by Natalia GONTAREVA, Cand. Sc. (Phys. & Math.), Yevgenia KUZICHEVA, Dr. Sc. (Biol.); Institute of Cytology (St. Petersburg), Russian Academy of Sciences
How could living matter originate and evolve on this planet of ours?
Biologists, planetary scientists and researchers in many other disciplines are looking for the answer.
New approaches to this formidable problem are suggested by the St. Petersburg-based RAS Institute of Cytology.
The simplest biomolecules-predecessors of life (the picture borrowed from the Internet site of NASA).
Articles in this rubric reflect the authors' opinion. - Ed.
Astrobiology is a comparatively new interdisciplinary science that involves astronomy, chemistry, molecular biology, paleontology and ecology. Studying the formative process of the basic structures of living matter, that is molecular biopolymers, is one major trend in the research quest. The terrestrial form of living matter, the only one known to us, must have been synthesized from a common precursor. Now what was the head source of organic matter? Was it formed here on earth? Or perhaps imported from the cosmic void? How did the initial structural components combine into molecules to give birth to systems capable of gaining energy from the ambient environment and using it for replication (reproduction)?
Looking into the origins and chemistry of organic and inorganic compounds as well as the power sources and microenvironment of the primordial earth, we shall be able in the long run to get a clear idea about the context of incipient life*. Furthermore, proceeding from the putative primary source of organic components and from alternative pathways of protein, nucleic acid and lipid synthesis, we could be able to track the course of subsequent evolution. Laboratory experiments modeling the physicochemical processes in the early stages of the history of our planet are assigned a major role in this regard. The polymerization of primitive molecules led to another step in the evolutionary ladder, the birth of a protocell capable of self-replication (self-reproduction). This event finally gave rise to a common ancestor (precursor) of every living organism.
Another important question bears on a hypothetical exchange of biological material among different planets, for life must have been born outside the earth and then delivered in by outer space bodies. This hypothesis should be substantiated-we should know the amount of transported organic matter and get to learn more about meteorites and other "delivery vehicles".
The theory of prebiological evolution describes in details the process whereby primary biological structures came into being, with simple molecules evolving into composite ones, the supermolecules. This theory is based on the ideas formulated in the early 1920s by a young biologist, Alexander Oparin (1894 - 1980), subsequently elected to the USSR Academy of Sciences. He postulated that simple organic compounds became stock components for cell formation. Biological life is the result of general evolution of matter throughout the universe; one stage of this process was the evolution of matter from primitive gaseous compounds to polymer molecules and multimolecular systems. According to Acad. Oparin, small organic molecules essential for primitive forms of life appeared in the earth's protoatmosphere enriched in methane and poor in oxygen.
That atmosphere saw intensive chemical conversions of primordial matter under the effect of the ultraviolet radiation of the sun, electric discharges and vigorous volcanicity. Carbon dioxide, water vapor, methane, ammonia and other gases came to be implicated. Acad. Oparin viewed origination of biological life on Earth as a logical sequel to the evolution of carbonic compounds. The buildup of a definite amount of biomass carried in by meteorites and cosmic dust particles (especially during intensive meteor showers) and the formation of the primary atmosphere were the start-out conditions for the next, higher stage of chemical evolution.
These ideas were upheld by Dr. Stanley Miller of the University of Chicago who, in 1953, conducted a model experiment with a mix of methane, ammonia and water acted upon by electric discharges. He obtained four out of the extant twenty amino acids via the intermediate cyanohydrogen and formaldehyde (formalin). Such kind of lab synthesis succeeds if we take a gas mixture containing a significant percentage of hydrogen. Many other studies simulating the prestine earth medium likewise confirmed the possibility of organic synthesis.
Another hypothesis, the theory of panspermia (postulating the probability of life having been imported from the Solar system and a stellar system other than that of the sun) was advanced back in 1884 by the Swedish physicist and chemist Svante Arrhenius (Nobel Prize, 1903; corresponding member of the St. Petersburg Academy of Sciences, elected to the USSR Academy of Sciences as honorary member in 1925). The latest scientific discoveries corroborate the fidelity of his idea. Apart from different kinds of cosmic radiation, i.e. the energy base sustaining the nascent molecular structures, there must have been a "direct delivery" of organic material from the universe-of the selfsame simplest biomolecules which grew into complex structures. Since the young Earth had no atmosphere, the natural carriers, i.e. meteorites and cosmic dust particles, could get freely onto its surface and thus raise the concentration of organic molecules in the upper layers of the lithosphere. The presence of carbonaceous compounds within meteorites is a fact confirmed by radioastronomical observations and studies of the "celestial" matter landing on our planet.**
Diverse carbon-containing compounds-from methane down to fullerenes*** - possessing a complex three-dimensional structure were discovered in different regions of interstellar space. They are thought to be originating on particles of interstellar dust clouds under the effect of cosmic radiation. Such molecules could
* See: V. Parmon, "Autocatalysis: Go-ahead to Life", Science in Russia, No. 4, 2004. - Ed.
** See: A. Litvak et al, "Cosmis Wanderers", Science in Russia, No. 2, 2003. - Ed.
*** Fullerenes - an allotrope of carbon (alongside diamond and graphite). Allotrope-one or two forms of a chemical element. - Ed.
The time scale of cosmic, chemical and biological evolution-a period of the slackening of meteor showers and the formation of DNA is all too short by a geologist's yardstick.
have been synthesized long before the birth of life on our planet and even before it, the Planet Earth, came into being. That is to say, even before the Earth gained a stable lithosphere, the Universe had accumulated a huge amount of organic matter which, once reaching the terrestrial surface, could touch off abiogenic synthesis (one not implicating live organisms) of supermolecules.
Analysis of cosmic dust samples both from the upper layers of the atmosphere and from the ice of Greenland and the Antarctic shows that each day the earth captures 50 to 100 tons of suchlike particles-in the form of micrometeorites alone! This fact allows for the possible participation of meteorites in bringing composite organic material to the infant earth of 4.2 to 3.9 billion years ago, when the meteor shower was dozens of times more intensive than today. Carbonic micrometeorites could bring in as much as 1020g carbon in the 300 mln years of heavy meteoric bombardment, or more than its present content within the surface biomass of our planet (ca. 1018g).
Hitting a planet's surface, a large meteorite knocks out organic particles which, speeded up to high velocities, can be carried to great distances within the Solar system. Transported this way, low-weight molecular structures and bacterial spores exhibit high resistance to Shockwaves and UV radiation at low temperatures. The simpler a biological system, the higher its survival chances under extreme conditions.
A circumterrestrial orbit offers fine opportunities for studying these and related problems: it is a medium of diverse energy effects, such as ultraviolet, cosmic rays, radiation and many other factors. All that is absorbed, either completely or in part, by the terrestrial atmosphere. At the earliest stages of our planet's existence, when the protective gaseous envelope was as good as absent, the above energy effects could operate at large and trigger reactions of prebiotic synthesis, that is what is coming to pass in the outer void today. Considering the inchoate phases of molecular synthesis, we kind of backslide to the situation obtained 4 billion years ago as any type of radiation could reach the Earth's surface. All these cumulative effects present in circumterrestrial orbits are of much interest to us in terms of their action on live structures and bioorganic molecules. The interaction of the latter stimulated by cosmic radiations may provide a clue to the genesis of biological life from a set of initial molecules. Still and all, primary polymers are not living systems yet, though they are the principal targets of chemical evolution, a stepping stone in the transition to biological evolution.
Medusa, a cosmic warderer back to Earth.
A circumterrestrial orbit gives an excellent opportunity for studying the action of cosmic radiation on the synthesis of bioorganic compounds. If a UVflux in the rarefied interstellar medium for photons with an energy of more than 6 eV is equal to 108 photons cm-2 * s-1, in a near-earth orbit its intensity is much higher, amounting to 9 * 1015 photons cm-2 * s-1, and the total value of such radiation in two weeks is up to 108,864 * 1017 photons * cm"2. To gain such a dose of radiation in the highly rare field interstellar medium, a space object should travel for 3.45 * 106 years. Consequently, the unique conditions of circumterrestrial space make it possible to get within just two weeks a radiation dose equal to one absorbed by chemical structures within 3 mln years in the rarefied interstellar space! Complexification of the molecular structure occurs thereby, which means that more complex compounds compared with to stock ones could have reached the earth aeons ago.
In this fashion near-Earth space helps to model the formative process of biomolecules under the action of natural energy sources in conditions close to those on the primordial earth. That is why in our astrobiological experiments we attempted to trace the pathways of the synthesis of peptides and nucleotides as the stock material for nucleic acide and proteins. As objects we took nucleosides (adenosine, deoxyadenosine, thymidine and uridine) as well amino acids (glycine and triptophane). All these substances were exposed in ET orbit in the form of dry films obtained from a solution of the stock material supplemented with inorganic phosphate so as to check on the possibility of nucleotide synthesis. The orbital stations SALYUT-6 (1978), SALYUT-7 (1985) and MIR (1985) carried the Medusa (Jellyfish) apparatus designed and manufactured at our research institute; it was mounted on the outer cover of the spacecraft to have the assayed samples exposed to the direct action of outer space effectors. Just before an experimental session a special blind was manually removed from the apparatus which, with the end of the experiment, was dismantled and brought into a spacecraft module. (Both operations were carried out by crew members during their regular walks in the raw space.) Back on Earth the samples were examined for the presence of new more complex compounds, also within proteins and nucleic acids. The upgraded version of Medusa - Persei (Perseus) - was used in international experiments aboard the orbital station MIR (1999).
An outer container with dry preparations of the samples was mounted on the biosatellites Kosmos-2044 (1989) and Bion-11 (1996 - 1997). Fixed to the orbiter's
Persei (Perseus) - an upgraded model of the Medusa (Jellyfish) apparatus.
outer cover immediately before the launch up, the container opened its lid in an automatic mode upon entry into orbit. During all these experiments control samples were stored within space vehicles and thus were safe from any external effects barring zero gravity.
These experiments furnished data proving the possibility of prebiotic synthesis in outer space under conditions similar to those obtainable in the radiation atmosphere of the early Earth. The final solution was found to contain native nucleotides present within nucleic acids - namely the 2'-, 3' - and 5' - nucleoside mono-phosphates. The 5' - nucleoside monophosphate predominated in the end mixture in all the cases. The same regularity holds in natural conditions: nucleotides present within a cell in a free form contain a phosphate group in the 5' position, since enzymatic reactions involving the synthesis or decay of nucleic acids produce nucleoside-5'-phosphates as intermediates. Upon irradiation of amino acids the suspensions were found to contain polypeptide compounds identified as di-, tri- and tetrapeptides, which confirms the fact of the structural complexification of the stock material.
The limit of stability of organic structures against the effects of the hostile medium was studied along with the conditions for the synthesized molecules persisting stable. An essentially new condition of all these experiments was that they were conducted in the absence of water, an inhibiting factor for chemical reactions. It stands to reason that synthesis in a water or gas medium proceeds far more readily, and with a higher yield of the end product. However, neither on the primordial Earth nor in outer space could there be any liquid water! Another "first"-the use of inorganic substrates of extraterrestrial origin like lunar soil and meteorite particles to test their protective and even catalytic properties. We studied lunar basalt brought from the Mare Fecunditatis by the automatic probe Luna-16 (1970), and we studied particles of the Allende and Murchison
Persei on board the MIR space station, a space view.
meteorites that fell onto the earth surface in 1969 - we got their fragments from South America and Australia, respectively. Both samples are assigned to one and the same class of carbonaceous chondrites, which are characterized by an enhanced concentration of carbon.
Interpreting the results of our in-flight experiments in the light of the life origin theory, we can say: it would be quite legitimate to assume that pre-biological live structures could have originated here on Earth. But there is also another possibility: the synthesis of new compounds under space flight conditions shows that such living structures likewide could have been imported to the terrestrial lithosphere from the cosmic void. This way the two horns of the life genesis problem-terrestrial and extraterrestrial-come together, not contradicting but rather complementing each other. Presuming that the amount of organic material came to be replenished both through the home, terrestrial resources and through those of ET origin, we shall be able to explain the fairly rapid course of chemical evolution (105 - 107 years) predating biological evolution that engendered the present-day forms of life.
Many unknowns are still there related to the pathways and modes of self-organization of matter on earth, its conversions to primary live structures. Whatever the results of our quests could be, the very birth of biological life on the Planet Earth is a unique gift of Mother Nature, and we ought to be grateful for a chance to learn a modicum of her secrets.
The Moscow Kremlin memorial complex has just marked its bicentenary. A momentous jubilee for millions both in this country and abroad, let alone historians, archeologists as well as art and architecture adepts. Today the Moscow Kremlin is the treasure-house of world significance, a depositary of precious relics, Old Russian books, and state regalia. Its architectural, artistic and cultural monuments are dazzling in their splendor. Welcome to the stories contributed to our magazine by leading researchers of the Moscow Kremlin museums.
View of the Moscow Kremlin. A lithograph from I. Daziaro's publication. First half of the 19th cent
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