AS RELATED IN THE PRECEDING CHAPTERS, WE FINALLY HAVE an answer to a question—where do we come from?—that has haunted humans ever since they gained the capacity to wonder. We now know that our origin is lost in the darkness of time. It goes back to such a remote past that our minds fail even to imagine it. Almost four billion years, four million millennia, 40 million centuries! Just to count them, at the rate of one century per second, we would need more than a year. In those immensely distant times, there appeared on our young planet, freshly recovered from the violent impacts and convulsions that heralded its birth, the primitive form of life from which, with all other living beings on Earth, we have descended. The line that links us to this ancestral form is uninterrupted. But it can be recognized only retrospectively.
During more than two billion years, life wove its occult networks of unicellular organisms over the surfaces of lands and seas, in the dark depths of oceans, and in the hidden crevices of rocks, radically upsetting the natural balances that governed the planet and recycling the elements through a new chemistry, still subject to the laws of nature, but channelled along strange pathways by molecules of its own making that turned into its rulers. From this pullulation, there emerged, through an astonishing metamorphosis, the eukaryotic cells that allowed life to escape from its prokaryotic shackles.
Surprisingly, it took these cells more than one billion years to discover the new, collaborative mode of existence that was to prove so enormously successful. It is only after life had already covered the major part of the distance that separates its beginning from the present time that eukaryotic cells born from the same parental cell started to form organisms in which vital functions were distributed among different cells. Next to plants and fungi, pluricellular, aerobic, heterotrophic, mobile living beings arose: the animals.
In the thick bush of evolutionary ramifications that grew from the first animals, a line can be distinguished a posteriori. It first goes through a string of sponges, jellyfish, polyps, and marine worms. Then, letting those primitive forms fan out laterally with all the invertebrates that arose from them, including crustacea, insects, molluscs, echinoderms, and many others, the line leads to the first vertebrates. After that, leaving the fish in the seas, it comes out of the water with the first amphibians, follows some of these in their transformation into reptiles, and pursues its progression up to the early mammals, letting amphibians and reptiles, together with the birds that emerged from the latter, diversify on their own. Among the mammals, the line comes to the primates and continues its course, by way of the big anthropoid apes, up to a species close to present-day chimpanzees. From this stage, reached some six million years ago, a final stretch leads, by way of a series of intermediary forms pale- oanthropologists are beginning to identify, to the modern human species: Homo sapiens sapiens.
This long, tortuous evolutionary pathway leading from the first ancestral form of life to the human species is landmarked by a large number of bifurcations, or forks, each of which signals an evolutionary step where our line diverged from another that either died out or branched into some other living group. How many such bifurcations there were is difficult to estimate, but they must have numbered at least in thousands. As we have seen in Chapter 7, modern science tells us that each of those bifurcations was the consequence of an accidental genetic modification that happened, by chance, to occur in an environment favorable to the selection of the mutant form. According to this description, we are the outcome of thousands of fortuitous events, each of which has involved a dual role of chance, at the genetic and at the environmental level. This much is accepted by a vast majority of scientists. The significance of this fact is, however, diversely appreciated.
For a number of experts, perhaps the majority, the message from science is inescapable. It was all an incredible piece of luck. The emergence of the human species was an extremely improbable event, so improbable as almost surely to be unique. Even if there should be life in other sites in the universe, which is far from certain, the likelihood of its leading to human beings or even to any conscious, intelligent, humanlike beings, is vanishingly small. The event, we are told, is so unlikely that it might very well not have happened on our planet either, where only an extraordinary combination of circumstances made it possible. This view, which has been expressed in various terms by such leading evolutionists as George Gaylord Simpson,1 Ernst Mayr,2 Stephen Jay Gould,3 and many others, was summed up by Monod, when he completed the sentence cited in Chapter 3, “the Universe was not pregnant with life,” with “nor the biosphere with man.”4
A small but increasingly vocal minority goes one step further. Initiated in France, in the wake of Teilhard de Chardin’s evolutionary mysticism, this movement has also gained the Anglo-Saxon world. The emergence of humankind, according to these modern defenders of finalism, is not just highly improbable, it is plain impossible, at least by way of strictly natural processes. There are just too many unlikely coincidences piling up. Without outside help, chance “wouldn’t have a chance,” however many opportunities the universe, or even trillions of universes, would supply. Sum up the thousands of fortuitous occurrences—which means multiplying their probabilities—that traced the road from the ancestral form of life to humankind, and you have long passed the point where improbability turns into impossibility. Yet, it happened. So, there must be “something else.”
This cryptic entity has been ascribed to some yet-to-be-discovered principle of unknown nature, vaguely described by words such as auto-poiesis, self-organization, complexity law, or informational force, or even identified as a mysterious manifestation of quantum mechanics. Some do not hesitate to see it as the hand of God actually manipulating genes.
We have already seen, in Chapter 3, how intelligent design has been invoked by Behe to explain the “irreducible complexity” of biochemical systems. The same kind of argument, based on “integrative complexity,” is applied to biological evolution by the New Zealand scientist Michael Denton in his 1998 book Nature’s Destiny, significantly subtitled “ How the Laws of Biology Reveal Purpose in the Universe.”5 Denton accepts the Darwinian explanation and does not explicitly call on “something else,” but he comes perilously close to it, with words such as “directed evolution,”6 “ingeniously contrived,”7 or “preordained.”8 In discussing the development of the characteristic lung of birds, he finds it “hard not to be inclined to see an element of foresight in the evolution of the avian lung, which may well have developed in primitive birds before its full utility could be exploited.”9 The whole blueprint, he believes, is written in detail into the DNA, presumably by a Creator who knew what He was doing and where He was going.
Divine rigging or intervention in evolution is not, however, seen by all believers as a mandatory correlate of religious faith. In Finding Darwin’s God,10 a book written largely in response to Behe’s Darwins Black Box, the American biologist Kenneth Miller, who, like Behe, identifies himself as a practicing Catholic, gives a detailed and incisive critique of Behe’s arguments and cogently exposes their fallacy. Miller himself has no problem accepting the neo-Darwinian theory in all its aspects, including the utter contingency and unpredictability of the human species. To Miller, in fact, contingency is part of God’s plan. It is written into the laws of physics and forms a necessary condition for human beings to be free to accept or refuse God’s will.
It should be added that not all scientists agree with the basic premise that the origin and evolution of life were highly improbable events that, to take place, required a nudge either from chance or from a Creator. Some, mostly astronomers or cosmologists, go so far as to claim that the probability of intelligent life arising naturally is so great that a serious effort deserves to be made to establish contact with at least one of the many civilizations that must exist “out there.” As we shall see in Chapter 17, holders of this view have made their case with sufficiently convincing vigor to gain the commitment of considerable resources to a search for extraterrestrial intelligence (SETI), which aims at detecting signals sent by some distant civilization.
The issue, clearly, hinges on a quantitative estimate of probability. Depending on the value you assign to the likelihood of the overall process that has led to humankind, you are in the camp of the “impossibilists,” the “improbabilists,” the “probabilists,” or the “obligatorists.” Simple gut feelings are not enough for such a decision. We must look more closely at the notion of chance itself, as it applies to evolutionary phenomena. Chance does not operate in a vacuum; it is always subject to constraints.
Chance does not mean an unlimited number of possibilities. It simply signifies that the choice among existing possibilities is governed strictly by their probabilities. The number of possibilities is always finite. At heads or tails, it is two. At dice, it is six with one die and 21 with two dice.11 At roulette, it is between 36 and 38 depending on the number of zeros. At a lottery, it is ten million if the numbers have seven digits. At the game of bridge, it is 5×1028 (50 billion billion billions). The number of possibilities may be very high, but it is never infinite. This means that a given result can always be achieved with near certainty, even though it depends strictly on chance, if chance is solicited often enough. This, of course, has to be physically possible. Within realistically acceptable limits, all depends on the number of opportunities chance is offered to produce a given event, as compared to the probability of the event.
Take a perfect coin and let it be tossed by a robot strictly ruled by the laws of chance. The probability of the coin falling on the heads side is one in two. Have it tossed a second time, and the probability of its falling on the heads side is again one in two. However, the probability of the coin’s falling on the heads side twice in succession is ½ × ½, that is, one in four. It follows that, with two tosses, the coin has three chances out of four of falling at least once on the tails side. With three tosses, the probability of its falling each time on the heads side is ½ × ½ × ½, or one out of eight. Hence, the coin has seven chances out of eight of falling at least once on the tails side. Pursuing this kind of reasoning, one readily finds that it suffices to toss the coin ten times for the probability of its falling at least once on each side to be 99.9 percent. We approach certainty.
One easily calculates in the same manner12 that a 99.9 percent probability of winning a bet is reached by throwing a die 38 times or by giving a roulette wheel about 250 spins. Even a seven-digit lottery number has a 99.9 percent chance of coming out if some 69 million drawings are done. The number of times the cards must be dealt at bridge to guarantee a given distribution with the same probability is likewise finite and theoretically computable, even though it is beyond the capacities of my hand calculator and, also, beyond the boundaries of any realistic physical possibility.
In conclusion, chance does not exclude inevitability. The probability of an event, however improbable, can become close to certainty if the event is given a sufficient number of opportunities to take place. This is of little help to those who play games of chance, because the games are always rigged so as not to give chance enough opportunities. But it may be relevant to the evolutionary game.
The number of mutations a given genome can undergo is large, but it is finite and, in many cases, not excessively large relative to the number of individuals at risk—often millions, if not billions or more—and to the time—up to millions of years—evolution has available for its experiments. Contrary to what is often assumed, evolution rarely has to wait very long for some favorable mutation to occur. More often than not, mutations are present in a population all the time or come up regularly, waiting, so to speak, for the environment to provide them with an opportunity to prove useful.
This assertion is easily demonstrated in the case of simple point mutations, that is, those in which one base is replaced by another. Considering only replication errors, which are known to occur with a frequency of about one wrongly inserted base in one billion, it is readily calculated13 that the probability of finding a given point mutation in a clone of cells produced by successive division from a single cell becomes 99.9 percent after about 34 generations, that is, less than one day for bacteria, and about one month for animal cells.14 To take a more concrete example, consider red-blood-cell renewal in the bone marrow of an adult human individual. The probability of a given point mutation taking place in the course of this process reaches 99.9 percent after only about two hours. It is fortunate that most of those mutations are harmless and that, in addition, evolution has provided elaborate repair mechanisms to offset their effects. There are, however, rare cases in which an accident of this sort may lead to some anomaly, such as cancer.
It may be argued that such calculations are of limited value because point mutations are likely to be of little importance for evolution, which probably depends mostly on more extensive genetic rearrangements whose probability is much more difficult to evaluate. It is therefore significant that all we know of evolution in action tends to confirm the richness of the mutational field.
Take, for example, the many cases of drug resistance. In only a few decades, the appearance of “superbugs” resistant to penicillin and most other antibiotics has grown into a worldwide health problem. One could argue that bacteria, with their huge numbers and fast multiplication rates, make a special case. But the problem is not limited to prokaryotes. The malarial parasite, Plasmodium falciparum, which is a eukaryotic protist, has become largely resistant to chloroquine, the drug that was produced during the last war as a substitute for the no-longer-available quinine, and it is now rapidly developing resistance against mefloquine, the latest antimalarial created in the 1960s by the U.S. Walter Reed Army Research Institute. Even more impressive, in 1948, the Nobel prize in medicine was awarded to the Swiss chemist Paul Müller “for his discovery of the high efficiency of DDT as a contact poison against several arthropods.” Malaria, it was triumphantly announced, was going to be vanquished by the simple means of killing off the mosquitoes that transmit the parasite.15 Today, the use of DDT is increasingly being questioned, not only because of its noxious effects on the environment, but also because mosquitoes in the affected countries have largely become resistant to it. These, it should be noted, are not microbes; they are complex animals, which, in less than 50 years, developed widespread resistance against the insecticide.
It is clear that the resistance mutations cannot have occurred in response to exposure to the drugs, except in a purely fortuitous fashion, should the drug, for example, have mutagenic properties. Any truly adaptive response would imply some sort of intentionality, which is strictly ruled out by our knowledge of molecular biology (see Chapter 7). No, the mutations were always there in some individuals or happened frequently, but only exceptionally were they of any use under natural conditions. Developing in the vicinity of a penicillium mold might be such an exception for a microbe. Barring such exceptions, it is clearly we who, by putting the drugs in the environment, have given the resistance mutations an opportunity to prove useful and thus to spread.
Many other natural phenomena illustrate the abundance of mutations. Mimicry is a good example. All are familiar with those insects that look for all the world like the leaf or branch they are sitting on, or with those fish that blend so well with the sea floor that an observer can hardly distinguish them. Such instances have often been brandished by adversaries of Darwinism as typical proofs of copying adaptations that cannot possibly be explained by natural selection acting on fortuitous genetic changes. Some instruction, it is claimed, must have been involved for such amazing similarities to arise. This need becomes less compelling once it is realized that natural mutations are so frequent and varied as to cover an immense spectrum of potentialities.
The many instances of artificial selection, which so impressed Darwin, are another case in point. Just think how, in a mere few thousand years, wolves have been led, by simple breeding methods, to produce fox terriers, shepherd dogs, St. Bernards, poodles, dachshunds, greyhounds, Pekingese, and all the other canine friends that humans surround themselves with. Who could have imagined, looking at the wild species, that they had it in them?
As another illustration of the enormous potential of chance mutations, it is a well-known fact among molecular biologists that almost any desired trait compatible with the cells’ general organization can be elicited in a population of growing cells by sufficiently stringent culture conditions. Also impressive are the successes of the “carpet-bombing” technique of creating useful variants, mentioned in Chapter 7. Just throw your X rays or mutagenic substances indiscriminately, and you have a good chance of obtaining what you want, for example, a mold strain producing high yields of penicillin. There is even evidence that natural selection has retained a mechanism of this sort, whereby bacteria enhance the mutability of parts of their genome under stressful conditions, where survival of the population may depend on some rapid genetic readjustment.
In a slightly different vein, the immune system offers another example of the natural exploitation of genetic lavishness. The facts are well known. You are exposed to a foreign substance (antigen) carried, for example, by some pathogenic bacterium or virus, and, in a matter of two to four weeks, your body has built specific proteins (antibodies) that bind to the foreign substance by the kind of structural complementarity mentioned in Chapter 1, thereby serving to combat the invader. The same result is achieved preventively in vaccination, by exposure to an antigen-bearing organism previously rendered harmless by some appropriate treatment.
The remarkable thing about immunity is that it works with almost any foreign material of some chemical complexity. Hence the long- accepted assumption, seen as almost self-evident, that the foreign substance “instructs” the immune system to manufacture the complementary protein. In fact, this is not so. We now know that the immune system, in the course of its maturation, creates a wide array of genetically diverse cells programmed to make a correspondingly wide array of antibody proteins. All that exposure to the foreign antigen does is to trigger the multiplication of those few cells in the array that display the appropriate antibody on their surface and thereby advertise their ability to synthesize it. This is how the antigen boosts the production of the antibody. Thus, in the time it takes the immune system to reach maturity—some six months after birth—enough variation has been created (by a combinatorial program of genetic rearrangements) to cover billions of possibilities. The similarity with evolution is clear. At the start, there is wide variation, subsequently followed by selection.
Summing up this part of our analysis, we arrive at the seemingly paradoxical conclusion that, even though the mutations that allow evolutionary bifurcations are purely accidental occurrences, the chance factor is, nevertheless, largely abolished by the continually replenished plentifulness of the mutational field. In many cases, the decisive role is played by the environment, not the mutations. There are exceptions, no doubt, but, on the whole, what we witness of evolution in action tends to support this view. This still leaves chance an important role, however, by way of the environmental conditions that serve to screen genetic variants. Before addressing this question, we must look at another important aspect of genomes as targets of natural selection.
In a primitive organism, almost any mutation may be the start of a new evolutionary line. But, as complexity increases, the number of possible productive genetic changes decreases. To take a somewhat shaky simile, starting with a Ford model T, you have a wide range of evolutionary possibilities. Just look around while driving on a highway, preferably in Europe, where diversity is of worldwide origin, and you quickly get a sampling of the many different directions car evolution has taken. But with a Ferrari, the number of options is obviously restricted. There are thus inner constraints that limit further evolution.
In the case of genomes, these constraints are of two kinds. First, as already mentioned in the preceding chapter, the number of genes likely to be involved in evolution becomes progressively reduced to a limited set of “supergenes” as body-plan complexity increases. In addition, the number of possible changes of these genes that are compatible with the continuation of evolution does itself decrease as well, since the mutant form must be viable and capable of producing enough progeny under the prevailing conditions if it is to be retained by natural selection. The greater the commitment, the fewer the options satisfying this requirement. Many evolutionists have commented on this fact, which is often reflected in a progressive acceleration of evolutionary change. The remarkably rapid transformation of an ancestral chimpanzee into a fullblown human is a case in point. Apparently, once some key step was taken, there was little choice left but to continue ever faster in the same direction.
It has long been known from the fossil record that a world-wide catastrophe must have occurred some 65 million years ago, causing the extinction of the dinosaurs and of many other living species. In 1978, two American physicists, Luis Alvarez and his son Walter, found evidence that led them to suggest that the fall of a large asteroid was the phenomenon responsible for the cataclysm. This hypothesis has since been amply confirmed, and the impact site of the asteroid has even been located in what is now Chicxulub in the Yucatan Peninsula, in Mexico. This event is often cited as evidence of the far-reaching effects chance environmental circumstances may exert on biological evolution. But for a major blow from outer space, dinosaurs might still roam Earth, mammals might still be leading a modest and inconspicuous existence in the shadow of the big reptiles, and we would not be here to take notice of the fact.
Another often-quoted example is the formation, some six to seven million years ago, of the Great Rift Valley, which split a good part of East Africa. According to a theory proposed by the French paleoanthro- pologist Yves Coppens, codiscoverer, with the American Donald Johanson, of the famous skeleton known as Lucy,16 this event may have played a major role in the emergence of the human species, by cutting off a group of apes from the forest and forcing them to adapt to the savannah, where bipedalism became a condition of survival and hands were freed to develop new skills. Had not Earth’s crust cracked there and then, it is said, our ancestors might still be up in the trees.
Those are just two better-known instances of the no doubt numerous cases where some step in our evolutionary history has been crucially influenced by chance environmental conditions, providing incontrovertible proof, in the eyes of many leading evolutionists, of the unpredictability of evolution, in general, and of the utter contingency of the human condition, in particular. Long taken as irrefutably supported by modern science, this view is perhaps not as certain as it is often made to appear. Evolution proceeds in two directions, and these are affected in very unequal fashion by the environment.
Every evolutionary step depends, as we have seen, on a genetic change that, in the case of a multicellular organism, has affected a germ cell that will later be involved in the generation of a fertilized egg. In order to play a role in evolution, this change must be reflected in a modification of the organism’s developmental program and it must be transmissible, that is, the modified individual must be able to produce progeny. Whether the change will eventually give rise to a new branch depends on the environment, including the necessity for reproductive isolation mentioned in the preceding chapter. What will happen also depends on how significantly the body plan is affected by the genetic modification. Here is where the distinction between the two directions of evolution, which I like to call horizontal and vertical—roughly equivalent to what is often termed microevolution and macroevolution—comes into play.
Most often, genetic changes are minor and do not basically modify the body plan. They are, so to speak, variations on the same theme. More than 750,000 insect species are known and several million may remain to be discovered. Even though they may be as different as a butterfly, a scarab, or a praying mantis, they are all built according to the “insect” blueprint. This type of evolution, which is the one I designate as horizontal, is largely responsible for biodiversity, that is, the extraordinary variety of forms evolution has created on given models, be they of a grass, insect, fish, or mammal.
In horizontal evolution, chance plays the star role. It enjoys an almost free rein, flitting over a multitude of special environments where small isolated groups are allowed to develop certain peculiar traits closely connected to the local scene. The cases of mimicry alluded to earlier are typical examples. Darwin’s famous finches are another. In the course of his voyage to the Galapagos Islands, Darwin observed that, on each island, finches had differently shaped beaks, adapted to the kind of food the island offered. These various species, Darwin reasoned, must descend from the same ancestral species, which, in the reproductive isolation offered by each island, evolved separately according to the local conditions.
Innumerable such examples could be summoned. Horizontal evolution is indeed the realm of contingency, largely shaped by the accidents that create, fortuitously and unpredictably, conditions favorable to the expression of one among the many mutations it is continually presented with. (Remember drug resistance.) This is how each type of organizational pattern has produced, by progressive horizontal ramification, thousands or more different species adapted to a wide variety of ecological niches. Even here, however, as I shall point out later, certain privileged directions, manifested by convergent evolution, may be discerned.
But evolution has not just composed variations on the same theme; it has also created new themes. It has done so through modulation from existing themes, by way of genetic modifications that were both rarer and more exacting than those involved in horizontal evolution and that have led to much more important, sometimes even spectacular, rearrangements of the body plan, while all the time remaining compatible with the survival and reproductive success of the new forms.
In many cases—not all, since examples of regressive evolution are also known—this type of evolution, which I term vertical, involves an increase in complexity. I use this word intentionally, rather than “progress,” which, because it implies a judgment of value, understandably upsets a number of contemporary thinkers. Even the word “complexity” is the object of many learned philosophical discussions. For my part, I shall stick to the intuitive meaning most of us attach to it, as opposite of simple. And I shall take it as self-evident that vertical evolution, that is, the one that involves significant changes in body plan, has, with time, produced living beings of increasing complexity. The two directions I singled out as privileged in the preceding chapter, leading to increasingly efficient reproductive means in both plants and animals and to increasingly intricate polyneuronal networks in the animal line, clearly illustrate the course of vertical evolution towards increasing complexity. Or, to take an even less subjective piece of evidence, the fact that evolution has, in both kingdoms, produced an increasing number of differentiated cell types should demonstrate to almost anyone’s satisfaction the reality of vertical evolution toward increasing complexity.
Vertical evolution, thus defined, is subject to much more stringent inner constraints than horizontal evolution. It is intuitively evident that transforming a body plan—say, from a fish to an amphibian—must be more difficult than just modifying it in some trivial fashion—such as turning an ancestral fish into a sole or a mackerel—especially because each intermediary form has to be viable and able to reproduce. Going back to our simile of car evolution, it is easier to convert a model T into a Jeep or a Ferrari than into a helicopter, especially if the conversion is to occur through a series of intermediary forms each of which is fit for transporting passengers with reasonable efficiency.
In concrete terms, this means that there are fewer genetic options that can propel evolution vertically than can do so horizontally. The difficulty increases—and, thus, the number of options decreases—as complexity increases. This, also, is evident. A simpler body plan is likely to be more flexible and more tolerant to change than a more complicated one.
All this goes to show that there is, written into the very structure of genomes, more inevitability in vertical than in horizontal evolution, and that the degree of inevitability increases with the progression of vertical evolution. This seems particularly true for the evolution of the nervous system of arlimals toward increasing complexity, which is almost independent of the environment, since it is hard to imagine an environment in which possession of a better performing brain would not be an advantage for an animal. In this sense, progression toward complexity may be viewed as obligatory in so far as it is genetically and environmentally possible.
This point is very relevant to the origin of humankind. I mentioned earlier two major fortuitous events—the fall of the asteroid that killed off the dinosaurs and the opening of the Great Rift Valley, believed to have isolated our ancestors from the forest—that may have played a key role in human evolution and are often alleged as proof of the contingent character of humanity. The argument is impressive, but not irrefutable. It is quite possible—some might even say probable—that environmental vagaries have done no more than determine the moment when an inevitable evolutionary development actually took place. Perhaps mammals were bound to supplant dinosaurs at some stage for reasons linked to the intrinsic properties of the two types of animals, and the asteroid only precipitated an event that would have occurred sooner or later. Similarly, had not the Great Rift Valley created conditions suitable for hominization, some other geographical upheaval would have done so at a later date. The same could be true of many of the other circumstances that have allowed a decisive step in the long, vertical progression that led to the human species.
“Wind back the tape of life…; let it play again from an identical starting point, and the chance becomes vanishingly small that anything like human intelligence would grace the replay.” This oft-quoted passage from Stephen Jay Gould’s best-selling Wonderful Life17 has become for much of the educated public the dire but unassailable message from modern biology, underlining the utter contingency of the human condition.
The British paleontologist Simon Conway Morris is not impressed with the message. An expert, as was Gould, on the Burgess Shale, a geological site in the Canadian Rockies about 530 million years old and rich in fossils of bizarre animals (which forms the main topic of Gould’s book), Conway Morris devotes much of a recent work18 to demonstrating that the Burgess Shale fauna is by far not as weird as is claimed by Gould. At the end of his book, Conway Morris addresses the “rerunning-the- tape” image. He considers the argument both trivial, in that it simply repeats what has always been known of the contingency of historical events, and false, to the extent that it applies to the main directions of evolution. “In fact,” he writes, “the real range of possibilities and hence the expected end results appear to be much more restricted. … Within certain limits the outcome of evolutionary processes might be rather pre-dictable.”19 In addition to some of the constraints already mentioned above, Conway Morris lists, in support of his contention, the increasing number of known instances, some truly astonishing, of convergent evolution.
Animals that have been separated and left to evolve independently for up to several hundred million years are found to develop into extraordinarily similar types. Anteaters have the same intricate specializations in North America, South America, Africa, and Oceania. So have felines dependent on hunting or herbivores built for speed. It is the case also with underground mammals. According to a world authority on the topic, the Israeli biologist Eviatar Nevo, the 285 known species of these animals share in astonishing fashion “regression, progression and global convergence”.20
What these and many other similar cases of convergent evolution tell us is, first, that there are not so many solutions to, for example, the problem of surviving on ants and, especially and more surprisingly, that, under sufficient selective pressure, the same solution will be found time and again by different species. Even trivial aspects of horizontal evolution may apparently be subject to this kind of channelling. “Rerunning the tape”—an image we should be careful not to take too literally, in any case—no doubt would not produce exactly the same script but might well come sufficiently near the original story to make it perfectly recognizable.
Watch a mare giving birth. The animal shows signs of only minor discomfort. After delivery, the newborn foal staggers to its feet in a matter of minutes and lurches toward its mother to partake of its first meal. Now move to the delivery room in a hospital and watch the human version of the same scene. It takes the mother hours of excruciating pain—with a number of attending risks that often proved fatal before the advent of modern medicine—to give birth to a baby that is entirely helpless and will need many months to reach a stage where it can match the newborn foal’s autonomy. Faced with these two contrasting scenes, even the most confirmed Darwinian may wonder how natural selection ever retained a mode of giving birth as hazardous as the human one. The key to the riddle, at least as I see it, is; a better brain.
In humans, development of the brain takes place inside the womb until the size of the head has reached the utmost limit compatible with passage through the birth canal. Even then, the brain is still at a very immature stage and continues to undergo outside the womb a developmental process that, in other mammals, is largely completed in utero. It is thanks to this delayed development—the technical term is “neoteny”—that the human species has been able to achieve its unique brain size and to acquire the extraordinary accompanying mental attributes. No other fact could illustrate in a more striking fashion the force of the selective pressures favoring vertical evolution towards increasing polyneuronal complexity.
Since Haeckel, the course of evolution is, for excellent reasons, represented by a tree. For the German zoologist-philosopher and for the other early disciples of Darwin, imbued as they were with Victorian triumphalism, this tree rose majestically towards a summit that nobody doubted was dominated by humankind, the uncontested master of creation.
Today, the point of view has changed. It has become politically correct to put the emphasis on the canopy of the tree and on the millions of terminal twigs that compose it. The human species, it is pointed out, is no more than one of those twigs, on par with plague bacilli, amoebae, orchids, scorpions, baboons, and all the other species that form the thin living pellicle that surrounds Earth. Like the others, the human twig is the outcome of some four billion years of evolution, the result of thousands of accidental mutations screened by natural selection according to the caprices of environmental conditions. Only our insolent vanity gives the twig a special significance, which is justified by no objective reason. On the contrary, if any discrimination is to be made, it should, as mentioned in Chapter 8, favor the much more ancient and sturdier bacteria.
Calling on science to denigrate the human species is part of the so-called deconstructionist, post-modernist trend, which blends science, philosophy, sociology, and politics into an ideological mixture inspired by a negativistic, relativistic vision of knowledge, which goes so far as to deny the very existence of objective reality. I do not feel qualified to discuss this topic, which has been addressed by many contemporary authors. Remaining at the level of my competence, I shall limit myself to pointing out that the image proposed for the tree of life is a perversion of the scientific facts on which it is allegedly based.
Like all trees, the tree of life has grown simultaneously in two directions: vertical and horizontal. It is true that if one looks only at the canopy, one is first struck by the biodiversity created by horizontal evolution. But this is only a very superficial vision. The early naturalists already distinguished similarities in the diversity of life and classified it in a series of hierarchical divisions. With the advances of paleontology and, more recently, of molecular biology, this hierarchy has been found to correspond to a historical reality and to reflect a series of stages in the growth of the tree. The lower branches, which detached earliest from the trunk, end up in the simplest forms of life. As we move upward, and the proportion of evolutionary time taken by vertical growth increases, terminal twigs bear increasingly complex forms of life. Thus, even the tree’s canopy, when properly examined, already appears as hierarchically organized.
The inner structure of the tree is not directly visible but has been revealed by modern science. As could be suspected, the visible terminal twigs are derived, by horizontal ramification, from hidden master branches that have detached from the trunk in the hierarchical order dimly revealed by the canopy. Many key bifurcations in this development have now been recognized, and the properties of the organisms that occupied them are beginning to be appreciated.
This fact calls for a remark. In everyday language, it is often said that birds descend from dinosaurs, reptiles from fish, or humans from monkeys. Used for simplicity’s sake, such expressions may be quite misleading if one doesn’t pay attention. Today’s reptiles were not born from today’s fish. They share with todays fish a common ancestor that lived some 400 million years ago. This ancestor was the starting point of a bifurcation of which one branch continued to diversify into fish, whereas the other evolved progressively up to another critical bifurcation leading, on one hand, to amphibians and, on the other, to reptiles. These long-gone transitional forms make up the trunk of the tree of life. They could have been very different from their present-day descendants. All we can do is try to reconstruct them with the information provided by the descendants and by fossils and vestigial organs.
The tree of life manifestly grows in two directions, vertically toward complexity and horizontally toward diversity. It has a top, albeit in the form of a thin terminal twig among millions of others. It is obvious that the human twig occupies this top, at least if the brain is adopted as the criterion of complexity. Considering all that the human brain has produced, one must have a distorted set of values to refuse to accept this criterion.
If our situation on top of the tree of life should leave little doubt, this is no reason for bragging. All we know of the history of life makes it likely that our eminent position is only temporary It was occupied three million years ago by a young female called Lucy21 and, three million years earlier, by the last common ancestor we share with chimpanzees. What form of life will occupy it in the future is anybody’s guess. The astronomers tell us that Earth should be able to sustain life for at least 1.5 billion years, possibly up to 5.0 billion years.22 If the tree of life goes on growing vertically, it could reach more than twice its present height. Our imagination is totally incapable of foreseeing what kind of being may emerge from such a process. Note that this development does not have to happen through further growth of the human twig. There is plenty of time for a more promising vertical line to start from another twig while our own twig withers. This thought should inspire a solid dose of humility, together with a reappraisal of some our most cherished notions concerning our significance.