to
our DNA, every person now alive is descended from a relatively small
group of Africans who lived between 100,000 and 200,000 years ago.
When the Berkeley geneticists first published their findings,
the idea that all people share a common origin caught the public’s
attention. A particularly notorious illustration on the cover of
Newsweek showed a topless black Eve offering an apple to a black
Adam. But what became known as the “Out of Africa” hypothesis also
was roundly attacked. Some anthropologists found holes in the
analyses. Multiregionalists raised a host of objections. By the early
1990s, the attacks had placed the hypothesis in some disrepute.
The idea’s rehabilitation has received less attention, but it
is nevertheless solid. As geneticists began looking at other segments
of DNA, they found similar signs of coalescence as they had found in
the mitochondria. At the same time, the holes in the original genetic
analyses were filled and new analyses con.rmed the original findings.
Equally important, other scientists began to look at the
fossil and archaeological evidence in a new way. A single origin fits
the pattern observed in other animals, for which multiregional
evolution is unknown. And an African origin for modern humans would
explain why anatomically modern fossils found on that continent are
older than those found elsewhere. In the most thorough computerized
analysis of fossilized skulls ever done, Marta Lahr, a
paleoanthropologist from Sao Paulo, Brazil, who is now at Cambridge
University, found little support for the multiregional
hypothesis. “The bulk of the chronological and genetic data indicate
a single origin of all modern humans in Africa,” she concluded.

In a way, the details of how modernity in humans arose are not
important. What must count as one of the most profound biological
insights of all time is the recognition of our remarkable genetic
similarity. About 7,500 generations have passed since our ancestors
lived on the savannas of eastern Africa. In evolutionary terms,
that’s the blink of an eye. The chimpanzees living on a single
hillside in Africa have more than twice as much variety in their
mitochondrial DNA as do all the 6 billion people living on the earth,
because today’s species of chimpanzees have been in existence much
longer than have modern humans.
With the appearance of modern humans, the large-scale
evolution of our species essentially ceased. Since then, humans have
expanded as a single and relatively well mixed population. Our
physical characteristics have changed slightly. We have become
somewhat smaller and lighter, especially when the invention of
agriculture changed the demands made on our bodies. But our basic
body plan was set more than 100,000 years ago. Since then, we have
been in a period of evolutionary stasis.
Everyone on earth today is equally distant from the early
modern humans of eastern Africa. In that respect, no one group,
including the Bushmen, is more closely related to our ancestors than
any other. The same number of generations separates Australians,
Canadians, and Ethiopians from early modern humans.
But different groups have different genetic histories, and
those histories hold clues to the past. That’s why the Bushmen are
especially intriguing. According to the archaeological and genetic
evidence, one of the earliest expansions of modern humans was into
southern Africa. The ancestors of the Bushmen were living there when
Europe and Asia were still populated with archaic humans and when the
Americas were completely devoid of humans. What’s more, in their
southern African cul-de-sac, the ancestors of the Bushmen appear to
have been relatively isolated. Certainly they mixed with people from
farther north, and undoubtedly they changed over the millennia. But
of all the histories described in this book, that of the Bushmen is
the most straightforward.
Though it is pure speculation, maybe the Bushmen, because of
their antiquity and isolation, offer a window onto the early history
of modern humans. Perhaps they retain some of the characteristics of
our early modern ancestors. In that case, if groups of humans have
undergone any important evolutionary changes since the appearance of
modern humans in Africa, maybe evidence of those changes can be found
by comparison with the Bushmen.
I’ve spent only a few days in the backcountry of southern
Africa; I can’t claim to know the Bushmen well. But I’ve spent enough
time with them to confirm what people who have spent years with the
Bushmen say — they are not fundamentally different from anyone else.
I’ve seen the affection the Bushmen lavish on their children, who
grow up at the center of extended families of aunts, uncles, and
cousins. I’ve seen their sardonic sense of humor, which they turn as
easily on themselves as on the visitors who come to ask them
questions. I’ve seen the intelligence they apply in their hunting and
gathering, and the aesthetic sense they bring to their music and
dance. The Bushmen seemed to me no different from other people,
though of course they live in very different circumstances. So if
they are at all representative of early modern humans, then humans
haven’t changed much over the past 150,000 years. And that’s exactly
the conclusion geneticists have been drawing from our DNA.

Copyright © 2002 by Steve Olson. Reprinted by permission of Houghton
Mifflin Company.