Replacement parts, and glue

What should have to be in the most simple living being construction kit ?

Simply the most simple elements from which we could assemble any creature, of the bacterium to the elephant, and a good glue to hold all this together.

These replacement parts, they are the molecules of the alive. They are constituted with atoms, mainly with carbon, oxygen, hydrogen and with nitrogen of whom you, I, the neighbours, the grass and the bacteria are making. On hundred sorts of different atoms which finds in the universe, 24 are necessary for molecules constituents the living beings.

The glue, it is necessary energy to bind together these atoms in the molecules which constitute the main thing of our environment. We shall return in detail on the molecules of the living beings, but already we can quickly get acquainted with the most representative of replacement parts necessary for the assembly of a bacterium for example: we find 4 molecular families:

-Amino acids, containing some nitrogen, which are bricks with proteins

- Nucleic acids, rich in phosphates, allow to build of the DNA and the other molecules capable of containing, to reproduce and to pass on an information

- the sugars which allow the functioning of cells, their recognition and the consolidation of their membranes

- the phospholipids which constitute membranes allowing to separate the inside outside from individuals

Many of these molecules contain one or several carbon cycles, forming of which can inform us about the very first stages having led to the elaboration of the living beings. But well before to form cycles, it was already necessary to have some carbon to be assembled! It is for this level that begins our adventure, which is going to show us that the history of our universe and that of the life are indissolubly connected.

At the very beginning, there is more than 10 billion years, only two atoms were available in great quantities: the atom of hydrogen (the majority party) and his neighbour the atom of helium (that one does not find in the human beings). The first act of the life comedy is going so to consist in obtaining a big variety of atoms, and more molecules from the only hydrogen. For the glue, we shall see that she is available in quantity, and under many forms. Our history is going so to begin there is 12 billion years, in a region of the universe or immense clouds of hydrogen are contracting. Let's approach: our galaxy is being born...

At galactic scale

An enormous mass of gas (75 % hydrogen, 25 % of helium), vaguely spherical and in rotation is flattening to form a disc. This globe is not homogenous: it is crossed by instabilities, nodules, gas currents which mixed constituents locally . The first stars which formed on the borders of this cloud were at the origin of globular clusters, which form a spherical halo around the galactic disc.

Much later, the first stars are going also to ignite in the galactic disc.

At present, our galaxy appears as an immense pancake of 100000 light years (LY) of diameter and 1000 of thickness, looking like enough in one fried egg : an almost spherical centre and a flattened disc divided into several arms. Everything around the disc, in a sphere of 100000 LY of diameter, we find globular clusters constituting extremely old stars heap spherical. The galactic disc owes mainly its aspect to stars and to clouds of gas which, under the influence of stellar brilliances, become brilliant, although one also finds there clouds of dust dark. These various clouds are confined in galactic plan, on a thickness of 150 LY only. Even so, our description is not complete: we see the galaxy only the objects which emit or intercept the light, and number of dark celestial bodies is still us, probably, unknowns. What we see of the galaxy is comparable to would seen of a forest under the weak moonlight: some sparklings on sheets let with us suppose the presence of the other invisible structures at the moment. By studying the way of which form galaxies, certain researchers even suppose that our complete "visible" galactic environment owes its shape to forces from the other side with the universe, a " ghost universe " symetric of our, has ever invisible for our eyes.

Our sun describes an orbit to 30000 LY of the galactic center, which it goes through in 300 million years. This orbit is not perfectly elliptic, and our star navigates so inside a slender ring in which he can get closer, during an orbit, a star which he will not meet any more afterward... The sun has already made 15 times the tour of the Galaxy at the speed of 220 km/s! As our star turns more quickly than the spiral arms of the Galaxy, he crosses successively numerous regions more or less rich in interstellar material. At present, the sun and numerous of his neighbours (Sirius, Vega, Altaïr, Aldebaran) are in what one calls the "local bubble", an almost spherical space of 500 LY of diameter which contains little interstellar material and fits between denser molecular clouds.

Stars, manufacturers of raw material for alive bodies

Our galaxy originally contained only hydrogen, helium and some tracks of lithium and béryllium. Not of what to build the least human being, nor the least solid planet... Then where from came the varied atoms which constitute our planet and his inhabitants? Simply from stars which made them with the hydrogen and from the available helium.

When a star is born, a big mass of hydrogen condenses, its diameter decreasing, its density increases until the point where in the heart the atoms of hydrogen are so tight and so shaken as, in spite of the aversion that they are inspired, their nucleus "sticking": they are merging, are transformed in helium by releasing an enormous energy. This energy production tends to blow up the star, the mass of which provokes, on the contrary, contraction.

Result: the star remains so as to balance these two forces: any star is so on the tightrope, and navigates all its life enters Charybdis of the explosion and Scylla of the collapse gravitationnel. When a star lacks energy, she contracts, and if she has too much, she expand.

In the end of their life, the small stars as our Sun go out slowly, not finding any more in their heart of hydrogen to be merged. They will have made only helium and, towards the end of their life, a little carbon. But the first stars which formed while galaxy was still in contraction possessed a more important mass, having condensed in an environment where material was abundant. These big stars (more than 8 solar mass) contain enough material and by contracting they can realize new stickings, new fusions between atoms: during million years, having merged hydrogen to helium, they begin "sticking" the nucleus of the helium atoms together to form carbon, neon, oxygen, magnesium, silicon...

However, more heavy atoms are to be assembled and more energy is necessary for the star to merge them: she contracts so, more and more, but these successive fusions owed less and less for a long time: The mechanism eventually warmed, the star merges elements one after the other takes a structure of it in "onion skin", where at every level corresponds a fusion, a manufacture of a precise atom. Until what? Until the moment when in the heart of the star is going to accumulate some iron. It is the wrong way of fusion: the iron being the most stable atom, an enormous energy is needed to merge it. This energy, the star have not. Then, brutally, fusion stops in iron core but continues in the various layers of the star. The iron atoms accumulate in the center. When the iron mass becomes too big, atoms are too much squeezed and, subjected to an enormous pressure, they collapse to form a huge, almost incompressible atomic nucleus: the external layers of the star rush in big speed on this heart on which they bounce with an incredible force, generating shock waves and soundwaves of a fantastic energy: in some microsecond, the star explodes and forms a supernova. produced energy is so important as a fraction of the iron merges to give a continuation of heavier elements, as the lead, the titane, the gold and the uranium for example. So, the gold of your rings results from the atmosphere of a star in explosion. This explosion scatters in the space atoms constituting the star: the unused hydrogen, the helium and a whole continuation of atoms heavier as the carbon or the oxygen. The supernova is also a source of shock waves which will be able to begin the contraction of the other gas clouds. In their tour, these, enriched in heavy elements, will form a second generation of stars, varied sizes. The supernovae explosion digs so "holes" in the galactic environment, concentrating interstellar material at the level of the limit of gigantic bubbles, where new stellar generation will form.

If the giant stars did not live for a long time, some dozens of million years, the lighter stars, as the sun, can merge in any peace of mind for 10 billion years. The huge stars present so the advantage to quickly enrich interstellar environment and to provoke indirectly the collapse of the other gas clouds: they begin chain reactions ending in the appearance of a big variety of different stars. As a result, at the beginning of the forming of the galaxy, very numerous giant stars had to undergo an explosive end even when the galactic disc was not formed: they were at the origin of numerous shock waves and of turbulences involved in the forming of the main plan of the galaxy.

The enrichment of the interstellar environment is not only the fact of the explosive stars: among the least massive, fusions can stop before reaching the iron stage, especially at the level of the carbon synthesis, which ask a half million years. When the star will not work any more, its star wind will scatter in the space its envelope of hydrogen enriched with helium and carbon. One often reads, in the books of astronomy, that only supernovae enrich interstellar environment in heavy elements. It is not completely exact: the stars which are just a little more massive than the sun uses the carbon, the oxygen and the nitrogen to maintain the fusions which give them some energy. Also, nuclear fusion is not the only mechanism to make new atoms: nitrogen, for example, is making by atoms of oxygen with effect of the star heart radiations . This process, called spallation, allows to make several different atoms, of which some will decompose because they are unstable: one calls them radioactive. Also, the explosion of a star is not the only way for communicating heavy elements to the galactic environment: the majority of stars emit some material under the shape of a " stellar wind " which, towards the end of their life, is enriched in elements as the carbon. So, in the course of the successive generations of stars of varied size, the clouds of gas grow gradually rich in heavy atoms, collapse, condense, form the another generation of star which, in the tour, enriches environment: during 7 on 8 billion years, every stellar generation increases the accumulation of carbon, oxygen and nitrogen in the gas clouds of the galactic disc. When this quantity becomes sufficient, numerous different molecules will be available in the space, and solid planets are going to be able to form, with stars, and surround themselves with an atmosphere.

Oxygen and carbon start up the life

The living beings whom we know possess all the same chemical composition: they are made mainly with carbon, hydrogen, oxygen and nitrogen. Add to it some elements of a big biochemical importance as the phosphor, the iron, the magnesium, the zinc.... The quantitative proportions of some atoms contained in the human body (at this level, one almost finds the same figures with a bacterium or a lettuce!) results mentioned in the following table give us:

Why are oxygen and carbon so important for the living beings?

The oxygen is of a big interest for two reasons: first of all, an atom of oxygen joining with two of hydrogen gives us the water molecule, which composes often more than 75 % of the mass of the human beings. This quite simple present molecule, we shall see him, exceptional properties. Furthermore, numerous rocks composing the telluric planets are oxides. The forming of planets, water and human beings requires so presence in the interstellar environment of a minimal quantity of this atom.

Then, this element is formed only with the big stars, the others, we saw it, stopping their syntheses at the carbon stage. As a result the galactic availability of the oxygen conditions the appearance of telluric planets and living beings.

The carbon have characteristics which make of this atom the indispensable "skeleton" of the molecules of the living beings: it is capable of lending 4 electrons to the other atoms and also bound to 4 neighbours for forming molecular buildings of a surprising variety, going from the simple methane, CH4, in thousands atoms constituting a protein or the DNA. Connections established with the carbon are also very interesting by their force: this one is enough raised so that atoms remain connected in molecules without scattering in the least increase of temperature and weak enough so that " partners exchanges ", chemical reactions can take place without asking for too much energy. The galactic availability of the carbon is of more relatively high: interstellar environment grows rich easily in this element.

Besides gases, interstellar environment is rich in dusts of which we shall see importance during phenomena ending in the forming of complex molecules. Most of the particles of interstellar dust originate from the particles of silicates which condense in the atmosphere of the supergiant stars, said M class. The pressure of radiation caused by the stellar radiation propels in the space these grains, their temperature being any more of ten Kelvin (-263 °C). Initially amorphous, they adopt then a configuration partially or totally crystalline. It is from these grains that are going to condense the interstellar dusts. Although these processes take place in a poor density environment, grains get be grow by incorporating then by modifying the elements which they meet. The carbon, under shape of graphite, resulting also stellar atmospheres, also enters in the composition of the interstellar dust. The old stars making also in their atmosphere an complex organic molecules: they are a source of benzene and other molecules, including until 6 atoms of carbon ( polyacetylens).

Hydrogen, heavier atoms, and dusts are available in great quantities in vast regions of the galaxy, the clouds of gas from which new stars and their procession of planets form. Carbon of stars, hydrogen of the universe and oxygen of supernovae, with the nitrogen of the stellar atmospheres, forms the essential part of the atoms constituting all the known living beings. All are so, in the atomic point of view, the threads of stars. We show afterward that this filiation does not stop there, and that we are more confidentially still connected to the cosmos which contains us.

Can the other atoms replace the carbon for the living beings?

It was proposed that the silicon, which presents common chemical characters with the carbon, can possibly replace this one in an extraterrestrial "biochemistry". However, as the nucleosynthesis of the silicon takes place after that of carbon in supernovae, interstellar environment grows rich more quickly in carbon. Furthermore, the silicon forms chemical connections which are more solid than those formed with the carbon: the solidity of the chemical connections of the silicon would imply the existence of very resistant forms of life but with very slow evolution and reproduction. Now, the absence of evolution, it is the death... However, in a particular energy environment (temperatures raised during long periods, strong pressure) The silicon could, due to the solidity, present advantages allowing him to be in the base of reactions leading to complex molecular buildings, maybe to forms of life, but which would remain very dependent on the constancy of physical conditions insuring the possibility of chemical reactions " in the silicon " at reasonable speeds. It is also possible to envisage a "infra-chemistry" occurring at the level of the atomic nucleus, with complex almost-alive groups could appear during intervals of extremely reduced times. This theoretical hypothesis not being able to be tested, she can not, in the current status of our technology, establish a valid track of search.

When the clouds of gas of the galactic plan receive shock waves from supernovae or undergo effects of tide due to the merger by our companions' galaxy smaller than her; they begin a condensation ending in the forming of a group of stars, varied masses, but providing all of a cloud of rather homogeneous composition. These clouds spread out on several A.L's dozens. Contains cold spherical regions where occur numerous chemical reactions, enriching environment in varied molecules, some di-hydrogen H2 for example. Of these zones 100000 stars of sun type will be able to be born more. For it, the magnetic field, which tends to scatter the cloud molecules, should have been overcome by the gravity of the neutral molecules which it contains, what requires normally some million years. However, this process is very strongly accelerated by the shock waves which cross the plan of the galaxy.

The stars forming is not so a continuous process: she becomes established by successive waves creating stellar "generations" in an environment more and more rich in heavy atoms. Except the periods of stellar forming, one considers that every year only 5 in 6 solar mass condense in stars. The oldest stars contain hardly 0,1 % of "heavy" atoms while our sun counts 1,5 % of CNO. The youngest stars of the galaxy contain until 3 % of heavy elements, showing that the enrichment of the interstellar environment continues.

Four " stellar generations " were identified in our galaxy. Our sun belonging to the third, one can suppose that it is from this time that the quantity of available heavy elements permit the start up of life.

For our Galaxy, the molecular clouds and most of the stars of big mass are situated in the plan of the galaxy. One supposes that disturbances of gravitational origin, implying maybe Magellan's clouds, two galaxies satellites of our, were at the origin of 3 or 4 big periods of stellar forming implying 3 stadia of enrichment of the galactic environment in heavy elements. One can consider that these elements were sufficiency available there is 5 or 6 billion years about: it is in this time that I place " start up" stellar for the appearance of the " alive phenomenon ". It supplies us a deadline, although very indistinct, for the appearance of the life in our galaxy. Indeed, our sun is a member of this stellar generation, and although one can also imply the stars of the previous generation, formed there is 9 billion years, nothing confirms us that the heavy elements were then rather plentiful to allow life. A contrario, the fact that you read this book proves us that the following stellar generation contained enough of it so that the life appears...

This idea that the availability of the heavy elements is a key condition of the evolution of star systems towards the forming of planets, then life forms, is confirmed by two observation facts :

- study with the Hubble space telescope of 34000 stars of the globular cluster 47 Tucanae did not allow to discover there any planet. Statistically, around thirty must be discovered to it. Now, globular clusters are constituted with very old stars (more than 10 billion years!) who formed in a very poor heavy elements environment .

-Most of the stars surrounded with planets which were discovered are particularly rich in heavy elements. It can have been caused by " the ingestion " of planets, but our sun does not show such an enrichment, although is accompanied by inhabited planets. It could so be a member of first stellar generation having allowed the appearance of the life.

However, recent results bring detection in particular galaxies, LCG ( Light Compact Galaxies) of a high content in heavy elements. These galaxies are in about ten billion years light of us, what indicates to us that the conditions of forming and development of the life were able to be more premature in the other galaxies than in our Galaxy. We show indeed that the process which leads to the life starts begin well before planets completely formed...

An element of answer to " Fermi's paradox "

I defend in this work the idea that the conditions of appearance of the life on Earth involve the existence of other forms of life in the galaxy. The big physicist E. Fermi exclaimed: " if there are intelligent extraterrestrials, where are they? ". he expressed so the idea as beings more advanced than us should have contacted us.

I shall return on this problem in the end of this work, but we already can notice that " sisters stars " formed together in the same cluster scatter because of their appropriate speed, but remain confined all the same for a long time in the same galactic region. Content in heavy elements being similar in these regions of the Galaxy, the appearance of the living beings is synchronized in a volume corresponding at least to that of the stellar cluster of departure. If one also takes into account the other "local" conditions in the galactic scale (shock waves from close, vague supernovae of stellar forming, enrichment of the interstellar environment), we see that for a wide region of the galaxy "stellar" conditions connected to the appearance of the life are roughly speaking synchronized.

In this galactic volume, the living beings evolve in different rhythms, but who can be comparable over long periods: as a result, possible extraterrestrial intelligence are about approximately at the same level as us, what explains partially why we never met them, apparently .

The specialist corner Interstellar material is very rare in the center of the galaxy. The ionized hydrogen is concentrate in a zone situated at 1200 L.Y of the center, while the molecular hydrogen forms a flat layer in the biggest part of the disc, curving only towards its edges. Average density of interstellar clouds is of 10 atoms by cm3, forming groups of 15 L.Y. of about diameter, containing 50 solar masses (SM). Certain complexs as Orion's cloud can exceed 100000 SM. These clouds form the stars of population 1, which include 2 in 3 % of heavy elements while the stars of population 2 do not contain it sometimes even 0,1 %. When a star of mass superior to 1,5 SM forms, the carbon which it produces allows her to merge H in He according to the carbon cycle, using this last one as catalyst. The functioning of the star core is then different. When central temperature reaches 100 million Kelvin, reaction 3a merges 3 He's nucleus in C. In superior temperatures, the heavy nucleus merge among them or arrest particles has until 2 billion degrees where the formed new elements are immediately destroyed, and at nuclear balance occurs .

We are all extraterrestrials:

the galactic origin of the molecules of the alive.

To the atoms at the organic molecules.

The sun forms in a region rich in complex molecules.

We are in the region of the galaxy where to be born the sun, there is 5 billion years. Solar system looks like then a sort of fried eggs: a central proto-sun, doubtless of spiral shape, and a acretion disc, breaking up in rings, which turn around him. This disposal is rather frequent, and numerous are the stars which are surrounded with planets or with acretion discs (b pictoris, 68 Ophiuchi, BD 31643...). It is likely that our star emitted then two material jets at the level of its poles. She constituted then a X-rays source, resulting from his magnetic activity, from shock waves sweeping solar system on forming and of varied particles establishing an intense solar wind. The present molecules then in the pre-solar cloud are more complex than one could believe him. When the sun and the stars of his generation born, they use a gas which does not contain isolated atoms but also some molecules of which are very complex! So, according to shocks, the carbon is bind with the other available atoms (especially hydrogen).

Among the present molecules, one finds some methan ( CH4), some water and the ammoniac ( NH3). These elements react between them, according to the temperature, according to equations:

These reactions occur towards the right-hand side with weak temperature (far from the sun) and in the other sense in the warmer regions.

The presence of dusts form also a substratum on which occur chemical reactions which use the energy of the stellar radiation. These dusts (0,1 mm, or smaller still), by moving closer atoms and molecules before their interaction, play the same role as the mineral catalysts (platinum sponge for example) used in chemistry, and what you find in the line of escape of your car. The formed molecules result mainly from combination with hydrogen.

Dusts form fluffy structures containing ice, silicats and carbon based molecules (hydrocarbons mainly). These particles are going to join to form two types of solid bodies: the telluric planets near the sun and, farther, the future comets core.

Among the two, the light elements chased away by the pressure of radiation of the young sun are going to condense on telluric "nucleus" to form jovian planets, provided with numerous satellites in respectable dimensions.

€ interstellar environment is a source of complex molecules.

Which are the molecules which can be synthetized in "interstellar" conditions and which we shall find at the beginning of the forming not only of solar system, but also for all the stars (several billions!) of this generation?

The observation of the current interstellar clouds allows us to bring valid answers to these questions. There are two main types of molecular clouds in the spiral arms of the Galaxy:

- Hydrogen clouds under molecular shape H2, temperature 10 - 20 K (-263 in-253 °C). They do not contain stars (so no source of energy), and not or few heavy elements. They can not be a source for syntheses of organic materials.

-Giant molecular clouds, containing H2 and CO as well as other numerous molecules and dusts. They have a nearby size of 200 L.Y., density of the order of 100 million molecules / ml and are associated to young stars of which radiation, source of energy, ends in the forming of ionized hydrogen. These clouds (called regions H2) are especially observed towards the galactic center and in the spiral arms, and the most known is Orion M42's nebula. They are among them that are born at present stars, and it is so in comparable environment that formed our star, the Sun.

The study of the constituents of the interstellar clouds started at the beginning of the last century, with the works of J. Hartmann. One can identify the present molecules in these clouds by their spectrum, that is the way their constituents of which absorb or re-emit the light of the stars which enlighten them. Every atom of an element absorbs and re-emits light in a characteristic way. So, molecules containing these atoms have toward the light a characteristic behavior which depends mainly on their chemical composition, but in which their shape and their orientation can also play a role.

One can also study interstellar environment in laboratory by reconstituting him and by analyzing the molecules which one manages to recreate there. This more experimental approach is led, for example , by Mayo Greenderg's team to Leyde's university, in Netherlands.

The observation of the big molecular clouds which strew the disc of our galaxy (and others!) allowed to astronomers to go from surprise to surprise. Although at the time of the forming of the sun content in heavy elements must be a little less raised, the composition of these clouds gives us an indication on the contributions of extraterrestrial molecules which were able to be present at the beginning of the history of our planet. There is around thirty years, many thought that the interstellar clouds contained only a ten most simple molecules, limited to 2 or 3 atoms. More pushed studies upset these conceptions: one counted at present more than 120 different molecules, of which especially some water, some methan, the ammoniac, the carbon monoxyde, the free radicals (OH, H3 +) participating at the syntheses of the other molecules, but also much more complex molecules, whose only the astrophysician Fred Hoyle had anticipated existence in the end of the 70s.

Marginalized in the astronomical community because of its ideas on the origin of the universe (he opposed, almost lonesome , to the model of the big bang today accepted everywhere), the deductions of this last one met, at best, only a polite interest. At present, there is not one month without one announces the detection of more and more complex molecules in the interstellar environment, molecules than until now astronomers had considered limited to the Earth...

So, hydrocarbons (HC3N , CH3CN , CH3CHO), alcohols (methanol, ethanol) and by-products of the cyanide form easily: the ices covering silicats, containing water, methanol and hydrocarbons, play the role of molecular trap which prevents the dispersal of the molecules which can recombiner under the effect of the U.V. stellar radiation. A gas cloud situated near the galactic center even revealed to contain glycolaldehyde ( C2H4O2), also found in meteorites, which can by chemical reaction lead to glucose or ribose. In laboratory, one was able to obtain cyclic molecules of quinone, which are at the base of the plant pigments, and which were identified afterward in molecular clouds. The isopren CH2=C ( CH3) CH=CH2 is a molecule which forms easily in the interstellar environment. By polymerizing, she gives birth to the terpenes which constitute the base of molecular structures establishing the first membranes capable of isolating the cells of their environment.

As the atoms of carbon join easily to form hexagonal cycles, he can also form molecules of complex shape, that one regroups under the term of fullerens: they are assemblies of hexagonal carbon cycles curling up in the space to form spheres (" buckyballs " ), tubes or volumes of more irregular shape. These molecules, that one finds on Earth in the exhaust gases of Diesel engines, present remarkable physical characteristics which give them a big interest for the elaboration of alive structures: they can constitute "jails" trapping monomer or more simple molecules; or to join between them to form some flakes acting as catalysts, accelerating chemical reactions involving other molecules.

The process of forming of the interstellar molecules sometimes can require more than 1500 intermediate stages, but often involves ion H3 + which react with the other present elements in the interstellar environment. The severe cold who there and stellar radiation allow the ice to adopt a conformation, called amorphous, in which it possesses the properties of a very sticky liquid allowing varied chemical reactions. Syntheses can so come true on and inside the grains of interstellar material and end in the forming of amino acids: besides the glycin, serin and the alanin discovered in molecular clouds, 17 amino acids were identified in contemporary meteorites of the time of forming of solar system, as well as quinones and amphiphile molecules comparable to those establishing the cells membranes. It is likely that the radiations to which were subjected the interstellar amino acids is at the origin of the current imbalance among the various enantiomers of these in the living beings. As we shall see him, this imbalance is a "fossil" indicating us the extraterrestrial origin of these molecules.

The specialist corner One can wonder that in zones with low temperature ( interplanetary and interstellar space), chemical reactions can occur easily. In fact, in these conditions, the kinetics of reactions do not obey any more Arrhenius's law: low temperature slows down molecular and atomic excitement, what allows forces electrostatics, important because the interstellar molecules (free radicals) are often charged to act to move closer to elements to react. The experimental studies of the molecules forming in the interstellar environment show that from molecules present to the gas state between solid particles it forms successive layers of materials containing numerous carbonated molecules: there is annexation of the carbon in a matrix containing some water under the shape of amorphous ice of high density. Energy necessary for reactions is supplied with U.V. photons and these can a start a reheating of the speck of dust of several dozens Kelvin's . This reheating is going to modify the distribution of hydrogens bonds in the amorphous ice, which density decreases, favoring the movement of the dissolved molecules and their interactions. If reheating is too important, it can lead to a partial evaporation of the organic materials of the grain following a chain reaction involving the free radicals trapped in the matrix.

The molecules of primitive solar systems

Silicates composing the specks of interstellar dust contain an important proportion of metal sulphides, oxides and clays. Due to their fluffy structure, these composed offer an enormous surface with regard to their volume, on which the syntheses can develop. These composed have a big importance for the continuation of events. Let us reason at the moment about the example that we know best, that of our solar system.

There is 5 billion years, while forming planets, comets and asteroids, the protosolar nebula already contains so numerous organic molecules: water, alcohols, amino acids as well as potential catalysts. Temperature, density and pressure decrease as one goes away from the Sun. Essentially, planets formed and are cooling. The telluric planets, warmed under the triple effect of their contraction gravitationnelle, the destruction of their radioactive elements and the impacts of meteors, see their heaviest elements collecting to form a nucleus, surmounted with a fluid coat and with a crust during solidification, surrounded with an atmosphere mainly composed by carbon dioxide and a little nitrogen, doubtless very thick atmosphere. The origin of this gas envelope is double:

-Micrometeorites, often of cometary origin, bombarded extensively the fluid basaltic crust of the planet. They brought to it volatile elements. These gases trapped in the deep rocks, under pressure, get free during volcanic effusions. This process is even observable today, and leads in one massive release of carbon dioxide and steam.

-The impacts of meteors but especially cometary nucleus vaporize on the surface of the planet big quantities of water as well as the other elements as the nitrogen, for example. This last one, finely analyzed in samples taken from the Moon, revealed to result, under "organic" shape (bound to carbon) or of nitrures, to the most external regions of young solar system.

The planets forming has nothing indeed of a quiet process of condensation: the young planets undergo an intense bombardment of which, in our close environment, the lunar surface marked with craters and with lava fields carries this track. Our satellite results also from an impact: there is 4,5 billion year, an enormous body, of the Mars size, collided tangentially the Earth in so violent a way as a big part of the external envelope of our planet was extracted and put into orbit, giving birth to the Moon. In this occasion, our planet lost its first atmosphere, of which only remains today the water bound to the rocks of the ground mantle. The other impacts so spectacular are probably involved in the slowing down of the rotation of the planet Venus or the modification of the Martian atmosphere. Until 3,9 billion years, collisions were extremely frequent in the solar system, taking sometimes the aspect of indescribable disasters: between 4 and 3,85 billion year, the Earth underwent 20 times more impacts than the Moon. In spite of the atmosphere, 10 - 20 of these impacts had to dig craters of more than 2000 km diameter and produced enough energy for vaporize all the water of the planet! Afterward, while volcanic activity released numerous gases, of which some steam, comets also enriched the Earth in water while asteroids and especially micro-meteorites bring carbonated compound and water. It was the same for all the telluric planets (excepted Mercury, excess near the Sun). This bombardments period continued during more than 300 million years during which the only micro-meteorites brought to our planet more than 100000 billion tons carbon , what represents more of 100 times the quantity of this element at present used in the Earth biomass.

The duration of geologic periods has to however lead us to qualify our comments: it would not be necessary to believe that term "bombardment" implies major shocks with big meteors once a day, far from there! T.S Culler's works, of the university of California, based on the analysis of the lunar materials returned by missions Apollo, showed that we are even at the moment in a period of strong meteor impacts ; of a comparable intensity, even superior, to the one that struck our planet at the dawn of the life... Human history is too brief so that we realize it, but these impacts were able to, we shall see it, to play an important role in the history of the life. As for life appearance, we have to conclude from it that she did not come true in an environment having million years of tranquillity, on the contrary!

We are going to be then confronted with a problem: " the ceaseless bombardment " which underwent the Earth seems to coincide with the development of the life: far from benefiting quiet oceans, it seems although life began in chaotic conditions, under a real carbon shower from the interstellar environment, disturbed periodically by impacts capable of volitilizing all the water of the oceans of the planet! This problem is often chastely evacuated by determined biochemists who have, regrettably , no knowledge in astronomy...

Where discovers, in astonishment, a too old life on a too young planet.

Our consideration of astronomical realities begins to carry its fruits: during years, the biochemists did not care about the cosmic neighborhood of the primitive Earth: they supposed that, apart the composition of the atmosphere, the other factors of the environment were similar those us to know. They elaborated so legally various theories for origin of life most of which require quiet oceans (or puddles tepid) where pre-biotic molecules assemble quietly in the course of the million years... It is necessary to say that the found fossils gave us for the life a aging not to more than 3 billion years. This vision considerably modified these last years, when the discovery of tiny fossils first repelled the existence of first directly detectable life forms in 3,5 billion years, then in 1996 when the study of the sediments of the Akilia island, in Greenland, showed that life was already developed there is 3,85 billion years!

Which are the testimonies of a life in this time?

They are interesting, because based on a technique which we shall find repeatedly, and which carries the barbaric name of isotopic fractionary. What is it? For the same atom, there are different "variants", calling isotops. They are differentiated with the number of neutrons contained in the atoms nucleus, which has no influence on their chemical properties, but on their mass: more an atom contains of neutrons, and more it is heavy. The most famous isotope is in doubt the 14 carbon (14C) which is used to date the prehistoric objects. The main thing of the carbon is under the shape 12C, but it exists also , because of the stellar origin of this element, the 13C in our environment. There where things become interesting, is that the living beings do not use indifferently the two carbon isotops : their metabolism uses more easily the light 12C than the heavy 13C. The living substance is so enriched in 12C, and this enrichment persist after the death of bodies, in the sediments which contain them: it is the oldest known chemical "fossil" indicating life, because one does not know process not appealing to living beings and provoking such an isotopic enrichment. The report 12C / 13C can even supply us indications on the type of metabolism which was used by the living beings, because it differs as they used the energy of the light (photosynthesis) or that of compound minerals (as the methane for example).

Life would have developed so in a chaotic environment, on Earth regularly struck by impacts of meteors and comets, subjected to a sun brilliance different from the one that we know.

Indeed, at the Earth youth corresponding that of our Sun. During the time of the solidification of the Earth, our star had the aspect of a mass of gas in contraction, 30 times more voluminous as at present, with a surface temperature of 3500 ° C and, even though nuclear reactions had not yet begun in its heart, it was 150 times more brilliant as at present! When life began to leave tracks on Earth, just after the period of the most violent bombardments, our star had begun to merge the hydrogen and its luminosity had strongly decreased, at the point to be appreciably lower (15 %) than today. Our planet received so less solar energy in the past.

Radiations emitted by stars as the Sun and absorbed by the interstellar dust are of a big importance because they can modify the geometry of certain molecules which will participate in the elaboration of first forms of life.

Recent results obtained by the observation and the analysis of zircon (this mineral, almost indestructible, is used in jewellery to imitate the diamond) even showed that there was already some liquid water, and a differentiation between continents and oceans there is 4,3 in 4,4 billion years, is hardly 200 million years after the forming of the planet, in full period of exogenous bombardments.

Another problem arises when one examines the oldest "direct" fossils: the most old observable forms of life are cyanobacterias, already very evolved micro-organisms, and even too much evolved! Indeed, the genetic analysis shows that these organisms would have had no time to develop during the time which separates them from the forming of the planet! So as to suppose the existence of a very high rate of mutation accelerating strongly the speed of the evolution (that remains possible), one is once again in the presence of a blatant contradiction between evolved life forms found on a too young planet to have allowed their evolution!

We have so only to pull the conclusions which are necessary, to explain why life was able to seem so fast and in such conditions. Let us examine the different ways of the possible, let see us of what are made the living beings and let us compare with the various theories which were elaborated to explain the emergence of the life on our planet.

Why did life seem so fast?

There are several manners, connected among them, for explain the speed with which first life forms appeared on Earth:

-it is much more simple to obtain life that we think. It appears easyly, in a short time.

-The replacement parts of the living beings were already available, and only assembly had made on Earth, in a fast way.

-The first living beings arrived already ready on the Earth, which they contented with sowing. We shall see that it is more difficult than it appears to reject this possibility, and than it ensues from it fantastic consequences.

Before leaving at the discovery of the possible previous history of the life, let us see which are the specificities of the living beings which we know, and which indications these last ones can supply us on their origin. We shall see that many surprises wait for us at this level, and that the advances of the microbiology are going to carry some supplementary knocks in our self respect!