The following is an excerpt from James Shreeve's The Neandertal Enigma. Shreeve raises an interesting question and provides the basis for considering if separate species of homo sapiens could have evolved. It is not uncommon to hear or read anthropologists who tend to differentiate Neandertal into a separate branch of homo sapiens distinct as a species. This is one of the rare cases where the basis for the differentiation is so well defined.
The most common definition of a biological species ... 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. ....[In other words] the chromosomes are incompatible, or perhaps recombine into an offspring that is itself incapable of breeding, an infertile hybrid, like a mule.
Mayr's "biological concept of species" is much truer to the living, natural - world than any notion based merely on differences in morphology. At the same time, it is easy to see why paleoanthropologists despair over trying to apply it 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 does an estrus cycle get preserved? What does an infertile hybrid, reduced to a few fragments of its skeleton, look like? How does a chromosomal difference turn into stone? Perhaps Neandertal and modern human males, hyraxlike, sported wildly divergent penises. Or perhaps the Neandertals had forty-eight chromosomes to our forty-six. There is no way of knowing. To my mind, the continuity of the morphological differences through time between the two humans of Mount Carmel hints that they were biologically different species. How] much more satisfying it would be, however, if we could point to some feature, something as clean and hard as a bone, and say, "There-that's the thing that kept them biologically apart, no matter how much they were the same in other ways."
This is probably impossible, but there is another way of looking at species that might offer a tiny window of hope. The "biological species concept" is a curiously negative one- what makes a species itself is that it doesn't mate with anything else. Thus a species is defined not by any activity or property inherent to itself, but by the limits imposed around it by isolation mechanisms. This is a little like defining a pie filling by calling it "what's inside the crust." What if we could turn this definition inside out and look at the pie filling itself?
A few years ago, a South African biologist named Hugh Patterson uncovered what he believes is a subtle but debilitating flaw in the biological species concept. Most evolutionists-including Ernst Mayr-believe that a new species can originate only when a small population of individuals becomes geographically isolated from others of their kind, probably in a habitat unlike the one to which they are adapted. A classic example is a flock of birds who are blown off course and end up stranded on a new island. In most cases, the stranded population goes extinct. But if it manages to adapt and even flourish, its descendants may come to live in the same region with the descendants of its parent population. If the two populations are able to interbreed and produce fertile young, then no | matter how different they may have become in other respects, they are still the same biological species. On the other hand, if isolation mechanisms have evolved to prevent interbreeding, then no matter how similar they look or act, they are a new species.
There is a great deal more to the process than is outlined here, but even this truncated summary is enough to hint at the hidden fault Hugh Patterson saw lurking in the biological species concept. If a population must be geographically isolated from its parent population in order to become a new species, how will it evolve reproductive isolating mechanisms against interbreeding with individuals of the parent population, since none are around to be isolated from? Let's say that a flock of birds- call them tree-nesting whippersnippers-is blown onto a desert island where no whippersnippers existed before. Ten thousand years later, the island rings with the song of whippersnippers. But since there are few trees, all of them make their nests on the ground. If some of these groundnesting whippersnippers should follow an errant gust of wind back to the land of their tree-nesting kin, the difference in nesting sites might look like an "isolating mechanism" against their interbreeding. But reproductive isolation can't be the cause of their divergence, because there weren't any tree-nesting whippersnippers on the desert island to evolve reproductive barriers against. The real reason for the evolutionary change was not the presence of tree-nesters, but the scarcity of trees. In other words, the emergence of isolation mechanisms might be an effect of a population's divergence. But it is hard to see how it could be the defining cause.
With this problem in mind, Hugh Patterson turned the biological species concept inside out, proposing a view of a species based not on whom it doesn't mate with, but on 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 array of adaptations, spanning every level in the life of an organism, 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 and their recombination into an offspring that is itself fertile. 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 a 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. The mating signal transmitted is received only | by male members of the species Canis familiaris, who are well equipped by evolution with olfactory sensors to respond to it. Many bird species use vocal signals to attract and recognize the opposite sex, but only of their own. "A female of one species might hear the song of the male of another, | or witness his courtship display," explains Judith Masters, a colleague of Patterson's at the University of the Witwatersrand in Johannesburg, "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."
Similarly, a taxonomist may be at a loss to discriminate between various firefly species based on the anatomy of the males. But on a summer night, a female firefly will have no such trouble. The males of her species will signal to her with a precise pattern of light flashes, distinguished from that of other species by their color, frequency, and duration. To her, the bioluminescent machismo of the other males is just so much noise. It is part of a wholly different "species mate recognition system." According to Masters, "a nice way of putting the difference [between Patterson's and Mayr's concepts of species] is that the recognition concept emphasizes the characteristics that the organisms themselves use to identify who is a member of the species and who isn't."
A species' mate-recognition system is extremely stable compared to adaptations to the local habitat. An individual 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 sized beak. But a sparrow who sings an unfamiliar song will not attract a mate, and thus is not going to have any young at all. He will be plucked from the gene pool of the next generation, leaving no 1 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 told me.
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 adapted to, even the powerful evolutionary inertia of the mate-recognition system may be overcome. This sea change in reproduction may be accompanied by new ecological 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 same would apply, of course, to human species. So perhaps Neandertals and modern humans were able to coexist in the Levant for tens of thousands of years without interbreeding because they were not sending and receiving the same mating signals. We have at least to consider the astonishing possibility that there were once two kinds of intelligent human beings, mutually uncomprehending in the only area that could merge them into one.
One tantalizing advantage of looking at species this way is that there may be a trace of real, biological species differences lingering in the fossil record. The only traits that determine whether two organisms really represent different species are the features associated with their mate recognition systems. In most cases, mating signals do not show up as fossils. But some species, notably those who signal to potential mates mostly by sight, may have a bit of their mate-recognition system inscribed in their anatomy. Elisabeth Vrba of Yale has shown that antelopes and other bovids wear their species' identification tags on their heads in the shapes and sizes of their horns and antlers. ("The only difference between a screw-horn goat and a non-screw-horn goat is a screw horn," comments Ian Tattersall.) These hard cranial ornaments show up very conveniently in the fossil record, making identification of extinct species a much easier task.
Neither Neandertals nor early modern humans sported antlers, of course. Nevertheless, human mate-recognition systems are overwhelmingly visual. "Love comes in at the eye," wrote Yeats, and the upright, bipedal posture of hominids provides a lot of sexual signals for the eye to take in. The human penis, for example, is much larger than that of all other great apes. The breasts of the female are swollen out of all proportion to those of apes. But the locus of the human body that lures, captures, and holds the eye most of all is the face. In one recent study, when American women were asked what part of a man's body they looked at first, 77 percent answered either the face or the eyes. The man's buttocks came in a distant second, with 12 percent. Among men, 64 percent were attracted first to the woman's facial region, 21 percent to her chest. Human faces are exquisitely expressive instruments. Behind our facial q 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 twenty-two expressive muscles on each side of the face can be called on to produce ten thousand 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 as it does on the features of a Iycienne in a Parisian cafe: a bashful lowering of the gaze to the side and down, followed by a furtive look at the other's face and the 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-its shape and outline, the geometric arrangement of the features, their relative proportions and placement. " '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 Neandertals 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 Neandertal's "classic" facial pattern-the mid-facial thrust
Our species shares the use of the face as the focus of mate recognition signals with many other primates. "It is a common Old World anthropoid ploy," says Judith Masters. "Cercopithecoid monkeys have a whole repertoire of eyelid flashes. Forest guenons have brightly painted faces with species-specific patterns, which they wave like Rags in the forest gloom. Baboons yawn ostentatiously, and show their canines. Good old evolution tinkering away, providing new variations on a theme."
.....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 browridges shading it all 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 evolved to help its bearer survive in a particular habitat. 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 to a totally separate, thoroughly Neandertal mate-recognition system?
Although it is merely a speculation, the idea fits some of the facts and solves some of the problems. Certainly the geography of the European continent was such t