The causes of the emergence, in basal Cambrian, of the major part of the current metazoa phyla, except for Bryozoa, enters within our probabilistic interaction model framework between the environmental evolution and the biological evolution.

Before analyzing the essential characteristics of the Cambrian fossiliferous "explosion", let us briefly summarize the biological events which precede this " explosion ".

In our current state of knowledge, one can fix the life origin, on the earth (approximately 4,6 billion years old), towards 3,85 billion years, dates when the first rocks and the first traces of life on the Akilia island, in the Groenland southwest (ratio carbon isotopes 12 and 13 Mojzsis 1996) are to be found.

The archaean fossils are of two types: 1) microfossils: procaryotic primitive bacteria and primitive filamentous cyanobacteria (Warrawoona Group in Western Australia 3,5/3,4 B.Y.).; primitive bacteria and unicellular cyanobacteria (Big True Group in South Africa 3,4 B.Y.). 2) stromatolites made up of calcium carbonate (Warrawoona Group, probably towards 3,5/3,4 B.Y.; Pongola Supergroup and Bularvayan Group in South Africa 3,1/2,8 B.Y.).

As during the Archaean one, Proterozoic fossils (2,5/0,543 B.Y.) are of two types: microfossils and stromatolites.

In lower Proterozoic (2,5/1,6 B.Y.): Guntflint Chert (North America, 2,1/1,8 B.Y.): various advanced procaryotic bacteria and filamentous cyanobacteria; Fortescue Group (Australia Western, 1,8 B.Y.): various advanced procaryotic bacteria and filamentous cyanobacteria (size of approximately 10 microns); the stromatolites become more abundant (Fortescue Group, 1,8 B.Y.)(FreemanLynde 1998). From 1,87 B.Y., eucaryote microfossils evolve. They differ from the procaryotes by larger dimensions (40 to 60 microns on average), thicker cellular walls, a core and chemical specific compounds (Butterfield 1998). Recent search (to be confirmed) would have revealed the eucaryote cells presence there is 2,7 B.Y. (Roger Summons 1999).

In middle Proterozoic (1.6/0,9 B.Y.), life diversifies; microfossils: first Acritarchs appearance (planktonic eucaryote algae cysts), multicellular algae (carbonated prints; possibility in Little Belt Mountains Montana 1,4 B.Y.; they extend to Spitzberg, in China, in the Indies, in Canada 1,2/0,7 B.Y.). With Neoproterozoic (0,9/0,543 B.Y.), metazoa first traces appearance (tracks, burrows) then of the ediacaran fauna (0,565/0,543 B.Y.). The stromatolites are very abundant. In Phanerozoic, they become restricted and confined in the hypersalin media (Freeman Lynde 1998).

If the biological events are summarized, one notes that, over a long period (approximately 2 B.Y., of 3,8 with 1,8 B.Y.), life on earth was a procaryotic unicellular microbial life. Then, for a long following period (of 1,8 in approximately 1 B.Y. Butterfield 1998) a unicellular eucaryote life (Holland Bengtson 1994) developed, jointly with a crisis of many groups of procaryotic bacteria. Since 1 billion years approximately, traces of metazoa fossils or fossiliferous traces appeared. The multicellular algae appear earlier (1,4/1,2 B.Y. Butterfield 1998).

In final Neoproterozoic, the Ediacaran fauna appears between 0,565 and 0,543 B.Y. Initially discovered in the Ediacara hills (Australia), the Ediacaran fauna was found since then in more than 30 localities, in all the continents, except the Antarctic. According to the molecular data, the metazoa would derive from unicellular organisms connected to choanoflagellates, going back to approximately 1 B.Y.. The oldest traces of the fossils of worms would be old approximately 0,565 B.Y. Researchers of Tübingen, Yale and Jadovpur (India) found tunnels which would have been dug, there is approximately 1 B.Y., by organisms similar to worms. This assumption, supported by Seilacher and its colleagues (1998) is nevertheless discussed. The Spongiae (phylum of Porifera) found in the Neoproterozoic sediments are the most primitive metazoa organisms.

Ctenophora and Cnidaria (jellyfishes, seaanenomes, corals), diploblastic primitive metazoa like Porifera, have, like them, 11 types of specialized cells whereas the worms, triploblastic organisms more evolved, equipped with a coelome, have 55 roughly of them. The vendian fossil organisms major part, besides some hard skeletons bits, are organisms with soft body. For the 2/3, it is of Ctenophora and Cnidaria; for 1/4 of the flat worms. And some primitive arthropods. Vendian fauna was interpreted like a different reign from the animal kingdom (McMenamin supported by Seilacher, disputed by Runnegar 1997), like made up of protozoa, lichens (Retallack 1994), like a group close to the Cnidaria, etc... Ediacaran fauna was practically extinct at the Precambrian and Cambrian border, there are 0,543 B.Y. but there is not solution of continuity between Neoproterozoic and Cambrian (Erwin, Valentine, Jablonski 1997). The first more advanced metazoa traces are located at this level (Trichophycus pedum International Subcommission one Cambrian Stratigraphy 1997), before the S.S.F.

In lower Cambrian many tiny metazoa with shells (approximately 1 to 5 mm) and with low diversity appear, the S.S.F. (Small Shelly Fossils). They are widespread in the world (Australia, India, China, Mongolia, Siberia, Iran, North America, etc...). This appearance and that of ediacaran fauna, starting from 0,565 B.Y., represent 2 major pulsations which characterize the emergence macroscopic animals at the Neoproterozoic end and lower Cambrian, followed by the " cambrian explosion " (Grotzinger 1998). In basal Cambrian, Anabarites, Protohertzina and primitive molluscs appear. As basal Cambrian progresses, the organisms with skeletons and distinct members become increasingly abundant. One thus often spoke about " tommotian explosion ". In Tommotian (0,530/0,527 B.Y.), new phyla are listed: Brachiopods and Porifera (Archaeocyaths reefs), Lapworthella, many phyla with soft body. On the floor Atdabanian (0,527/0,525 B.Y.), the Arthropods (Trilobites which will dominate Cambrian) and primitive Echinodermata emerge. In middle Cambrian, the Burgess Shale fauna (towards 0,520 B.Y.) will be made up of animals with soft body and animals with hard parts. The modern marine phyla majority are represented there (except Bryozoa). One deducts approximately 37 (Erwin, Valentine, Jablonski 1997) among whom of Porifera, Annelida, Priapulids, Onychophora, Molluscs, Arthropods, Coelenterata, Echinodermata, Chordata and of many extinct phyla with soft body (Pamela Gore 1997). Perhaps one estimates at one hundred the phyla number appeared in Cambrian (Valentine; Clark 1997). At Cambrian, diversification is at the phyla level and not genera (approximately 400 genera in Cambrian for 1500 in Ordovician, Pamela Gore 1997). Cambrian arises thus more like a period of evolutionary experimentation, metazoa radiation and lines diversification that like a period of speciation. Metazoa Cambrian radiation constitutes one of the history of the live most significant events with the apparition of most of the plans of organization associated with the most essential innovations with the living organisms (mineralized skeletons, intestines, jaws, gills, etc...)..

One can schematize the " cambrian explosion " in 4 significant periods:

1) The vendian period, former to the cambrian explosion and which announces it (0,565/0,543 B.Y.) characterized by the sudden emergence of the metazoa with soft body.

2) The S.S.F. appearance (Small Shelly Fossils) characterized by many tiny organisms with shells, spicules and a low diversity (0,543/0,530 B.Y.).

3) Tommotian-atdabanian radiations (0,530/0,525 B.Y.) with the emergence of most modern lines and many extinct descendants lots (with hard skeleton and soft body) and their biological innovations.

4) The Burgess Shale fauna (towards 0,520 B.Y.) who prolongs tommotian-atdabanian radiations and the evolution of the lines which appeared there (with hard skeleton and soft body)..

The " cambrian explosion " causes can be classified in two categories:

a) Intrinsic causes: genetic flexibility, metazoa Hox genes complexification (1 in the sponges, 6 in the flies, 8 in the arthropods, 38 in the mammals... Erwin, Valentine, Jablonski 1997), existence of free ecological niches, apparition of predation.

b) Extrinsic causes, environment modifications : oceans chemical composition (carbon, sulphur), nutriments level variations, phosphogenese, limestone formations, increase in the oxygen rate in the atmosphere and oceans.

A purely intrinsic cause can be excluded. The " cambrian explosion " concerning all taxa simultaneously would seem incomprehensible in the case of a purely intrinsic cause.

We propose to integrate the " cambrian explosion " phenomenon within the probabilistic interaction model framework between the environmental evolution and the biological evolution.

We schematized the " cambrian explosion " in 4 significant periods. We will try to correlate, according to our model, the environment stimuli variations and their probabilistic influence on the living organisms. Among the environment stimuli whose influence on the organisms is dominating and who evolved considerably during about thirty million years of the cambrian process (of approximately 0,549 B.Y., top of ediacaran fauna Grotzinger 1998 to 0,520 B.Y., Burgess Shale fauna), we retain two essential ones: chemical elements O and Ca.

The influence of the level of stimulus molecular free oxygen in atmosphere or dissolved in the oceans is of primary importance in the living organisms physiology and morphology and in particular of the metazoa. We will study, in chapter IX, the interaction between the PO2 P.A.L. evolution (PO2: molecular oxygen partial pressure compared to the P.A.L. Present Atmospheric Level) since the earth origin and the biological evolution since the life origin.

Let us point out the complex relations which exist between the metazoa and free molecular oxygen. In the aerobic organisms cells, molecular oxygen is the last chain link of the electrons conveyors, the respiratory chain. The aerobic organisms react (or use) to oxygen present, in dissolved form in water or gas form in the atmosphere, according to two principal respiratory modes: watery breathing (cutaneous and branchial) and air breathing (cutaneous, trachean and pulmonary). Certain devices mix watery and air breathings, " tracheobranchiae " of watery insects, larva of Ecdyonurus (Epheremoptere), Phanorbes " water lungs " (Basommatophorae) (Turquier 1994). Just as environment calcium or other chemical or physical parameters (iodine, sulphur, carbon, electromagnetic or sound waves, etc...), oxygen can be regarded as an environment stimulus to which the organisms react. According to our model, the animal evolution is correlated, chronologically, in a probabilistic way, during geological times, with the free molecular oxygen content present in the hydrosphere and the earth's atmosphere. PO2 P.A.L. having passed, the Archaean one at our days from 0 to 100 %, concomitantly with this stimulus variation of free molecular oxygen, the organisms reaction evolved, this evolution appearing primarily by morphological, physiological innovations and in particular of the diversified respiratory systems.

The aerobic eucaryote cells appearance towards 1,8 B.Y. implies a PO2 P.A.L. at least of 1 % (Chapman, Schopf 1983), that of the metazoa, at least of 7 % PO2 P.A.L. (Raff). The dissolved oxygen in water use, for the aquatic animals breathing, is conditioned by many parameters, as well physical as anatomical, physiological or biochemical. Among the physical parameters, let us quote the oxygen solubility coefficient in water (or capacitance Dejours 1994) (34,1 ml l-1 in distilled water with 15 ° C balanced against air) and the temperature (Krogh 1941). The oxygen diffusion in the tissues, which obeys the 1st Fick law, depends on the distance parameter L between 2 points A and B, of the various compartments contact surface, of their partial pressure difference, the Krogh (K) diffusion constant. The parameter L importance was formalized by Harvey (1928). The simple diffusion of oxygen in animal tissues cannot be effective, according to the Harvey modelisation that for animals of small size or when forms or special devices (foliaceous aspect, developed pseudopodia, complicated surfaces) make it possible to respect the Harvey constraints (Turquier 1994). The cutaneous respiration can increase its effectiveness by significant improvements (external convection renewing water: lashes, whips, appendices; internal convection: circulatory apparatus, blood or hemolymph, respiratory pigments). These multiple devices can be combined or not in the species with branchial or pulmonary breathing. Branchial breathing, with its essential characteristics (great exchange surface, thinned epithelium, external convection, branchial and intern ventilation, coelomic or blood circulation, respiratory pigments ), constitutes, in the aquatic animals, the most effective device to absorb oxygen, for a big size animal. Whereas, in the Invertebrates, the external gills are widespread, they are relatively rare and often transient in some Fishes larvae (Polyps, Dipneustes) and in the Amphibia (Turquier 1994). The oxygen quantity dissolved in fresh water and sea water is much weaker than that which exists in the air, 5,79 ml l-1 in sea water and 7,22 ml l-1 in fresh water with 15° in balance with the atmospheric air, for 209,5 ml l-1 in the atmospheric air (Schmidt Nielsen 1977). The animals with air breathing use, to absorb the oxygen atmosphere, apart from specific cutaneous devices (adult Amphibia, terrestrial Oligochaeta, etc...) primarily the trachean and pulmonary systems, independently or combined (mixed breathing of the Spiders).

The importance of stimulus O2 varied during geological times. In chapter IX, we tried to establish an approximate scale, within the current knowledge limit, the increase in the partial free molecular oxygen pressure, compared to the present, PO2 P.A.L., since the earth origin. The data we have being full of uncertainties, one must regard this scale levels more as probable magnitude orders than like rigorous PO2 rates.

The geochemical data provide us indications on the oxygen rates, PO2 P.A.L. since the earth origin until Phanerozoic.

In Hadean (4,6/3,9 B.Y.), the earliest atmosphere of the earth and the intense volcanicity degazifications of this time do not contain free molecular oxygen.

The nitrogen isotopic composition in the microfossils, towards 3,4/3,5 B.Y., just as that of carbon in this period sediments indicate as O2 missed almost atmosphere (Beaumont and Robert 1998).

From middle Archaean to middle Proterozoic (2,9 to 1,6 B.Y.), 3 principal phases proceed:

1) The uraninite (UO2) and pyrite deposits (FeS2) to Precambrian, until towards 2,3 B.Y., indicate atmosphere not oxydative conditions with PO2 P.A.L. weak rates, probably going from 0,0005 to 0,005; between 2,25 and 2,05 B.Y., one notes a sudden increase in the PO2 P.A.L rate (Holland 1998).

2) B.I.F. deposits (Banded Iron Formations), alternating magnetite rich and low layers (Fe3O4), occur approximately 2,8 to 1,8/1,7 B.Y. with abundance peaks between 2,5 and 2,00 B.Y., coinciding with the increase, in the fossil files, of the producing O2 cyanobacteria towards 2,3 B.Y. and the uraninites disappearance. The B.I.F. deposits also indicate not oxydative atmosphere conditions.

3) The Red Beds (Red Formations) of hematite (FeO3) deposits appear towards 2,00 B.Y., indicating oxydative atmosphere conditions and lead to the B.I.F. disappearance towards 1,8/1,7 B.Y. and to the ozone layer O3 development (Levin). The increase in the molecules O2 in the atmosphere allows the emergence of the oldest known aerobic eucaryote cells, approximately 1,9 B.Y. old (Gryptania spiralis Pan Terra 1996).

Between 0,750 and 0,550 B.Y., the carbon isotopic composition suggests an appreciable increase in the PO2 P.A.L. rate before the cambrian era (Hoffmann, Kaufman, Halverson 1998). The geochemical data clearly indicate a rise in the PO2 P.A.L. rate right before the Vendian macroscopic animals appearance. They also show a phytoplankton dynamic evolution at the Precambrian/Cambrian border (Knoll 1996).

Biological arguments consolidate the correlation between the PO2 P.A.L. level and the evolution of the living organisms. Many data were compiled (Rhoads, Morse 1971) on the relation between the dissolved oxygen level and the benthic fauna presence in the basins on low oxygen level, in the Black Sea (Bacescu 1963), the Gulf of California (Parker 1964), the Santa Barbara Basin (Emery, Hulsemann 1961) and the San Pedro Basin (Hartman 1955, 1966). They showed that faunas can be classified in three facies correlated at different PO2 P.A.L. rates. With a value < 0,1 ml/l (approximately 0,01 PO2 P.A.L.), the marine sediments are benthic metazoa primarily deprived ; for a value ranging between 0,3 ml/l and 1 ml/l (between approximately 0,03 and 0,10 PO2 P.A.L.), benthic faunas are made up mainly of small soft body species; when the level is higher than 1 ml/l (approximately 0,10 PO2 P.A.L.), faunas are relatively varied and made up many species which secrete limestone skeletons. Similar relations were observed in the Saanich creek in the Vancouver Island (Tunnicliffe 1981). Rhoads and Morse proposed that these relations between the emergence of the faunas and the dissolved oxygen rates in these basins are regarded as analogues with the metazoa groups development during the Proterozoic last stages and the Phanerozoic first stages.

This interpretation is in phase with the probabilistic interaction model between the environmental evolution (increase in the PO2 P.A.L. rates) and the biological evolution (procaryotes, eucaryotes, metazoa with soft body, metazoa with exoskeleton, etc...) concomitant, developed with chapter IX. 

According to this model, as the PO2 P.A.L. rate grows in the atmosphere and the oceans, to final Neoproterozoic, as it confirms all the geochemical and biological data which we have just examined, new possibilities and new suitable reactions, arise in particular metazoa. Only most probable, i.e. those which have the most chances (in the mathematical meaning) will continue.

Thus, the PO2 P.A.L. rate < 0,10 approximately reached at the first significant period that we quoted, the vendian period, allows and supports the metazoa emergence with soft body and cutaneous watery breathing, Ediacaran fauna made up 70.% of Coelenterata (especially Cnidaria) often with medusoids forms. These structures made it possible at these organisms to absorb oxygen, by epithelial diffusion, with weak concentrations of about 0,07 PO2 P.A.L. (Rudolf, Elizabeth Raff). Organisms with flattened forms like the marine worm Dickinsonia (1 m length for a maximum thickness of 6 mm) could have taken enough oxygen at rates between 0,06 and 0,10 PO2 P.A.L. (Bruce Runnegar 1982). This rate also allows the collagen production necessary to the conjunctive tissue constitution (Towe 1970).

The second significant period that we distinguished is the appearance towards 0,543/0,530 B.Y. of the S.S.F. It is rise, with a PO2 P.A.L rate > 0,10 approximately, everywhere in the world, of the calcium phosphate and carbonate shells organisms production. The major part of the probabilistic favorable factors (six out of seven) to the calcium biomineralization are then joined together: 1) hot and not very deep water higher level of supersaturated oceans in calcium++ ions and aragonite and calcite (Holland 1998); 2) hot or tropical temperature (the continents are not assembled in Pangaea; they are mainly localised with tropical latitudes; no continents with the poles from where not glaciers; globally hot temperatures; Pamela Gore 1997); 3) orogenetic calm and weak volcanic activity; 4) pH relatively neutral; 5) favorable O2 level (for a PO2 P.A.L. level > 0,10, many species secrete limestone skeletons, Rhoads and Morse 1971); 6) intact food chain (at the PC/C limit, phytoplankton, protists, foraminifera and radiolaria, algae, etc... radiations - Jerry Lipps).

The Tommotian-Atdabanian third period (0,530/0,525 B.Y.) sees the soft body and mineralized skeleton metazoa phyla explosion. The probabilistic factors favorable to the calcium biomineralization which engaged the S.S.F. emergence play full. In parallel, increase in PO2 P.A.L. at that time, attested by the geochemical and biological data, makes possible the larger organisms constitution and makes it possible to circumvent the first Fick law and the Harvey constraints for the cutaneous respiration. Branchial breathing, with its characteristics (see higher) gives to the big size animals new effective devices to collect oxygen.

The fourth period, the Burgess Shale fauna, develops all the potientalities and the probabilities appeared in basal Cambrian and amplifies the metazoa phyla radiations.

Thus, two environment essential parameters, in basal Cambrian, will allow the release of a phyla: "explosion" : 1) probabilistic factors favorable to the calcium biomineralization for the mineralized skeletons organisms (six out of seven) 2) the threshold reached by PO2 P.A.L. (probably > 0.10) allowing the larger and more advanced organisms construction than the epithelial breathing animals. It is the conjunction, at this earth history period , of the two parameters necessary to the rise of the macroscopic fauna which allows, by the possibilities and supports, by the probabilities, the basal Cambrian "explosion". The combined existence of these two factors, missing at the Precambrian, thus explains the release, at this time and not two hundred million years front or after, of the "cambrian explosion" which relates to simultaneously all the phyla, as well for the mineralized skeletons construction as for the biological innovations (intestines, external or interns gills, jaws, etc...).

It is probable, that apart from these two "cambrian explosion " fundamental parameters that we have to try to highlight, other factors could play a role which is not negligible: 1) the trophic calcium chain intensification is corroborated, on the one hand by the superior Precambrian and Cambrian significant phosphate deposits indicating one phosphogenese general period (Cook, Shergold), on the other hand by the " carbonate belts " which one finds, at the same time, along several continents (Palmer); 2) flexibility and genetic complexifications could develop with leisure in still virgin ecological niches. These elements allowed the blossoming, in Cambrian, of all open possibilities primarily by the biomineralization factors and the PO2 P.A.L. threshold (perhaps one hundred phyla) and the perennisation, by the probabilities law, only the phyla profiting from the most favorable biological characteristics.