The appearance of life on Earth

@robot

Published in 

Nature

 · 18 Dec 2022

Introduction

As our most attentive readers will have noticed, this is a moment, in the history of the exploration of the other bodies of the Solar System, of transition between the phase of hypotheses and that of confirmations regarding the presence of some form of life on them. The images of the space probes Galileo and Mars Global Surveyor answer several questions, opening up new ones from time to time (as, after all, the evolution of science and knowledge requires): the images of Mars and Jupiter's satellites, in fact, do nothing but give new life to theories about the origin of life: just think of the oceans of Europa and Callisto or the water present, in the distant past, on Mars. Probes are also being planned or already on the way that have the express purpose of investigating the prebiotic chemistry of Solar System bodies, such as the well-known Cassini with regard to Saturn's satellite Titan.

With a series of articles, beginning with this one, we will try to investigate, initially limiting ourselves to the Earth and the inner planets (excluding Mercury), the necessary and sufficient conditions for the "triggering" of the fundamental processes that from non-life forms, according to current theories, led to life (we will not, therefore, deal with the origin of life, but with those processes that led to the prebiotic state that had in itself the potential to generate it); we will go on, then, to analyze the variables that might condition life elsewhere in the Solar System and in other stellar systems scattered throughout our own and other galaxies. For many of the hypotheses analyzed, evidence for and evidence against will be reported.

Part One

That of the origin of life is a rather difficult subject to deal with, since it is based mostly on theoretical speculation, and only on a few fragmentary experimental evidences. Unfortunately, in fact, the geological and climatic vitality of our planet (volcanic eruptions, tectonics, meteorite rains and cometary nuclei, etc.) and, above all, the tumultuous and continuous evolution of the "life phenomenon", have erased all traces that could have "told" the leap from non-life to life: fossils of very ancient living things may be found, but never that of the non-living in the act of beginning to live.

Today, however, molecular biology and genetics, as well as chemistry and physical-chemistry, are the very powerful weapons that researchers can exploit to try to peel back the veil that separates us from the solution of the enigma: with these tools it is possible, in fact, to narrow the field of hypotheses more and more until we arrive at a verisimilar reconstruction of how things actually happened.

Before we begin, however, we must try to define, in broad strokes, the "life phenomenon", since the word life is only an abstract term. Since we are invading the field of philosophy, we will help ourselves with the Treccani Encyclopedia. A living being, to be defined as such, must have at least 5 characteristics:

1. originate from another living thing;
2. have a certain form;
3. grow and build itself everything it needs, taking it from its surroundings;
4. exchange matter and energy with the environment and be able to react to stimuli;
5. being born, growing, reproducing and then dying.

This is important because many people, treading a bit too far, consider even complex molecular aggregates to be "living", as long as they are able to meet these conditions.

In general, a good discussion always starts at the beginning. Why, then, in our Solar System, did life as we know it (for what little is known about it, in fact, there could be an equal or different kind of biological life on Mars or, why not, on Uranus) develop only on the planet Earth?

To answer this question, several indispensable factors must be analyzed, at least 4 of which must occur simultaneously: the right temperature, the right size, the presence of liquid water (at least at certain stages of the planet's life), and that of a satellite to stabilize its obliquity.

TEMPERATURE

First, let's talk about the temperature. This parameter depends on both solar irradiance and internal heat (mantle pressure on the core and radionuclide decay) of a given planet. Regarding the former, being too close or too far away from the parent star is, of course, a major handicap for the development of life; at considerable sacrifice, we are able to withstand temperatures on the order of -40 °C at one extreme and +50 °C at the other (on Earth, however, there are bacteria that can live among Antarctic ice and bacteria that can withstand temperatures above 110 °C as well). For these reasons, a planet that "desires" to harbor life must necessarily "choose" to form at a reasonable distance from a star, in what might be called the "life belt": for the Solar System, this distance corresponds, to a good approximation, to the area between Venus and Mars (and Earth is right in the middle).

Internal heat, on the other hand, is important primarily because it "sustains" ideal conditions for the continuation of the "life phenomenon", helping to keep the planet geologically alive and providing significant amounts of usable energy for the microorganisms underlying complex ecosystems. In addition, a planet that keeps its interior liquid, thanks to the high temperature of its core, will surely have a magnetic field, an important factor in protecting life forms from solar radiation and cosmic rays, which are highly ionizing and therefore "hostile" to life.

We shall see better later how decisive the temperature factor proved to be for the development of life.

THE SIZE

Second, the size of our planet. Indeed, if the Earth was, for example, the size of Mercury, the weak gravitational force would have prevented the planet from holding any kind of atmosphere. At the other extreme, if the Earth had been the size of Jupiter and with an atmosphere as dense and extensive as that of the giant planet, the great pressure and temperature below the atmospheric cap would most likely not have allowed the development of any kind of biological life on the ground.

THE PRESENCE OF LIQUID WATER

Life also requires a large amount of water to sustain itself (driven dehydration leads to death) and, fortunately, on Earth this is present in abundance. But why is there no liquid water on Mars or Venus?

At this point it is thermodynamics, specifically the study of phase transitions (i.e., the transitions between the various states in which a molecule can be found, i.e., solid, liquid or vapor), that comes to our aid. A molecule such as water is characterized by a well-known state diagram

The state diagram of water

that is, a Cartesian graph based on an analysis of the behavior of the molecule in question as pressure and temperature change. From such a graph, it can be inferred that liquid water is found only at well-determined conditions of temperature and pressure. Comparing these parameters with those found, for example, on Venus (ground temperature around 490 °C and pressure around 90 atm) or Mars (average temperature around -60 °C and pressure around 6 thousandths of an atmosphere) explains the lack of liquid water on these planets.

As far as Mars is concerned, the matter is a bit different, since on the red planet liquid water did exist in the past (thank you, Mars Global Surveyor!); however, the conditions for the persistence of that state have changed over time and led to its loss at the surface, although the presence of water in the solid state in a layer (permafrost) at an unknown depth (a few meters to hundreds of meters) below the crust is likely.

A RELATIVELY LARGE SATELLITE

A final aspect, which on superficial examination might seem negligible, is the presence of a satellite of sufficient mass to be able to stabilize the angle formed by the tilt of the planet's axis of rotation with respect to the perpendicular to the orbital plane (this angle is called obliquity).

A drastic change in Earth's obliquity (an angle between 22.1° and 24.5°, currently about 23.5°) would, in fact, produce climatic upheavals that would result in the extinction of many living species (or almost all, depending on the magnitude of the change). A strong instability of planetary obliquity could have prevented, therefore, the development of life on Earth. The Moon, on the other hand, contributes to making the precession motion high enough to prevent spin-orbit resonances, which would be responsible for the large oscillations in obliquity. What about the other planets in the solar system?

Let's keep it in the "neighborhood" (Mercury and the planets outside the asteroid belt are excluded by the failure of the other conditions, but the large satellites of the giant planets are not excluded, as we shall see); Venus, being closer to the Sun, has had its obliquity stabilized by the solar tidal force, while Mars is a good example of a planet potentially suitable for life, but without a large satellite.

Phobos and Deimos, almost certainly only captured asteroids, do not have the mass to hope to stabilize obliquity: the precession of Mars is such that it experiences gravitational perturbations from the other planets in resonance with its rotation, and this causes high variations in obliquity. Computer simulations have shown that in a few million years, for the obliquity of Mars the variations would be ± 15°, while for the Earth, without the Moon, ± 10° (according to other studies, with a rotation speed about one and a half times the present one estimated for the Earth 4.5 billion years ago, the obliquity would have been, without the Moon and on timescales of the order of 10 million years, characterized by rapid and chaotic variations of even 60° - 80° in amplitude!).

Mars, as already mentioned, has been very rich, in the past, in liquid water, while today we can see what such an unstable climate has led to (along, however, with other factors); on the other hand, ever since the Earth-Moon system (given the close relationship, one can rightly speak of a "double planet") was formed, the Earth's obliquity has known fluctuations of only ± 2°, and this slightest variation has been enough, in all probability, to give rise to situations far from devoid of consequences (just think of the influence they are believed to have had on the evolution of humankind) known as ice ages!

A LITTLE BIOCHEMISTRY

Before continuing to discuss the origin of life, it is necessary for the reader (don't get me wrong) to resign himself to a quick smattering of biochemistry, or else he will not understand it at all. One concept is fundamental: biological life is based on carbon chemistry. An example of this comes from the four classes of molecules fundamental to the development of life on Earth: carbohydrates, lipids, amino acids and nucleic acids. The molecules that delimit the cellular environment are lipids, which have an end capable of interacting with aqueous solutions (hydrophilic), formed by a residue of a three-carbon atom alcohol (glycerol), followed by a "tail" that instead hates water with all its strength (hydrophobic), also consisting of a skeleton of carbon atoms, often very long (usually around 20 carbon atoms, but there are also longer ones). Their characteristic arrangement in cell membranes is a double layer with hydrophilic heads on the outside (in red) and hydrophobic tails on the inside, as shown in the next figure (the unit of measurement is equivalent to 1 millionth of a millimeter; follow-up figure: the structure of a cell membrane).

Amino acids, in addition to acting, when isolated, as precursors for the biosynthesis of a large number of molecules, originate, bound to each other, an infinity of proteins essential to life (enzymes, for example, without which even the simplest living creature would be inconceivable, are made up of huge protein molecules based on carbon, hydrogen, oxygen, nitrogen and other elements in smaller quantities).

Nucleic acids, which form both the genetic code (DNA and, in some viruses, RNA) and the molecules designed to convert that code into proteins (the three RNA families), are made up of long chains of repetitive units, the nucleotides (see figure-Nuc-Nuc. gif -), consisting of two types of carbohydrates formed by a skeleton of five carbon atoms (deoxyribose for DNA and ribose for RNA), a phosphoric derivative and any one of five nitrogenous bases (heterocyclic molecules derived from a purine or pyrimidine v. figure nuclacid.gif -), which distinguish one nucleotide from another (adenine, guanine, cytosine and thymine for DNA; the same, but with uracil instead of thymine in RNA).

As can be seen from what has just been expounded, life without carbon is like pasta with sauce without ... pasta (for a change, pasta consists almost entirely of starch, a polymer of glucose, a sugar with six carbon atoms).

THE BIRTH OF THE EARTH AND ITS ATMOSPHERE

A decisive weight in determining which of the theories seeking to explain the origin of life is more scientifically acceptable rests with the composition of the Earth's primordial atmosphere.

It is important, in fact, to try to clarify, especially in the case of theories calling extraterrestrial material into play, whether the primordial atmosphere was of the reducing or oxidizing type, since the presence of organic molecules of interplanetary origin or the synthesis of such molecules in the Earth's atmosphere can only be explained by admitting the absence of free oxygen in the atmosphere itself (reducing atmosphere). Let us see, then, how this originated and, then, by what criteria some assume a primitive atmosphere of the reducing type and others of the oxidizing type.

The nebula surrounding Beta Pictoris imaged by HST-WFPC2; it is obvious that the size of the protosolar nebula was much larger than it is today (The photo was taken from the website of the Padova Observatory).

Approximately 4-5 billion years ago, in an area of the Galaxy somewhat larger than that occupied by the Solar System today, there was a protosolar nebula in which two distinct processes took place: the segregation of the "dusty" part from the gas (hydrogen and helium for more than 98%) and a subsequent process of planetary aggregation. The matter constituting the primordial cloud, however, was not divided equally between the inner and outer planets, and this was for two orders of reasons:

• the action of the Uv radiation flux and the very strong particle "wind" generated by the young Sun during the T-Tauri phase, which would have pushed most of the gas outward;
• the formation of the giant planets, which would have prevented, due to their powerful gravitational force, further gas pull from the forming rocky planets in the inner region of the Solar System.

The stellar wind during the T-Tauri phase imaged by HST-WFPC2
(The photo was taken from the website of the Padova Observatory).

Before the Sun entered the T-Tauri phase, therefore, the Earth must have had an atmosphere similar to that of Jupiter (i.e., consisting of hydrogen, helium, methane - CH4 -, ammonia - NH3 - and water - H2O - in the vapor state)(1) which, as it continued to accumulate, would have contributed to the rise in temperature at the surface until it became magma, in which some of the atmospheric gases would have dissolved. After the initiation of the T-Tauri phase, which blew away the primordial atmosphere, the Earth would have remained bare of atmosphere for some time before it created a whole new atmosphere for itself, between 4.4 and 4.2 billion years ago, through evaporation of ice brought by the last planetesimals (comets and asteroidal bodies), outgassing and volcanism; this atmosphere contained mainly H2O (vapor), molecular nitrogen - N2 - and carbon dioxide - CO2 -, not forgetting the release of gases dissolved in magma during previous phases. At this point, the decisive role of water in the formation of the present atmosphere and that of life should be strongly emphasized.

The inner planets
(Photo was taken from the website of the Padova Observatory)

In the first part, it was mentioned that fundamental parameters for the development of life are, among others, the temperature of the planet and the presence of liquid water; and it is here that the differences with Venus and Mars become clear.
Earth had a lower temperature than Venus because it was farther from the Sun (internal heat matters relatively since, given the similarities between the two planets, it can be considered to be of roughly the same magnitude), so the water vapor produced by outgassing, volcanic activity and evaporation of ice carried by planetesimals was able to reach saturation in the atmosphere, condensing and forming oceans. The formation of large quantities of liquid water was most important because it was able to initiate a primordial "carbon cycle," one of the fundamental biogeochemical cycles for life on Earth: in the oceans, in fact, the vast majority of CO2 was dissolved, in the form of carbonates (according to some calculations, 50 times the amount of CO2 present in the atmosphere would currently be dissolved in the oceans). This did not, therefore, lead to the condition on Venus, on which the higher temperature prevented the condensation of water vapor in oceans and the dissolution in them of CO2, which remained in the atmosphere acting as a thermal blanket!

On Mars, however, the temperature (dependent on solar radiation and internal heat) would have been too low to allow liquid water to form if the planet had not been able to rely on carbon dioxide (mainly that of endogenous, volcanic origin). This ensured a mild temperature on the ground due to both the greenhouse effect and increased atmospheric pressure on the ground; but when volcanic activity and outgassing phenomena came to an end (Mars is in fact too small to "sustain" volcanic activity for long), the surface temperature also dropped, leading to the disappearance of liquid water (according to some, there were many cycles of water appearance and disappearance, the last of which occurred about 300 million years ago; others, however, argue that "seasonal" water is still present on the red planet). It can therefore be concluded that the presence of water on Mars lasted as long as the heat released by radioactive isotopes in the interior of the planet acted as an engine for volcanic phenomena.

So far the theoretical hypotheses about the origin of the primordial atmosphere. But was the atmosphere of the oxidizing or reducing type?

H. C. Urey, of the University of Chicago, Nobel laureate in chemistry, supposed that the primordial atmosphere was reducing (rich in hydrogen-containing gases such as CH4 and NH3), and this inspired Miller's experiment, which we will discuss in a bit. According to recent laboratory experiments and computer reconstructions of the atmosphere, however, solar-derived UV radiation would, if the atmosphere was reducing, have destroyed the hydrogen-containing molecules in the atmosphere because they were not filtered out by the ozone screen (if there was no oxygen...), causing the free hydrogen to be lost to space.

These results suggest that the most abundant components of the atmosphere were carbon dioxide (CO2) and molecular nitrogen (N2), ejected from volcanoes and rocks by outgassing: this type of atmosphere, which is oxidizing, would not have favored the synthesis of either amino acids or other molecules essential for the development of life. However, other studies indicate that clouds might have protected hydrogen-containing gases from ultraviolet radiation; at the same time, solar wind and cosmic rays might have promoted the synthesis of free hydrogen-and, consequently, CH4 and NH3-from the dissociation of water molecules.

The following scenario could be considered quite likely.

it has been said that the primordial atmosphere, at some point, was saturated with water vapor, and that this began to condense, forming the oceans. This mechanism (condensation by saturation) continues as long as the atmospheric temperature remains above -50°C: at this value, in fact, water vapor spontaneously condenses, so higher than this limit it cannot really rise (this temperature is reached by the Earth's atmosphere at about 11 km above sea level).

On this gaseous layer of H2O, solar ultraviolet rays acted undisturbed; those of appropriate wavelength (less than 2000 Å) initiated a dissociative phenomenon known as "water photolysis", releasing oxygen. The latter, which does not condense at -50°C, partly passes over the water vapor layer in the form of ozone (O3), absorbing the UVs and protecting the gaseous water vapor layer before they reached it (it is believed that this protection mechanism became effective when oxygen reached a partial pressure value in the atmosphere of at least 1/1000th of its present value). In this regard, another important difference between Earth and Venus is that, on the latter, solar UVs were able to destroy all the water vapor in the atmosphere, releasing hydrogen (which largely escaped to space) and oxygen (which formed oxides on the crust). On Earth this did not happen mainly because most of the HO was preserved from photolytic decomposition due to the fact that it was able to pass to a liquid state precisely because of the phenomenon of saturation condensation.

Oxygen, however, does not absorb UVs of wavelengths of 2500-3500 Å (which destroy biological molecules); these, therefore, could reach as far as the crust, and could only be shielded by a water layer of a few meters. This would exclude life on land or in small puddles, but not the formation of complex organic molecules by Uvs; the depth of the protective layer of water, then, did not have to be excessive, otherwise the scarcity of light of appropriate wavelength would not have favored the appearance of photosynthesis. To stop the biologically lethal fraction of UV at least at the surface level of the ocean, thus making it all available to life, the partial pressure of free oxygen had to rise to about 1/100^th of its present level.

The contribution to this increase in free oxygen probably came from the appearance of the first benthic (i.e., deep-sea) oxygen-removing forms (both heterotrophic forms and the early photosynthetic autotrophs). Some new hypotheses, then, complicate (or simplify, depending on one's point of view) the picture by calling into question a contribution of certain types of molecules, quinones, that are believed to have been present in the primordial atmosphere. These compounds may have played a dual role in the development of life: given their chemical structure-dependent ability to absorb Uv, they may have protected the planet's surface prior to the advent of the more efficient and abundant ozone; they may, moreover, given their ability to accumulate and give up electrons by simple oxidation-reduction processes, have become part of the primordial life forms capable of carrying out a primitive photosynthetic mechanism.

THE PREBIOTIC STATE: THE LABORATORY

The fact that, under certain physicochemical conditions, the fundamental "building blocks" of life are formed in space or on Earth (the aforementioned amino acids, which placed in sequence form proteins; nucleic acids; lipids; etc. ) is certainly not the genius brainchild of a science fiction writer; these elements have indeed been unearthed, to keep us within the confines of our Solar System, on comets and asteroids that "rained" down on our planet, but laboratory experiments, in which conditions similar to those reigning on our planet some 4 billion years ago were reproduced, have also achieved remarkable results.

Such is the case with the world-famous (those who have never heard of it cast the first test tube!) experiment by S.L. Miller (1953), in which our hero placed in a glass bubble a mixture of water, ammonia, methane and molecular hydrogen (to reproduce a reducing-type atmosphere, such as that hypothesized for our planet so many billions of years ago), subjecting it both to the fire of a stove (to simulate high temperature) and to high-potential electrical discharges (to simulate lightning); orbene, after a few weeks, analyzing that "stinking broth" (as he called it), Miller found many organic compounds in it, including some amino acids (these present, however, as racemic mixtures, that is, composed 50% by the D and L enantiomers of the various amino acids).
(Insight: Miller's experiment).

This was followed by many other experiments, substantially similar to Miller's (always starting with a mixture of simple inorganic and organic substances), two of which deserve some attention. One of these dates back thirty-five years (1965), and was carried out by Ponnamperuma; the "variation on the theme" is the exposure of the "broth" not to the action of heat and electricity, but to that of ultraviolet rays (somewhat as happens in interplanetary space); result: Ponnamperuma found in this mixture even nucleotides, those molecules that, as already mentioned, are the basis of the structure of DNA and RNA. The second, however, was the work of Juan Oró (1961), who demonstrated how it was possible to prepare amino acids from simple mixtures of hydrogen cyanide and ammonia in aqueous solution: with these simple chemical reactions, moreover, the most abundant of the complex molecules present turned out to be adenine. The latter, as already mentioned, is one of the four nitrogenous bases that form DNA and RNA; it also forms the backbone of molecules, such as adenosine triphosphate (ATP, the main energy provider in the vast majority of aerobic organisms), FAD and NAD (flavin-adenine-dynucleotide and nicotinamide adenine-dynucleotide, respectively, electron transporters in metabolic reactions), which are practically ubiquitous between the pages of a biochemistry textbook.

Other studies indicate that the other nitrogenous bases contained in nucleic acids can also be formed by reaction between hydrogen cyanide and two other compounds, dianogen (N=C-C=N) and cyanoacetylene (H-C=C-C=N), present in a reducing atmosphere. All this leads to the conclusion that, under certain conditions, the most important constituents of proteins and nucleic acids were present on Earth at the time of the transition from nonliving to living.

Miller's has the merit of being the progenitor of a whole series of experiments that have led scholars to a fair knowledge of the prebiotic stage, that is, of those elements that, when joined together, had the potential to originate life, although, as already mentioned, one is still not certain of the correctness of the assumptions on which the experiments themselves are based.

One consideration: many may be skeptical about the possibility that the "key" molecules of biological life, such as amino acids and nucleotides, could be found on comets or asteroids. Specialized publications in Astronomy, however, continually report experimental data obtained by this science's own means of investigation, and a brief account of some of these data will be the subject of a forthcoming article.

THE "ALL-PURPOSE" RNA

We left, in the last issue, scientists halfway around the world grappling with the following nagging question: was the egg (the protein) or the chicken (the DNA) born first? In order to get around the chicken and egg obstacle, the role of RNA in the primordial "soup" began to be considered preponderant: this nucleic acid would have been responsible for catalyzing all the reactions essential for the survival of the "Progenitor," then becoming capable of forming proteins by joining multiple amino acids. To do this, RNA would have had to perform at least two tasks that are no longer incumbent on it today, namely, that of duplicating itself without bothering the proteins and that of presiding over every step of protein synthesis (in practice, it would have had to be both chicken and egg at the same time!).

The "RNA world" hypothesis, as opposed to the "DNA world" hypothesis (the "protein world" hypothesis, as we have seen, has several points in its favor, including the fact that RNA ribonucleotides are easier to produce than DNA deoxyribonucleotides; the latter, however, are more stable in aqueous solution, so it is plausible to imagine that DNA intervened later and, being better adapted to it, has since assumed the role of genetic "repository" and, therefore, the focus of reproduction.

Another plus point lies in the discovery, in 1983, of ribozymes (enzymes formed from RNA, different, therefore, from the enzymes found in living things, consisting of amino acid sequences, i.e., proteins): these molecules are capable of cutting and stitching together RNA fragments; starting, then, from mixtures of short nucleotide sequences (oligonucleotides) that were believed to be present in the primordial "soup," ribozymes were found to use the energy of a triphosphate group (a close relative of ATP), just as occurs in our cells. Also to be taken into account as circumstantial evidence is the fact that, in cells, presiding over protein synthesis are particular organelles, ribosomes, consisting largely precisely of RNA.

This model of an RNA molecule from the protozoan Tetrahymena piriformis shows "paired" regions (consisting, that is, of two complementary strands joined to form a double helix - red stretches -) that give the molecule a three-dimensional shape, complicated by further interactions (regions in blue), which make this molecule capable of "cutting away" a useless part. It is precisely this cutting operation that makes this RNA similar to an enzyme.

The image is taken from "The Sciences" No. 221, 01/1997.

But how did self-replicating RNA form?
The simplest hypothesis assumes that RNA nucleotides were formed when direct chemical reactions led to the binding of ribose to nitrogenous bases and phosphate. The ribonucleotides thus produced then joined to form polymers, at least one of which was found to be capable of regulating its own reproduction.

Unfortunately, in the absence of enzymes it is very difficult to produce ribose. This can be produced easily by reaction between molecules of formaldehyde, but a mixture of sugars is formed in which ribose is always a by-product. In addition, attempts made to synthesize nucleotides directly from their components under prebiotic conditions have resulted in purine nucleosides (ribose and a purine base - adenine or guanine - lacking, however, the phosphate group), but not, in the absence of enzymes, in pyrimidine nucleosides (ribose and cytosine or uracil).

One may assume, returning to the solid surface "thread," that there were inorganic surfaces acting as catalysts that could ensure the formation of only the correct nucleotides and then their polymerization, but so far no substrate has proved ideal.
It is possible, quite simply, that the steps in the reactions that led from inorganic molecules to ribose have yet to be discovered: some researchers were able to limit the number of intermediates in ribose synthesis from formaldehyde polymerization only by replacing one intermediate with a phosphorylated one, resulting in phosphorylated derivatives of ribose, which shows that the prebiotic "soup" had the potential to produce such molecules.

More difficult is to understand how RNA could have begun to duplicate itself without the aid of proteins. In some experiments in which oligonucleotides were mixed with free nucleotides, it was observed that the latter were arranged on the oligonucleotides and joined together to form new oligonucleotides (this succeeds only with "dextrorotatory" nucleotides, whereas it is plausible that, in the prebiotic broth there were raceme mixtures, that is, formed from dextrorotatory and levorotatory isomers); however, it was never possible to form an oligonucleotide complementary to a starting one without the proteins.

As can be seen, this theory of the origin of life has its pros and cons, but so far neither current of thought has been able to prevail.

HYDROTHERMAL VENTS

Some 20 years ago, in the seabed off the Galapagos, some structures were discovered that were likened to "hydrothermal vents"; these are cracks in the seafloor in contact, at depth, with hot rocks or magma; from these cracks in the crust seawater penetrates, which heats up and is "erupted" violently from the vent.

Instead, these places, inhospitable to most living things, are the focus of a very well-developed ecosystem that is independent of inputs from outside the mouths; in fact, colonies of bacteria and Archaeobacteria are present as primary producers (terminology in use in Ecology by which organisms at the base of the food chain are identified); these, which under conditions in which the temperature is well above 100°C reproduce optimally (they are thermophilic or hyperthermophilic), use various inorganic compounds (sulfur, iron, hydrogen, methane, etc.) to produce food. ; they are, therefore, chemoautotrophic, i.e., non-photosynthetic organisms) produced by the interaction of seawater with hot magma or rocks, reclaiming energy from the donation of electrons obtained from the above compounds (reduced) to oxidized compounds dissolved in seawater. It has been seen that the concentration of bacteria increases, going from the outer wall to the inner wall of these "black chimneys," from 40 percent to over 90 percent!

In the linked paper (Insight: The chemistry of hydrothermal vents) it is clearly shown how these structures can disrupt the homeostasis of a system such as the ocean floor by bringing in and removing new materials; in fact, it has been observed that ions such as magnesium and sulfate are removed and there is the input of helium (which is used as a marker in studies of the currents generated by these structures), manganese, iron, hydrogen, carbon dioxide, sulfur, etc.

To complete the discussion of the food chain and ecosystem maintained by these structures, it must be said that the fauna present is very peculiar; from bacteria, on which progressively larger organisms feed, to giant tube worms (over 3 m), invertebrates such as mollusks and crustaceans (also quite peculiar) and the vertebrates that feed on them. All very far from the nearest light source!

According to proponents of the theory of the origin of life in hydrothermal vents, life, in these places, could have formed for several orders of reasons, including: protection from the impacts of extraterrestrial bodies on the surface (we are near submarine ridges formed by the meeting of tectonic plates); the possibility that the geothermal heat feeding them facilitated reactions between primordial molecules, leading to more complex molecular structures; and the discovery of a particular class of microorganisms, the Archaeobacteria, common, as mentioned above, in places where:

• the temperature approaches (or is higher than) the boiling temperature of water;
• high concentrations of salts are present;
• there is abundant sulfur present;
• there is some acidity;

If possible, oxygen is also absent (almost all conditions met by submarine hydrothermal vents).

Some scholars willingly accept this theory, while others think that hydrothermal vents hosted life only after its origin; still others, however (this is the case of the oft-quoted Miller), strongly reject this theory by calling into question a non-lived life span of the vents (they last a few decades, before closing), or by appealing to the fact that temperatures of even 200-300°C should destroy biological molecules. Proponents of the theory respond to Miller that, a few billion years ago, submarine (and surface) hydrothermal vents were most likely hundreds of times more numerous than today, making the "duration" factor less stringent, and counter the temperature objections with similar reasoning to what we have already seen regarding extraterrestrial body impacts.
This theory has numerous supporters, and is, as we shall see, one of the strengths of hypotheses about the presence of life elsewhere in the Solar System, such as on Jupiter's Galilean satellites (except Io).

HELP FROM THE SKY

This hypothesis, already mentioned several times in the course of the column, predicts that the ease of synthesis of even complex organic molecules in the interplanetary environment could have had an effect on the origin of life on Earth, but not to the point of forcing it in a particular direction. This "help" could have been provided to the planet in numerous ways, including direct impacts of bodies containing organic material (a hypothesis already analyzed) and the passage of our planet through interplanetary dust clouds.

This second scenario has been proposed to circumvent the problem of the destruction of organic material carried by meteorites or cometary nuclei by the heat developed by passage through the atmosphere and/or impacts, and it predicts that the Earth, in its wanderings around the Sun, has collected like a vacuum cleaner enormous amounts of dust, which is not destroyed in the atmosphere; if it is accepted that a certain percentage of this dust contains organic material (on average, the carbon content is close to 10 percent, with peaks as high as 50 percent) and multiply this amount by the tons of material raining down on our heads every year (currently, excluding meteorites, more than 30,000 tons of dust, belonging to the "zodiacal cloud," rain down on Earth every year; it is obvious, therefore, to think that, a few billion years ago, this amount was thousands of times higher), we conclude that, in the first billion years, a considerable amount of organic material "parachuted" onto the Earth.

Let us now broadly consider the amount of material from meteorites, which, at present, accounts for about one hundredth of that of dust; looking at the Moon (on Earth the traces of medium-sized meteorite impacts, such as craters of about 100 meters, last only a few thousand years), it can be concluded that in the fateful first billion years the cratering of the satellite took place at a rate thousands of times higher than at present (two statistical data(3):

1. adding up all the craters, from the smallest to the largest, found on the inner planets, it must be assumed that enough meteorites contributed to their formation to constitute a few percent of the meteorites found in the asteroid belt today;
2. it would have taken about 50 percent of the present-day constituents of the asteroid belt to crater to the same level as the Moon all the inner planets; quite a lot of matter, eh?).

Among meteorites, those carrying a good "baggage" of organic matter (on average, around 7 percent) are carbonaceous (or carbonaceous) chondrites, lithoid meteorites containing spherules (called chondrules) consisting of iron and magnesium silicates; these meteorites account for about 5 percent of all those falling to Earth.
One representative of this class of meteorites is the well-known Murchison meteorite, which fell in Australia in 1969, from which more than 200 kg of material was recovered. Chemical analysis of this meteorite established a carbon content of 2 percent and a nitrogen content of 0.16 percent; importantly, some 30 amino acids were identified in this meteorite (Insight: Organic Molecules: the Murchison meteorite and Miller's experiment).

The objectors to these results (basically those who believed in contamination by terrestrial material) were defeated by several "pieces of evidence," including the fact that non-"organic" amino acids, i.e., not found in terrestrial proteins, were also found, and the fact that the amino acid mixtures were almost racemic (mixtures of about 50 percent D and L enantiomers), whereas on Earth there are only L enantiomers (refer to the section "A bit of biochemistry"). In addition to amino acids, then, several biological compounds were found (think, for example, of nitrogenous bases, components of nucleic acids).

To these data should be added those on the chemical composition of comets. Take, for example, three of the most famous comets: the Halley, which has been passing over our heads for more than two millennia, and the Hyakutake-Hale-Bopp duo, the last two major comets to visit us.

The nucleus of a comet is an inconsistent agglomeration of volatile ices and dust (in this regard, Fred Whipple's "dirty snowball" model is famous). Volatile ices (which, under the right conditions, go directly from the solid to vapor state, i.e., sublimate) are composed mostly of water, then carbon dioxide (CO2), methane (CH4), ammonia (NH3), hydrogen cyanide (HCN); most of these molecules are trapped inside three-dimensional "cages" formed by the solid water molecules, and remain there until the latter sublimate, releasing them.

Dust, on the other hand, is formed by iron and magnesium silicates, as well as an organic polymer similar to that found in some chondrites. The ices and dust give rise to the phenomenology observed at the perihelion passage of these objects, the comet tail.
This is due to the action of "radiation pressure," exerted by solar photons on cometary material, and the interplanetary magnetic field, both phenomena acting on charged particles (ions, such as CO+, CO2+, H2O+, etc.), formed in turn by the action of solar UV radiation on cometary molecules and dust.

This "photochemical breakdown" operated by the Sun is also important because, by splitting molecules, it produces chemical species that can react with each other to form more complex compounds. Organic compounds such as ethane (C2H6), formaldehyde (HCHO), formaldehyde (HCHO), formic acid (HCOOH), acetic acid (CH3COOH), acetaldehyde (CH3CHO), methanol (CH3OH), acetylene (C2H2), methyl cyanide (CH3CN) and formamide (NH2CHO), in addition to the aforementioned hydrogen cyanide; in short, there is everything "The Biochemist" needed to give free rein to his imagination, including the source of energy (solar UV).

Returning to the frequency of lunar craters, some 20 years ago the amount of cometary material that fell on the primordial Earth was estimated at 1017 tons; based on Halley's data (about 10 percent of this comet's material is organic), readers of good will can by themselves derive the amount of organic compounds that rained down from the sky and add it to that from meteorites and dust: lots and lots!

Two other studies would confirm that space is a very suitable place for the synthesis of biological molecules. The first is a French experiment(4) that dealt with the reactions between certain radicals (-CN and -CH) and various atoms and molecules (O2, NH3, various hydrocarbons and acetylene, very common not only on comets but also in interstellar clouds) at temperatures only a few degrees above absolute zero. It has been observed that it is possible for complex molecules to form at low temperature from even neutral, unreactive molecules, and not only from more reactive molecular species, such as ions and free radicals; this could cause all theories about the rate of chemical reactions in relation to temperature to be revised.

In the second case, however, rather than an experiment, these are hypotheses based on observational data(5); some astronomers have discovered, in a region of the Orion nebula that is the site of intense star formation (similar to the primordial Solar System), circularly polarized electromagnetic radiation. As already mentioned in the section "A Little Biochemistry," polarized light is electromagnetic radiation in which the electric vector, instead of propagating in all directions, propagates in only one plane perpendicular to the direction of motion; well, in the case of circularly polarized light, the vibration in this plane rotates steadily, like the hands of a clock, as the motion proceeds (somewhat like a screw in a piece of wood).

Let us now consider two facts: first, ultraviolet radiation, as we have had occasion to say several times, can be energetic enough to "break" chemical bonds; second, amino acids absorb electromagnetic radiation in the ultraviolet range. The two optical isomers of an amino acid can absorb circularly polarized radiation differently if the wavelength of the latter is close to that of the absorption band of the amino acid itself. Thus, we have that, when passing through a racemic mixture (consisting of 50 percent L and 50 percent D stereoisomers), a circularly polarized UV radiation can induce the destruction of a good portion of the molecules that rotate the plane of the polarized light in the same direction as its electric vector and that, by what has already been said, absorb such radiation more efficiently than the other stereoisomer.

Some calculations show that, if there had been a period of circularly clockwise polarized light emission in the forming Solar System, there might have been an abundance of laevorotatory amino acids, as opposed to dextrorotatory ones, in interplanetary space (the proportion of D stereoisomers would in fact have been a tad diminished), of the same magnitude as that observed in the amino acids found in the Murchison meteorite.

Some more data, this time about the oft-mentioned interstellar clouds. These jumbles of gas and dust are estimated to contain, in the form of PAH, the polycyclic aromatic hydrocarbons made famous by the Martian meteorite ALH84001, more than 20 percent of the carbon in the entire galaxy. In an experiment(6) of organic synthesis under conditions similar (temperature, pressure, chemical composition and UV irradiation) to those reigning in clouds, complex molecules such as alcohols, ethers, ketones, amines and quinone compounds were produced from water ice containing PAH; the latter, already found when the composition of the primordial atmosphere was mentioned, are highly represented in cellular structures and are involved in the electron transfer processes that occur during photosynthesis and oxidative phosphorylation (a process, the latter, from which eukaryotic cells derive much of their energy). When we consider that the Solar System, and thus the Earth, was formed by the condensation of one such cloud...

All together, these data give for certain the arrival on our planet of organic material synthesized in the interplanetary environment. Somewhat more debated, however, is the role these molecules played in the development of life on Earth. The various currents of thought hypothesize:

• (a) a direct role (the "Progenitor" originated from the chemical evolution of these molecules);
• (b) an indirect, "directing" role (the molecules in question would have somehow "conditioned" molecular evolution, "forcing" its turn toward life);
• (c) an intermediate role (they would have participated, with complex molecules already evolved independently on Earth, in the appearance of the "Progenitor").

CONCLUSIONS

As has been mentioned above, the fact that one hypothesis is more plausible than another does not exclude the latter. Given the multitude of environments and ways in which it is possible for "something" resembling a life process to be produced, it seems plausible that, of the three points mentioned above, the last two are the most worthy of consideration. In all likelihood, we will never know how things actually turned out, but, personally, I like to think of a scenario (we have already discussed this in the section "From non-life to life") in which complex organic molecules of terrestrial and extraterrestrial origin were forced, at first, to "fight" each other, no holds barred, in order to maintain the high honor of natural selection; the distant descendants of this war must have "realized," later, that it was not so dishonorable to cooperate, to "co-evolve," but rather that it was a winning strategy.

Unfortunately, the peace must have lasted very little, since the complicated aggregates of molecules that were being formed were more efficient than others in hoarding nourishment, energy, in replicating themselves: thus here was a new battle on the road to the origin of the "Progenitor," with other winners and other losers.
This scenario was, from time to time, modified in favor of some or others by environmental variations that helped to select, from time to time, the complexes best suited to the conditions of the moment, which therefore "reproduced" most successfully. In the end, this strenuous struggle (which continues to this day, both at the molecular level and at the organism level, and which has the name "evolution") must have resulted in a "something" that possessed all the characteristics common to every living thing: the "Progenitor," the last common ancestor of all the inhabitants of the planet.

In light of all that we have analyzed in these pages, according to the most optimistic, the "life phenomenon" would be nothing more than an obligatory step in the chemical-physical evolution of both the molecules that rained down from the sky astride comets and asteroids and those that formed on Earth thanks to the very conditions that reigned there in the early days.

"The spontaneous appearance of a single-celled organism from a random combination of chemical compounds is as likely as the assembly of a Boeing 747 by a tornado passing through a junkyard."

This sentence, uttered by Fred Hoyle, a British astronomer, may sound very plausible, not to say unquestionably true, but, as the research stands at present, it certainly appears that that tornado first assembled the Boeing, and then also filled it with passengers!