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ORIGIN OF LIFE: A GEOLOGIST'S VIEW
by Igor REZANOV, Dr. Sc. (Geol. & Mineral.), S. I. Vavilov Institute of the History of Natural Science and Technology, Russian Academy of Sciences
Life on earth is commonly believed to have originated about 3.5 to 3.8 billion years ago in the seas - under conditions close to those of today. But according to fresh evidence, the origins of life go back to even earlier times than that. First, we learned this from an article published in 1989 by our paleontologist V. Koshevoi about the mummified microorganisms he had discovered in the oldest rocks (more than 4 billion years old) of Eastern Siberia's Aldan shield. Those were the fossil remains of cyanobacteria, peridineans and other organisms some of which have no rank in present- day taxonomy. Afterwards Russian scientists Stanislav Zhmur, RAS Corresponding Member Alexei Rozanov and Vladimir Gorlenko detected fossilized microorganisms in meteorites, likewise more than 4 billion years old. This was the subject of a report carried by our magazine (A. Rozanov, "Microorganisms in Meteorites", Science in Russia, No. 6,1998). All these findings enable a new approach to the problem of the origin of life.
LOOKING 4.5 BILLION YEARS BACK
Oldest rocks have been best studied within the confines of the Aldan shield in Siberia where they occur in great abundance. R. Cherkassov, who is a native Siberian researcher, has shown, proceeding from geological survey data, that its profile reveals three major series, or strata: the lower (quartzite-gneiss), 2-2.5 km thick; the middle (gneiss) - 2.6 km thick (where V. Koshevoi found bacterial relics) and the upper (gneiss-carbonate) series, 1 to 1.5 km thick. The gneiss rocks predominant in this section (erstwhile basaltic lavas) alternate with the less common primary sedimentary rocks - quartzites, high-clay shales and carbonates. These rocks were metamorphosed (recrystallized) in the past at high temperature (over 700 0 C) and pressure (ca. 10,000 atm.) due to the superdense primary atmosphere composed of hydrogen - an atmosphere that persisted ever since the accretion (increase in size) of our planet.
Using isotope analysis methods, research scientists have determined the age of the Aldan shield at 2 to 4 billion years. This time has seen manifold phases of local granulite granitization. Since the earliest such phase occurred about 4 billion years ago, the accumulation of rocks and their granulite metamorphism must have begun even earlier - 4 to 4.4 billion years ago. This view was articulated in 1982 by Lazar Salop, a Russian geologist and expert in the Pre-Cambrian.
The Aldan profile contains information on the character of the biosphere of the earth in the initial 500 million years of its existence. At that time our planet performed nearly all biochemical functions proper to it today That is why the amount of graphites tends to increase higher up along the profile; judging by their isotope composition, these graphites are of biogenic origin. As to the widely occurring ferruginous quartzites and phosphorus - and manganese-containing carbonates, they were formed with the participation of bacteria. And then the very isotope composition of sulfur is proof of the biogenic circulation of this chemical element. And if we add to that the variety of microorganisms whose fossil remains have been detected in the Aldan shield's graphites, we shall see that life on earth appeared soon after the planet's accretion, that is about 4.5 billion years ago, to give rise to a powerful biosphere.
THE MOON TELLS ITS TALE
Three stages are manifest in the early history of the moon, our planet's natural satellite. At first - some 4.5 billion years ago - a "magmatic ocean" covered the lunar surface, that is when the outer shell of this celestial body was heated to the melting point of silicates, and feld spars "came up" to form a crust on it. The subsequent 500 mln years did not witness any significant events on the moon. But then, about 4.5-3.9 bln years ago, a heavy shower of asteroids and meteorites hit its surface to form giant craters there. This "bombardment" slackened somewhat, and about 3.8-3.1 bln years ago a considerable part of the lunar surface happened to be under basalt outflows-the moon lost its activity.
Although our next-door heavenly neighbor shows no traces of life, be it in recent history or in the dim and distant past, it helps us in identifying the conditions essential for the birth of life. Towards the end of their accretion both the earth and the moon were hot bodies with a molten outer shell. The vigorous eruptive processes brought basalt lavas cum gases (hydrogen, oxygen oxide and dioxide, methane and the like) - both necessary and sufficient for the inception of life-to the surface.
But life did appear on the earth, while on the moon it did not. Why? Because our planet with a mass 100 fold as big as that of the moon could retain part of its primary hydrogen
Articles in this rubric reflect the opinion of the author. - Ed.
"blanket". However, some initial stages of organic compounds synthesis must have been proceeding on the moon as well at the expense of the gases escaping from its interior. At any rate, American chemists discovered glycine, serine, aspartic, glutamic and other amino acids in regolith samples brought in by the Apollo lunar probe.
...NEVER WAS A STORY OF MORE WOE THAN THIS OF PHAETHO
The solar system comprises" nine planets. Yet way back in the early 19th century the German astronomer Heinrich Wilhelm Olbers, proceeding from the Titius-Bode empirical rule of planetary distances, hypothesized: there must have been yet another planet between the orbits of Mars and Jupiter; afterwards that planet disintegrated and vanished.
In 1861 Gabriel Auguste Daubree, a French geologist and foreign member of the St. Petersburg Academy of Sciences, postulated: meteorites reaching the earth could be relics of a terrestrial type planet. This view was upheld by Russian scientists, Academician Alexander Zavaritsky (1884-1951) for one: some stone meteorites, he noted, do not differ from terrestrial basic rocks (basalts, diabases, gabbro) in the content of rock-building oxides.
Most of the meteorites found on the earth in the last 30 years have been assayed for their mineralogical and chemical composition. They are thought to be debris of a large planet according to the magmatic differentiation pattern possible only within a planetary body. Another bit of evidence - diamonds detected in some of these space wanderers. The presence of tiny gas bubbles containing water and hydrogen as well as other indications show that these diamonds were once formed within a planet, for a pressure of 60 kilobars and more is needed for that.
A further study of meteorites allows to reconstruct the composition and dimensions of the erstwhile tenth planet of the solar system as well as
the correlation of the masses of its three component parts, that is the ferrous core, the chondrite mantle (one containing minute spheric particles, the chondrules) and the achondrite (basaltic) crust; this can be done proceeding from the number of meteorites: 7 percent of them were of iron, 84.6 percent - of chondrite and 8.3 percent - of achondrite. That planet, dubbed Phaetho(n), had a 100 km-thick crust and a rather small ferrous core. Phaetho was closest to Mars and probably of about the same size, judging by the diamonds detected in every type of chondrites. Which means that the pressure within Phaetho's mantle must have been above 60 kilobars. According to a Mars model, this value is attained at a depth of 500 km or so. Yet Phaetho could not have the upper part of its chondrite mantle as deep as that, for otherwise the mass of its crust would have to be increased by 30 percent. So we should assume that the tenth planet of the solar system had about the same dimensions of its crust, mantle and core as Mars. But what about the pressure of 60 kbars? It could reach that value due to the hydrogen atmosphere surrounding that ferrous-silicate celestial body.
In planets the relative mass of hydrogen increases the farther they are from the sun. Thus Mercury and Venus, which are the closest to the sun, did not have this gas in the age of their accretion. On the earth the total amount of hydrogen made up about 0.5 percent of its mass. As to Phaetho, which was two astronomical units (this unit is equal to 149.5 x 10 9 m) from the sun, the relative mass of hydrogen was 100 fold as high; and on Jupiter, which is 4 a.u. away, this figure is 100х100 as high. Phaetho perished because of the thermal dissipation of its hydrogen atmosphere. And since the external pressure on that planet decreased as a result, the internal pressure of fluids pent up within its ferrous- silicate core ultimately destroyed it. According to Academician Alexander Zavaritsky, who has been studying the structure and crystallization of various types of meteorites, the destruction proceeded in
several stages. First Phaetho was stripped of its achondrite crust. But the planet held on in that condition for a long time, and its surface cooled. Thereupon eruptive processes set in - a scenario characteristic of planets possessing a cool atmosphere alongside interior depths heated up to the melting point of substances confined therein. Eruptive processes (volcanism) brought forth the hot molten magma and gases to the surface. The magma crystallized little by little, a process that occurred simultaneously with the hydration of silicates. And the gases, entering into chemical reactions, gave rise to a wide range of hydrocarbon compounds.
VOLCANISM, THE FIRST PHASE OF INCIPIENT LIFE
The bacterial forms of life must have originated on two planets at least - on the earth and on Phaetho. Both had similar conditions at that epoch: a very dense (ca. 10 kbars) hydrogen atmosphere and pervasive volcanic activity.
Hence the physical and chemical conditions of biopoesis - when inanimate, nonliving matter engenders living matter. The physical conditions: pressure - no less than 10 kbars, and sharp temperature fluctuations, from 1,000 0 C in molten lava to 0 0 C as it solidifies. The chemical conditions: the presence of a hydrogen atmosphere; and a little water associated with volcanic gases. Besides the molecular hydrogen and water, the other chemical components essential for biopoesis were carbon oxide and dioxide, ammonia, methane, hydrogen sulfide, hydrochloric and hydrofluoric acids together with small amounts of boron, bromine and phosphorus.
Two Russian scientists, Yevgeni Markhinin and Nikolai Podkletov of the RAS Institute of Volcanology, have made a study of present- day products of volcanic eruptions and found large amounts of organic matter present in volcanic ashes. These are, above all, saturated and aromatic hydrocarbons, including those associated with sulfur, chlorine, oxygen and nitrogen (hundreds of thousands of tons ejected in one single eruption) and also the principal components of proteins and nucleic acids making up a living cell: porphyrines, amino sugars, amino acids and pyrimidines (dozens of tons). Volcanic eruptions on the young planet earth and on the half-destroyed Phaetho ushered in the first phase of the inception of life - there came to be a wide range of organic compounds, the building blocks for more complex polymers.
How all that could occur is shown by experiments conducted in the 1960s by the American biochemist Sidney Fox and coworkers. They took an anhydrous mixture of amino acids and heated it up to 170 0 C, obtaining compounds similar to native proteins - with as many as 18 amino acids out of the 23 occurring in present- day organisms. Washing the hot mixture of artificial polymers in water or water solutions, Dr. Fox detected numerous microspheres; like in a living cell, their envelopes reacted to changes of external pressure.
Now I come to what is most important in the problem of living matter origination (biopoesis). What triggered off the protein metabolism, and what was the energy source needed for that?
On the semidestroyed Phaetho life cropped up on the surface partly covered with hot molten magma, i.e. at sharp temperature differences. The high temperature that gave rise to polymers then destroyed them concurrently with their subsequent proliferation. To survive the incipient biosphere had to protect itself against overheating. Chemical processes in this case obey the principle formulated by the French physicist and chemist Henri Louis Le Chatelier (1850-1936), foreign member of the St. Petersburg Academy of Sciences: if some effect acts upon a system in equilibrium, processes within this system will shift the equilibrium toward minimization of the effect. In our case the nascent biosphere responded to overheating by touching off a process that lowers the temperature of the medium - the endothermic (endoergic) reaction. That was the production of oxygen from the available chemical compounds (2H 2 + 2CO 2 -> 2CH 2 O + О 2 ). Since overheating recurred now and then, the biosphere upgraded the mechanism enabling polypeptide macromolecules to withstand high-temperature conditions.
Eruptive activities, however, ceased every now and then, and polypeptide chains had to brave low-temperature conditions. So the system (biosphere) switched in a reverse process - the oxygen pool was expened on the oxidation of hydrogen, sulfur, iron, manganese and other elements. This reaction caused a release of chemical energy. As I see it, the metabolism of what was to evolve later into bacteria was predicated on redox reactions stimulated by temperature fluctuations. At that stage it was still inanimate, nonliving matter, mind you. In time polypeptides developed optimal mechanisms of reduction and oxidation which, via selection, produced a universal energy-accumulating mechanism materialized in ATP (adenosine triphosphate).
And now we come to the next stage of biopoesis - namely, the formation of a genetic code that could replicate the existing sequence of amino acids capable of metabolism. In that case, too, it was the natural selection that did most of the job. Once polypeptides had built up defenses against temperature fluctuations, another problem came to the fore - how to safeguard this protoprotein (polypeptides) against random destruction and annihilation. But as said above, the protein synthesis was proceeding alongside the abiogenic synthesis of amino sugars, pyrimidines and other components of nucleic acids. The latter "learned to memorize" the protoprotein and induced its repeated synthesis upon destruction. In this fashion each amino acid came to be supplied with a definite set of molecules to speed up its resynthesis; in other words, a process of complementary self- reproduction came into being. Natural selection resulted in the fur-
ther complexification of this mechanism and ultimately, in a genetic code.
Superhigh pressure played an all-important part in the process of biopoesis: it led to a dramatic acceleration of chemical reactions. It is owing to this factor that the multistep process of synthesis, destruction, selection and resynthesis could be realized within a short "reprieve" granted by the fates to the planet Phaetho between the two phases of its disintegration. The shock waves produced by the debris of that hapless tenth planet falling on its own surface also stimulated the synthesis of amino acids and other hydrocarbons.
And now we are going to touch on the most intriguing problem related to the origin of life - the emergence of asymmetrical (chiralic) organic molecules: the "left" in amino acids and the "right" - in sugars. The author of this article believes that such asymmetry in organic molecules was caused by superhigh pressure. At 10 kbars most of the original and synthesized organic compounds are crystallized. And the crystals thus obtained are deformed as a result of such pressure - the distances and angles of their component atoms are changed. Due to the anisotropic (different in different directions) compressibility, conditions must have been created favorable now to the "left", now to the "right" molecules. Be that as it may, an asymmetric organic medium - proper to bacteria and man alike - took body and form at the genetic code formative stage.
Now, how could life appear within inanimate matter after all? First, owing to the ability of carbon bonded to hydrogen, oxygen and other elements to form complex molecules associating into polymers composed of thousands of atoms. Second, because of the planetary volcanism supplying gases, ashes and magma, and responsible for temperature contrasts. Third, due to reversible redox reactions resulting in energy absorption and release alike. And last, the high pressure of a hydrogen atmosphere activated chemical reactions and possibly produced an asymmetric medium.
Another essential process stimulating the birth of living matter was the production of oxygen as a response of protoproteins to temperature rises. The oxygen synthesis was the first energy- consuming reaction that later evolved into bacterial metabolism. At that stage it was thermosynthesis whereby the ambient heat is consumed-not the now common photosynthesis. Turning back to our Phaetho: its distance from the sun was three times as long as that of the earth. Consequently, Phaetho could get only 10 to 15 percent of the solar energy that the earth was getting. If we add to that the superdense atmosphere that screened out much of the sun's radiation, it will become obvious: the mechanism of photosynthesis could not function on Phaetho. As to the earth, it likewise had a very dense atmosphere screening most of the luminous energy. However, the terrestrial volcanism supplied sufficient - even excessive - amounts of energy. Photosynthetic bacteria came much later, when the temperature on the terrestrial surface dropped, and their precursors had to look elsewhere for energy sources.
This is how I visualize the inception of life on the earth and on Phaetho. Just imagine a pitch-dark eternal night and the terrific pressure of a hydrogen atmosphere, jets of magma and gases bursting forth from beneath, and a shower of ashes pouring from above. As the temperature fell and the lava hardened, amino acids, sugars and other hydrocarbons gave rise to polymers - a thin "film of protolife" nestling on the lava or in volcanic ashes. This film often perished at temperature rises, but some of it could survive in exceptionally rare instances when an adequate mechanism of cooling was developed. In one or in several places the primordial life embryo could via selection go through all the stages of evolution and gain a genetic code that enabled assembly of a polymer (from amino acids) capable of metabolism. Little by little the "film ofprotolife" spread to cover the entire surface of the earth and Phaetho to engender a biosphere. Its components, making use of redox chemical reactions, were further upgraded and formed cyanobacterial mats analogous to what we know today on earth. They passed further evolution already in shallow bodies of water.
WHERE ELSE LIFE POSSIBLE?
Considering the above, we can name two essential conditions for bacterial life emerging on this or that celestial body: 1) vigorous volcanic activity, and 2) a dense hydrogen atmosphere. The first condition holds quite often - during their accretion planets heat to the melting point of their outer shells (the moon is an ocular example). The second condition applies less often, for in this case two other factors are involved - the size of a heavenly body and its distance from the sun. Small celestial bodies cannot retain a hydrogen atmosphere. Again, the selfsame moon furnishes a graphic example: although it was the stage of eruptive activities too, and had amino acids and other hydrocarbon compounds formed, no life appeared there, for its highly dense hydrogen atmosphere vanished soon.
Needless to say, no atmosphere and no hydrosphere could be formed around asteroids, for they had no volcanism and thus remained barren.
The presence of a primary hydrogen atmosphere on massive planets is a function of their distance from the sun. The hydrogen atmosphere that once enveloped Mercury and Venus, which are the closest to the sun, was gone toward the end of their accretion - the solar wind virtually knocked it out. As to planets situated at very long distances from our luminary (Jupiter, Saturn), they have retained an excess of hydrogen, which is likewise counterproductive to biopoesis.
It is in the interim, in between these extreme cases that a hydrogen atmosphere could in part survive toward the end of a planet's accretion; this as well as a favorable atmospheric pressure on the surface was conducive to the germination of life. Within the solar system such was the case of the earth and Phaetho, a planet later blown to pieces. The planet Mars might still be having some forms of bacterial life. The hydrogen atmosphere of Mars makes up about 3 percent of that planet's mass. The Martian primary atmosphere was quite commensurate with that of the earth, but its pressure was half as much, and its dissipation did not destroy Mars. The existence and disintegration of the hydrogen atmospheres of the earth and Mars is confirmed by the fact of a regular decrease of a flux nonradiogenic isotopes of inert gases (blown away together with hydrogen). Volcanism manifested itself very early on Mars, probably simultaneously with that of the earth. So Mars had two essential conditions for the origin of life - volcanism and a hydrogen atmosphere. The third condition, too, is available, and this is water. Finally, a cogent argument for the existence of bacterial life on the Red Planet is furnished by the abundance of iron and sulfur oxides there. It would be logical to relate the formation of these compounds to the biogenic generation of oxygen. Well, and the low temperature on contemporary Mars could not kill every token of life: for instance, here on earth a vast number of live bacteria have been found in permafrost (perpetually frozen ground).
So: life appeared on three planets - the earth, Mars and Phaetho - in the first 500 million years of the solar system's existence. Yet on Phaetho it perished together with that planet. And on Mars it is suppressed due to the low temperature of its surface.
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