Genetics of Original Sin (3 page)

In
Part II
, I discuss such key processes as metabolism, reproduction, and development before addressing the central theme of the book: natural selection. I end with a brief consideration of some of the evolutionary mechanisms that have been proposed besides natural selection.

The scene thus set, I move on, in
Part III
, to the extraordinary saga of the human adventure, which, initiated a few million years ago in the heart of Africa, has developed, sustained by a stupendous expansion of the brain, at an increasingly dizzying pace, leading, in the last centuries and, even more so, in the very last decades, to the fantastic success of our species and to the mortal menaces it causes to weigh on the future, the ultimate consequences of our “original sin.”

Then, in
Part IV
, I sketch potential solutions toward redemption or, at least, the chance for it, which I see in the specifically human power to act against natural selection. But to exercise this power, we will have to find in the resources of our minds a wisdom that is not written in our genes.

W
hat is life? What are its principal properties? What reasons do we have for believing there is only one kind of life on Earth, issued from a single ancestral root? How did life arise? What are the main steps of its history? Such are some of the questions that I try to answer in this first part of the book. I do so in descriptive and phenomenological form, postponing until the following part an examination of underlying mechanisms.

A
ll known living organisms are descendants from a
common ancestral form of life,
often represented by the acronym LUCA (Last Universal Common Ancestor). Put forward as an affirmation, not just a theory or hypothesis, this statement may strike many readers unacquainted with modern biology as almost incredible, if not objectionable or even contrary to their most sacred beliefs. An explanation is in order.

For most of their history, humans have seen Earth as the center of the universe, their privileged abode. In the second century, the Greek mathematician and astronomer Ptolemy integrated this “geocentric” view into a coherent theory that placed the Sun, the planets, and the stars in concentric spheres surrounding Earth. The Ptolemaic system dominated thinking for about fourteen hundred years, until the Polish astronomer Copernicus (1473–1543) rejected it in favor of the “heliocentric” view, which has the Sun in the center and the Earth and
other planets circling around it. This view ran the risk of being seen as heretical at the time; it conflicted with the biblical account that Joshua “stopped the sun” to allow the Israelites to win the Battle of Gibeon against the Canaanite kings. For this reason, Copernicus prudently refrained from having his theory publicized until after his death. This may have been a sound decision, considering the fate that befell the Italian Galileo (1564–1642), almost one century later, when his advocacy of the Copernican view led, in 1633, to his condemnation by the Catholic Church, which, although yielding to the evidence much earlier, officially revoked this condemnation only some three and a half centuries later.

Since the time of Galileo, the Sun itself has lost its central status. It has been found to be only one among some one hundred billion stars in our galaxy, which has itself been dethroned by the observations of the American astronomer Edwin Hubble, who discovered, in the 1920s, that the distant celestial objects then known as “nebulae” are actually other galaxies, of which about one hundred billion are believed to exist.

While the status of our planet was progressively relegated from the center of the universe to the backyard of one in one hundred billion stars in one of one hundred billion galaxies, the “anthropocentric” view of a universe made for humans was also shaken. It all began with increasing realization that Earth is not, as was long believed, a fixed setting created for our human adventure. Earth was found to have a history of its own.

This awareness dawned in the eighteenth century, from observations in places where flowing water has cut through rocks to expose their structure—the Grand Canyon is the most spectacular
example—showing that the ground beneath us is stratified in layers of different texture and composition. The layers may be flat or curved, or inclined. Some contain the shells of marine animals embedded in the rock; think of marble, for instance. This telling clue enforced the astonishing conclusion that these layers had once been under water, where they had slowly formed through sedimentation of sand and dust particles, in which dead animals became buried, their bodies rotting, leaving only the mineral shells. As time went by and new layers accumulated on top, the old ones were pushed deeper and deeper, exposed to increasing heat and pressure, solidifying into rocks. Some of these rock layers were later driven upward by underground movements, to finally rise above the level of the waters in which they were born, even building mountains.

These observations allowed terrains to be classified in terms of their relative age. These ages could not be known in absolute terms at that time, but it was clear that they must cover considerable durations, possibly measured in as much as millions of years, a span of time almost unimaginable in those days. But the facts were there. Mountains obviously do not arise overnight, not even in a few millennia. The Alps have probably not changed much since Hannibal's army crossed them on elephants, more than two millennia ago, to take the Romans by surprise. Today, thanks to a method known as radioisotopic dating, geological times have been measured and found to be even longer than was first pictured, covering up to several billion years.

Although rudimentary in comparison with present-day geological knowledge, these early findings were sufficient to throw a revealing light on another set of observations that had long intrigued nature watchers, those dealing with fossils. Many such vestiges had been discovered by amateur naturalists
over the years in various places. Their origin was a matter of lively debate. They were readily identified as the remnants of dead plants and animals, many of which, however, seemed to be different from any extant species known. Whether the living precursors of the fossils were organisms that still lived elsewhere in unexplored areas or had become extinct was much discussed. Some even considered that fossils were the remains of victims of the Flood. Putting some order into all this fantasizing, a crucial piece of information was provided by geological observations revealing that the complexity of the organisms whose fossils were found in a given terrain was related to the age of that terrain. The younger the terrain, the more complex the fossilized remains. These findings showed that life, like the Earth, also has a history, in the course of which organisms of increasing complexity progressively appeared.

The notion of the “fixity of species” was, however, so dominant at the time that the significance of these observations was not immediately appreciated by most scientists. Only a few were sufficiently perceptive, as well as daring, to conceive the revolutionary hypothesis that life started with very simple forms that progressively evolved into forms of increasing complexity. This so-called transformist hypothesis was first formulated at the end of the eighteenth century, simultaneously in France, by Jean-Baptiste de Monet, chevalier de Lamarck (1744–1829), and, in England, by Erasmus Darwin (1731–1802), the grandfather of the famous Charles.

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 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.

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.