View Full Version : The Neanderthal Peace

Wednesday, June 30th, 2004, 09:40 AM
The Neanderthal Peace

By James Shreeve

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 café in 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 café was 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 café, 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 café raise 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 cliché to 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 café in 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. Voilà. 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, Hélène 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. Cortés 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 lycéenne in a Parisian café: 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.