All Neanderthal mandibles, for example, lack 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 are grossly enlarged, indicating tremendous torque in the bite. Between the last two molars and the upward thrust of the rear of the jaw, gaps of a quarter of an inch - an architectural nicety - shift the business of chewing farther toward the front. These and other features make a jaw uniquely, quintessentially Neanderthal; no other member of the human family before or since shows the same pattern.
As for Neanderthal appearance, the stereotype of a muscled thug is not completely off the mark. Thick-boned, barrel-chested, a healthy Neanderthal male throw an average American football linebacker 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 from the planet. 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 marvellous a part of the human story as anything that has happened since.
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 figurines, and the beads 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 brow ridges; a domed braincase; and a slender, lightly built frame.
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 were found in a communal grave. All exhibited the anatomical characteristics of modern human beings. Scattered 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 readymade 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. 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 co-exist, at least not for any appreciable amount of time. But recent archaeological discoveries in the Middle East have challenged that neat supposition. They suggest that Neanderthals, rather than being the low-tech ancestors of modern humans, may have inhabited the same land at the same time - and for far, far longer than had previously been thought.
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. Mount Carmel lies in the Levant, a tiny hinge of habitability between the sea and the desert, linking the two great land masses of Africa and Eurasia - a region rich in archaeological finds finds that have helped make sense of human evolution.
At the cave of Kebara in 1983, archaeologist Lynne Schepartz 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. 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. Moshe, as he is affectionately known, was found on his back, his right arm folded over his chest, his left hand on his stomach in a classic attitude of burial.
A few miles inland from Kebara, on a hill in lower Galilee, is Qafzeh. Here, in 1965, a young French anthropologist found a veritable Middle Paleolithic cemetery of distinctly modern humans. Though some Neanderthal bones were found among them, the tools found with all the bones were pretty much the same.
This confounded one of the accepted beliefs about the difference between Neanderthal and Cro-Magnon cultures. 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."
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, no painted caves, beaded necklaces, or other evidence of exploding Cro-Magnon culture.
The paradox of the Levant is a teasing one. At Kebara and nearby Qafzeh, Neanderthals and more modern humans appeared to use the same tools and share an identical culture. One did not "evolve" into the other, accompanied by a sudden flourishing of technology and art, as the old theories suggested. The question was, did the two live in the Levant at exactly the same time, distinct in appearance but otherwise acting as a single community?
The key to the riddle lay in precisely dating the flint tools and human remains found in the Levant caves. In the early 1980s, a new technique called thermoluminescence (TL) was used to do just that. The method 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 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, Helene Valladas - the French archaeologist using the technique - had squeezed an age of 60,000 years out of the burnt tools found beside Moshe at Kebara. The shocker came the following year, when she 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. By comparison, clearly Neanderthal remains in Spain had been reliably dated at just 30,000- 40,000 years old. Clearly, if modern humans were inhabiting the Levant tens of thousands of years before the Neanderthals, they could hardly have evolved from them. If the dates are 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.
IF THE two human types co-existing on mount Carmel cannot be distinguished on the basis of the time at which they lived, or by their tools, why should we think of them as separate anyway? Some early excavators in the Levant didn't. They 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'," maintains Geoffrey Clark of Arizona State University, "whatever those terms mean in the first place - which I don't think is much."
Yet this theory of "oneness" is flawed. 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 behaviour - however alike the archaeological 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. Certainly, they never interbred.
This is yet another puzzling component in the Levantine riddle. 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.
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. The moderns arrive very early at Qafzeh and the cave of Skhul nearby, and never lose their modern aspect. 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.
If these humans were truly contemporaneous, then how on earth did they fail to do so? 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 but biologically distinct.
The evolutionary barriers that prevent species from wantonly interbreeding are called "isolating mechanisms". These obstructions to breeding 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 oestrous cycles, or the barrier might come into play after mating: incompatible chromosomes, or offspring that cannot breed, an infertile hybrid like a mule.
But there is another possible explanation for why Neanderthals and moderns failed to interbreed. Long before a sperm cell gets near a receptive egg, the two sexes must recognise 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. As any dog owner knows, a bitch in heat lures males from all over the neighbourhood - but the scent doesn't draw squirrels, tomcats, or teenage boys. Many birds use vocal signals to attract and recognise the opposite sex, but only of their own species. The human mate-recognition system is overwhelmingly visual. "Love comes in at the eye," wrote Yeats, and the focus of the human body that lures the eye most is the face - a trait our species shares with many other primates. 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.
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.
But the mating display we call flirtation plays the same on the face of a New Guinean tribeswoman and a lyceenne in a Parisian cafe. A host of other sexual signals are communicated facially - the downward tilt of the chin, the glance over the shoulder, the parting of the mouth. The underlying message is communicated by the anatomy of the face itself, and it is that - 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, is, of course, the face.
The Neanderthal's "classic" facial pattern - the mid-facial 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 extra-thick brow ridges 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?
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.
If lack of mate recognition lay behind the 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 co-exist through millennia, doing the same humanlike things but without interbreeding, simply because the issue never came up.
The idea seems scarcely imaginable. Continuity believers cannot credit the idea of two human types co-existing in sexual isolation. Replacement advocates cannot conceive of such a long period of co-existence without competition, if not outright violent confrontation. They would rather see Neanderthals and moderns pushing each other in and out of the Levant in a 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 - that 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.
! This is an extract from 'The Neanderthal Enigma' by James Shreeve, published by Viking on 2 May at pounds 20.