Lamarck’s theory of inheritance of acquired characters lost a lot of ground—though not without a struggle—after natural selection was proposed by Darwin. In the beginning of the twentieth century, the Austrian biologist Paul Kammerer announced that he had succeeded in causing toads that copulate on land to acquire the “nuptial pads” whereby the males that copulate in water grip the slippery body of the female. He claimed to have accomplished this transformation simply by forcing the animals to copulate in water, generation after generation. Accused of falsifying his results, Kammerer even-tually committed suicide. This did not prevent the British writer of Hungarian origin Arthur Koestler from defending Kammerer’s memory and presenting him as a victim, in The Midwife Toad, published as late as 1971, well after the advances of molecular biology had invalidated the foundations of Lamarckism.
A much more dramatic instance of Lamarckian fraud was perpetrated under Stalin by the Soviet agricultural biologist Trofim Lyssenko, who claimed to have converted winter wheat into the more productive and faster-growing spring wheat by the simple device of “vernalization,” which depends on soaking and chilling the grains. When this claim was contested by geneticists, Lyssenko used the apparent agreement between his theory and Marxism as an argument to have the dissenters condemned, with catastrophic consequences for the future of genetics and of agriculture in his country. This scandalous affair caused a number of prominent leftist scientists in the West to cut ties with Soviet Russia and resign from the Communist Party.
DNA cannot be a vector of Lamarckian heredity
The inheritance of characters acquired by the parents’ experience was ruled out as a possible evolutionary explanation by the findings of modern molecular biology, at least as concerns transmission via DNA. There is no way whereby such an acquired trait could travel up to DNA and become hereditarily encoded in it. The pathway from gene to trait is strictly one-way. Crick has gone so far as to call this rule the “Central Dogma.” This choice was unfortunate, since science knows no dogmas, but it underlines the strict incontrovertibility of the rule. The Darwinian mechanism, on the other hand, is perfectly compatible with modern biology and supported by it.
Lamarck still has a few rearguard defenders, especially in France, where he has become something of a national symbol in the old struggle against Anglo-Saxon cultural hegemony. Not so long ago, after giving a lecture in a French university town, I was treated to a lengthy harangue by an elderly biology professor, extolling the qualities of Lamarck and lamenting his abandonment, which the professor saw as a betrayal in favor of the Englishman Darwin.
Cases of Lamarckian heredity that do not involve DNA exist
Note, however, that the ban against Lamarckism concerns only DNA-mediated heredity. Recent years have seen discovery of several other forms of heredity susceptible to a Lamarckian explanation. Biological membranes offer a revealing example. Cellular membranes grow by accretion, that is, incorporation of new components into preexisting membranes. In this process, the pattern of the recipient membrane influences the “choice” of the new constituent to be inserted. A membrane modified by usage could thus induce the insertion of a different constituent and change the pattern of the new material added to it. This material would, by the same mechanism, transmit this modification to progeny, with as consequence the inheritance of an acquired character, in Lamarckian fashion. Another, similar, example concerns the pattern of implantation of swimming appendages, called cilia, on the surface of some protists named paramecia. Let this pattern be changed by environmental influences, and later progeny will display the new pattern.
The discovery of prions reveals a particularly dramatic case of shape transmission. Prions are infectious agents of protein nature that owe their pathogenicity to their abnormal shape. Their molecular sequence is normal, but the manner in which the chains are folded is abnormal and—accounting for the infectious nature of the molecules—can be transmitted to normal molecules by contact. Mad cow disease and its human version, Creutzfeldt-Jakob disease, which made headlines a few years ago, are typical instances of prion diseases. The importance of this kind of transmission in normal heredity is not known but could be significant. Future work will tell.
Yet another instance of possible Lamarckian inheritance is represented by a set of phenomena recently grouped under the term “epigenetics,” a term long used in an entirely different meaning by developmental biologists and neurobiologists (see chapter 16). In its new meaning, epigenetics refers to a number of inheritable traits that are not written into the DNA sequences but accompany the DNA in germ cells and influence subsequent events in the fertilized egg. Such traits include the blockage of certain bases by chemical groups (for example, methyl groups) or the manner in which DNA is combined with the local proteins, or histones, in the chromosomes. Some of the most exciting new findings are being made in this area.
Genetic drift accompanies evolution without selection
Another non-Darwinian form of inheritance concerns traits that are transmitted without being selected. Many genetic mutations seem close to neutral, with little selective value, whether positive or negative, and just accompany the others by inertia, so to speak, because of the simple fact that their elimination is not sufficiently advantageous. The Japanese theoretician Motoo Kimura has developed a mathematical theory, under the name of “genetic drift,” that explains how mutated genes are inherited in this way. Many drifting genes exist. The best proof of this is given by the genetic diversity exploited by molecular phylogenetics. We have seen (chapter 1) that this technology uses the sequence differences among genes that play the same role in different organisms. The very fact that such differences exist, and actually do so on a large scale, is proof that many different versions of the same gene can exist and, apparently, perform satisfactorily. Whether these mutations are truly neutral is, however, debatable. For example, modern medicine has identified a number of human genetic variants that affect the probability of falling victim to a disease, such as diabetes or breast cancer. Such “risk genes” are not neutral; they may significantly influence the life span of the individuals concerned. On the other hand, to the extent that the genes do not affect fertility—the diseases they influence often break out late, after the individual has ceased to be reproductively active—the genes are indeed neutral with respect to natural selection.
Self-organization could theoretically drive evolutionary events
An evolutionary factor, unrelated to heredity and believed by some of its proponents to be as important as natural selection, lies in the ability, attributed to certain living systems, to spontaneously settle from an initially unstable situation into a stable, organized pattern. Variously referred to as “self-organization,” “autopoiesis,” “order out of disorder,” and the like, the phenomena underlying this ability have been the object of many ingenious theoretical studies but little experimental work so far.
Were some key evolutionary steps guided by “intelligent design”?
One last alternative to natural selection has been proposed under the name of intelligent design. It warrants attention, not because it offers a valid scientific explanation of evolution, which it does not, but because of the political and educational issues it has generated, especially in the United States but also, in recent years, in France and other countries. Intelligent design should not be confused with creationism. Theoretically, intelligent design presumes no biblical roots. Many of its defenders accept evolution. Some even accept natural selection. All they claim is that natural selection does not account for everything and that certain evolutionary steps cannot be explained naturally and must have required supernatural intervention.
This view is not new. It was known in the past as finalism, or teleology, a doctrine close to vitalism that sees life as a goal-directed process (which implies someone or something that does the directing). This is, indeed, the appearance life gives to the observer and is reflected in the common language: we say that the lungs are for breathing, t
he stomach and intestine for digestion, and so on. Replacing such statements by “the lungs, or the stomach, happen to be such that they can function in breathing, or in digestion” is counterintuitive but more consonant with current thought, which sees these organs as emerging by natural selection because they were accomplishing functions useful to survival and multiplication of the organisms in which they first appeared. The term “teleonomy” is often used, in opposition to “teleology,” to qualify the apparent purposefulness of biological structures.
Defenders of intelligent design use several arguments. One, by the American biochemist Michael Behe, is “irreducible complexity,” the property of systems made of several parts, each of which needs specific features to fit within the whole. He gives the “simple mousetrap” as a model of irreducible complexity and goes on to cite complex biochemical systems, such as blood coagulation or motor organelles like cilia and flagella, as examples of assemblages that could not have come together without the assistance of some entity that conceived the machinery, foresaw its use, and designed the various parts accordingly. Two centuries earlier, the English theologian William Paley made a similar argument in his famous watchmaker analogy, which served as basis for his Natural Theology; or, Evidences of the Existence and Attributes of the Deity Collected from the Appearances of Nature, which was first published in 1802: you find a watch and deduce that there must have been a watchmaker.
The New Zealander Michael Denton cites as evidence of intelligent design certain evolutionary transitions, such as the passage from reptile to bird. He points out that so many changes had to take place concurrently, in skin, bones, lungs, and other organs, that they could have happened only under the instructions of a designer who knew what the final product would look like.
William Dembski, an American mathematician, evokes the familiar sequence paradox. The calculation is simple. Take twenty different kinds of letters (representing the twenty kinds of amino acids with which proteins are constructed) and use them in all possible combinations to make words. The number of different possible words, known as the “sequence space,” increases with the length of the words. It is four hundred for two-letter words, eight thousand for three-letter words, in general, 20n for words of n letters. Many protein “words” have lengths of three hundred or more amino acid “letters,” which corresponds to a minimum of 20300, or 10390 different words. There is simply no way such a number could be represented, except by filling an entire paragraph with the figure 1 followed by 390 zeros, which is essentially meaningless. It dwarfs the biggest natural numbers we can think of, such as the number of stars in the universe (1022) or even the number of elementary particles in the universe (1042). Even the number of different fifty-letter words (2050, or 1065) already exceeds this range. For Dembski—and for a number of theoreticians who have made the same calculation before him—the conclusion is clear: proteins could not, without guidance, have reached the “infinitesimally minuscule” spot they occupy in their “immeasurably immense” sequence space.
At first sight, such reasonings may be appealing and even seem compelling. Take a case such as mimicry (chapter 7). To an observer faced with an insect that can hardly be distinguished from the leaf on which it sits, the simplest explanation that comes to mind is that of an agency that modeled the insect as a copy of the leaf. The natural explanation, which posits a very large number of small steps in which the ancestral insect each time became a little more leaflike, enough so to gain a selective edge over the others, is, in a way, much harder to swallow, especially if one adheres to the notion of a creator. A God who created life and endowed it with its properties should know enough about genetics and its applications to be able to carry out the changes that make a beetle look like a leaf and help it avoid predators.
It is thus no wonder that intelligent design has been so warmly received among various religious circles, including those that reject biblical creationism. The Catholic Church, for example, has recently been leaning in this direction, first in the words of the Austrian cardinal Schönborn, later endorsed by the pope himself. This is understandable and even logically consistent. For one who believes in a God capable of granting prayers and even performing miracles, directly or through persons of exceptional saintliness, it is not difficult to imagine this God occasionally giving evolution a nudge in a direction of his choice, especially if, as is firmly asserted by Catholic doctrine and several other theistic religions, one believes this choice to include the advent of humankind. But that is not the question. It is, even for the believer, to know whether the nudge was necessary or whether events could have happened naturally. We shall see that the advances of sciences do not favor intelligent design.
In the United States, the issue has become political, even legalistic. The question is not so much whether intelligent design is right or wrong, but whether its teaching in schools as an alternative to natural selection contravenes the separation between church and state. From the didactic point of view, this way of handling the situation ignores the fact that intelligent design is advocated by a significant number of people and the object of much public debate. My preference is to allow discussion of intelligent design and thereby demonstrate why it fails and why Darwinian natural selection succeeds in showing how evolution works. But such discussions would perhaps put an unfair burden on teachers and would probably be even more controversial than banning the subject outright from the curriculum.
Refuting intelligent design is easy and does not require any specialized knowledge. Intelligent design is simply not a scientific theory. Science is based on the working hypothesis that things are naturally explainable. This may or may not be true. But the only way to find out is to make every possible effort to explain things naturally. Only if one fails—assuming failure can ever be definitely established—would one be entitled to state that what one is studying is not naturally explainable. The entire history of science, including the dramatic advances of biology in the last fifty years, is there to validate and consolidate the naturalistic postulate. Now is hardly the right time to abandon this cornerstone of the scientific endeavor. Yet, this is exactly what intelligent design does, by stating a priori that certain evolutionary events are not naturally explainable, thus ruling out the possibility of ever finding the explanation if there is one (see also chapter 2). Just tell this to the students and let them draw their own conclusions. It has nothing to do with religion.
At the scientific level itself, the arguments put forward in support of intelligent design can easily be dissected, as many critics have done, and shown to rest on oversimplified views of the evolutionary process, which ignore the immense times taken, the circuitous pathways followed, and the large numbers of individuals and generations involved. Complex biochemical systems, for example, have arisen from simpler ones by way of many steps that are only now beginning to be identified. Similarly, evolutionary pathways that have often been viewed as almost miraculous are now being elucidated. The history of the eye, for example, which has evoked so much wonder and puzzlement, has been largely reconstructed, starting from a small light-sensitive spot in some primitive organism and branching out into at least six different directions.
As to the sequence paradox, we have seen in chapter 4 that the structures of present-day proteins and other observations indicate that these molecules started as short chains. It is conceivable that the molecules were short enough for all possible sequences, or almost, to be realized (by way of their genes) and submitted to natural selection. Combination of the sequences retained by this first screening could have yielded an essentially complete set of the longer sequences achievable with the starting sequences, leading once again to pruning by natural selection, and so forth, up to the sequences of hundreds of amino acids that prevail today. In other words, proteins would not have reached in one shot the “infinitesimally minuscule” spot they occupy in their “immeasurably immense” sequence space, as assumed by Dembski, but by a stepwise pathway subject at each stage to natural sele
ction.
Much remains to be discovered, but so much has already been found that one can only be urged to look for more, rather than give up and invoke supernatural influences of unknown nature.
III
The Human Adventure
Introduction
W e enter the last episode of the history of life, the one that concerns us most directly, as we are its outcome (most likely provisional, as evolution is far from finished). In this part of the book, I briefly retrace the main steps of hominization, paying special attention to the development of the brain, which is its most outstanding event, and to the latest developments that have led to today’s world, totally dominated—and threatened—by the inordinate success of the human species.
I conduct this analysis in the light of the notions that are available on the earlier history of life and on its mechanisms, with as main source of illumination the “beacon” of natural selection. I thus arrive at the book’s central theme: “original sin” reinterpreted in the light of knowledge, namely the genetic flaw imprinted into human nature by natural selection.
9
The Emergence of Humans
L et us go back about three and a half million years. On the human scale, this is an immense time span: thirty-five hundred millennia, five hundred times the duration of the whole of recorded human history. But, in the framework of the history of life (more than three and a half million millennia), or even of animal life (six hundred thousand millennia), it is little more than a brief coda.
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