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.
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.
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anniversary of the publication of this book was celebrated in France with some prominence in 2007, in spite of its outdated character.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).
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.
From this naturalistic perspective, the products of cosmic chemistry are seen as landing in a milieu where some started interacting with each other to produce molecules of increasing size and complexity, which then interacted to produce polymolecular assemblages of increasing size and complexity, up to forming entities that could be defined as “protocells,” or primitive cells. Here is the snag. Nobody has so far succeeded in reproducing such a situation, or even a small part of it, in the laboratory.
One problem is that there is no agreement yet on what may have been the milieu in which it all started. Some believe sunlight may have been needed as a source of energy. Others think that life originated in the darkness of deep waters. Debates also occur between proponents of a “hot cradle” and proponents of a “cold cradle.” According to the latter, the constituents of life are much too fragile to be able to survive long enough in a hot environment. The former, on the other hand, have been influenced by the fact that all the bacteria identified as most ancient by molecular phylogenies are thermophilic, that is, adapted to very hot environments.
Another point of disagreement concerns the relationship between the early chemistry that led to life and present-day biochemistry. Many specialists, impressed with the total reliance of biochemistry on the activity of protein catalysts, or enzymes, which obviously are too complex to have been present at the dawn of life, argue that there is no relationship between the two chemistries. Some, however, including myself (see chapter 4), see biochemistry as flowing congruently from that early chemistry.
As to the temperature that surrounded life’s birth, I adopt the opinion, held by a number of experts in the field, that life probably started in hot volcanic waters, perhaps in one of those underwater formations, called deep-sea hydrothermal vents, or black smokers, that spew overheated, pressurized, sulfurous, metal-laden waters from fissures in the bottom of oceans and have been found, against all expectations, to harbor many strange forms of life.
My reason for subscribing to this view does not, however, rest on the thermophily of the most ancient bacteria, which are probably late products of a long evolutionary process and may not tell much about the conditions under which life first arose. I see volcanic waters as the likely site where life originated because present-day processes of biological energy transfer universally use derivatives of two central compounds, inorganic pyrophosphate and hydrogen sulfide, that are produced naturally only in a volcanic environment.
Whatever the pathways involved in those early stages, they must have been chemical in nature, which means that they were bound to happen under the physical-chemical conditions that prevailed where they took place. Chemistry deals with strictly deterministic, reproducible phenomena. Were it not so and should chemical processes involve even a tiny element of chance, there would be no chemical laboratories, no chemical factories. We could not afford the risk. Thus, if early events leading to the origin of life were chemical, as they must have been if our premise of a natural—and not supernatural—origin of life is correct, then they were bound to occur under the prevailing conditions. It follows that if the same conditions should occur elsewhere in the universe, one would sensibly expect life to emerge similarly there, an implication of interest with respect to a related question much in vogue: Does extraterrestrial life exist?
This conclusion holds for all the early events in the origin of life, up to the appearance of the first information-bearing molecule capable of being replicated, that is, of inducing the making of copies of itself by whatever chemical machinery is responsible for making such molecules. As we shall see in chapter 7, this ability automatically entails the occurrence of selection, adding a new, chancy element to chemical determinism.
In present-day life, the function of storing information in replicable form is universally fulfilled by DNA. There is, however, strong reason to believe that, in the origin of life, DNA has been preceded in this function by RNA, which also preceded proteins. To look into the arguments that support this opinion would take us into too many details. I mention it only in order to emphasize the crucial importance of the appearance of RNA in the origin of life, a veritable watershed separating a first stage exclusively dominated by chemistry from a second stage in which selection was added to chemistry (fig. 2.1). Experts in the field recognized this and have, in the last forty years, expended enormous efforts to try reproducing RNA synthesis under plausible early-Earth conditions. Many interesting results have been recorded; but the problem remains unsolved. Nobody has yet succeeded in making RNA in the laboratory, even under much more advantageous conditions than probably existed at the beginning of life. It could be said that a solution to this problem represents the “Holy Grail” of research on the origin of life.
Making RNA would be only one of a long series of steps. Another, of fundamental importance, would be the birth of proteins, most likely through the operation of RNA molecules, which are universally responsible for the synthesis of proteins in present-day living beings. Proteins would inaugurate enzymes and, with them, metabolism. I shall return to these questions in greater detail in chapter 4, devoted to metabolism. In addition, the function of storing genetic information in replicable form would have to be transferred from RNA, with which it first arose, to DNA, which accomplishes it today. Finally, at some undetermined stage, these systems would have to become enclosed within an envelope, or membrane, to give rise to the first protocells.
Fig. 2.1. The origin of life. This process may be defined as the chemical pathway, so far unknown, that has led from certain organic products, which form everywhere in the universe, to the last universal com-mon ancestor (LUCA) of all life on Earth. This pathway may be divided into two stages separated by the appearance of RNA (or, in a more general fashion, of the first replicable information-bearing molecule). The first stage must have depended exclusively on chemistry. In the second, selection was added to chemistry (see chapter 7).
All these events must have taken place when life first arose on our young planet. How and in what order is totally unknown. Also unknown is the manner in which the first protocells progressively evolved into the last universal common ancestor of all living beings, or LUCA, which, must, by definition, have possessed all the main properties living organisms have in common and no doubt inherited from this common ancestor.
There are plenty of challenges for future investigations. Whether these challenges will ever be successfully overcome cannot be predicted at this time. Today, the prospects seem bleak. On the other hand, experience has shown that a single breakthrough sometimes suddenly opens immense fields to scientific exploration. This has happened time and again, often with people exclaiming a posteriori: “Why didn’t I think of that?”