Who is alive?
It is there first question if one wants to take into account the variety of the alive. When shall we be able to speak about life? Problem is not as simple as it appears, and classically one proposes that a living being is capable at the same moment of being born, of developing, of reproducing, of feeding, of waste producing and finally to die. This aspect of things remains rather restrictive and, if one applies him really, leads to impasses: a bacterium reproduces by dividing, she does not die so: immortal, she would not be alive! Also, if one considers that cellular division is a shape of death, then cells do not reproduce, and are not alive!
Even imperfect, this set of conditions allows to eliminate certain processes which mimic life without for all that, if one holds it there, to be a part of it. So, the crystals which appear in a saturated solution are capable of growing, feeding elements of their environment, to multiply possibly, and " die" if they are dissolved.
The viruses status puts also problem: they are not capable of reproducing, but one can not deny a relationship more than sure with the other lifeforms. It is necessary indeed to see that environment possesses an enormous importance in the definition of the life: once in a cell, a virus can be considered as living, and not outside. Such molecule capable of reproducing in test tube will show itself completely sluggish in natural environment etc.... This importance of the environment can be illustrated by the example of the parasites of computers that are computer viruses. If one considers an environment purely numeric, cybernetic, of which physical manifestation is only the modification of the magnetic orientation of a fragment of hard disk or the electric state of a constituent of memory, we can validly support that some lines of code which constitute)a computer virus are well near, in this particular " biotope ", of the life. It's the same for the "agents", these programs which are crossing internet network to find and to select useful information for search engines.
The living beings today
The study of the biochemistry of the living beings confirmed us, these last 50 years, what had allowed to suppose the study of the morphological and anatomical resemblances of the various groups, as well as the indications of the paleontology: all the alive forms possess a comparable biochemistry, based on similar molecules and on a unique transmission system of the information from a generation to the other one.
Thus Implies that all these forms of life divert from a homogeneous group of primitive organisms, maybe only one, maybe more. The biologists who classified the living beings, what is very difficult, say themselves that their classification went to the smallest, " the most simple " beings which they called less evolved, towards the most evolved, namely naturally mammals and, quite at the top, the human being. This conception, which one still finds in number of popularization works, is a staggering anthropocentrism: contrary to what one could think, the jellyfish " is not less evolved " that the human being, on the contrary: the various species of this sticky animal managed to cross major crises and cataclysms of the Earth history which would erase of its surface all the group of mammals!
If we want to interrogate us validly about the life, it is not necessary to raise us on an improbable pedestal and to look with disdain the slugs, the froths and the bacteria: at evolutionary point of view, the least bacteria which swarms in your bowel possess 3 billion years beforehand on the first multicellular living beings !
Don't forget this fact, and let us see so which are the various groups in which one can class, in a very general way, the living beings:
These three groups are a part of what one calls Eucaryots: their cells are big, complicated, and contain a group of internal compartments. The three other groups, constituting procaryots, are much more simple, more evolved and more resistant. They different many of the eucaryots; it is even possible that they do not possess the same origin, what explains himself, we shall see him, in our scenario for the genesis of the life.
Nanobes was first identified in carbonates precipitating at the level of warm thermal springwater by Robert L. Folk, emeritus professor of geology in the Texas university, in 1992. Observed under the shape of strands or of spheroïds, their biologic nature is only suspected then . Some scientists* identify then similar structures, interpreted as being the fossils of primitive lifeforms, always associated strongly to minerals such as clays, sulphites, silicates (that one finds in the interstellar dust).
These " biomimetic structures " are involved in the mineralization processes, that one had always believed up to here to be abiotic. They were also identified in the blood, at the level of kidney stones, atheromatic patches and, probably , in dental plaque. However, their study and their difficult identification in human pathologies are at the origin of a debate. Reason is not only scientific , but involves, as often, quarrels of bell towers: the nanobes, identified at first on purely morphological criteria, sees their biologic status denied by researchers considering them artefacts or formings of strictly mineral origin. The discoverer of nanobes, Pr Folk, indicates that these are so numerous in certain rocks samples as it is very possible that the biomass of nanobes surpasses that of all the alive species in the surface of the Earth. Indeed, these "bodies" present the characteristic of power to resist to very strong pressures and temperatures.
The biologic status of nanobes is however established in 1998 by a study appeared in the American mineralogist review, and written by Pr. P. J.R. Uwins and his team of Queensland university. Based on nanobes resulting from stonewares collected during off shore oil drillings in more than 4000 m under the oceanic surface, for temperatures about 150°C and pressures of 2000 atm!
Obtained results show:
I got in touch with Pr. Uwins who had the kindness to inform me of the current searches, in process of publication, according to which the biologic nature of nanobes does not make any more any doubt. Although the preparation of these bodies very resistant and hydrophobe for the electronic transmission microscope is very difficult, equips her with this researcher obtained, by improving its techniques, the following results:
A big quantity of these bodies was able to be obtained by culture, what will allow to have enough subject for the study of their genetic material.
It is so many unknown micro-organisms until this day... French scientific environment is, for this discovery, about an eloquent silence... Alone trace of it that I found is a paragraph of about fifteen lines in n ° 251 of the review Scientific American, concerning the possible infectious origin of kidney stones, and a brief article in "Science and life" of January, 2000. Why this lack of enthusiasm? Simply because nanobes is too small to contain the molecular machinery assuring the life of bacteria. Their biochemistry must so be considerably different from that of the other organisms that we know...
Another element explains maybe this embarrassed silence: nanobes a lot looks like structures discovered in several meteorites, of which we shall speak again, and which were considered as not convincing for extraterrestrial life for, in particular , their insufficient size... One is in the presence of a bundle of convergent indications aiming to establish the existence of primitive forms of life, connected to minerals, and widely spread in all universe. We shall see that this suits perfectly to my conceptions giving to the life on our planet an essentially extraterrestrial origin with a strong implication of minerals components.
Our size and the limits of our vision led us to see, during millenniums, in the surface of the Earth only pluricellular organisms. Invention and perfection of the optical microscope allowed to discover the profusion of micro-organisms, particularly bacteria, of which implication in infectious pathologies was a powerful stimulant of the search. The electron microscope allowed to discover viruses, then the last evolutions of the sweeping electron microscope showed these last years the existence of nanobes. As our observation techniques progressed, the importance of the human being, then the mammals, then the pluricellular organisms, decreased. If one is interested among numbers of species, individuals or in their dispersal in all the environments, micro-organisms constitute the main thing of the life on Earth. They are they who supplied some energy to the ancestors of cells eucaryotes, they who began to produce the oxygen of our atmosphere, they who allow the animals to degrade the plants molecules, they who make, in our bowel, certain molecules which are indispensable to us. Besides the industrial uses which we make it, micro-organisms are the major representatives of the life on Earth, and they " took departure " there is about 4 billion years, evolving and perfecting alone 3 billion years... When one realizes that a 6 times lesser duration has be enough to spend to first toweling to the human, measures the evolutionary road gone through by the ancestors of bacteria, and how much it is difficult for us to imagine their ancestor: this one is doubtless more different from a current bacterium than a toweling of a human being!
Attempts of enumeration (indistinct by nature) of the number of cells of micro-organisms, excluding nanobes, give the fantastic figure of 5x1030 bacterias on our planet, more than 90 % living in the basement of the oceans or the continents! This fantastic number is itself a problem, because the quantity of available carbon would not allow the renewal of these bacterial populations at the speed currently know. Pr. Whitman and Wiebe, of the university of Georgia, USA, deducts from it that the deep bacteria owe be capable of using sources of energy up to here unknown. Consolidating this hypothesis, the team of F. Chapel, of the US GEOLOGICAL SURVEY, identified in 2002 January a community of alive micro-organisms 200 m under the ground of the Idaho, in warm sources, and pulling their energy of the hydrogen contained in rocks (what leads them to reject some methan!).
One often reads in popularization works, but also in number of treaties addressing a more restricted public, than evolution would possess a "sense", always going towards the construction of more and more complex structures. This dogma of the increasing complexity meets also in other domains, astrophysics for example. However, for the living beings, it is erroneous, it is an optical illusion due to the undue valuation that we grant to the forms of life which look like us. If one looks at the temporal evolution of forms of life on Earth, one notices that has to leave of a certain level of indispensable organization so that one speaks about living substance (that the evolutionist Stephen J Gould, paleontologist at Harvard university, calls the " wall of the complexity "), the development of complex forms of life, pluricellulaires, does not constitute the ruler but rather an exception: What evolves, it is especially the number, the adaptation and the distribution of micro-organisms. Metazoans constituting only a diverticule certainly interesting for us, concerned primarily, but little significant in the point of view of the variety of alive forms. Furthermore, metazoans remain confined on very specific environments, because they are more fragile, and their evolution is slower than the procaryots.
The viruses case deserves a specific exam: one considers at present that these "beings" constitute the most finished shape for the procaryots evolution, limiting itself then to the simple perpetuation of a genetic information in depends on the other cells. The fact that viruses are incapable to reproduce alone persuaded that they could not be the ancestors of the other living beings, but nothing forbids to think that they could result that a secondary "involution", their parasitic way of life, having made losing them ancestral characteristics for their reproduction. Similar "regressions" are usually for animals leading a parasitic life, and of which there is sometimes only a simple reproductive organ. Bacteria and archaebacterias can be infested by viruses, what gives evidence of their age. The study of viral genes reveals also that these last ones are not only very former but as well were able to play a major role in the organisms evolution , going to replacing bacterials or eucaryots genes, unless they derive from it: reports between the ancient viruses and first lifeforms are complex, but indubitably real (a big part of the "dumb" ADN of our own cells is very probably of viral origin).
Numerous micro-organisms support life conditions of an exceptional hardness
Procaryots does not have sexual reproduction about the sense where we understand it: for them, no genes admixture and reorganization of a new genome but direct exchange of DNA fragments . Added to their fantastic capacity of proliferation which still accelerates their evolution, those genetic exchanges allowed them to adapt itself to strictly all the environments which we were able to discover on our planet, and to resist to a stay of some years on the Moon.
If we want to know how appeared all
the living beings, to begin by the most simple, bacteria, it is
going to need to interest us in their constituent elements, the
molecules of the life. We shall see that some of them correspond
to those that we have already met in the interstellar environment,
and that this correspondence is a supplementary indication of
the extraterrestrial origin of life.
Polymers, a structure
The most important of these molecules are very big sizes: one speaks about macromolecules. They are formed by the chain of smaller molecules, the monomothers. By simplifying a lot , one can distinguish 3 big groups of macromolecules characterizing the alive:
There is a very strong and very former link between proteins and nucleic acids. In the molecular plan, one can say that these two species living in symbiosis, each are indispensable to the survival and to the perpetuation of the other one. The mechanism of the proteins synthesis is complex, but we can give it very main lines because, preserved at all the known living beings, it supplies us a supplementary "fossil" for the origin of the life.
Let us see how things happen nowadays, and what it implied originally:
The chain of nucleic acids in DNA forms a code which allows to correspond to a suite of 3 nucleic acids an amino acid. The unwinding of DNA's helix and its reading require numerous proteins. Among these last ones, certain ( the RNA POLYMERASES ) make a partial copy of the DNA in an RNA called messenger ( mARN). DNA's fragment which was read corresponds mostly to a gene. This gene is going to allow the production of a protein, if is necessary. The mRNA contains vast regions without meaning, called introns. It curls up on itself, cuts these introns and resticks these extremities. This process, typical of eucaryotic cells , is called splicing. Then, the mRNA is going to be read with other RNA bound to proteins in organits called ribosoms. At the level of this multi-molecular complex, ribosomal RNA, of which we shall speak again, going to allow to associate to every triplet of base another RNA, called transfer ( tRNA) connected to a particular amino acid. Reading the chain of the triplets of the ARN ends so in the synthesis by the ribosoms of a amino acids chain, in other words a protein. A single mRNA can be read several times, by several ribosoms at the same time: this allows to amplify the message emitted with the DNA.
The DNA role gave place to numerous fantasies, and many errors roam on its subject in numerous popularization books. It is necessary indeed to see that this molecule does not appear, as one says it often, how a book which it is enough to read and to decode to understand: the DNA of animals as the man is in big useless part! It contains only hardly 10 % of codifying sequences for proteins, lost in a jumble of acids nucléiques of which we ignore role and importance. Certain sequences are probably involved in the control of the shape of the molecule, of which we shall see interest.
Rather than a book, it would be necessary to imagine the eucaryotic DNA as a Japanese phone book in which one would have stuck, on places random chosen, some fragments of a english text... If one wants to make a computer analogy, I would say that DNA is a very bad hard disk, where information is quite split up in multiple and remote blocks, separated by unimportant zones (at least seemingly). To be able to be translated, this molecule is forced to acrobatic contortions, allowing "to neutralize" the not codifying sequences. Bacteria possess a more evolved DNA, cleared of all the polluting sequences and completely effective: often, it did not stop being transcribed in mRNA that this last one is already read by ribosoms and translates into proteins!
Information traffic in cells
One of the dogmae of the molecular biology is that information in cells must circulates in one-way: of the DNA towards the RNA , then towards the proteins. The retroviruses discovery wore a first blow in this conception: they have shown that RNA could give of the DNA, what opened an immense way of search and experiment to the molecular biology. On the other hand, to imagine a communication, a transfer of information, between protein and DNA would open the door to the old conceptions of the heredity of acquired characters, and is felt as a heresy, conferring an ambiguous status on the one that would dare to take advantage of it to explain phenomena resisting to the classic analysis. An example of us it given by the discovery of the infectious proteins, the prions, by the biochemist S. Prusiner: The fact that a protein can pass on an information ( its shape), reproduce in a sense and make disease was accepted back, many protests having welcomed the allocation of the Pr Prusiner's Nobel prize , and some, particularly in CEA (ATOMIC ENERGY AUTHORITY), in France, where our opening of spirit is proverbial, are anxious still absolutely to incriminate virus as classic as discreet. This biochemical dahu (a french Loch Ness monster!) have a name: the virino.<>
And nevertheless, if a direct passage of information between protein and RNA, a " reverse translation " does not seem (or any more, nuance is of importance!) possible, it is unmistakable that every second a current of information circulates proteins towards the DNA: number of proteins are bound at this molecule and controls its activity according to informations, at the same moment, from the inside but also from outside the cell. Only concession in the current dogma is that the various proteins of the gene expression control do not modify the sequence of the ADN at answer to the variations of the environment, preventing as well as an acquired characteristic becomes hereditary. However even this rule is not strictly respected: the enzymes which insure the DNA repair during its replication are not infallible, and this alterations are, exactly, an acquired characteristic passed on in a hereditary way...
It is indeed necessary to see that alterations occur apparently at random, what limits their adaptative reach: the cell does not react to a given environment, but to a situation. This effect of the cellular environment in the genetic material appears completely in the techniques of cloning: when one integrates the nucleus of a skin cell ,for example, into the cytoplasm of an ovocyte, it is this last one who is going to act on the ADN to reprogram it and to allow the gene expression which are going to lead to the forming of an embryo and replace the genes activated in the cell of origin of the nucleus. In that case, the ADN behaves less as a program than as a data bank. The reading of these data can moreover make at random, without excessive determinism: The Dr Jean Jacques Kupiec, in the laboratory of genetic and molecular virology of the Cochin hospital, thinks that the molecules which control the genes activity, during the embryo's development, are moving randomly along DNA's molecule, as a pearl sliding forward or behind on a thread. This process would be at the origin of the differentiation of cells in different types (skin cells, liver cells, ect) which would be maintained by stabilizing information exchanged between cells. This conception opens the way to the possibility of transforming, at adult state, a type of cell into the other one. This transformation, which would have made roar to laugh a jury of thesis ten years ago, is actually searched , obtained and studied actively now because of its potential therapeutic implications.
Another example of the plasticity of the genetic information is supplied by HOM genes, which drive embryonic development. It turns out that these genes are similar between very remote evolutionary bodies , as the fly and the mouse for example. Simply, vertebrates have 4 copies of these genes, and the flies, only one. This is a " orthology " (genetic homology in very different groups). There or it becomes interesting, it is that the mouse genes can perfectly replace those of the fly, and lead to the development of the nervous system of a fly!
The genes which regulate the brain development are even more surprising: named "otd" at the fly, they exist at the mouse under shape of several copies, the "otx" genes. Imagine that one deprives genes otd of an embryo of fly: all the front of nervous system does not develop. Imagine then that one replaces genes otd (fly) by otx (mouse): this was made, and one obtains then normal flies! In other words, the same DNA who to the mouse guides the mouse brain forming is going, to a cell of fly, to guide the manufacture of a fly brain...
Shape it's function!
The alive molecules pull their specificity of their shape, but also movements which these last ones authorize or forbid, they are dependent on their topology. This "shape dictatorship" was experimentally verified for proteins, of which the folding is more topology affair than chemical properties. Molecules of different sequences but possessing similar forms can so possess identical activities.
So that a molecule is capable of auto-replication, its shape is primordial, as well as the forces distribution exercised by the various molecular groupings which compose it. These forces can direct and attract basic molecules, and molecular shape allow the link of the different molecules, to facilitate their reaction: it is the functioning mode of the enzymes of our cells. To do it, the experimental attitude which consists in trying to obtain synthesis molecules capable of self-replication shown the superiority of nucleic acids: the molecules of synthesis which can catalyse their reproduction, to evolve and quarrel the resources of their environment contain nucleic acids . It is the case of the ARNI (Adenin ribose naphtalen imide) which can replicate or join with itself, or of the diaminotriazine xanthene thymine (DIXT) which can be bound and engender by recombinaison other endowed molecules of autoreplication or not. This shows the interest of the adenin, present molecule in the two compound precedents, and which the synthesis mechanisms are well known in various abiotic environments.
Also, the activities of the DNA are widely dependent on the shape which can adopt this molecule: nodes and twist which she forms allow to expose or to hide regions codantes, what participates in the regulation of the genes expression. There are indeed several variants different from the DNA's double helix. If the most known is said B shape , it is other one, Z shape, where the direction of the axis of the molecule is modified. One was able to show that proteins are capable of making transition between this two topology of the DNA molecule. Experimentally, one can still be well more far in the DNA deformation: by provoking the twisting strand of this molecule, the researchers of the superior normal school of Paris even arrived at a real " DNA reversal ", bases normally buried in the centre of the molecule meeting itself exposed outside of this one.
The fact which nucleic acids form "ribbons" capable of curling up on themselves because of the connections of electric nature between nucleic acids, by adopting a particular shape, explains that if their morphology depends partially on their sequence, their activity always depends on their shape. It's the same for proteins, shape of which is also capable of transferring an information. Only difference seems to be that information passed on with nucleic acids sequences is everlastingness while that resulting from proteins is unstable, and limits itself in the middle internal of the individual, without being passed, mostly , to the individuals of the following generation.
A strange, very badly known, liquid: water
If one excepts molecules situated in the lipidic cells membrane, most of the biologic polymers are in solution in a common, but strange, liquid, water. Indeed, if one refers to this chemical formula, the water should boil around 0°C and to solidify only around -100°C. Why behave it otherwise? Every molecule water molecule tends to hang on weakly by all the means to his neighbours. In ambient temperature (20°C), it changes several thousand time partners by second but the forces group generated with all molecules which fidget together allows these last ones to stay joined: water remains liquid in common temperature. This molecular excitement is so complex as at the moment the precise structure of the water in the liquid state is unknown, what can seem surprising: complexity can hide among the most common substances!
Water molecules tend to hang together, they are transitory bound also in all the soluble molecules: an sodium ion , for example, is surrounded with a real armor of water molecules which take turns permanently in its neighborhood, forming a solvatation coat which increases its diameter. This water armor is not without consequence on biologic processes: a protein contains numerous chemical, more or less charged groupings, and behaves in the water as a mobile force field, certain regions of which attract the molecules of water while the others tend to repel them. Also, protein molecules stir and all this chemical domains permanently exchanges energy under electromagnetic shape. As a result the presence of the water is not neutral for the lifes molecules: She conditions their shape but plays also a poorly known role in their mutual exchanges. The water molecules can so decrease or increase the "relief" of a molecular region, several thousand time by second, and intermolecular relations always make through this water armor of which one begins hardly to suspect importance.
The water ice possesses also interesting properties: the molecules of water are more or less congealed there according to a dozen possible configurations, what allows a big variety of behavior of this solid and assures conditions varied for the molecules which it can imprison. These last ones, among others, will so be able to keep a certain mobility in the interstellar environment , what facilitates sometimes chemical reactions to very low temperature. This fact was verified by S. Miller, who obtained has to leave of a solution of cyanide of ammonium forgotten 27 years in a deep freeze at -80°C puric and pyrimidic bases entering the composition of the RNA.
The metabolism, an energy dynamics.
The ancestral heterotrophics, not finding any more to feed, would have then been reduced to the state of spores, which in the course of the evolution would have been able to succeed in the most simple and in the most effective of the molecular buildings of the " simili-life ": viruses.
The Reproduction, a fate
An living body can not, because of the physics laws, aspire to the total immortality. Although the phenomena of the senesence and ageing are extremely little known, the unicellular organisms found a means very simple to survive: an individual copies out his genome, then divides to give 2 of it (or a copy of itself, it is according to the adopted point of view!). At molecular plan, thus implies a transmission of a genetic information by replication. A molecule is capable of passing on an information, in fact, if it is capable of reproducing its shape. On the support of this genetic information, debate stay opened:
· One can emit the hypothesis that, as nowadays, they are nucleic acids who were always used for genetic information support. For a long time, the researchers wondered who enzymes or nucleic acids had seemed in first: we saw that these two families of molecules seem indispensable. This small game of the hen and the egg lasted until 1986 when the biochemist T. Cech observed a protozoon that the RNA can be its own enzyme, catalysing itself this division. Thus, the RNA was supposed to be the first biologic macromolecule appeared on Earth.
It presents several interests: this molecule possesses due to its structure an activity on the proteins which exercises at present in complex strutures, ribosoms, in which the RNA catalyses the forming of peptidic bonds between amino acids. Others RNA can cut proteins. Furthermore, these molecules possess a certain capacity of evolution in answer to their environment: the ARN can so become resistant for the ribonucleasic activity which, normally, splits it.
Although the usage of nucleic acids polymers are today generalized at the living beings, nothing insures that it was always so. It is indeed difficult to obtain an ARN in prebiotic environment , the only nucleic acid easily made being the adenin. Besides, the replication of the RNA requires intermediaries onnected to phosphates, what is with difficulty reproducible without enzymes...
Another problem is the weak probability of the molecular replication in liquid environment, the nucleotides asking only to scatter instead of preparing very wisely on strand models of the molecule. It seems although RNA are too evolved molecule, and although he had to divert from a more simple ancestor. Nevertheless, this nucleic acid must have been involved rather quickly in vital processes.
· Genetic information was able to be of electromagnetic nature: the researchers C. Woese and G. Wachtershauser are at the origin of this conception with, at different degrees, the crystals reproduction as base of the alive reproduction. We shall see that this approach is by far the most interesting, because as declares it G Wachtershauser " created order (biologic) from the order (crystals) and not of the chaos ". The distribution of electric loads in minerals would have directed the synthesis of the first molecules, and as this distribution is preserved, with certain variations, when the crystal develops, it supplies a physical support for a primitive reproduction.<>· The genetic information support has only not enough importance: what's counts, it is shape. I would say that this hypothesis, personal, could decline under the name of " three band ": crystals, proteins and nucleic acids are capable of passing on forms. I make so the hypothesis that they worked together to reproduce first "life" forms : crystals supply an environment capable of reproducing with variations, entailing an nucleic and amino acid mixture bound to its surface. We shall see that crystals in question can those be contained in the interstellar dust, and that the beginning of the life-origin process, if it is not totality, was able so to take place in the space.
A iconoclast hypothesis: proteins only
The majority of the researchers consider that the ARN is the first original organic molecule. Problem, it is that in Science (as somewhere else, but this is another history) one owes (or rather should not have to) not take advantage of the number to assert the truthfulness of a comment, but only experimental facts: the truth is not democratic. therefore this hypothesis: and if we had had, at the beginning of life, a more simple, protein information, being passed simply by contact with oligonucleotids? This idea would have seemed absurd there is little, until the discovery of the first-rate prion, this extremely resistant protein (regrettably!) capable of passing on the "information" without nucleotidic material.... The first autoreplicative molecule it would have not been able to be a " paléoprion ", passing only a shape, from which would ensue, later, a function ? It is an opened question.
Now that we saw which were the living beings molecules, let us see which are different theories explaining their origins. We shall see if they are capable of supplying a coherent frame allowing to explain the elaboration of the molecular machinery which equips the current living beings, even though this gives only an deformed image with billions years of evolution of what she was originally.
The origin's theories
There are two big "chapels"
in the contemporary biology, different by their approach of the
problem. More than the details of their arguments, what divide
them are almost philosophic conceptions: to the upholders of
an purely endogenous origin of life, which they very often limit
to the Earth, oppose the zealots of an essentially exogenous origin
(I am one) , who they are favorable to a wide dispersal of alive
In 1950, chemist Harold Hurey proposes a reducing primitive atmosphere, rich in methane and in ammoniac. he bases himself for it on the study of meteorites and the atmosphere of the giant planets. Three years later, his student Stanley Miller has the idea to reconstitute in laboratory the environment of the primitive Earth. Without boasting of the idea, he tinkers an experimental apparatus with closed circuit containing some water, the various gases of the atmosphere and the source of energy, mimics lighting of the thunderstorms of the primitive atmosphere. The circulating mixture , sterilized, to reproduce the phenomena of evaporation and condensation. One would have expected to preserve only the molecules which one had put in it, but Miller had the surprise to discover brown deposits formed on the wall of the device. By analyzing this substance, he discovered that it was about amino acids: These protein's bricks had formed under the influence of the light and the heat in a reconstruction of the primitive ocean! This experiment had a big echo, because it showed that the molecules of the alive could indeed , experimentally, be synthetising in the conditions thought of being those of the primitive Earth. Many adversaries at the idea of a "easy" forming of life with simple prebiotic molecules arrested in this experiment, and did not follow following developments. Some of their arguments fall so. The Miller's experiment was redone many a time, by varying the atmosphere of departure, the intensity of the source of energy but one always obtains amino acids. In the end of the 50s, S Fox shows that these last ones get organized in small spheres, the protenoïds. However, those can not reproduce, so to evolve.
At present, numerous biochemists (Meli, Vergne, Maurel) study molecules synthetized in prebiotic conditions, which make link enters RNA's information capacities and protein's enzymatic activity. So, N6 ribosyl-adénine is a potential analogue of the amino acid histidine, of which it reproduces chemical behavior. The other by-products N6 or N3-ribosyl-purine were able to be essential links between the world of proteins and the RNA world . As we shall see him afterward, RNA bound with amino acids was able to be the ancestors of the modern tRNA, participating so at the emergence of genetic code.
We live in a world the atmosphere of which contains 20 % of O2, a corrosive gas from which the living beings protect themselves by varied devices. For example, some enzymes as catalase, peroxydase or superoxid-dismutase allow the metabolism to resist to the noxious action of the oxygen and its by-products. For unprotected anaerobic bacteria, oxygen is always a mortal poison. Without oxygen, no ozone to protect the surface of the planet of the solar U.V. which arrive at the ground, bringing energy necessary for the chemical syntheses with available mineral elements. There are many indications which show that the oxygen was absent in the primitive atmosphere, at the beginning of the life:
The chemical reactions which allow the living beings to get back some energy from the molecules of their food are 4 different types:
The multiple republications of the Miller's experiment led between 1960 and 1980 us showed also that nucleic acids can form in Miller's " primeval soup " and accumulate. But we have a problem: we saw that the syntheses of proteins and DNA's molecules are interdependent: how can nucleic acids reproduce without proteins? And where from come proteins if there are no nucleic acids to produce them? One met himself then in the uncomfortable situation to know who, hen protein or egg DNA, had been born the first...Another difficulty was going to show itself: the primitive atmosphere, we saw him, had no composition foreseen by Urey: it was richer in carbon dioxide and nitrogen. Although a reducing atmosphere would have been able to become established under the influence of the solar radiation, temperature and pressure in the surface of the Earth must be more raised thought Miller. In this conditions, synthesis of prebiotic molecules others than some amino acids becomes much more difficult. Nucleic Acids, in particular, are hardly obtained.
The researchers lived so in this prospect when, in 1980 , the biochemist T. Cech, studying a protozoon, show that the RNA is capable of repairing and of copying out alone, without protein's intervention: it is at the same moment gene and enzyme. As a result, it seems although one likes an element of answer to our dilemma: it is " the egg " RNA which would have first formed. This idea is presented in 1986 by W. Gilbert under the name of " RNA's world ": first organisms would have been simple molecules of autoreproductive RNA. Secondarily, they would have used protein's molecules to reproduce more easily. This set RNA + proteins surrounds itself then with a lipidic membrane which concentrates and protects these molecules. Afterward, the RNA is replaced by the DNA, more stable.
It does not make any more doubt today than indeed the ADN succeeded the ARN in the living beings: biochemically, DNA is a modified RNA, and our cells always make this transformation. Main problem, it is how to obtain this RNA of departure! Spontaneous forming, in solution (we shall see importance of this term) of this molecule is not very probable, but possible.
It is however to make the risked hypothesis to believe that the primitive autoreproductive molecules looked like ours. Another molecule, more simple than the RNA, could have played its role, it is at least that thinks L. Orgel, of the Salk institute. Indeed, the self replication capacity does not necessarily require a big complexity: the chemist J. Rebek obtained experimentally molecules of synthesis capable of autoreproduire, but which are more simple than RNA's least strand!
is a fragile molecule.
Initially, one thought that this molecule, appeared in solution,
had concentrated step by step in small tepid puddles. We saw that
the conditions of the primitive Earth do not allow such processes.
If one preserves the hypothesis of a primitive RNA, a controversy exists as for the thermic conditions of the environment where this molecule was synthetized. As one thought that the most primitive life forms were archaebacterias living in warm places, one guessed that the RNA had an origin implying high temperatures. But, in 1999, the study of the ribosomal RNA shows that these last ones do not contain enough of guanine ( G ) and cytosine ( C ) to have resisted to high temperatures (54 % instead of minimum 60 %). Indeed, G and C are bound by 3 H-links while the two other bases are only by two. As a result, a warm environment is going to select the links ( most solid: 3 ) which should be more numerous than double connections. The last common ancestor (if he had it it one and the only one!) had so to live in temperate environment. However, a re-examination of the conditions of this study as well as the other considerations let understand that the ancestral RNA well takes its origin in a warm environment.This is interesting seen that all the earth's surface was " a warm environment! " (One supposes a temperature of 80°C on the ground!). This controversy allow to think that the idea of a synthesis of autoreproductive RNA in solution, constituting the base from which molecular evolution leads to the living beings is doubtless erroneous, because this molecule would not have been able to form in this way in the conditions of the primitive Earth, there is 4 billion years.
Before examining the other ideas, let us return one moment on the historic Miller's experiment. We saw that the gas mixture which it used was not adequate for describe the original terrestrial atmosphere. However, the results which he and his(her) successors obtained are unmistakable. Strangely, nobody to my knowledge pointed out that if Miller's atmosphere did not apply to the Earth, it was convenient on the other hand perfectly for the description of the atmospheres of jovian planets Jupiter and Saturn: the gas envelopes of these last ones contain methane, ammoniac and water. Energy does not miss there, for example because of intense auroras borealis, and of gigantic thunderstorms. Into their cloudy coats, temperature and pressure increase gradually. One deducts from it that one meets the conditions of the experience(experiment) of Miller, and that fatally amino acids and nucleic acids had to form in this enormous atmosphere. Thus, bacterial life forms, living in suspension in the atmosphere would have been very well able to develop. On our own planet, such atmospheric bacteria were recently discoverings. On Jupiter, huge gravity (3 times terrestrial gravity) were able to forbid the appearance of too massive forms of life, these being entailed towards the centre of the planet, in pressures and the incompatible temperatures with the preservation of complex molecular structures. A bacterial life in the atmospheres of planets joviennes is in agreement with all that one knows about the appearance of the life, but it was never considered with seriousness, colliding in an epistemologic obstacle: as we live in a telluric planet, we want in any force that life is limited to this planet type. It is not probably, and which knows if the delicate colours of the clouds of Jupiter do not result from molecules thrown rejected with bacteria? If it was case, we could indeed observe, since centuries, even in the smallest telescopes, the evidence of extraterrestrial life for which we look everywhere else while it bursts us eyes... It is, according to me, more likely, as we shall see him, even if Jupiter do not shelter endogenous life, they have however receiving microbial " space travelers " from the Earth or Mars.
Difficulties for finding an environment enough protected so that first life forms resist to the destructive influences of radiations and the falls of meteors led to consider an environment which, at first sight, had considered totally unfit for life: the abyss.
This conception suits perfectly to what one knows about the family tree of cells, and we saw that physical conditions at the time of the appearance of the life militate in favour of primitive organism appeared to temperatures close of 100°C. There are however some shadow zones, the most widened concern nucleic acids: it is has little near sure that RNA appeared before DNA. But the RNA is easily destroyed at high temperature, while the DNA resists to it! It is for it that the researcher P. Forterre questions this hypothesis. However, a "warm" origin of the life remains probably because the "cold" regions seem to have been more than rare at this time... on Earth!
The nucleic acid problem was going
however to be resumed and developed by an original researcher
of whom we shall speak again more in detail, G. Wachtershauser.
Another difficulty is the duration of activity of sources hydrothermales: it is presently only some years before these sources of nutriments dry up: nothing indicates that this duration is sufficient to allow the appearance of first forms of life, but it's possible to admit that their development was more faster than generally believes.
Our rocky ancestors : a " stony dawn " at the dawn of the life.
The forming of the first nucleic acids is a huge problem, but a chemist working in the patent office, in Bern, and occupying the same function as certain Albert Einstein, reconsiders the problem of the emergence of the alive by proposing that the first demonstration of the life is not autoreproduction but establishment of a metabolism capable of getting some carbon and of using some energy: in 1988, Gunter Wachtershauser imagines so a mineral origin of the life.
At the surface of pyrit crystals, organic molecules settle to form " métabolists " which pull their energy of iron sulphide. They produce so some organic molecules with minerals. The growth of crystals of pyrite supplies a primitive division which allows the reproduction of the " métaboliste " by successive divisions. This primitive metabolism uses sulphurated compounds: C. Of Duve shows their interest and use by some organism to produce acids ribonucléiques, and so RNA. This way of synthesis is the only one presently plausible .
Before, A. Cairns-Smith had proposed a theory similar but based on the clay crystals of which allow a replication with errors: thus the possibility of an molecular evolution exist. If this researcher envisaged a mineral support subjected to cycles of drying and of hydration, at the edge of a primitive sea, or in rivers; G. Wachtershauser show that the syntheses of the " metabolist " demand heat, strong pressure and plentiful availability of sulphurated minerals: these conditions correspond to those reigning near deep hydrothermic sources. he places so the origin of the life at the level of hot abyss sources, quite as Corliss. However, others environment can have presented comparable characters: inside of the terrestrial rocks, surface under strong atmospheric pressure with volcanic eruptions or geysers...
The idea that minerals has their word to be said on the origin of first forms of life is extremely important: she allows, we shall see it, to explain why first forms of life are not dissolved in the "primeval soup" existence of which becomes more and more suspect. We shall see that link between mineral and organic worlds takes shape, according to me, in the clouds of interstellar dust, and strengthens afterward only until the emergence of the alive. The recent revealing by Folk and Uwins of the existence of nano-organisms connected to the processes of mineralization comes to confirm in a sensational way the pertinence of this conception.
It are facts and arguments of this community of thought which drove me to publish these pages. If they lead sometimes to excesses harmful to the scientific value, they have the advantage to supply us the bases of a new Copernician revolution aiming to establish the commonness of the life in the universe.
They are the kangarooes which were surprised! On Sunday , September 28, 1969, a brilliant flash of light streaked the sky of Australia, near the city of Murchison. Searchers got back on the ground a meteorite of 225 kg, the analysis of which was going to reveal many surprises: she showed that this rock contained in blackish inclusions called chondres 17 different amino acids. This meteorite does not result from the fragmentation of a bigger body but formed at the same time as solar system, and isotopic balance of certain elements, as the neon, show us besides that she come from a nearby region of explosive stars of the type Nova or Supernovae. We have to conclude, from it contrary to all expectations, that amino acids can form in the interstellar environment. More former meteors, studied with the modern techniques, revealed so to contain amino acids. 8 amino acids on the 20 used by the living beings were found in meteorites. Furthermore, some of these amino acids show the existence of an asymmetry between their chemical forms "right" D and "left" L, what is noticed for all living being, what use only forms L.
More than 500 organic molecules were identified in Murchison's meteorite: in December, 2001, the team of G. Cooper announced the detection of a simple sugar and of polyols as the glycerin. Previously, from the compound extracts of this rock, D. Deamer had obtained membranes constituted with organic molecules. These vesicles demarcate an middle which would be favorable to the concentration of prebiotic molecules. Experiments realized in Earth orbit ("biopan" program) showed that a layer of 5 microns of meteoritic dust was enough to protect the amino acids of the destruction. Thus, even meteorites of small size (+ of 5 microns) are able of having transported these molecules. This point is important, because micrometeorites are at the same moment extremely plentiful and particularly resistant: more than 80 % of them no melts when they cross over our atmosphere, the combustion of a part of the carbon contained in these meteors provoking a gas relaxation cooling the rock and assuring him so a protective "air conditioning" during its fall. Every year, these meteors bring 20 t of extraterrestrial carbon on our planet, what represents a hallucinating contribution of 10(17) tons of C since the forming of our planet (5 orders of magnitude above the quantity of carbon contained altogether living beings)!
the vague "ecologist" of the 60s and the
magnificent photos of the Earth floating in the space, a current
of idea "isolationist" crossed over sciences generally
and biology in particular. The idea which we " had only the
Earth " and it was necessary to protect her resulted, unconsciously
, in the necessity of describing all the Earth history without
appealing to extraterrestrial influences, badly known by the geologists
and the biologists and considered as stenches of old catastrophists
conceptions where got involved religious myths and works of first
geologists, at the beginning of the 19's century. So, never
forget that science is made by mans and women sensitive, sometimes
without their knowledge, to the modes which can accept or throw
back an idea independently of its scientific value. The subdivision
of the search as well as the absence of a real scientific culture
to most of the researchers urge them to slander systematically
what does not constitute the object of their searches: biologists
and geologists, united to discover the origin of the life, considered
then firmly that they had to study only what had happened on
Earth, the rest being considered as being without interest.
The study of meteorites was going to deliver more sensational information. Indeed, in 1998 , Pr. Folk song identified in the Murchison meteorite of formings being able to be the remainders of fossilized nano-bodies. Similar structures were also identified in other meteorites (Allende, ALH 84001, Tataouine). Naturally, controversy makes fury around the interpretation of observed structures: some see there the fossils of extraterrestrial life forms, the others of simple artefacts (" mirages ") or purely terrestrial contagions. Debate is far from being closed, what illustrates very well controversy concerning the ALH 84001 meteorite.
Let us note that another Martian meteorite, EETA 79001, released, after heater, carbon dioxide enriched in 12C. This enrichment isotopique is, when it is discovered on Earth, interpreted as being of biologic origin. Terrestrial contamination is always possible , it will be necessary, for to know more about it, to take the direction of sedimentary formings identified recently on the surface of the red planet.
Life comes from beyond ...
Let us approach
now a hypothesis, which a few ensues from the previous one, and the
aspect of which "
science fiction " does not have to mask interest. The main
idea is : life did not appear on Earth, but was brought to it.
This deposit, for the most reasonable defenders of this thesis,
was involuntary, while the others, not the least, involve a voluntary
sowing of the planet. Let us notice first of all that problem
is only moved, because it is necessary to explain the origin of
this exogenous life. Then, I have to admit that I had, during
years, not enough consideration for this hypothesis. It seemed
to me by too fantastic, and had been in the past too much compromised
with number of cheats and of delirious archeomans to preserve
this interest. It is the study of the general conditions of the
appearance of the life and the most recent discoveries which led
me to modify my conception and to hold as very likely the most
reasonable side of this thesis. Only problem being to balance
what, for the first living beings, was specifically endogenous,
terrestrial, and what could result from the interplanetary, even
First doubts come from the moon: when the Apollo 12 astronauts recovery on the Moon a camera of the automatic probe Surveyor, analysis showed that a bacterium at least had been able to, without damages, made journey Earth - moon and remain live ten years on this celestial body. In the 70s, the microbiologists went from surprise to surprise, discovering that bacteria could practically colonize any environment: the discovery of these "extreme" forms of life able of resisting to the heat, to the cold, to the radiations, to the absence of water demonstrated that microbial life was more resistant than more scientist had believed him.
In front of difficulties for clarifying the origin of the life on our planet, "alien" hypothesis made a return in force. Some used their scientific authority to dare the most mad hypotheses, and and so in 1980 Nobel prize F. Crick, co-discoverer with T. Watson and R. Franklin of the structure of the ADN, and L. Orgel suggest that the Earth and the other planets had been sowed voluntarily by extraterrestrial intelligences. We shall see that, without going also far, the hypothesis of a sowing of the Earth by micro-organisms can not be thrown back, or even the one according to our own planet was able to serve of " life source" not only for solar system, but can be for a part of the galaxy!
It is indeed more and more probably than bacteria can be extracted away from their planet of origin by meteor and remain million years alive before reaching other celestial bodies.
To take into account eventual extraterrestrial contributions of micro-organisms would allow to explain for example why several enzymes important of the metabolism need to work of a rather rare element on our planet, the molybden. However, this problem is secondary by comparison with others than the partial extraterrestrial origin of the life allows to resolve.
Now that we reviewed how was organized interstellar environment, how the primitive solar system was forming, seen which were constituents and general organization of the living beings and which were hypotheses relating to their appearance, we are enough armed to try to propose a synthetic vision which, by using the recent contributions of the astrophysics and the microbiology, is going to lead us to propose a general and synchronous appearance of the life in immense regions of our galaxy. The fact that one can not consider any more the Earth as being a planet biologically isolated will bring us to postulate the existence of a galactic isobiologic region , of a life network of astronomical size within which forms of life, at least microbial, are similar.