Like I say, in order to be descended from something, you have to be able to interbreed with the something.
The following article from Discover Magazine back around 95 or 96 is hard to find copies of. It describes the thing about humans never having interbred with neanderthals.
James Shreeve, The Neanderthal Peace:
For perhaps 50,000 years, two radically different types of human lived side by
side in the same small land. And for all those millennia, the two apparently
had nothing whatsoever to do with each other. Why in the world not?
I met my first Neanderthal in a cafï½in Paris, just across the street from the
Jussieu metro stop. It was a wet afternoon in May, and I was sitting on a
banquette with my back to the window. The cafï½was smoky and charmless. Near
the entrance a couple of students were thumping on a pinball machine called
Genesis, which beeped approval every time they scored. The place was packed
with people--foreign students, professors, young professionals, French
workers,
Arabs, Africans, and even a couple of Japanese tourists, all thrown together
by
the rain. Our coffee had just arrived, and I found that if I tucked my elbow
down when raising my cup, I could drink it without poking the ribs of a
bearded
man sitting at the table next to me, who was deep into an argument.
Above the noise of the pinball game and the din of private conversations, a
French anthropologist named Jean-Jacques Hublin was telling me about the
anatomical unity of man. It was he who had brought along the Neanderthal.
When
we had come into the cafï½ he had placed an object wrapped in a soft rag on
the
table and had ignored it ever since. Like anything so carefully neglected, it
was beginning to monopolize my attention.
"Perhaps you would be interested in this," he said at last, whisking away the
rag. There, amid the clutter of demitasses and empty sugar wrappers, was a
large human lower jawbone. The teeth, worn and yellowed by time, were all in
place. Around us, I felt the cafï½raise a collective eyebrow. The hubbub of
talk sank audibly. The bearded man next to me stopped in midsentence, looked
at
the jaw, looked at Hublin, and resumed his argument. Hublin gently nudged the
fossil to the center of the table and leaned back.
"What is it?" I asked.
"It is a Neanderthal from a site called Zafarraya, in the south of Spain," he
said. "We have only this mandible and an isolated femur. But as you can see,
the jaw is almost complete. We are not sure yet, but it may be that this
fossil
is only 30,000 years old."
"Only" 30,000 years may seem an odd way of expressing time, but coming from a
paleoanthropologist, it is like saying that a professional basketball player
is
only 6 foot 4. Hominids--members of the exclusively human family tree--have
been on Earth for at least 4 million years. Measured against the earliest
members of our lineage, the mineralized piece of bone on the table was a
mewling newborn. Even compared with others of its kind, the jaw was
astonishingly young. Neanderthals were supposed to have disappeared fully
5,000
years before this one was born, and I had come to France to find out what
might
have happened to them.
The Neanderthals are the best known and least understood of all human
ancestors.
To most people, the name instantly brings to mind the image of a hulking
brute,
dragging his mate around by her coif. This stereotype, born almost as soon as
the first skeleton was found in a German cave in the middle of the last
century, has been refluffed in comic books, novels, and movies so often that
it
has successfully passed from clichï½to common parlance. But what actually
makes
a Neanderthal a Neanderthal is not its size or its strength or any measure of
its native intelligence but a suite of exquisitely distinct physical traits,
most of them in the face and cranium. Like all Neanderthal mandibles, for
example, the one on the table lacked the bony protrusion on the rim of the
jaw
called a mental eminence-- better known as a chin. The places on the outside
of
the jaw where chewing muscles had once been attached were grossly enlarged,
indicating tremendous torque in the bite. Between the last two molars and the
upward thrust of the rear of the jaw, Hublin pointed to gaps of almost a
quarter inch, an architectural nicety shifting the business of chewing
farther
toward the front.
In these and in several other features the jaw was uniquely, quintessentially
Neanderthal; no other member of the human family before or since shows the
same
pattern. With a little instruction the Neanderthal pattern is recognizable
even
to a layman like me. But unlike Hublin, whose expertise allowed him to sit
there calmly sipping coffee while the jaw of a 30,000-year-old man rested
within biting distance of his free hand, I felt like stooping down and paying
homage.
Several years before, based on a comparison of DNA found in the mitochondria
of
modern human cells, a team of biochemists in Berkeley, California, had
concluded that all humans on Earth could trace their ancestry back to a woman
who had lived in Africa only 200,000 years earlier. Every living branch and
twig of the human family tree had shot up from this "mitochondrial Eve" and
spread like kudzu over the face of the globe, binding all humans in an
intimate
web of relatedness.
To me the Eve hypothesis sounded almost too good to be true. If all living
people can be traced back to a common ancestor just 200,000 years ago, then
the
entire human population of the globe is really just one grand
brother-and-sisterhood, despite the confounding embellishments of culture and
race. Thus on a May afternoon, a cafï½in Paris could play host to clientele
from three or four continents, but the scene still amounted to a sort of ad
hoc
family reunion.
But Eve bore a darker message too. The Berkeley study suggested that at some
point between 100,000 and 50,000 years ago, people from Africa began to
disperse across Europe and Asia, eventually populating the Americas as well.
These people, and these alone, became the ancestors of all future human
generations. When they arrived in Eurasia, however, there were thousands,
perhaps millions, of other human beings already living there--including the
Neanderthals. What happened to them all? Eve's answer was cruelly
unequivocal:
the Neanderthals--including the Zafarraya population represented by the jaw
on
the table--were pushed aside, outcompeted, or otherwise driven extinct by the
new arrivals from the south.
What fascinates me about the fate of the Neanderthals is the paradox of their
promise. Appearing first in Europe about 150,000 years ago, the Neanderthals
flourished throughout the increasing cold of an approaching ice age; by
70,000
years ago they had spread throughout Europe and western Asia. As for
Neanderthal appearance, the stereotype of a muscled thug is not completely
off
the mark. Thick-boned, barrel-chested, a healthy Neanderthal male could lift
an
average NFL linebacker over his head and throw him through the goalposts. But
despite the Neanderthal's reputation for dim-wittedness, there is nothing
that
clearly distinguishes its brain from that of a modern human except that, on
average, the Neanderthal version was slightly larger. There is no trace of
the
thoughts that animated those brains, so we do not know how much they
resembled
our own. But a big brain is an expensive piece of adaptive equipment. You
don't
evolve one if you don't use it. Combining enormous physical strength with
manifest intelligence, the Neanderthals appear to have been outfitted to face
any obstacle the environment could put in their path. They could not lose.
And then, somehow, they lost. Just when the Neanderthals reached their most
advanced expression, they suddenly vanished. Their demise coincides
suspiciously with the arrival in western Europe of a new kind of human:
taller,
thinner, more modern-looking. The collision of these two human
populations--us
and the other, the destined parvenu and the doomed caretaker of a
continent--is
as potent and marvelous a part of the human story as anything that has
happened
since.
By itself, the half-jaw on the table in front of me had its own tale to tell.
Hublin had said that it was perhaps as young as 30,000 years old. A few
months
before, an American archeologist named James Bischoff and his colleagues had
also announced astonishing ages for some objects from Spanish caves. After
applying a new technique to date some modern-human- style artifacts, they
declared them to be 40,000 years old. This was 6,000 years before there were
supposed to have been modern humans in Europe. If both Bischoff's and
Hublin's
dates were right, it meant that Neanderthals and modern humans had been
sharing
Spanish soil for 10,000 years. That didn't make sense to me.
"At 30,000 years," I asked Hublin, "wouldn't this jaw be the last Neanderthal
known?"
"If we are right about the date, yes," he said. "But there is still much work
to
be done before we can say how old the jaw is for certain."
"But Bischoff says modern humans were in Spain 10,000 years before then," I
persisted. "I can understand how a population with a superior technology
might
come into an area and quickly dominate a less sophisticated people already
there. But 10,000 years doesn't sound very quick, even in evolutionary terms.
How can two kinds of human being exist side by side for that long without
sharing their cultures? Without sharing their genes?"
Hublin shrugged in the classically cryptic French manner that means either
"The
answer is obvious" or "How should I know?"
Among all the events and transformations in human evolution, the origins of
modern humans were, until recently, the easiest to account for. Around 35,000
years ago, signs of a new, explosively energetic culture in Europe marked the
beginning of the period known as the Upper Paleolithic. They included a
highly
sophisticated variety of tools, made out of bone and antler as well as stone.
Even more important, the people making these tools--usually known as
Cro-Magnons, a name borrowed from a tiny rock shelter in southern France
where
their skeletons were first found, in 1868- -had discovered a symbolic plane
of
existence, evident in their gorgeously painted caves, carved animal
figurines,
and the beads and pendants adorning their bodies. The Neanderthals who had
inhabited Europe for tens of thousands of years had never produced anything
remotely as elaborate. Coinciding with this cultural explosion were the first
signs of the kind of anatomy that distinguishes modern human beings: a
well-defined chin; a vertical forehead lacking pronounced browridges; a domed
braincase; and a slender, lightly built frame, among other, more esoteric
features.
The skeletons in the Cro-Magnon cave, believed to be between 32,000 and 30,000
years old, provided an exquisite microcosm of the joined emergence of culture
and anatomy. Five skeletons, including one of an infant, were found buried in
a
communal grave, and all exhibited the anatomical characteristics of modern
human beings. Scattered in the grave with them were hundreds of artificially
pierced seashells and animal teeth, clearly the vestiges of necklaces,
bracelets, and other body ornaments. The nearly simultaneous appearance of
modern culture and modern anatomy provided a ready-made explanation for the
final step in the human journey. Since they happened at the same time, the
reasoning went, obviously one had caused the other. It all made good
Darwinian
sense. A more efficient technology emerged to take over the survival role
previously provided by brute strength, relaxing the need for the robust
physiques and powerful chewing apparatus of the Neanderthals. Voilï½ Suddenly
there was clever, slender Cro-Magnon man. That this first truly modern human
should be indigenous to Europe tightened the evolutionary narrative: modern
man
appeared in precisely the region of the world where culture--according to
Europeans--later reached its zenith. Prehistory foreshadowed history. The
only
issue to sort out was whether the Cro-Magnons had come from somewhere else or
whether the Neanderthals had evolved into them.
The latter scenario, of course, assumes that modern humans and Neanderthals
didn't coexist, at least not for any appreciable amount of time. But the
jawbone from Zafarraya challenged that neat supposition. Even more damaging
were some strange findings in the Mideast. Recent discoveries there too
suggest
that Neanderthals and modern humans may have inhabited the same land at the
same time, and for far, far longer than in Spain.
In Israel, on the southern edge of the Neanderthal range, a wooded rise of
limestone issues abruptly out of the Mediterranean below Haifa, ascending in
an
undulation of hills. This is the Mount Carmel of the Song of Solomon, where
Elijah brought down the false priests of Baal, and Deborah laid rout to the
Canaanites. In subsequent centuries, armies, tribes, and whole cultures
tramped
through its rocky passes and over its fertile flanks, bringing Hittites,
Persians, Jews, Romans, Mongols, Muslims, Crusaders, Turks, the modern
meddling
of Europeans--one people slaughtered or swallowed by the next but somehow
springing up again and gaining strength enough to slaughter or swallow in its
turn.
My interest here is in more ancient confrontations. Mount Carmel lies in the
Levant, a tiny hinge of habitability between the sea and the desert, linking
the two great landmasses of Africa and Eurasia. A million years ago a massive
radiation of large mammals moved through the Levant from Africa toward the
temperate latitudes to the north. Among these mammals were some ancestral
humans. Time passed. The humans evolved, diversified. The ones in Europe came
to look very different from their now- distant relatives who had remained in
Africa. The Europeans became the Neanderthals. Then, still long before
history
began to scar the Levant with its sieges and slaughters, some Neanderthals
from
Europe and other humans from Africa wandered into this link between their
homelands, leaving their bones on Mount Carmel. What happened when they met?
How did two kinds of human respond to each other?
Reaching the Stone Age in Israel is easy; I simply rented a car in Tel Aviv
and
drove a couple of hours up the coastal road. My destination was the cave of
Kebara, an excavation hunched above a banana plantation on the sea-weathered
western slope of the mountain.
Inside the cave the present Mideast, with all its political complexities,
disappeared--here there was only a cool, sheltered emptiness, greatly
enlarged
by decades of archeological probing. Scattered through the excavation were a
dozen or so scientists and students; an equal number were working at tables
along the rim. The atmosphere was one of hushed, almost monkish
concentration,
like that of a reading room in a great library.
The Kebara excavation began ten years ago, picking up on the previous work of
Moshe Stekelis of Hebrew University in the 1950s and early 1960s. Stekelis
exposed a sequence of Paleolithic deposits and, before his sudden death,
discovered the skeleton of an infant Neanderthal. A greater treasure emerged
in
1983. After Stekelis's time, the sharp vertical profiles of the excavation
crumbled under the feet of a generation of kibbutz children and assorted
other
slow ravages. A graduate student named Lynne Schepartz was assigned the
mundane
task of cleaning up the deteriorated exposures by cutting them a little
deeper.
One afternoon she noticed what appeared to be a human toe bone peeking out of
a
fused clod of sediments. The next morning her whisk broom exposed a pearly
array of human teeth: the lower jaw of an adult Neanderthal skeleton.
Stekelis's team had missed it by two inches.
Lynne Schepartz was no longer a graduate student, but she was still spending
her
summers at Kebara. I found her and asked her how it felt to uncover the
fossil.
"Unprintable," she said. "I was jumping up and down and screaming."
She had reason to react unprintably. Her discovery turned out to be not just
any
Neanderthal but the most complete skeleton ever found: the first complete
Neanderthal spinal column, the first complete Neanderthal rib cage, the first
complete pelvis of any early hominid known. She showed me a plaster cast of
the
fossil--affectionately known as Moshe--lying on an adjacent table. The bones
were arranged exactly as they had been found. Moshe was resting on his back,
his right arm folded over his chest, his left hand on his stomach, in a
classic
attitude of burial. The only missing parts were the right leg, the extremity
of
the left, and except for the lower jaw, the skull.
Schepartz led me down ladders to Moshe's burial site, a deep rectangular pit
near the center of the excavation. On this July morning, the Neanderthal's
grave was occupied by a modern human named Ofer Bar- Yosef, who peered back
up
at me from behind thick glasses, magnifying my sense that I had disturbed the
happy toil of a cavernicolous hobbit. He seemed evolved to the task, nimble
and
gnomishly compact, the better to fit into cramped quarters.
Bar-Yosef told me that he had directed his first archeological excavation at
the
age of 11, rounding up a crew of his friends in his Jerusalem neighborhood to
help him unearth a Byzantine water system. He had not stopped digging since.
Kebara was the latest of three major excavations under his direction. "My
daughter has been coming to this site since she was a fetus," he told me.
"She
used to have a playpen set up right over there."
Throughout his career, Bar-Yosef has dug for answers to two personal
obsessions:
the origins of Neolithic agricultural societies and-- the point where our
obsessions converge--the twisting conundrum of modern human origins.
The story in the Levant never really made much sense. In the old days, back
when
everybody "knew" that modern humans first appeared in western Europe, where
the
really modern folks still live, you could identify a hominid by the kind of
tools he left behind. Bulky Neanderthals made bulky flakes, while svelte
Cro-Magnons made slim "blades." Narrowness is, in fact, the very definition
of
a blade, which in paleoarcheology means nothing more than a stone tool twice
as
long as it is wide. In Europe a new, efficient way of producing blades from a
flint core appeared as part of the "cultural explosion" that coincided with
the
appearance of the Cro- Magnon people. Here in the Levant, however, the
arrival
of anatomically modern humans was marked by no fancy new tools, not to
mention
no painted caves, beaded necklaces, or other evidence of exploding Cro-Magnon
couture. In this part of the world, how modern a hominid looked in its body
said nothing about how modernly it behaved.
Just a couple of bus stops up the coastal road from Kebara is the cave of
Tabun,
with over 80 vertical feet of deposits spanning more than 100,000 years of
human occupation. The treasures of Tabun, like those of Kebara, are
Neanderthals. Literally around the corner from Tabun is another cave, called
Skhul, where some fairly modern-looking humans were found in the 1930s. And a
few miles inland from Kebara on a hill in lower Galilee is Qafzeh, where in
1965 a young French anthropologist named Bernard Vandermeersch found a
veritable Middle Paleolithic cemetery of distinctly modern humans. But though
the bones in these caves include both Neanderthals and modern humans, the
tools
found with the bones are all pretty much the same.
In 1982, Arthur Jelinek of the University of Arizona made an inspired attempt
to
massage some sense into the nagging paradox of Mount Carmel. As in Europe
later
on, he argued, tools get thinner along with the bodies of the people who make
them. Only in this case, the reduction is front to back rather than side to
side.
The fattest flakes, he showed, came from a layer near the bottom of Tabun
cave,
where a partial skeleton of a Neanderthal woman had turned up; if flake
thickness was indeed a true measure of time, then she was the oldest in the
group. The next oldest would be the Neanderthal infant that Stekelis had
found
at Kebara. The modern humans of Skhul yielded flake tools that were flatter.
And the flattest of all belonged to the moderns of Qafzeh cave. Although the
physically modern Skhul-Qafzeh people might not have crossed the line into
full-fledged, blade-based humanness, they appeared, as Jelinek wrote, "on the
threshold of breaking away.
"Our current evidence from Tabun suggests an orderly and continuous progress
of
industries in the southern Levant," he went on, "paralleled by a
morphological
progression from Neanderthal to modern man." According to this scenario, the
Neanderthals simply evolved into modern humans. There was no collision of
peoples or cultures; two kinds of human never met, because there was really
only one kind, changing through time.
If Jelinek's conventional chronology based on slimming tool forms was right,
the
fossils found in Qafzeh could be "proto-Cro-Magnons," the evolutionary link
between a Neanderthal past and a Cro-Magnon future--and thence to the present
moment. But the dating methods he used were relative, merely inferring an age
for the skeletons by where they fell in an overall chronological scheme. What
was needed was a new way of measuring time, preferably an absolute dating
technique that could label the Mount Carmel hominids with an age in actual
calendar years.
The most celebrated absolute dating method is radiocarbon dating, which
measures
time by the constant, steady decay of radioactive carbon atoms. Developed in
the 1940s, radiocarbon dating is still one of the most accurate ways to pin
an
age on a site, so long as it is younger than around 40,000 years. In older
materials the amount of radioactive carbon still left undecayed is so small
that even the slightest amount of contamination leads to highly inaccurate
results. Another technique, relying on the decay of radioactive potassium
instead of carbon, has been used since the late 1950s to date volcanic
deposits
older than half a million years. Radiopotassium was the method of choice for
dating the famous East African early hominids like Lucy, as well as the new
"root hominid," Australopithecus ramidus, announced in 1994. Until recently,
though, everything that lived between the ranges of these two
techniques--including the moderns at Qafzeh and the Neanderthals at
Kebara--fell into a chronological black hole.
In the early 1980s, however, Hï½ï½e Valladas, a French archeologist, used a new
technique called thermoluminescence, or TL, to date flints from the Kebara
and
Qafzeh caves. As applied to these flints, the technique is based on the fact
that minerals give off a burst of light when heated to about 900 degrees. It
is
also based on the certainty that past humans, like present ones, were
sometimes
careless. In the Middle Paleolithic, some flint tools happened to lie around
in
the path of careless feet, and some tools got kicked into fires, opening up
an
exquisite opportunity for absolute dating. When a flint tool was heated
sufficiently by the fire, it gave up its thermoluminescent energy. Over
thousands of years, that energy slowly built up again. The dating of fire-
charred tools is thus, in principle, straightforward: the brighter a bit of
flint glows when heated today, the longer since the time it was last used.
By 1987, Valladas and her physicist father, Georges, had squeezed an age of
60,000 years out of the burnt tools found beside Moshe at Kebara. That number
pleased everybody, since it agreed with time schemes arrived at through
relative dating methods. The shocker came the following year, when Valladas
and
her colleagues announced the results of their work at Qafzeh: the "modern"
skeletons were 92,000 years old, give or take a few thousand.
Several other Neanderthal and modern human sites have since been dated with
TL,
and the one at Qafzeh remains not only the most sensational but the surest.
Key
sites in the Levant have also been dated by a "sister" technique called
electron-spin resonance (ESR). Large mammal teeth found near the Qafzeh
skeletons came back with an ESR date even older than Valladas's
thermoluminescent surprise. The skeletons were at least 100,000 and perhaps
115,000 years old. "People said that TL had too many uncertainties," Bernard
Vandermeersch told me. "So we gave them ESR. By now it is very difficult to
dispute that the first modern humans in the Levant were here by 100,000 years
ago."
Clearly, if modern humans were inhabiting the Levant 40,000 years before the
Neanderthals, they could hardly have evolved from them. If the dates are
indeed
correct, it is hard to see what else one can do with the venerated belief in
our Neanderthal ancestry but chuck it, once and for all.
Case closed? On the contrary, the dates only twist the mystery on Mount Carmel
even tighter. Presuming that the moderns did not just come for a visit
100,000
years ago and then politely withdraw, they must have been around when the
Neanderthals arrived 40,000 years later--if the Neanderthals as well weren't
there to begin with: the latest ESR dates for the Tabun Neanderthal woman
place
her there 110,000 years ago. Either way, two distinct kinds of human were
apparently squeezed together in an area not much larger than the state of New
Jersey, and for a long time--at least 25,000 years and perhaps 50,000 or
more.
Rather than resolving the paradox, the new dating techniques only teased out
its
riddles. If two kinds of human were behaving the same way in the same place
at
the same time, how can we call them different? If modern humans did not
descend
from the Neanderthals but replaced them instead, why did it take them so long
to get the job done?
At Kebara, I took the paradox with me to mull over outside, on a still summer
afternoon, where the horizon manifested the present moment in the silhouette
of
an oil tanker, far out at sea. If the names "Neanderthal" and "modern human"
are meaningful distinctions, if they have as much reality, say, as the oil
tanker pasted onto the horizon, then they cannot be blended, any more than
one
can blend the sea and the sky. But what if they are mere edges after all,
edges
that might have had firm content in France and Spain but not here, not in
this
past; edges whose contents spilled over and leaked into each other so
profusely
that no true edges can be said to exist at all?
In that case, there would be no more mystery. The Levantine paradox would be a
trick knot; pull gently from both ends and it unravels on its own. Think of
one
end of the rope as cultural. Every species has its own ecological niche, its
unique set of adaptations to local habitats. The "principle of competitive
exclusion" states that two species cannot squeeze into the same niche: the
slightly better adapted one will eventually drive the other one out.
Traditionally, the human niche has been defined by culture, so it would be
impossible for two kinds of human to coexist using the same stone tools to
compete for the same plant and animal resources. One would drive the other
into
extinction, or never allow it to gain a foothold.
"Competitive exclusion would preclude the coexistence of two different kinds
of
hominid in a small area over a 40,000- or 50,000-year period unless they had
different adaptations," says Geoffrey Clark of Arizona State University. "But
as far as we can tell, the adaptations were identical at Kebara and Qafzeh."
Clark adds to the list of common adaptations the use of symbols--or lack of
it.
Perhaps Neanderthals lacked complex social symbols like beads, artwork, and
elaborate burial. But so, he believes, did their skinny contemporaries down
the
road at Qafzeh. If neither was littering the landscape with signs of some new
mental capacity, by what right do we favor the skinny one with a brilliant
future and doom the other to dull extinction?
This leads to the morphological end of the rope. If the two human types cannot
be distinguished on the basis of their tools, then the only valid way of
telling a Neanderthal from a modern human is to declare that one looks
"Neanderthalish" and the other doesn't. If you were to take all the relevant
fossils and line them up, could you really separate them into two mutually
exclusive groups, with no overlap? A replacement advocate might think so, but
a
believer in continuity like Geoffrey Clark insists that you could not. He
thinks the lineup might better be characterized as one widely variable
population, running the gamut from the most Neanderthal to the most modern.
The
early excavators at Tabun and Skhul saw the fossils there as an intermediate
grade between archaic and modern Homo sapiens. Perhaps they were right. "The
skeletal material is anything but clearly 'Neanderthal' and clearly 'modern,'
"
Clark maintains, "whatever those terms mean in the first place, which I don't
think is much."
This view preserves the traditional idea of continuity but abandons the
process:
there was no evolution from one kind of human to another--from Neanderthal to
modern--because there was, in fact, no "other." But for all its appeal, the
"oneness" solution to the Levantine paradox is fundamentally flawed. Nobody
disputes that the tool kits of the two human types are virtually identical.
But
it does not logically follow that the toolmakers must be identical as well.
Middle Paleolithic tool kits are associated in our minds with Neanderthals
because they are the best known human occupants of the Middle Paleolithic.
But
if people with modern anatomy turn out to have been living back then, too,
why
wouldn't they be using the same culture as the Neanderthals?
"If you ask me, forget about the stone tools," Ofer Bar-Yosef told me. "They
can
tell you nothing, zero. At most they say something about how they were
preparing food. But is what you do in the kitchen all of your life? Of course
not. Being positive people, we are not willing to admit that some of the
missing evidence might be the crucial evidence we need to solve this
problem."
Whatever the tools suggest, the skeletons of moderns and Neanderthals look
different, and the pattern of their differences is too consistent to dismiss.
As anthropologist Erik Trinkaus of the University of New Mexico has shown,
those skeletal differences clearly reflect two distinct patterns of behavior,
however alike the archeological leavings may be. Furthermore, the two
physical
types do not follow one from the other, nor do they meet in a fleeting moment
before one triumphs and the other fades. They just keep on going, side by
side
but never mingling. In his behavioral approach to bones, Trinkaus purposely
disregards the features that might best discriminate Neanderthals and moderns
from each other genetically. By definition, these traits are poor indicators
of
the effects of life-style on bone, since their shape and size are decided by
heredity, not by use. But there is one profoundly important aspect of human
life where behavior and heredity converge: the act that allows human lineages
to continue in the first place.
Humans love to mate. They mate all the time, by night and by day, through all
the phases of the female's reproductive cycle. Given the opportunity, humans
throughout the world will mate with any other human. The barriers between
races
and cultures, so cruelly evident in other respects, melt away when sex is at
stake. Cortï½ began the systematic annihilation of the Aztec people--but that
did not stop him from taking an Aztec princess for his wife. Blacks have been
treated with contempt by whites in America since they were first forced into
slavery, but some 20 percent of the genes in a typical African American are
"white." Consider James Cook's voyages in the Pacific in the eighteenth
century. "Cook's men would come to some distant land, and lining the shore
were
all these very bizarre-looking human beings with spears, long jaws,
browridges," archeologist Clive Gamble of Southampton University in England
told me. "God, how odd it must have seemed to them. But that didn't stop the
Cook crew from making a lot of little Cooklets."
Project this universal human behavior back into the Middle Paleolithic. When
Neanderthals and modern humans came into contact in the Levant, they would
have
interbred, no matter how "strange" they might initially have seemed to each
other. If their cohabitation stretched over tens of thousands of years, the
fossils should show a convergence through time toward a single morphological
pattern, or at least some swapping of traits back and forth.
But the evidence just isn't there, not if the TL and ESR dates are correct.
Instead the Neanderthals stay staunchly themselves. In fact, according to
some
recent ESR dates, the least "Neanderthalish" among them is also the oldest.
The
full Neanderthal pattern is carved deep at the Kebara cave, around 60,000
years
ago. The moderns, meanwhile, arrive very early at Qafzeh and Skhul and never
lose their modern aspect. Certainly, it is possible that at any moment new
fossils will be revealed that conclusively demonstrate the emergence of a
"Neandermod" lineage. From the evidence in hand, however, the most likely
conclusion is that Neanderthals and modern humans were not interbreeding in
the
Levant.
Of course, to interbreed, you first have to meet. Some researchers have
contended that the coexistence on the slopes of Mount Carmel for tens of
thousands of years is merely an illusion created by the poor archeological
record. If moderns and Neanderthals were physically isolated from each other,
then there is nothing mysterious about their failure to interbreed. The most
obvious form of isolation is geographic. But imagine an isolation in time as
well. The climate of the Levant fluctuated throughout the Middle
Paleolithic--now warm and dry, now cold and wet. Perhaps modern humans
migrated
up into the region from Africa during the warm periods, when the climate was
better suited to their lighter, taller, warm-adapted physiques. Neanderthals,
on the other hand, might have arrived in the Levant only when advancing
glaciers cooled their European range more than even their cold-adapted
physiques could stand. Then the two did not so much cohabit as "time-share"
the
same pocket of landscape between their separate continental ranges.
While the solution is intriguing, there are problems with it. Hominids are
remarkably adaptable creatures. Even the ancient Homo erectus- -who lacked
the
large brain, hafted spear points, and other cultural accoutrements of its
descendants--managed to thrive in a range of regions and under diverse
climatic
conditions. And while hominids adapt quickly, glaciers move very, very
slowly,
coming and going. Even if one or the other kind of human gained sole
possession
of the Levant during climatic extremes, what about all those millennia that
were neither the hottest nor the coldest? There must have been long stretches
of time--perhaps enduring as long as the whole of recorded human
history--when
the Levant climate was perfectly suited to both Neanderthals and modern
humans.
What part do these in-between periods play in the time-sharing scenario? It
doesn't make sense that one human population should politely vacate Mount
Carmel just before the other moved in.
If these humans were isolated in neither space nor time but were truly
contemporaneous, then how on earth did they fail to mate? Only one solution
to
the mystery is left. Neanderthals and moderns did not interbreed in the
Levant
because they could not. They were reproductively incompatible, separate
species--equally human, perhaps, but biologically distinct. Two separate
species, who both just happened to be human at the same time, in the same
place.
Cohabitation in the Levant in the last ice age conjures up a chilling
possibility. It forces you to imagine two equally gifted, resourceful,
emotionally rich human entities weaving through one tapestry of
landscape--yet
so different from each other as to make the racial diversity of present-day
humans seem like nothing. Take away the sexual bridge and you end up with two
fully sentient human species pressed into one place, as mindless of each
other
as two kinds of bird sharing the same feeder in your backyard.
When paleoanthropologists bicker over whether Neanderthal anatomy is divergent
enough to justify calling Neanderthals a separate species from us, they are
using a morphological definition of a species. This is a useful pretense for
the paleoanthropologists, who have nothing but the shapes of bone to work
with
in the first place. But they admit that in the real, vibrantly unruly natural
world, bone morphology is a pitifully poor indication of where one species
leaves off and another begins. Ian Tattersall, an evolutionary biologist at
the
American Museum of Natural History, points out that if you stripped the skin
and muscle off 20 New World monkey species, their skeletons would be
virtually
indistinguishable. Many other species look the same even with their skins
still
on.
The most common definition of biological species, as opposed to the
morphological make-believes paleontologists have to work with, is a succinct
utterance of the esteemed evolutionary biologist Ernst Mayr: "Species are
groups of actually or potentially interbreeding natural populations that are
reproductively isolated from other such groups." The key phrase is
reproductively isolated: a species is something that doesn't mate with
anything
but itself. The evolutionary barriers that prevent species from wantonly
interbreeding and producing a sort of organismic soup on the landscape are
called isolating mechanisms. These can be any obstructions that prevent
otherwise closely related species from mating to produce fertile offspring.
The
obstructions may be anatomical. Two species of hyrax in East Africa share the
same sleeping holes, make use of common latrines, and raise their young in
communal "play groups." But they cannot interbreed, at least in part because
of
the radically different shapes of the males' penises. Isolating mechanisms
need
not be so conspicuous. Two closely related species might have different
estrous
cycles. Or the barrier might come into play after mating: the chromosomes are
incompatible or perhaps recombine into an offspring that is incapable of
breeding, an infertile hybrid like a mule.
It is easy to see why paleoanthropologists despair over trying to apply Mayr's
biological concept of species to ancient hominids. The characteristics needed
to recognize a biological species--the isolating mechanisms--are not the kind
that usually turn up as fossils. How can an estrous cycle be preserved? What
does an infertile hybrid, reduced to a few fragments of its skeleton, look
like? How does a chromosomal difference turn into stone?
But there is another way of looking at species that might offer hope. The
biological-species concept is a curiously negative one: what makes a species
itself is that it doesn't mate with anything else. A few years ago a South
African biologist named Hugh Patterson turned the biological-species concept
inside out, proposing a view of a species based on not with whom it doesn't
mate but with whom it does. Species, according to Patterson, are groups of
individuals in nature that share "a common system of fertilization
mechanisms."
With reproduction at its core, Patterson's concept is just as "biological" as
Mayr's. But he turns the focus away from barriers preventing interbreeding
and
throws into relief the adaptations that together ensure the successful
meeting
of a sperm and an egg. Obviously, sex and conception are fertilization
mechanisms, as is the genetic compatibility of the two parents' chromosomes.
But long before a sperm cell gets near a receptive egg, the two sexes must
have
ways of recognizing each other as potential mates. And therein, perhaps, lies
a
solution to the mystery of Mount Carmel.
Every mating in nature begins with a message. It may be chemically couched:
eggs
of the brown alga Ascophyllum nodosum, for example, send out a chemical that
attracts the sperm of A. nodosum and no other. It may be a smell. As any dog
owner knows, a bitch in heat lures males from all over the neighborhood. Note
that the scent does not draw squirrels, tomcats, or teenage boys. Many birds
use vocal signals to attract and recognize the opposite sex, but only of
their
own species. "A female of one species might hear the song of the male of
another," explains Judith Masters, a colleague of Patterson's at the
University
of Witwatersrand, "but she won't make any response. There's no need to talk
about what prevents her from mating with that male. She just doesn't see what
all the fuss is about."
A species' mate-recognition system is extremely stable compared with
adaptations
to the local habitat. A sparrow born with a slightly too short beak may or
may
not be able to feed its young as well as another with an average-size beak.
But
a sparrow who sings an unfamiliar song will not attract a mate and is not
going
to have any young at all. He will be plucked from the gene pool of the next
generation, leaving no evolutionary trace of his idiosyncratic serenade. The
same goes, of course, for any sparrow hen who fails to respond to potential
mates singing the "correct" tune. With this kind of price for deviance,
everybody is a conservative. "The only time a species' mate-recognition
system
will change is when something really dramatic happens," Masters says.
For the drama to unfold, a population must be geographically isolated from its
parent species. If the population is small enough and the habitat radically
different from what it was previously, even the powerful evolutionary inertia
of the mate-recognition system may be overcome. This change in reproduction
may
be accompanied by new adaptations to the environment. Or it may not. Either
way, the only shift that marks the birth of a new species is the one
affecting
the recognition of mates. Once the recognition threshold is crossed, there is
no going back. Even if individuals from the new population and the old come
to
live in the same region again--let's say in a well-trafficked corridor of
fertile land linking their two continental ranges--they will no longer view
each other as potential mates.
The human mate-recognition system is overwhelmingly visual. "Love comes in at
the eye," wrote Yeats, and the locus of the human body that lures the eye
most
of all is the face--a trait our species shares with many other primates. "It
is
a common Old World anthropoid ploy," says Masters. "Cercopithecoid monkeys
have
a whole repertoire of eyelid flashes. Forest guenons have brightly painted
faces with species-specific patterns, which they wave like flags in the
forest
gloom. Good old evolution tinkering away, providing new variations on a
theme."
Faces are exquisitely expressive instruments. Behind our facial skin lies an
intricate web of musculature, concentrated especially around the eyes and
mouth, evolved purely for social communication--expressing interest, fear,
suspicion, joy, contentment, doubt, surprise, and countless other emotions.
Each emotion can be further modified by the raise of an eyebrow or the slight
flick of a cheek muscle to express, say, measured surprise, wild surprise,
disappointed surprise, feigned surprise, and so on. By one estimate, the 22
expressive muscles on each side of the face can be called on to produce
10,000
different facial actions or expressions.
Among this armory of social signals are stereotyped, formal invitations to
potential mates. The mating display we call flirtation plays the same on the
face of a New Guinean tribeswoman and a lycï½nne in a Parisian cafï½ a bashful
lowering of the gaze to one side and down, followed by a furtive look at the
other's face and a coy retreat of the eyes. A host of other sexual signals
are
communicated facially--the downward tilt of the chin, the glance over the
shoulder, the slight parting of the mouth. The importance of the face as an
attractant is underscored by the lengths to which humans in various cultures
go
to embellish what is already there. But the underlying message is
communicated
by the anatomy of the face itself. " 'Tis not a lip, or eye, we beauty call,
/
but the joint force and full result of all," wrote Alexander Pope. And it is
that "joint force"--over generations--that keeps our species so forcefully
joined.
This brings us back to the Levant: two human species in a tight space for a
long
time. The vortex of anatomy where Neanderthals and early moderns differ most
emphatically, where a clear line can be drawn between them and us by even the
most rabid advocate of continuity is, of course, the face. The Neanderthal's
"classic" facial pattern--the midfacial thrust picked up and amplified by the
great projecting nose, the puffed-up cheekbones, the long jaw with its
chinless
finish, the large, rounded eye sockets, the extrathick browridges shading it
like twin awnings--is usually explained as a complex of modifications
relating
to a cold climate, or as a support to heavy chewing forces delivered to the
front teeth. Either way it is assumed to be an environmental adaptation. But
what if these adaptive functions of the face were not the reason they evolved
in the first place? What if the peculiarities evolved instead as the
underpinnings of a totally separate, thoroughly Neanderthal mate-recognition
system?
Although it is merely a speculation, the idea fits some of the facts and
solves
some of the problems. Certainly the Neanderthals' ancestors were
geographically
cut off from other populations enough to allow some new mate-recognition
system
to emerge. During glacial periods, contact through Asia was blocked by the
polar glaciers and vast uninhabitable tundra. Mountain glaciers between the
Black and Caspian Seas all but completed a barrier to the south. "The
Neanderthals are a textbook case for how to get a separate species,"
archeologist John Shea told me. "Isolate them for 100,000 years, then melt
the
glaciers and let 'em loose."
If mate recognition lay behind a species-level difference between Neanderthals
and moderns, the Levantine paradox can finally be put to rest. Their
cohabitation with moderns no longer needs explanation. Neanderthals and
moderns
managed to coexist through long millennia, doing the same humanlike things
but
without interbreeding, simply because the issue never really came up.
The idea seems scarcely imaginable. Continuity believers cannot credit the
idea
of two human types coexisting in sexual isolation. Replacement advocates
cannot
conceive of such a long period of coexistence without competition, if not
outright violent confrontation. They would rather see Neanderthals and
moderns
pushing each other in and out of the Levant, in an extended struggle finally
won by our own ancestors. Of course, if the Neanderthals were a biologically
separate species, something must have happened to cause their extinction.
After
all, we are still here, and they are not.
Why they faded and we managed to survive is a separate story with its own
shocks
and surprises. But what happened on Mount Carmel might be more remarkable
still. It is something that people today are not prepared to comprehend,
especially in places like the Levant. Two human species, with far less in
common than any two races or ethnic groups now on the planet, may have shared
a
small, fertile piece of land for 50,000 years, regarding each other the whole
time with steady, untroubled, peaceful indifference.
