Highly controversial at the time it was first proposed, this theory has since been abundantly confirmed and is now established fact. Behind the extraordinary diversity of life-forms that make up what is known as the “biosphere,” there lies an impressive set of similarities that all point to a single origin.
All living beings share a number of basic properties
All living organisms, from the simplest bacteria to humans, consist of one or more cells, which are microscopic entities enclosed by a membranous envelope and endowed with the ability to subsist under appropriate conditions, to grow, and to multiply by division.
All cells are constructed with the same molecular building blocks—mostly sugars, fatty acids, amino acids, nitrogenous bases, and a few mineral components—which are themselves assembled into the same kinds of large molecules, including polysaccharides (carbohydrates), lipids (fats), proteins, and nucleic acids (DNA and RNA).
All cells manufacture these constituents by the same chemical mechanisms—the bacteria in our gut and the nerve cells of our brain make their proteins in the same manner. All cells depend on the same types of metabolic reactions and use similar mechanisms to extract energy from the environment and convert it into work. There are differences, of course—plants derive their energy from sunlight, animals from the combustion of foodstuffs—but very quickly the two mechanisms converge into a common pathway (see chapter 4).
Even more impressive, all cells use the same genetic language. They all use DNA as repository of their genetic information, replicate this DNA by the same mechanism whenever they prepare to divide, and execute the instructions stored in the DNA by the same processes.
DNA (deoxyribonucleic acid) molecules consist of long chains made of a very large number of small molecular units, called bases, of which there are four different kinds, represented by their initials: A, for adenine, G, for guanine, C, for cytosine, and T, for thymine. The order, or sequence, in which the bases follow each other specifies the molecule’s information content, just as the sequence of letters in a word specifies the word’s information content. Our words are short but can carry large amounts of information because they are constructed with an alphabet of twenty-six letters. The DNA “alphabet” has only four “letters,” but DNA “words” are very much longer than ours, often consisting of thousands of “letters.” Their information capacity greatly exceeds that of our vocabularies.
The sole function of DNA is the storage of genetic information in a form capable of being copied by a mechanism, called “replication,” which takes place every time a cell prepares to divide into two daughter cells, each of which will contain one of the two DNA copies. This phenomenon ensures the hereditary transmission of genetic information.
For this information to be turned into action, it must be transferred to RNA (ribonucleic acid), a closely related molecule, likewise constructed with an “alphabet” of four “letters”: A, G, C (as in DNA), and U (for uracil, a substance very like T). The synthesis of RNA molecules on DNA templates is appropriately called “transcription” (the two languages are very similar).
The RNA molecules arising in this way carry out several functions. Many act as “messengers” instructing the synthesis of proteins, which, through their structural and catalytic properties, are the main agents that carry out the instructions transcribed from DNA to RNA. This function was first considered the principal, if not the only biological role of RNA. Later, however, it was discovered that some RNA molecules exert a catalytic function in some important processes, including the synthesis of proteins. Even more recently, it has been found that much of the DNA that does not code for messenger or catalytic RNAs, rather than being “junk,” as was believed, actually codes for a large number of small RNA molecules endowed with a variety of regulatory functions. This has become one of the most fecund fields of research.
Proteins are also long molecular strings but made with twenty different kinds of units, called amino acids. They are molecular “words” made with an “alphabet” of twenty “letters.” In the synthesis of proteins, known as “translation” (the two languages are totally different), the sequence of bases in the messenger RNA, which itself reflects the sequence of bases in the corresponding DNA, dictates the sequence of amino acids in the synthesized protein, according to a “dictionary,” called the “genetic code.” With minor exceptions due to late changes, this code is the same throughout the living world. Life is truly one; all forms of life are related.
The history of life is written into molecular sequences
To top it all, for those still not convinced by all those proofs, there is the incontrovertible evidence provided by the comparative study of the sequences of DNA genes, or of their RNA transcripts, or of their protein translation products. We have just seen that the information held by these molecular “words” is determined by the order, or sequence, of their molecular “letters,” their spelling, so to speak. The last few decades have witnessed the development of techniques of extraordinary efficiency for deciphering those sequences, to the point that many entire genomes have now been sequenced, including the human genome, which contains some three billion “letters,” the equivalent of about 150 volumes of the Oxford Concise Dictionary! This technology has revealed that genes that carry out the same function in different organisms show many sequence similarities, many more than could be accounted for by chance. The genes are unmistakably related and are all derived from a single ancestral gene by a pathway that has involved a number of changes in sequence (mutations), somewhat like words whose spelling has changed over time.
Not only have the sequence similarities been illuminating, by showing the single ancestry of many genes. The sequence differences have also been revealing. They have allowed the reconstruction of what is known as the “phylogenetic” (from the Greek phylon, race) history of the genes, that is, what amounts to their “genealogical tree,” by a technique that uses the number of sequence differences between two forms of the same gene (belonging to two different organisms) as a measure of the time that has elapsed since the two genes separated from their last common ancestor and started evolving separately. Etymological research follows a similar line.
This method has been applied to a very large number of genes and continues to be applied more and more. Its results have confirmed—and sometimes corrected—a number of the conclusions derived from the study of fossils; especially, they have enormously enriched those conclusions. Indeed, the beauty of comparative sequencing is that it can throw light on the evolutionary history of any organism, not only those that have left fossil remnants. Fossils remain invaluable clues, of course, as illustrated by a number recently unearthed in China that have revealed several “missing links.” But the innumerable organisms, such as soft-bodied animals and, especially, bacteria and other unicellular organisms, that have disappeared without trace can be reconstituted by the magic of molecular sequencing.
The history of life is written in the genes of extant organisms. It is written in very fine print, which it has been our generation’s privilege to discover how to decipher. The general conclusion of all that has been learned is clear and indisputable: all known living organisms are descendants from a single common ancestral form.
Biological evolution is an established fact
The notoriously cautious language of science is rarely so affirmative. But, in the present case, with all the debates that surround the so-called theory of evolution, it is necessary to speak out unambiguously. Evolution is not a theory, contrary to what is often stated, sometimes even by scientists. Evolution is a fact. It was a theory two centuries ago, when Lamarck and Erasmus Darwin first proposed it, just as heliocentrism was a theory in the days of Copernicus and Galileo. Evolution is no longer a theory, just as heliocentrism is no longer a theory; it is a fact. The Catholic Church has recognized this with remarkable promptness, as compared to the Galileo affair. On October 22, 1996, at a meeting of the Pontifical Academy of Sciences, Pope John Paul II solemnl
y announced that “evolution is more than an hypothesis.” He did admittedly retreat somewhat to make a special case for the creation of the human soul; and his successor has retreated even further by leaning in favor of the so-called theory of intelligent design (see chapter 8). Nevertheless, biological evolution is not negated by the Catholic Church. Such is not the case for several other religious groups.
Opposition to evolution on religious grounds is widespread
Ever since Darwin, the notion of evolution has provoked opposition from religious groups. At one end of the spectrum are a number of fundamentalist Protestant Churches, especially in the United States, that deny evolution because it conflicts with what is written in the Bible, held to be directly inspired by God and, therefore, literally true. In line with this belief and in spite of all the evidence to the contrary, they persist in affirming that the world was created by God in seven days, some five thousand years ago, as written in Genesis. They will not recognize that the Bible, at least the early parts of it, was written almost three thousand years ago by human beings, possibly inspired by God if that is the cherished belief, but using the knowledge and language of their time.
At the other end of the spectrum are the defenders of so-called intelligent design, who pretend to invoke no explicit religious motivation but merely claim that purely natural phenomena cannot account for all evolutionary events. There will be more about this “creationism in disguise” in chapter 8.
Between these two extremes, there is a form of creationism—sometimes referred to as “old Earth” creationism, as opposed to the “young Earth” variety based on a literal interpretation of the Bible—that similarly denies evolution and advocates instant creation of living species, but on a more flexible time scale, consistent with the fossil record. This form of creationism is more widespread and professed by members of other Christian churches, including some conservative Catholics, notably in Poland, and also by many Muslims and by a number of Orthodox Jewish scholars.*
Defenders of this form of creationism often accept so-called micro-evolution, which takes place within existing genera, but deny “macro-evolution,” the kind whereby new species arise from old ones. They use a number of allegedly scientific arguments to affirm, for example, that there is no valid proof of a descent of birds from reptiles or of reptiles from fish. As in the days of Darwin, the origin of humankind from chimpanzee-like ancestors is the most contested aspect of modern evolutionism. This is where the official voice of the Catholic Church departs from the scientific account of evolution. The biological descent of humans is accepted; but creation of the human soul is seen as a special event.
Creationism is not just a religious creed. It claims to be a science, which deserves to be taught alongside evolutionary biology, or even in place of it. Its proponents have built powerful organizations in pursuit of this goal. They fight in court and try to convince legislatures to give equal weight to what they call “creation science” in school curricula, or even to abolish the teaching of evolutionary biology. They provide teachers, not only in the United States, but also in other countries such as Poland or Turkey, with costly, beautifully illustrated “textbooks” in which the facts of life are reinterpreted within a creationist framework. They also try to propagate their ideas among the general public by, for example, filling the bookshops around the Grand Canyon with pseudo-scientific pamphlets describing this beautiful illustration of the Earth’s history as a recent product of the Flood.
Further discussion of this phenomenon, which is the object of numerous books and debates, does not belong in this book. Let it simply be said that it conveys the image of a Creator who deliberately filled the world with all sorts of false clues that lead scientists astray, including the geological strata, the fossils, the kindred DNA molecules with which phylogenetic trees are constructed, the radioactive isotopes that allow us to date the Earth’s history, and all the other pieces of evidence that rigorous and honest investigators have collected and used in their efforts to understand nature. Such an image of the Deity as a willful mystifier is hardly one a sincere believer is likely to defend.
*See J. Sechbach and R. Gordon, eds., Divine Action and Natural Selection: Science, Faith and Evolution (Singapore, 2009).
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The Origin of Life
The beginnings of life on Earth are shrouded in the darkness of a very distant past, going back at least 3.55 billion years—more than three and a half million millennia!—according to microscopic traces believed to be of fossilized bacteria, detected in rocks of that age. It is interesting to place this event within the framework of the history of our planet and of the history of the universe.
Life appeared on Earth shortly after the young planet had become physically able to harbor it
The Big Bang, the primeval explosion taken by most cosmologists to have sparked our universe into being, took place 13.7 billion years ago according to the most recent estimate. The solar system was born some 4.55 billion years ago—when the universe was already more than 9 billion years old—from a swirling cloud of gas and dust that gradually condensed into the central Sun and surrounding planets, including the Earth. This birth was a violent affair, which subsided only about 4 billion years ago, when the Earth became covered with bodies of liquid water and became, for the first time, physically capable of harboring life. Less than half a billion years later, maybe much earlier but leaving no record so far discovered, life was there. It is not impossible that life appeared as soon as the Earth was physically ready to receive it, or almost.
The origin of life is not known, but the only scientifically acceptable hypothesis is that it arose naturally
How life started is the object of much research and even more speculation. Instant divine creation is one possibility, not only advocated by creationists but also implicitly accepted by a large number of laypeople, perhaps a majority, who see life as due to some kind of “vital spirit” that was initially “blown” into matter and still goes on “animating” it in every living being. Everyday language is permeated with this belief.
Unlike strict creationism, this view, known as “vitalism,” is not incompatible with evolution; it dominated biology for a long time, especially in France, where it was defended by many famous scientists, including Lamarck, one of the fathers of evolutionism, the celebrated Louis Pasteur, and, more recently, many other biologists, influenced by the philosopher Henri Bergson, winner of the 1927 Nobel Prize for literature, whose major opus, L’Évolution créatrice, recognized evolution, as the title says, but saw it as the product of an “élan vital,” a vital surge. Remarkably, the one-hundredth anniversary of the publication of this book was celebrated in France with some prominence in 2007, in spite of its outdated character. Today, vitalism is rejected by most scientists, with the exception of the advocates of intelligent design, who espouse the related theory of finalism (see chapter 8). Thanks to the revolutionary advances of the last fifty years, we now understand and explain life entirely in natural terms.
The same can’t be said of the origin of life, which is unknown so far. It thus remains permissible, while rejecting vitalism, to imagine, as some do, that life was flipped into being by a Creator, who subsequently left it to function and evolve under its own power, although such a conception of the deity does not fit with the more usual one of an omnipotent God who, notably, can be asked to change the course of things. As long as the origin of life can’t be explained in natural terms, the hypothesis of an instant divine creation of life cannot objectively be ruled out. But this hypothesis is sterile, stifling any attempt to investigate the origin of life on Earth by scientific means. The only scientifically useful hypothesis is to assume that things, including the origin of life, can be naturally explained. If we start with the premise that they cannot, we may as well close our laboratories. Searching for an explanation that is taken, a priori, not to exist is futile (see also chapter 8).
The building blocks of life arise spontaneously throughout the universe
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So far, investigations based on this “naturalistic postulate” have failed to provide an answer to the problem of the origin of life but have achieved some progress. One of the most important findings of the last decades is that the small molecular building blocks of life, the sugars, amino acids, fatty acids, and nitrogenous bases from which are constructed the larger polysaccharides, lipids, proteins, and nucleic acids that make up the bulk of so-called living matter, arise spontaneously in various sites of our solar system and, probably, in many other parts of our galaxy, as well as in other galaxies. These astounding facts, which belie the traditional view of organic chemistry as the prerogative of living organisms, were recently revealed by the spectral analysis of the radiation coming from outer space, by the probing of comets with instruments borne by spacecraft, and, especially, by the analysis, with all the resources of modern chemistry, of meteorites that have fallen on Earth.
Thus, what most likely constitutes the first stage in the origin of life is known. It is provided by cosmic chemistry, which, in innumerable parts of the universe, spontaneously generates the basic building blocks of life. Note that cosmic chemistry is not bioselective. It makes a gamut of organic compounds, of which some happen to participate in the building of living organisms, whereas many others do not. Subsequent events in the development of life have entailed a selection among the potential building blocks provided by cosmic chemistry.
Genetics of Original Sin Page 3
