In: Evolutionary and Molecular Biology: Scientific Perspectives on Divine Action. R. J. Russell, W. R. Stoeger and F. J. Ayala, Editors. Vatican City State/Berkeley, California: Vatican Observatory and the Center for Theology and the Natural Sciences, pp. 79-99.

THE PHENOMENON OF THE EUKARYOTIC CELL

Julian Chela-Flores
International Centre for Theoretical Physics,
Miramare P.O. Box 586; 34100 Trieste, Italy and

Instituto Internacional de Estudios Avanzados,
(Universidad Simon Bolivar)
Apartado 17606 Parque Central,
Caracas 1015A,Venezuela

Abstract. In the context of cosmic evolution we present a brief discussion of the appearance of the first steps in evolution that led to intelligent life on Earth. Special attention is given to chemical and biological evolution. We emphasize the hypothesis that provided the planets of a given solar system have the appropriate volatiles (particularly water and oxygen), not only life, but eukaryogenesis (i.e., the appearance of nucleated cells) is bound to occur. Further, this conjecture has the merit that it is subject to experimental verification. In fact, the question of the possible existence and degree of biological evolution of microorganisms in the Solar System has been raised in the past, in spite of the negative results of the Viking Missions. In the special case of Mars the possible occurrence of eukaryotes during an earlier epoch was raised prior to the announcement of some preliminary evidence for prokaryotic life on Mars, a question that has not been settled yet. We maintain that there is a second environment in our own Solar System, the Jovian satellite Europa, in which the hypothesis of the ubiquity of eukaryogenesis may be tested in the foreseeable future. We discuss this possibility in some detail and mention a specific mission that, if approved, could in principle test the degree of evolution of the putative Europan biota.

1. From cosmic evolution to chemical evolution

1.1. INTRODUCTION

Long before the advent of science and philosophy, theologians were raising questions that we still cannot answer fully. Some of the deepest questions that have persistently remained with us since biblical times are: What is the origin of the universe? What is it made of? What is its ultimate destiny? How did life in general, and humans in particular, originate? Are we alone in the cosmos? The first philosophical attempts to answer these questions were made a few centuries before our own era by the ancient Greeks, notably by Thales of Miletus (who was active during the time of the Prophet Jeremiah and the destruction of Jerusalem in 587 B.C., which had been founded by King David almost half a millennium earlier). A significant contribution to the birth of science was made by another ancient Greek, Democritus of Abdera, one of the founders of atomism (who was active during the time of Ezra's reform, which, by the promulgation of the by-then-ancient Mosaic Law in 430 B.C., gave Judaism its distinctive character).
I feel that there is no better way to organize the information available to scientists, than to reconsider those questions as originally set in the antiquity, in order to appreciate, in an orderly manner, the considerable progress that has been achieved up to the present in our understanding of cosmological models and the appearance of intelligent life on Earth. In spite of the impressive progress of science, many fundamental questions remain unanswered.Therefore, before beginning our task it may be prudent to recall that Cosmology suggests that there is sufficient time available for science to progress to a stage in which further, and possibly better attempts that are now possible, will be made in order to search for answers of some of the deepest questions (mentioned above) which humans have been asking themselves from time immemorial.
Up to the present partial answers have often led to controversy between scientists, philosophers and theologians. We should be constantly aware of the limited scope of scientific method, a point that has been stressed by Bertrand Russell. Recognising a limit to the applicability of present day science, he expressed that:

almost all the questions of most interest to speculative minds are such as science cannot answer 43.

1.2. COSMOLOGICAL MODELS

Our first topic is suggested by the Book of Genesis ("In the beginning God created the heavens and the earth" : 1, 1). To approach this problem from the point of view of science, a preliminary step is to grasp the significance of the scale of time involved. For this purpose we must return to the first instants of cosmic expansion. Our sketch must be brief, as we already have available the previous review of Coyne at the 4th Trieste Conference14.
The American scientist Edwin Hubble discovered in 1929 that the velocity of recession of a certain galaxy under observation is proportional to its distance. This phenomenon is normally expressed in megaparsecs. The constant of proportionality, which is known as the Hubble constant Ho is, consequently, given by the ratio of the speed of recession of the galaxy and its distance; Ho represents quantitatively the current rate of expansion of the universe. The measurements of Ho 25 at present lie, in one case (Mould and Freedman) in the range of 55-61 km s -1 Mpc -1 and, in a second case (Sandage, Tammann, and Saha) Ho lies in the range of 68 to 100 km s -1 Mpc -1 . These results, and other current values, agree with a value of Ho in the range 65-82 km s -1 Mpc -1 (1 pc = 3.26 light years; 1 Mpc = 106 pc).
The implication of, for instance, Ho = 70 km s -1 Mpc -1 in the standard cosmological model (cf., the Friedmann model below) of an expanding universe implies an age of 9-14 thousand million years (Gyr), depending on the particular assumption we may adopt for the matter density present in the universe. Unfortunately, this state of affairs raises some difficulties, as there are galactic globular clusters whose age is estimated to be 13-17 Gyr (cf., ref. 24 for references). On the other hand, life on Earth extends back to some 4 Gyr before the present (BP); therefore, the chemical evolution scenario faces no particular difficulties with the above values of Ho .
The classical cosmological solution assumes isotropy and homogeneity in the equations of the theory of gravitation of Albert Einstein, known as General Relativity (GR) 2. Cosmological models may be discussed in terms of a single function R of time t. This function may be referred to, quite appropriately, as a 'scale factor' or, sometimes in reference to the particular solution of the GR field equations in a geometric background, the expression 'radius of the universe' may be preferred for the function R. As the universal expansion sets in, R is found to increase in a model that assumes homogeneity in the distribution of matter (the 'substratum'), as well as isotropy of space.
The functional dependence of R, as a function of time t is a smooth increasing function for a specific choice of two free parameters which have a deep meaning in the GR theory of gravitation, namely, the curvature of space and the cosmological constant. The functional behaviour of the scale factor R was found by the Russian mathematician Alexander Friedmann in 1922. This solution is also attri
buted to Howard Percy Robertson and A.G. Walker for their work done in the 1930s. Such a (standard) model is referred to as the Friedmann model.
In fact, R is inversely proportional to the substratum temperature T. Hence, since R is also found to increase with time t (cf., the previous paragraph), T decreases; this model implies, therefore, that as t tends to zero (the 'zero' of time) the value of the temperature T is large. (The temperature goes to infinity as T tends to zero.) In other words, the Friedmann solution suggests that there was a 'hot' initial condition.
As the function R represents a scale of the universe (in the sense we have just explained), the expression 'Big Bang', due to Sir Fred Hoyle, has been adopted for the Friedmann model. The almost universal acceptance of Big-Bang cosmology is due to its experimental support. As time t increases the universe cools down to a certain temperature, which at present is close to 3 K. The work performed during the 1960s by the engineers of the Bell Telephone Company Arno Penzias and Robert Wilson was rewarded with a Nobel Prize in Physics in 1978. They provided solid evidence for the "T = 3 K" radiation, which may be confidently considered to be a relic from the Big Bang.
Andre Linde envisions a vast Cosmos in a model in which the current universal expansion is a bubble in an infinitely old 'superuniverse'. In other words, in this model the universe is assumed to be a bubble amongst bubbles, which are eternally appearing and breading new universes. The scale parameter R evolves as a function of time t in such a cosmology, but as in the earlier model of Alan Guth, the Linde model differs from the Friedmann solution in the first instants of cosmic evolution. A word of warning, depending on the way the word "standard" is used, both the Linde and Guth models can be considered standard Big Bang models. Indeed, Guth proposed the original inflationary model which solved many of the problems inherent of the preliminary Big Bang models. In fact, the inflationary universe, as we are realizing, is a class of Big Bang models that incorporates a finite period of accelerated expansion during its early development. The inflation process releases a large amount of energy retrieved from the vacuum of spacetime. It should be pointed out that Linde also assumed inflation.

1.3. THE BIRTH OF THE STUDIES OF THE ORIGIN OF LIFE.

Science today aims at inserting biological evolution in the context of cosmic evolution. We will mainly attempt to convey the idea of an ongoing transformation in origin of life studies. This progress in understanding our own origins began about three decades ago, triggered by the success in retrieving some key biomolecules in experiments which attempted to simulate prebiotic conditions. Some of the main experiments of the 1950s and early 60s were done by organic chemists including Melvin Calvin, Stanley Miller, Sidney Fox, Juan Oro, Cyril Ponnamperuma and their co-workers.
These efforts have led to view the Cosmos as a matrix in which organic matter can be inexhorably self-organized by the laws of physics and chemistry into what we recognise as living organisms. However, in this context it should be stressed that chemical evolution experiments have been unable to reproduce the complete pathway from inanimate to living matter. In particular, the prebiotic synthesis of all the RNA bases is not clear and cytosine, for instance, may not be prebiotic and it may even have been imported from space . This aspect of chemical evolution has been repeatedly pointed out by Robert Shapiro 53. Thus, at present the physical and chemical bases of life that we have sketched remain a strong possibility, but further research is still needed.
Originally the subject began to take shape as a scientific discipline in the early 1920s 38, when an organist chemist, Alexander Oparin applied the scientific method of conjecture and experiments to the origin of the first cell 40, thereby allowing scientific enquiry to shed valuable new light on a subject that has traditionally been the focus of philosophy and theology.

1.4. THE GROWTH OF THE STUDIES OF THE ORIGIN OF LIFE.

The still ongoing second large step in the development of origin of life studies consists of taking the subject from the laboratories of the 'card-carrying' organist chemists, where Oparin had introduced it, into the domain of the life, earth and space scientists. In fact, the subjects of Exobiology and Bioastronomy have come of age due to the space missions of the last 30 years (Mariner, Apollo, Viking and particularly the most recent one, Galileo, which is currently sending valuable information on the Jovian system). Many further missions are being planned for the rest of this decade, and the beginning of the next century
New areas of research have come into existence as a consequence of this change of emphasis, such as planetary protection, whose aim is to prepare ourselves in advance for the retrieval of samples from Mars and Europa in the missions that may be scheduled, in the case of Mars, as early as the year 2003.
The experiments and sample retrieval from Europa are in the process of discussion. Indeed, a NASA and National Science Foundation (NSF) meeting to discuss such matters was organized in 1996 (Europa Ocean Conference, San Juan Institute, San Juan Capistrano, California). Planetary Protection has been pioneered by Donald DeVincenzi at NASA-Ames, John Rummel at the Marine Biological Laboratory and Margaret Race at the SETI Institute.
A second discipline that has arisen from the exploration of the Solar System is comparative planetology 33, which is needed in the formulation and development of devices which in principle may allow the search for Solar System relics of the earliest stages of the evolution of life.
The problem of the search for extraterrestrial life may be approached in order of increasing complexity: Alexandra MacDermott, from the University of Cambridge, has led in the search for extraterrestrial homochirality, SETH. (Homochirality refers to single-handedness observed in the main biomolecules, namely, amino acids, nucleic acids and phospholipids.) Secondly, we have argued in favour of the search for extraterrestrial single-celled organisms of a nucleated type. (For a discussion of several types of microorganisms we refer the reader to Sec. 2.3 below, in which the word of Greek origin 'eukaryote' shall be assigned to such nucleated cells. The search for such organisms shall be abbreviated as SETE, the search for extraterrestrial eukaryotes. )
Finally, at the other extreme, we have the search for extraterrestrial intelligence (SETI), a time-honoured subject that was pioneered by Frank Drake in the early 1960s 13, 14. The eventual success of SETI may depend on the adoption of truly significant enterprises, such as the extension of SETI to the far side of the Moon in the 100 km diameter crater Saha, as advocated by Jean Heidmann from the Observatory of Paris and Claudio Maccone from the G. Colombo Center for Astrodynamics in Turin. None of the extremely ambitious engeneering projects underlying this proposal seem beyond present technological capabilities, such as the construction of a 340 km road linking laboratories in Mare Smythii (at the equatorial level and at the Moon's limb) and crater Saha (103 o E, 2o S) 19. In fact, the only limitation forseen is the will of our society to maintain continued progress by providing adequate budgets to scientists and engineers involved in the main programmes of Bioastronomy and Exobiology during the coming decade.
To continue the overview of the origin of life studies, we must first return to cosmological models.

1.5. THE ORIGIN OF THE ELEMENTS

In less than one million years after the beginning of the general expansion, the temperature T was already sufficiently low for electrons and protons to be able to form hydrogen atoms. Up to that moment these elementary particles were too energetic to allow atoms to be formed. Once 'recombination' of electrons and protons was possible, due to falling temperatures, thermal motion was no longer able to prevent the coulomb interaction from forming atoms. This is the 'moment of decoupling' of matter and radiation.
Gravity itself has induced the coalesced nuclei of stars to initiate thermonuclear reactions that created all the heavier elements. At the supernova stage these elements are expelled for later new cycles of condensation into stars and planets. The Earth arose in this manner some 4.6 Gyr BP.
It is well known that stars evolve as nuclear reactions convert mass to energy. In fact, stars such as our Sun follow a well known pathway (the main sequence) along a Hertzprung-Russell (HR) diagram, which was introduced independently by Ejnar Hertzprung and Henry Russell in the early part of this century. Many nearby stars exhibit a certain regularity (such stars are called main sequence stars) when we plot luminosity (the total energy of visual light radiated by the star per second) versus its surface temperature or, alternatively, its spectroscopic type. We may ask, how do stars move on the HR plot as hydrogen is burnt? Extensive calculations show that main sequence stars are funnelled into the upper right hand of the HR diagram, where we find stars of radii that may be 10 to 100 times the Sun radius. Such giant stars are characterized by high luminosity and red colour, and hence are called 'red giants'.
It is instructive to consider a sample of stars, emphasizing some that are now known to be much more similar to our own Solar System 27, as they have Jupiter-like planets; such a property militates in favour of life having been provided a variety of favourable environments in our own galaxy
Stellar evolution puts a significant constraint on the future of life on Earth, since the radius of the Earth orbit is small (its eccentricity may be neglected), and the radius of the Sun is bound to increase as it evolves along the main sequence in the HR diagram, thus eventually ending life on Earth. The current estimate is that in another 4-5 Gyr life may continue on Earth before the Earth's orbit is no longer in a habitable zone. However, the implication of such duration of geologic time is indeed profound for our species, as we shall suggest in the next section.

1.6. IMPLICATIONS OF STELLAR AND BIOLOGICAL EVOLUTION

Homo sapiens has evolved, according to the standard view of paleoanthropology, in less than a few million years since the last common ancestor of the hominids, who must have lived at the Miocene/Pliocene transition, just some 5 million years before the present (Myr BP). Such fast evolutionary tempo has been effective only within a small fraction of 1% of the geologic time that is still available for natural selection to continue the process of evolution of all living species, including H. sapiens. (Such time is measured in billions of years).
Science is at present unable to anticipate the future of H. sapiens; but this has not deterred fiction writers to envisage a model for a future speciation event into the imaginary eloi and morlock species of the Homo genus 57.
What is clear at present, from a combined consideration of cosmic, stellar, biological and cultural evolution, is that the contemporary stage of human development will inevitably continue to evolve. This is due to the mechanism of natural selection, which is expected to continue for at least four billion years to play its role on all living organisms on Earth. (Once again, this would be true as long as the Earth remains within the habitable zone of the Solar System.)
Although the human brain is considerably larger than that of the great apes, it will still be subject to evolutionary pressures for over a thousand times as long as the period in which natural selection rose the primitive brain of the earliest hominids (the australopithecines, who lived in the Lower Pliocene Epoch) with cranial capacity from under 400 cc, to over 300% of that capacity in the most evolved early humans of the Pleistocene. This broad scenario for the possible future effects of natural selection on the Homo genus gives us an optimistic outlook on the future of life on Earth.

2. From chemical evolution to the origin of the single cells

2.1. CHEMICAL EVOLUTION IN THE COSMOS

Given the importance of the origin and evolution of the biomolecules, we feel that it is appropriate to begin with comments on the origin of some of the most important biomolecules. We have to recall that there has been considerable progress in our understanding of the interstellar organic molecules. There are also many indications that the origin of life on Earth may not exclude a strong component of extraterrestrial inventories of the precursor molecules that gave rise to the major biomolecules. About 98% of all matter in the universe is made of hydrogen and helium. The other five biogenic elements C, N, O, S and P make up about 1% of the cosmic matter. The abundance of biogenic elements would suggest that the major part of the molecules in the universe would be organic (i.e., such molecules would be compounds of carbon atoms); in fact, out of over a hundred molecules that have been detected, either by microwave or infrared spectroscopy, 75% are organic 35.
Once again, we should underline that chemical evolution experiments fare well in comparison with the observation of the interstellar medium. Some of the identified molecular species detected by means of radioastronomy are precisely the same as those shown in the laboratory to be precursor biomolecules.

2.2. FROM 'PRECURSOR-BIOMOLECULES' TO BIOMOLECULES

In our studies of the orgin of life we may encounter the major biomolecules: amino acids, nucleic acids, polysaccharides and lipids. Altogether, about 12 interstellar molecules have been shown to be precursors of the main biomolecules. Our insights into the origin of life are based on the formation of compounds of H, N, O and P. The evolution of organic compounds in the interstellar medium has been exhaustively studied. Interstellar gas and dust, ices and grains are the precursors of the Solar System, after a primordial condensation process. Biomolecular precursors have been detected in the interstelllar medium, such as molecular hydrogen, water (the dominant ice component in dense clouds), and carbon monoxide, amongst others. Research in the field of what has been called 'cosmochemistry' began in the 1950s and extends right into the present, but will undoubtedly continue its robust progress in the future. Comprehensive reviews are given in the Trieste Series on Chemical Evolution 39, 40, 6, 7.

2.3. MODERN TAXONOMY AND TERMINOLOGY OF SINGLE-CELLED ORGANISMS

The focus of our attention in what follows will be a group of microorganisms that are either single-celled, or multicellular; their genetic material is enclosed inside a membrane different from the cell membrane. For this reason they are called eu-karyotes, from the Greek words 'karyons' (which means nucleus) and 'eu' (which means true).
Taxonomically the totality of such organisms is said to form the domain Eukarya, which contains kingdoms, such as the well-known Kingdom Animalia. (The most recent taxon introduced in the scientific literature at the highest level is called a 'domain'.)
On the other hand, there are unicellular organisms referred to as 'prokaryotes' that, as their name implies, normally lack a nuclear membrane around their genetic material. Generally they are smaller than eukaryotes. Well known examples are bacteria. However, not all prokaryotes are bacteria! There is a whole domain that encompasses a group of single-celled organisms which are neither bacteria nor eukaryotes. They are generally referred to as archaebacteria 21. In other words, all prokaryotes are encompassed in two domains: Bacteria and Archaea. They may be adapted to extreme conditions of temperature (up to just over 100 ° C), in which case they may be called thermophiles. They may also be adapted to extreme acidic conditions, in which case they may be referred to as being acidophilic. The alternative expressions of extremophiles, or hyperthermophiles are sometimes used for these organisms to distinguish different degrees of adaptability to such extreme ranges of conditions. As we said above, all archaebacteria are said to form the third domain of life, namely the domain Archaea, which is divided into several kingdoms.

2.4. EVOLUTION OF PROKARYOTIC CELLS IN THE PRECAMBRIAN

The main steps of chemical evolution on Earth should have taken place from 4.6 - 3.9 Gyr BP, the preliminary interval of geologic time which is known as the Hadean Subera. It should be noticed, however, that impacts by large asteroids in the early Earth do not necessarily exclude the possibility that the period of chemical evolution may have been considerably shorter. Indeed, it should not be ruled out that the Earth may have been continuously habitable by non-photosynthetic ecosystems from a very remote date, namely since approximately 4.44 Gyr BP54. The content and the ratios of the two long-lived isotopes of reduced organic carbon in some of the earliest sediments (retrieved from the Isua peninsula, Greenland, some 3.8 Gyr BP), may convey a signal of biological carbon fixation44 This reinforces the expectation that chemical evolution may have occurred in a brief fraction of the Hadean Subera, in spite of the considerable destructive potential of large asteroid impacts which took place during during the same geologic interval in all the terrestrial planets, the so-called heavy bombardment period.
An early date for the origin of life may be considered by interpreting the molecular analysis of ribosomal RNA (rRNA) in a variety of organisms in terms of the molecular clock hypothesis 16. This work in molecular biology suggests that life emerged early in Earth's history, even before the end of the heavy bombardment period. In those remote times life may have already colonized extreme habitats, which allowed at least two prokaryotic species to survive the large impacts that were capable of nearly boiling over an ocean. In subsequent suberas of the Archean (3.9 - 2.5 Gyr BP) life, as we know it, was present. This is well represented by fossils of the domain Bacteria, which is well documented by many species of cyanobacteria 46.
To sum up, we may not exclude from the geochemical data earlier dates, in the Lower Archean, for the first prokaryotic microflora, although some critical considerations from the point of view of geochronology still do not rule out the possible origin of life immediately after the end of the Hadean Subera 32. However, the earliest prokaryotic fossils that have not been subjected to regional metamorphism are those dating back to 3.5 Gyr BP 45. Eukaryotes - nucleated cells - arose in the next eon, which is called the Proterozoic (2.5 - 0.6 Gyr BP). This eon saw the progression from a largely anoxic atmosphere to a weakly oxic one about 2 Gyr BP. The increment of atmospheric oxygen was a key geophysical factor which allowed the formation of the eukaryotic cells.

3. From the age of the prokaryotes to eukaryogenesis

3.1. ROLE OF O2 AND Fe IN THE FIRST APPEARANCE OF EUKARYOTES (EUKARYOGENESIS)

Several lines of research suggest the absence of current values of O2 for a major part of the Earth history. There are some arguments, nevertheless, that militate in favour of Archean atmospheres with values of the partial pressure of atmospheric oxygen O2 about 10-12 of present atmospheric level (PAL).
Shale is a rock that has played a role in our understanding of biological evolution. Shale is mainly clay that has hardened into rock. The onset of atmospheric oxygen is demonstrated by the presence in the geologic record of red shale coloured by ferric oxide.
The age of such 'red beds' is estimated to be about 2 Gyr At that time oxygen levels may have reached 1-2% PAL, sufficient for the development of a moderate ozone protection for the microorganisms of the Proterozoic from ultraviolet (UV) radiation. In fact, UV radiation is able to split the O2 molecule into the unstable O-atom, which, in turn, reacts with O2 to produce ozone O3, which is known to be an efficient filter for the UV radiation.
The paleontological record suggests that the origin of the eukaryotic cell occurred earlier than 1.5 Gyr BP. Some algae may even date from 2.1 Gyr BP 18, a period comparable to the first onset of red beds. This is still rather late, compared to the earliest available prokaryotic fossils of some 3.5 Gyr BP 46.
However, if we keep in mind certain affinities between eukaryotes and archaebacteria (such as homologous factors in protein synthesis), we may argue that archaebacteria and the stem group of eukaryotes may have diverged at about the same time 42. This conjecture, combined with the lightest carbon isotope ratios from organic matter, imply that bacteria capable of oxidizing methane CH4 ('methylotrops') may have been using methane produced by archaebacteria that were able to produce it as a byproduct of their metabolism. From the age of such fossils a tentative date of 2.7 Gyr BP, or earlier, may even be assigned to eukaryogenesis.
There are Archean rock formations (which may be found up to 2 Gyr BP) that are significant in the evolution of life. These are laminated compounds of dioxide of silicon (silica) and iron. In reference to their laminated structure they are called "banded iron formations" (BIFs).
The period in which the BIFs were laid out ended some 1.8 Gyr BP. In the anoxic atmosphere of the Archean, iron compounds could have been dispersed over the continental crust. They could have absorbed some oxygen, thereby protecting photosynthesizers that could not tolerate oxygen. Such microorganisms in turn produced oxygen that combined with their environment to produce iron oxide (for example, hematite Fe2O3), which makes up the BIFs.
In strata dating prior to 2.3 Gyr BP it has been observed that there is an abundance of the easily oxidized mineral form of uranium (IV) oxide (urininite, for example the well-known variety pitchblende). This supports the conclusion that we had to wait until about 2 Gyr BP for a substantial presence of free O2. Once the eukaryotes enter the fossil record, its organization into multicellular organisms followed in a relatively short period (in a geological time scale).
Metazoans are hypothesized to have arisen as part of a major eukaryotic radiation in the Riphean Period of the of the Late Proterozoic, approximately 800-1,000 Myr BP 24, when the level of atmospheric O2 had reached 4-8% PAL. There is some evidence in the Late Proterozoic during the Vendian Period, for the existence of early diploblastic grades (Ediacaran faunas): These organisms were early metazoans with two germ layers, such as the modern coelenterates (jellyfishes, corals, and sea anemones).
Later on, when the level of atmospheric O2 had reached values in excess of 10% PAL, these grades were overtaken in numbers by triploblastic phyla as the level of atmospheric O2had reached 40% PAL. These organisms are called the Cambrian faunas, i.e., Early Paleozoic faunas, some 500 million years before the present (Myr BP), which were mainly metazoans with three germ layers). They constitute at present the greater majority of multicellular organisms, including H. sapiens.
We may obtain further insights from paleontology: acceleration in the evolutionary tempo is observed after the onset of eukaryogenesis, as it is clearly demonstrated by the microfossils of algae from the Late Proterozoic 25 and by the macrofossils of the Early Phanerozoic (Cambrian Period of the Paleozoic Era) 8. Such evolutionary changes within the first billion years of atmospheric oxygen rose the simple prokaryotes to eukaryotes, metazoans and metaphytes.

3.2. IDENTIFICATION OF EUKARYOTES WHOSE MORPHOLOGY IS NOT RADICALLY DIFFERENT FROM PROKARYOTES

Primitive eukaryotic organisms have been studied in detail and many difficulties are inherent in the eventual design of an assay to identify them in an extraterrestrial environment, a question which begins to be important in view of the forthcoming space missions In the present paragraph we discuss some properties of the organisms, either alive or fossilized, that may be difficult to distinguish in a robotic mission:

o One such taxon is the family Cyanidiophyceae. These organisms are rhodophytes, commonly known as red algae49. Joseph Seckbach from the Hebrew University of Jerusalem has argued eloquently that these acido-thermophilic algae may constitute a bridge between cyanobacteria and red algae50. In particular, Cyanidium caldarium has a primitive eukaryotic cellular structure. Other remarkable properties of C. caldarium is that it may live at temperatures of up to 57 oC and shows better rates of growth and photosynthesis when cultured in an 'atmosphere' of pure CO2 48.

o Silicification experiments of microorganisms have led to the identification of one artefact, which can cause confusion in the identification of the transition organisms 58. Frances Westall and co-workers at the University of Bologna have observed an artificial nucleus formed during the process of fossilization. If such a phenomenon is preserved in the natural fossil record, then it can lead to a confusion with the eukaryotes.

o Prokaryotic symbionts of the ciliated protozoan Euplotidium itoi has ultrastructure which is more complicated than the majority of prokaryotes 41. Strictly from the point of view of planning future SETE experiments, the most striking feature discovered by Giovanna Rosati and co-workers at the University of Pisa is the presence of 'basket tubules', consisting of the protein tubulin, which is normally present only in eukaryotes.

4. From eukaryogenesis to the appearance of intelligent life on Earth

4.1. MODERN TAXONOMY EMPHASIZES SINGLE-CELLED ORGANISMS

In evolutionary terms, we choose not to emphasize complex multicellular organisms. Instead, we have shifted our attention to the single-celled nucleated organism (eukaryote), whose evolution is known to have has led to intelligence, at least on planet Earth. This point of view is forced upon us by the present taxonomical classification of organisms into domains, which stresses single-celled organisms. Previously, an older taxonomical classification in terms of kingdoms as the highest taxon highlighted multicellular organisms, incorrectly in our view.
The older approach was due to biologists lacking an understanding of molecular biology, which only made its first appearance in the early 1950s. Biology has been able to provide us with sufficient insights into the cell constituents to permit the wide acceptance of a comprehensive taxonomy, which places Bacteria, Archea and Eukarya as the highest groups (taxa) of organisms. We can paraphrase Sir Julian Huxley's comments in the introduction of "The Phenomenon of Man" 55, by remarking that there is an evident inexhorable increase towards greater complexity in the transition from Bacteria to Eukarya.
To borrow the phrase of Christian De Duve11, we may say that the laws of Physics and Chemistry imply an 'imperative' appearance of life during cosmic evolution, a view which is not in contradiction with the relevant remarks of Sir Peter Medawar in his comment on Père Teilhard's work 30.

4.2 THE PHENOMENON OF THE EUKARYOTIC CELL

We are going one step beyond De Duve. We defend the hypothesis that not only life is a natural consequence of the laws of Physics and Chemistry, but once the living process has started, then the cellular plans, or blueprints, are also of universal validity: the lowest cellular blueprint (prokaryotic) will lead to the more complex cellular blueprint (eukaryotic). This is a testable hypothesis. Within a decade or two a new generation of space missions may be operational. Some are currently in their planning stages, such as the hydrobot/cryobot, which is aiming to reach the Jovian satellite Europa in the second decade of next century 56, 20. We shall present the rationalisation behind this effort in Sec. 6 below.
Closely related to the above hypothesis (the proposed universality of eukaryogenesis), concerns the different positions which are possible regarding the question of extraterrestrial life:

Is it reasonable to search for Earth-like organisms, such as a eukaryote, or should we be looking for something totally different?

We will discuss in turn tsome of the arguments involved. Firstly, the more widely accepted belief on the nature of the origin of life is that life evolved according to the principles of deterministic chaos. Evolutionary developments of this type never run again through the same path of events 23.
Secondly, the possibility for similar evolutionary pathways on different planets of the solar system has been defended recently 10. Indeed, even if some authors may consider this to be a remote possibility, Davies bases his work on the increasing acceptance that catastrophic impacts have played an important role in shaping the history of terrestrial life. Thus there may be some common evolutionary pathways between the microorganisms on earth and those that may have developed on Mars during its 'clement period' (roughly equivalent to the early Archean in earth stratigraphy). The means of transport may have been the displacement of substantial quantities of planetary surface due to large asteroid impacts on Mars.
Finally, even in spite of the second possibility raised above, many researchers still see no reason to assume that the development of extraterrestrial life forms followed the same evolutionary pathway to eukaryotic cells, as it is known to have occurred on earth. Moreover, a widespread point of view is that it would seem reasonable to assume that our ignorance concerning the origin of terrestrial life does not justify the assumption that any extraterrestrial life form has to be based on just the same genetic principles that are known to us.
In sharp contrast to the position denying that of common genetic principles may underlie the outcome of the origin of life elsewhere, De Duve has pointed out 11 a fourth point of view in the discussion of the various possible ways of approaching the question of the nature of extraterrestrial life. We may conclude that we all agree that the final outcome of life evolving in a different environment would not be the same as the earth biota. But De Duve has broken new ground when he raised the question as to how different would be the outcome of the origin of life elsewhere? This has led to a clear distinction that there is no reason for the details of the phylogenetic tree to be reproduced elsewhere (except for the possibility of biogenic exchange in the Solar System discussed above 10 ). The tree of life constituted by the earth biota may be unique to planet earth.
On the other hand, there is plenty of room for the development of differently shaped evolutionary trees in an extraterrestrial environment, where life may have taken hold. But

"certain directions may carry such decisive selective advantages as to have a high probability of occurring elsewhere as well" 11.

4.3. THE PHENOMENON OF MULTICELLULARITY

The current view on terrestrial evolution spanning from the first appearance of the eukaryotic cell to the evolution of metazoan organisms is as follows: In the Proterozoic Eon, a geologic period which extends from 2.5 to 0.545 Gyr BP, we come to the end of the Precambrian that had seen the first appearance of the eukaryotic cell. The Proterozoic was followed by the most recent eon, that has seen the spread of multicellular organisms throughout the Earth on its oceans and continents, the Phanerozoic, which extends from the end of the Proterozoic to the present. We should comment on some aspects of the current Phanerozoic:
o Intelligence as we understand it, had to await the Quaternary Era, when the first traces of intelligence arose in our ancestors. A more evident demonstration of the appearance of intelligent life on Earth than the habilines' tools, or the ceremonial burials had to wait till the Magdalenian 'culture' (in the archaeologic classification of southwestern Europe and north Africa this group of human beings flourished from about 20,000 to 11,000 years before the present (yr BP). The Magdalenians left some fine works of primitive art as, for instance the 20,000 year old paintings on the walls of the cave discovered in December 1994 by Jean-Marie Chauvet in southeastern France. Indeed, the birth of art in the Magdalenians' caves, is one of the most striking additions to the output of humans that entitle us to refer to the groups that produced these fine works, as cultures, rather than industries, a term which is reserved to the group of humans that produced characteristic tools, rather than works of art.
The purpose of this sketch has been to bring into focus the different branches of science, including the social sciences, which have contributed to stitch together a huge canvas depicting a great deal of knowledge that has been put together to provide partial answers to the very questions that humans have recorded since the most ancient times, particularly the the Israelites over two thousand years ago when the Old Testament of the Bible was written down. Those disciplines include anthropology, paleontology, prehistoric research, geochronology, biogeology and geochemistry, amongst others.

5. Extraterrestrial sources for life's origin: the evidence from two meteorites

5.1. THE CASE FOR A SETE PROGRAM

The problem of eukaryogenesis occupies a central position in a wide range of problems which concern the origin, evolution and distribution of life in the universe, ranging from chemical evolution to "exobiology" . A program for the search for extraterrestrial eukaryotes (SETE) is relevant to SETI, as the only form of intelligent life that we are familiar with is based on multicellular organisms of eukaryotic cells (Domain Eukarya). Rather than the empirical SETI approach, exploring the consequences of the laws of physics and chemistry may give us some insights on the question whether or not we are alone in the Cosmos. This has been attempted by De Duve 11: This concept of the constraint on chance militates against the older criterium of Monod 31 who, during his lifetime, had less information than we have accumulated now on the mechanisms that are behind the origin and evolution of life on Earth.
Indeed, chance (rate of mutations) and necessity (natural selection) do not imply that life elsewhere in the cosmos is unlikely. Thus, the prospect for detecting life in the newly discovered planets remains a possibility by detecting life-supporting volatiles, as strongly advocated in the past by Christopher McKay. The rationale behind this approach to the search of life is that we now know that a substantial fraction of the volatiles on Earth were deposited by the infall of comets in the past. It seems plausible that such a phenomenon may not have been restricted just to the planet Earth. We should add that many researchers now feel that searching for life is equivalent to searching for water, since as we shall argue in Section 6, water on a silicate body (either on a planet or on a setellite), nearby a star or even far from a star, but near a Jupiter-like planet, seem to be sufficient to engender life, as tectonic activity in the silicate body is triggered off by the vicinity of the large planet or star; this is related to the general consensus that neither oxygen nor photosynthesis are necessary to trigger off the simplest blueprint for a microorganism (prokaryotic) 12, 4.
The possibility of exogenous source of eartly volatiles was suggested by John Oro in the early 1960s. Current spectroscopic analysis of the nuclei of comets, such as the Hale-Bopp, have supported this point of view, since it has been observed that most of the precursors of the prebiotic molecules are present in comets. An alternative means of probing for extraterrestrial life is by directly detecting their radio messages, as persistently maintained by Frank Drake. Preliminary steps to search for intelligent life in the new planets have already been taken 1.

5.2. THE MURCHISON METEORITE: THE INVENTORY OF EARTHLY AMINO ACIDS MAY HAVE AN EXTRATERRESTRIAL COMPONENT

It has been clear for some time now that the extraterrestrial option should not be ruled out. Firstly, a significant illustration of the plausibility of an extraterrestrial origin of the precursors of the biomolecules is based on the meteorite which fell in the town of Murchison in Australia on September 21, 1969. The laboratory of Cyril Ponnamperuma was able to obtain a piece of it. At that time they were preparing for the first analysis of the lunar rocks. They were able to get the first conclusive evidence of extraterrestrial amino acids 38. The results obtained by Ponnamperuma's group have demonstrated the universality of the formation of some organic compounds which are essential for life today 38.
However, the above chemical evolution sketch of the origin of life is incomplete, a fact that is illustrated, for instance, with the question of the gases that were present in the early Earth atmosphere, a topic which is not settled 22. Besides, carbon dioxide must have been sufficiently abundant to have prevented the Archean Earth from freezing under the Sun at a lower level in the main sequence of the Hertzprung-Russell diagram (some 30% less luminous at the time of the origin of life). This is referred to as the 'faint young Sun paradox'. Through a greenhouse effect there would have been the appropriate temperatures for producing the microflora that we know must have been in existence some 3.5 Gy BP. Besides, it also remains to be clarified what was the relative importance of the precursors of life that were brought to Earth by comets, meteorites, and micrometeorites compared with the inventories that were part of the Earth as it formed out of our own planetary nebula.

5.3. THE ALLAN HILLS METEORITE: HAVE THERE BEEN PROKARYOTES IN MARS?

A second important meteorite for exobiology has been retrieved in 1984 by a NSF mission from the wastelands of Antartica in a field of ice called the Allan Hills. It weighs about two kilograms. A preliminary analysis of this meteorite (ALH84001) has been reported during the 1995 Trieste Conference on Chemical Evolution 28; subsequently a fuller report was made known to the public independently 29. This work has shown the presence of an important biomarker the polycyclic aromatic hydrocarbons (PAHs), which on Earth are abundant as fossil molecules in ancient sedimentary rocks, and as components of petroleum.
The presence of PAHs in the ALH84001 meteorite is compatible with the existence of past life on Mars. This result, which requires confirmation, underlines the importance in the future of formulating comprehensive questions (as suggested in this section), regarding life on Mars, to include the degree of biological evolution covering the complete range known to us, from simple bacteria to the eukaryotes.
It should be added that since the first report of preliminary evidence of life in the Allan Hills meteorite, some scientists have found chemical traces in a separate meteorite that seem to be consistent with the presence of life on Mars. They have found organic matter in two meteorites -- the Allan Hills one and in a second meteorite (named 79001) that crashed to Earth 600 Kyr BP. The rock itself was sedimented some 150 million years ago. Residues and chemicals in the rock that could only be formed by living organisms.
The meteorite had been considered earlier in the context of whether life existed on Mars. Experiments on parts of 79001 may be significant as the meteorite had become sealed in a glass-like substance before it came to Earth and was, to a certain extent, insulated from the Earth's organic matter. The main result is that amounts of organic material were found (up to 1,000 parts per million).
To be fair we should point out that there is still some skepticism on the fact that these trace elements found in Martian meteorites are sufficient evidence to say that it represents proof that there was life on Mars.

6. Habitability of Europa by eukaryotic life

6.1. IN WHICH SOLAR SYSTEM HABITATS CAN EXTREMOPHILES SURVIVE ?

It is useful to know in which atmospheres extremophiles may survive, as we now know of several Solar-System planets and satellites that have atmospheres, not all similar to our own. This consideration may help the reader to narrow down the possible Solar System sites in which to focus our attention in our search for 'clement' habitats. For example, four large planetary satellites in the outer Solar System are known to have atmospheres 17: Europa and Io (Jupiter), Titan (Saturn) and Triton (Nepune). Mars currently has a CO2 dominated atmosphere. The possibility of extending the biosphere deep into the silicate crust of Mars has some implications.
This question seems pertinent to exobiology: we cannot exclude the possibility that organisms, which have been found to inhabit deep in the silicate crust of the Earth, may been deposited with the original sediment, and survived over geologic time. McKay has considered this question in some detail from the point of view of geophysics 28. From all of these options we feel that the most promising candidate is Europa. The reasons shall be considered below.

6.2. FAVOURABLE SITES IN THE SOLAR SYSTEM FOR EXTREMOPHILES AND OTHER MICROORGANISMS

In the present 'Golden Age' of the search for life in the Solar System, there are two promising sites to be investigated:

o The first site is Mars, but the possibility of finding life elsewhere in the Solar System is not restricted to planetary habitats 5, 28, 29. As this possibility has been exhaustively discussed elsewhere, we shall focus attention on an attractive alternative.

o The second site is Europa, the Jovian satellite. The existence of internal oceans in the Jovian satellites goes back to 1980 when Gerald Feinberg and Robert Shapiro speculated on this possibility in the specific case of the satellite Ganymede; more recently, Shapiro has further refined the Feinberg-Shapiro hypothesis by suggesting the presence of hydrothermal vents at the bottom of the ocean in Europa, in analogy with the same phenomenon on Earth 51, 52. In fact, for these reasons Europa is a candidate for a SETE program, which we have proposed.

A large proportion of the spectroscopically detectable material on the surface of Europa is water 36, 37. According to the results on the 1979 Voyager fly-by, Europa showed a cracked icy crust suggesting that underneath there might be an ocean of water. Also, since Europa must be subject to strong Jovian tides, one would expect the ocean to be heated by the proximity to the planet. For these reasons it was estimated by Oro, Squyres, Reynolds and Mills that the temperature underneath the icy crust should be 4 oC 36. The preliminary results of the photographs of Europa, which have been retrieved from the Galileo Mission add some plausibility to the older conjectures regarding the Europa ocean.

6.3. CAN THERMOPHILIC ARCHAEBACTERIA EVOLVE IN THE EUROPA'S OCEAN?

From the similarity of the processes that gave rise to the solid bodies of the Solar System, we may expect that hot springs may lie at the bottom of the ocean. The main thesis supporting this expectation is that, as Jupiter's primordial nebula must have contained many organic compounds, then possibly, organisms similar to thermophilic archaebacteria can evolve at the bottom of Europa's ocean.
The argument of Oro et al., correctly pointed out the most important requirements for the maintenance of life in Europa; their work is in agreement with the above list (water, an energy source and organic compounds). However, the analogous Earth ecosystems considered by them excluded eukaryotes. The case mentioned above of the cyanidiophycean algae is a warning that we should keep an open mind while discussing the possible degree of evolution of Europan biota, or indeed of living organisms elsewhere.
We may add that up to the present the divergence into the three domains, arising from the evolution of the 'progenote' (the earliest ancestor of all living organisms), is not well understood. Indeed, plate tectonics has obliterated fossils of early organisms from the crust of the Earth, which is the only record available to us so far, with the possible exception of the SNC meteorites. (The initials are taken from the names of subgroups, namely, shergottites, nakhlites and chassignites.) These are a small group of basaltic meteorites from relatively young (120-1300 Myr BP) lava flows on Mars.

6.4. A CASE FOR EVOLUTIONARY TRENDS IN THE EUROPAN BIOTA

Several additional factors arising from current experience with eukaryotes may contribute to clarify the case for not excluding these microorganisms from microorganisms to be searched for in new Solar System environments.
o A critical step in the diversification of single-celled organisms may have been the loss of the ability to manufacture a cell-wall biomolecule (peptidoglycan). Without the constraint that this biomolecule imposes on cell shape, both Archaea and Eukarya have been able to diversify beyond the Domain Bacteria.
o In spite of the fact that eukaryotes and archaebacteria have both lost peptidoglycan, we can nevertheless distinguish between them, as all eukaryotes have ester lipids in their membranes, whereas archaebacteria have ether lipids.
o Earth-bound eukaryotes are not extremophiles, but their diversification may share a common thread with archeabacteria. Eukaryotes, in spite of not being able to exploit fully all the extreme environments, they may invade, to a certain extent, environments normally at the disposal of the extremophiles.
We have remarked that the identification of primitive eukaryotes is not straightforward; one additional difficulty is that the simplest criterium, namely the search for a membrane-bounded set of chromosomes, does not help to identify unambiguously eukaryotes, as there are prokaryotic organisms that do have a membrane-bounded nucleoid 15.
Finally, if we are granted the possibility that archaebacteria may be present at the bottom of the Europan Ocean (Sec.6.3), then there is really a case for testing the degree of evolution of such biota 4:
Because they are so foreign to our everyday experience and also far removed by catastrophes that may exterminate other ecosystems, the hot-spring environment has been assumed to be a sort of refuge against evolutionary pressures. If these ecosystems were true refuges against evolution, no extinctions would be expected. Hence no organisms that once dwelled in them would be expected to be completely missing from contemporary hydrothermal vents.
It has also been observed by in situ submarine investigation that hot-spring communities of animals are remarkably similar throughout the world. This 'refuge' concept has prevailed even amongst exobiologists that have speculated on the possible origin of life at the bottom of the Europan putative ocean.
Previous authors have only considered that archaebacteria may have also evolved in the bethnic regions heavily dependent on bacterial chemosynthesis. However, scientists at the Urals branch of the Russian Academy [26] have identified fossils from the earliest hydrothermal-vent
community found so far, dating from the late Silurian Period, over 400 Myr BP. This particular community has its own case of species extinction, as the fossils have been identified and correspond to lamp shells (inarticulate brachiopods) and snail-like organisms (monoplacophorans). The implication of this discovery is that since neither the above-mentioned brachiopod nor the mollusk inhabit contemporary hydrothermal vent communities, these environments have known their own cases of species extinction. This weakens considerably the refuge hypothesis, namely that any environment on Earth may serve as a refuge against the inevitable consequences of natural selection."
We cannot maintain with certainty today that the analogous conditions that may exist in Europa will induce the appearance of archaebacteria-like organisms. This is purely a question of an experimental, or observational nature. However, given the common origin of all the Solar System, the favourable conditions in Europa may have been conducive to the first steps in evolution, including the first appearance of archaebacteria.

7. Conclusions

We should underline that from the experience that we have gathered on Earth 3 we are aware of at least two important groups of abundantly distributed biomarkers which have been characterized from oils and sediments (the fossil triterpanes and steranes), whose parental materials are found almost exclusively in eukaryotes. Therefore, if the degree of biological evolution in Mars has gone as far as eukaryogenesis (or taken the first steps towards multicellularity) then, there are means available for detecting this important aspect of exobiology in future space missions (i.e., the search for the parental materials for steranes: tetracyclic steroids).
To sum up, the presence of archaebacteria-like organisms as the most likely biota in the Europa ocean is plausible 36, 12. We have argued that evolution should have occurred in Europa. We have based our work on the the fact that hydrothermal vents should not be considered to be refuges against evolution 26 (Sec. 6.4). This has led to our current attempt to identify parameters that may be considered to be indicators of the degree of evolution of putative Europan biota, both at the ice surface as well as in the putative ocean.
These conjectures may be put to the test in the not too distant future as we are now in the process of planning possible missions in which these ideas may be tested. Indeed, the present work has been stimulated by the possibility of designing for the early part of next Century, an advanced lander mission that may melt through the ice layer above the Europan ocean, in order to deploy a tethered submersible 20, 56, 4, which may allow a direct test for life and its evolutionary status.

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