Which component of the cell theory argues against life on earth evolving from inorganic molecules?

Life in Comets?

The ‘panspermia’ theory was due to the Swedish scientist Svants Arrhenius, whose work won him the Nobel Prize for Chemistry in 1903. Arrhenius believed that life on Earth was brought here in a meteorite, but the theory never became popular, because it seemed to raise more problems that it solved. The same sort of theme has been followed up much more recently by Sir Fred Hoyle and C Wickramasinghe, who claimed that comets can actually deposit harmful bacteria in the Earth's upper atmosphere, causing epidemics. Again there has been little support.

6 More on comets

A largely organic composition of comets and the concept of cometary panspermia was proposed and developed by Hoyle and Wickramasinghe (Hoyle and Wickramasinghe, 1981). This concept (cometary panspermia) involves comets as the collectors, amplifiers, and redistributors of microbiota, the origin of which is posited to be a cosmological event.

As we have already noted, spectroscopic identification of interstellar dust and molecules in space started in the 1970s continues to come into ever sharper focus and their biochemical relevance, once contested, is now conceded. The trend remains, however, to assert without proof that we are witnessing the operation of prebiotic chemical evolution on a cosmic scale rather than biology. With no progress in the quest to make life from nonlife in the laboratory (Deamer, 2012), and considering the superastronomical odds against the spontaneous emergence of the simplest biological system (Hoyle and Wickramasinghe, 2000), astronomical data from both interstellar dust and comets point to biology operating on a galaxy-wide scale. If biological evolution and replication are regarded as the only reliable facts—life always generates new life—this must be so even on a cosmic scale.

The Rosetta Mission to comet 67P/C-G has continued to yield a wealth of data that satisfy consistency checks for biology. Fig. 1.5 shows the close consistency between the surface properties of the comet and the spectrum of a desiccated bacterial sample.

The presence of complex organic molecules including the building blocks of life in comets is now amply confirmed; so it is reasonable to hypothesize that there is fully fledged microbial life in comets. The reigning paradigm, however, firmly rejects this possibility. Life related chemicals and prebiotic molecules are at long last permitted but fully fledged life still appears taboo. Serious inconsistencies are beginning to arise. The Rosetta Missions Philae lander has recently provided novel information about the comet 67P/C-G (Capaccione et al., 2015). Jets of water and organics issuing from ruptures and vents in the frozen surface are consistent with biological activity occurring within subsurface liquid pools (Wallis and Wickramasinghe, 2015). The most recent report of O2along with evidence for the occurrence of water and organics provides further evidence of such ongoing biological activity (Bieler et al., 2015). Such a mixture of gases cannot be produced under thermodynamic conditions, because organics are readily destroyed in an oxidizing environment. The freezing of an initial mixture of compounds, including O2, not in thermochemical equilibrium, has been proposed, but there is no evidence to support such a claim. On the other hand the oxygen/water/organic outflow from the comet can be explained on the basis of subsurface microbiology. Photosynthetic microorganisms operating at the low light levels near the surface at perihelion could produce O2 along with organics. Many species of fermenting bacteria can also produce ethanol from sugars, so the recent discovery that Comet Lovejoy emits ethyl alcohol amounting to 500 bottles of wine per second may well be an indication that such a microbial process is operating (Biver et al., 2015).

Next we turn to the recent report of the presence of the amino acid glycine and an abundance of phosphorus in the coma of comet 67P/CG (Altwegg et al., 2016). Fig. 1.6 shows the atomic mass spectrum of the coma gas examined by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) mass spectrometer. Very high ratios of P/ C≈10−2 are difficult to reconcile on the basis of volatilization of condensed material of solar composition with P/C≈10−3, particularly when we might expect inorganic phosphorus to be mostly fixed in refractory minerals. On the other hand the P/C ratio of biomaterial is close to that implied by the ROSINA data and would be explained if the material in the coma started off as biomaterial virions and bacteria (Fagerbakke et al., 1996).

The idea that microbial life cannot exist in comets still dominates scientific orthodoxy but its rational basis is fast eroding (Wickramasinghe and Tokoro, 2014). Adhering to a paradigm that has long past its sell-by date will surely be to the detriment of science in the long run.

2 How did life emerge on the primitive Earth?

Many hypotheses have been proposed to solve the riddle on the origin of life, how E-life emerged on the primitive Earth such as Panspermia hypothesis, space-origin hypothesis, hydrothermal vent hypothesis (Imai et al., 1999; Holm and Andersson, 2005; Chandru et al., 2013), RNA world hypothesis (Gilbert, 1986), coenzyme world hypothesis (Sharov, 2016), amyloid world hypothesis (Maury, 2015) and tRNA core hypothesis (De Farias et al., 2016), and so on. However, the evolutionary process from accumulation of organic compounds on the primitive Earth to the emergence cannot be rationally explained by any hypotheses including RNA world hypothesis (Ikehara, 2017).

On the other hand, I have proposed [GADV]-protein world hypothesis (GADV hypothesis) on the origin of life about 15 years ago (Ikehara, 2002), based on the analysis of databases of genes and proteins with the four conditions (hydrophobicity/hydrophilicity, α-helix, β-sheet, and turn/coil formabilities) for protein structure formation, which were obtained with data of extant microbial genes encoding water-soluble globular proteins (Ikehara et al., 2002; Ikehara, 2002). The hypothesis assumes that life emerged from [GADV]-protein world, which was formed by pseudo-replication of [GADV]-protein in the absence of any genetic function (Ikehara, 2009). I have asserted that the evolutionary process to the emergence of life can be explained according to the GADV hypothesis without any large contradiction. The GADV hypothesis, but not RNA world hypothesis, can also well explain three formation processes of the first protein, genetic code, and gene (Ikehara, 2017). On the contrary, the processes have not been addressed in other hypotheses, such as cofactor world hypothesis, amyloid world hypothesis, tRNA core hypothesis, and so on.

Here, I describe an evolutionary process from synthesis of organic compounds including amino acids on the primitive Earth to the emergence of life in more detail (Ikehara, 2002, 2005, 2016a, b): (1) Primitive atmosphere composed of CO2, H2, H2O, N2, CH4, NH3, and so on was formed. (2) Simple amino acids, as [GADV]-amino acids, were physically and chemically synthesized and accumulated on the primitive Earth. (3) Peptide catalysts mainly composed of [GADV]-amino acids were produced such as by repeated heat-drying process in depressions on the primitive Earth. (4) [GADV]-protein world was formed by pseudo-replication with [GADV]-protein(s), actually [GADV]-peptide aggregate(s). (5) Nucleotides and oligonucleotides were synthesized and accumulated in the protein world. (6) The first GNC genetic code encoding [GADV]-amino acids was established through complex formation of GNC-containing oligonucleotide or primeval tRNA with the corresponding [GADV]-amino acid. (7) Single-stranded (GNC)n genes were produced by joining of GNC triplet base sequence in the complex. (8) Successively, double-stranded (GNC)n genes were formed by synthesis of complementary strand to the single-stranded (GNC)n genes. (9) Finally, the first life emerged on the primitive Earth, after a sufficient number of double-stranded (GNC)n genes, which are required for the first life to emerge, were produced.

The reason, why the establishment processes of the three elements, the first gene, the first genetic code, and the first protein, could be explained with GADV hypothesis for the first time, would be because I introduced a new concept or protein 0th-order structure (Ikehara, 2002, 2005, 2014). Therefore it is expected that life emerged from [GADV]-protein world, but not from RNA world (Ikehara, 2017). Then, the emergence and evolution of E-life are discussed according to GADV hypothesis in the next section.

9.10.4.6 Other Worlds

Another possibility is that life evolved elsewhere in the universe and colonized our planet as propagules that could withstand long-distance transport in space within rocks or resistant pellicles. This concept of ‘panspermia’ goes back to the turn of the century and the Swedish chemist Svanté Arrhenius. A related idea is deliberate colonization of the Earth by advanced extraterrestrial civilizations (Crick, 1981). All manner of organisms could have been broadcast or dispatched, ranging from influenza viruses or unicellular bacteria to sophisticated aliens in space vehicles. Such views have some appeal in this age of space exploration but merely remove the question of the origin of life to another planet. The environment where that life evolved is likely to have been Earth-like in many respects because life has long been well suited to our planet. Thus, it remains useful to consider the origin of life from natural causes here on Earth.

4 The Origin of Life

There are numerous and diverse theories for the origin of life currently under serious consideration within the scientific community. A diagram and classification of current theories for the origin of life on Earth are shown in Figure 10.6. At the most fundamental level, theories may be characterized within two broad categories: theories that suggest that life originated on Earth (Terrestrial in Figure 10.6) and those that suggest that the origin took place elsewhere (Extraterrestrial in Figure 10.6). The extraterrestrial or panspermia theories suggest that life existed in outer space and was transported by meteorites, asteroids, or comets to a receptive Earth. In this case the origin of life is not related to environments possible on the early Earth. Along similar lines, life may have been ejected by impacts from another planet in the Solar System and jettisoned to Earth—or vice versa. Furthermore, it has been suggested in the scientific literature that life may have been purposely directed to Earth (Directed Panspermia in Figure 10.6) by an intelligent species from another planet.

The terrestrial theories are further subdivided into organic origins (carbon based) and inorganic origins (mineral based). Mineral-based theories suggest that life's first components were mineral substrates that organized and synthesized clay organisms. These organisms have evolved via natural selection into the organic-based life forms visible on Earth today. The majority of theories that do not invoke an extraterrestrial origin require an organic origin for life on Earth. Theories postulating an organic origin suggest that the initial life forms were composed of the same basic building blocks present in biochemistry today—organic material. If life arose in organic form then there must have been a prebiological source of organics. The Miller–Urey experiments and their successors have demonstrated how organic material may have been produced naturally in the primordial environment of Earth (endogenous production in Figure 10.6). An alternative to the endogenous production of organics on early Earth is the importation of organic material by celestial impacts and debris—comets, meteorites, interstellar dust particles, and comet dust particles. A comparison of these sources is shown in Table 10.5. Table 10.6 lists the organics found in the Murchison meteorite and compares these with the organics produced in a Miller–Urey abiotic synthesis. Organic origins differ mainly in the type of primal energy sources: photosynthetic, chemosynthetic, or heterotrophic. The phototrophs and chemotrophs (collectively called autotrophs) use energy sources that are inorganic (sunlight and chemical energy, respectively), whereas heterotrophs acquire their energy by consuming organics (Table 10.2).

TABLE 10.5. Sources of Prebiotic Organics on Early Earth

TABLE 10.6. Comparison of the Amino Acids in Murchison Meteorite and in an Electric Discharge Synthesis, Normalized to Glycine

Hydrothermal vent environments have been suggested for the subsurface origin of chemotrophic life. In the absence of sunlight these organisms must utilize chemical energy (e.g. CO2 + 4H2 → CH4 + 2H2O + energy). Alternatively phototrophic life utilizes solar radiation from the surface for prebiotic synthesis. These organisms with the ability to chemosynthesize and photosynthesize can assimilate their own energy from materials in their environment. One feature that the various theories for the origin of life have in common is the requirement for liquid water. This is because the chemistry of even the earliest life requires a liquid water medium. This is true if the primal organism appears fully developed (panspermia), if it engages in organic chemistry, as well as for the clay inorganic theories.

For many years the standard theory for the origin of life posited a terrestrial organic origin requiring endogenous production of organics leading to the development of heterotrophic organisms. This was generally known as the primordial “soup” theory. Recently, there has been serious consideration for the chemotrophic origin of life and at the present time the scientific community is split between these two views.

8 Implications of the Cosmic Origin of Life on Earth

The first implication of our study of genomic complexity is that the early appearance of life on Earth most likely stemmed from contamination with prokaryotes (bacteria or archaea or their predecessors, such as LUCA) from space. Thus, despite the fact that we don’t have a final answer, it makes sense to explore the implications of a cosmic origin of life, before the Earth existed. The idea that life was transferred to Earth by intelligent beings (i.e., “directed panspermia”) (Crick and Orgel, 1973) is unlikely because there was no intelligent life in our universe at the time of the origin of Earth by Fig. 1. The universe was only 8 billion years old at that time, whereas the development of intelligent life seems to require ca. 10 billion years of evolution.

Second, our analysis indicates that life took a long time, perhaps 5 billion years, to reach the complexity of prokaryotes. Thus, the possibility of repeated and independent origins of life of this complexity on other planets in our Solar System can be ruled out. Extrasolar life is likely to be present at least on some planets or satellites within our Solar System, because (1) all planets had comparable chances of being contaminated with microbial life, and (2) some planets and satellites (e.g., Mars, Europa, and Enceladus) provide niches where certain prokaryotes may survive and reproduce. If extraterrestrial life is present in the Solar System, it should have strong similarities to terrestrial microbes, which is a testable hypothesis. We expect that they have the same nucleic acids (DNA and RNA) and similar mechanisms of transcription and translation as in terrestrial prokaryotes. The ability to survive interstellar transfer was the major selection factor among prokaryotes on the cosmic scale. Thus, prokaryotic life forms were successful in colonizing the cosmos only if they were resilient to radiation, cold, drying, toxic substances, and highly adaptable to a broad range of planetary environments. In particular, photosynthesis or chemosynthesis is needed to be independent from organic resources. The similarity between terrestrial and extraterrestrial prokaryotes may appear sufficient to draw a unified evolutionary tree of life, though it may be complicated by later transfers between the planets, such as between Earth and Mars (Gordon and McNichol, 2012).

Third, we can anticipate repositories within our own solar system of samples of the life that might have first “contaminated” its planets. For example, the more distant, perhaps geologically inactive dwarf planets, beyond Pluto, such as Eris, Makemake, and Haumea (Holman and Rudenko, 2016), are close enough to be reached by current space technology. Their orbits have low enough eccentricity that they probably have never been close to the sun. Eris varies from 37.74 to 97.59 Au from the sun (Holman and Rudenko, 2016) and has a surface temperature (Alvarez-Candal et al., 2011; Howett et al., 2016) of 30°K. Thus, Eris is a deep freeze for any life that may have fallen on it, and probably also records, under pristine conditions, much of the history of our solar system. Rogue planets formed before our solar system could similarly record much of the history of our galaxy, including panspermia.

Fourth, attempts to reproduce the origin of life in laboratory conditions (Damer et al., 2012) may prove more difficult than generally expected because such experiments have to emulate many cumulative rare events that occurred during several billion years before organisms reached the complexity of the RNA world (Davies, 2003; Lineweaver and Davis, 2003; Sharov, 2009). Despite the success of copying an existing bacterial genome (Gibson et al., 2008), humans have so far failed to invent a single new functional enzyme from scratch (i.e., without copying it from nature) and have had limited success in imitating existing enzymes (Bjerre et al., 2008). Thus, it may prove hard to make a primitive living system that does not resemble anything that we observe on Earth.

Fifth, the environments in which life originated and evolved to the prokaryote stage may have been quite different from those envisaged on Earth. Thus, emulating conditions on the young Earth may not increase the chance of generating primordial living systems in the lab. Even a bigger mistake would be to use contemporary minerals in such experiments because most mineral species on Earth are directly or indirectly modified by life (Hazen, 2010). To define possible environments for the origin of life, we can extrapolate the evolution of minerals back in time to the initial cooling of the universe after the Big Bang. The major questions to ask are: (1) When did stars and planets form? (2) What was the elemental composition of stars and planets versus cosmic time? (3) How was the surface of those early planets stratified? (4) What atmospheres might those planets have had? It is reasonable to assume that life originated in the presence of water, as water is very abundant in space and is a byproduct of star formation. Young stars shoot jets of water into the interstellar space (Fazekas, 2011). Large quantities of water have been detected in space clouds (Glanz, 1998). Thus, it is reasonable to assume that water was present early in the young universe and could have supported the origin of life. Major chemical elements of living organisms (carbon, hydrogen, oxygen, and nitrogen) are among the most abundant in the Universe. Phosphorus is less abundant and may have been the limiting factor for the origin of life, although recent studies suggest that it can be effectively extracted from apatite or come from volcanic activity or space (Schwartz, 2006). Life may have started on planets around the first, low “metal” stars, where “metals” mean, to astronomers, higher atomic number elements that formed from hydrogen and helium during stellar nucleosynthesis. Such stars may have formed as early as 200 million years after the Big Bang (Zheng et al., 2012). High metallicity is a negative factor for life origin because Earth-like planets may be destroyed by giant planets (Lineweaver and Grether, 2002). The stars of such planets probably lasted only 4 billion years, so if life didn’t have a “false start” in such systems (Johnson and Li, 2012), it may have been propagated into interstellar space during the star's supernova event (Gordon and Hoover, 2007). Therefore, the time scale we are proposing for the origin and complexifying of life requires panspermia mechanisms (McNichol and Gordon, 2012) for life to persist.

And sixth, the original Drake equation for guesstimating the number of civilizations in our galaxy (Wikipedia contributors, 2017c) may be wrong (Haqq-Misra and Kopparpu, 2017), as we conclude that intelligent life like us has just begun appearing in our universe. The Drake equation is a steady state model, and we may be at the beginning of a pulse of civilization. Emergence of civilizations is a non-ergodic process, and some parameters of the equation are therefore time-dependent. Because the cosmic transport of life is most likely limited to prokaryotes, young planets have not had enough time to develop intelligent life. Another time-dependent process is the probability of interstellar transfer of prokaryotes, which we expect to have become more frequent as the total pool of prokaryotes in the galaxy increased with time. There are many modifications of the Drake equation, but if civilizations have just begun to appear, any version is of limited use. The answer to the Fermi paradox (Wikipedia contributors, 2017d) may be that we are among the first, if not the only so far, civilization to emerge in our galaxy. The “Rare Earth” hypothesis (Ward and Brownlee, 2003) need not be invoked. The linking of civilization to the lifetime of a particular star, such as our Sun (Livio and Kopelman, 1990; Webb, 2002), is also not necessary.

4.6 The MESSIER surveyor

Summary provided by David Valls-Gabaud on behalf of the MESSIER collaboration.

The proposed MESSIER Surveyor is a small-class space mission designed for exploring the last remaining niche in observational parameter space: the very low surface brightness universe. At a 900 km orbit, the mission will drift-scan the entire sky in six bands covering the 200–1000 nm range, reaching the unprecedented surface brightness levels of 34 and 37 mag/arcsec2 in the optical and UV, respectively. These levels are required to achieve the two main science goals of the mission: to critically test the LCDM paradigm of structure formation through (1) the detection and characterization of ultrafaint dwarf galaxies, which are predicted to be extremely abundant, but remain unobserved; and (2) tracing the cosmic web, which feeds dark matter and baryons into galactic halos, and which may contain the reservoir of missing baryons at low redshifts.

Remarkably, and even though the mission has not been designed for these goals, a large number of additional science cases appear as free by-products of the mission, which is perhaps not surprising given that the sky has never been surveyed to these depths in surface brightness. Hence, additional science cases range from cosmology to stellar physics through extragalactic physics, and include: the galaxy luminosity function, the intracluster and intragroup light, the cosmological UV/optical background, Ly-alpha galaxies at z = 0.65, the BAO scale at that redshift using emission-line galaxies, the recalibration of cosmological distance indicators, the actual extension of galaxies, the detection of warm molecular hydrogen at z = 0.25, stellar mass-loss envelopes, debris discs, and of course the first reference photometric UV-optical catalog which will have a unique legacy values. The entire astronomical community will benefit from the time-domain full-sky maps that MESSIER will provide.

To achieve these goals, the payload consists of an off-axis TMA telescope with a 50 cm primary mirror at f/2.3 which produces an ultrastable PSF with ultralow wings. No lenses can be used, as not only they create multiple scatterings, but they also produce Cerenkov radiation at levels larger than the faint surface brightness that must be detected. The design includes extreme baffling to ensure a minimal stray-light contamination. The flat focal plane array is divided into 12 independent CCDs, each designed to have a maximum quantum efficiency at the central wavelength of each filter (see Fig. 5.18) and a set of narrowband and broadband filters centered at 200 nm. The design is very robust, as there are no moving parts, and the cooling is passive, ensuring that the power consumption remains low. The CCDs are read in TDI mode to ensure flat field corrections below the 0.0025% level. Each detector is optimized to have QE>80% in each band. With a scale of 1 arcsec per pixel and a FOV of 4°×2°, MESSIER is 200 times better than HST in terms of surface brightness detection.

On the issues related to the origins of life, the MESSIER Science Team has identified the following science cases which will be free by-products of the maps produced by the mission:

The time-domain mapping of the Zodiacal cloud. MESSIER will provide a 2 degree-wide multi-band map of the Zodiacal cloud at a solar elongation of 90°, with a time sampling rate of one exposure every 90 min. This will enable the measure of local heterogeneities, asteroidal bands and cometary trails with unprecedented detail. The multi-band maps will allow the inversion of line of sight measures to constrain the properties of interplanetary dust particles and their growth. This is particularly important given that, at the time of the Late Heavy Bombardment, the Zodiacal cloud was much denser than today, and the small relative velocities of dust particles released by comets in the inner Solar System, combined with their fluffiness, would enable them to survive atmospheric entry and enrich in complex carbonaceous compounds the atmospheres of the early telluric planets. This could also lead to constraints on scenarios based on the panspermia hypothesis.

The detection of the remnants of the formation of planetary discs. The study of debris discs is closely coupled with exoplanets and will eventually place our Solar system in its proper context. Planetesimals around other stars are not directly detectable but their mutual collisions generate enough dust to produce a cross-sectional surface which is large enough to be observable either through thermal emission of large dust grains at midinfrared wavelengths and longwards, or observable through light scattered by small grains in the optical and UV. Surveys with the ISO, Spitzer, and Herschel satellites have discovered hundreds of dusty debris discs among the thousands of stars in our neighborhood (<25 pc). At optical wavelengths, only about two dozen debris discs have been discovered because of their faint brightness and because of the requirement for high-contrast imaging. The typical sensitivity in the optical is 24 mag/arcsec2 at 10 arcsec from the star with HST/STIS. MESSIER will offer a full-sky survey of debris discs in the UV/optical with a vastly superior sensitivity for discs larger than 5 arcsec in projected radius. The potential of MESSIER to detect debris discs in the MF490 band is simulated in Fig. 5.19 using the most complete catalog of nearby stars and shows that hundreds of debris discs around a variety of stars of different spectral types will be characterized by MESSIER.

The detection of the remnants of mass-loss episodes in stars. The emission from stars is not pointlike, as stars undergoing mass-loss get surrounded by circumstellar shells containing dust particles that scatter light from the central star itself, from nearby stars, or from the interstellar radiation field. Key for the origin of life is the understanding the chemical evolution of the Solar Neighborhood, and this depends on the mass lost by nearby red giants and supergiants, which dominate the mass return budget. Yet the actual mechanisms of mass-loss are not understood. Processes such as turbulent pressure generated by convective motions, combined with radiative pressure on molecular lines, might initiate mass-loss which coupled with Alfven waves generated by magnetic fields drive the episodes of mass-loss. Pulsations and radiation pressure on newly formed dust grains may take over in AGB stars, but the detailed understanding of these mechanisms is lacking. Similarly, the interaction with the surrounding interstellar medium creates shocks in the wind-ISM interface, which have only been serendipitously detected recently. Long tails such as the one detected around Mira (2.5° long) by GALEX, and shells around Betelgeuse and AGB stars have led to the concept of astrospheres. In the UV domain, the FUV emission is inferred to be H2 line emission collisionally excited by hot electrons in shocked gas, but much remains to be studied. By reaching unprecedented surface brightness levels, MESSIER will systematically detect all astrospheres larger than 5 arcsec in projected radius, providing UV-optical detailed maps to characterize their properties, bringing unique constraints on the actual chemical evolution on solar and nonsolar type stars.

Hypotheses and Theories on the Origin of Life

The traditional position of theology and some philosophy views the origin of life as the result of a supernatural event which is permanently beyond the descriptive powers of chemistry and physics. In its most general form, this view is not necessarily contradictory to contemporary scientific knowledge about prebiotic evolution, although the biblical descriptions of creation given in the first two chapters of Genesis, taken literally and not metaphorically, are inconsistent with modern knowledge.

Until the mid-seventeenth century, the prevailing opinion was that God created man together with higher animals and plants, but that simple forms of life such as worms and insects arise steadily from mud, waste, and putrefied matter during short periods of time. The physiologist William Harvey (1578–1657), who studied reproduction and development of deer, was the first to challenge this view by postulating that every animal comes from an egg (“omnia viva ex ovo”) a long time before Karl-Ernst von Baer (1792–1876) discovered the existence of human egg cells by microscopy. An Italian scientist, Francesco Redi (1626–1698), found Harvey's idea to be true, at least for insects; he found that maggots in meat arise from fly eggs. Later, Lazzaro Spallanzani (1729–1799) discovered that spermatozoa were necessary for the reproduction of mammals. Before Pasteur, Spallanzani also showed that living matter (“infusories”) does not originate from boiled fluids kept in closed containers. Although Redi's and Spallanzani's findings definitely proved that insects and larger animals develop from eggs, it remained obvious to a large majority that at least microorganisms, because of their ubiquity, are generated continually from inorganic material. The debate of whether life is spontaneously generated from non-living matter or not culminated in the famous controversy between Louis Pasteur and Félix-Archimède Pouchet (1800–1872) which Pasteur won triumphantly. He showed that even microorganisms in fluids come from germs floating in the air, and he also demonstrated that nutrient solutions could be guarded against these creatures by suitable sterilization such as filtration or boiling. However, contemporary scientists were not satisfied by Pasteur's experiments because a delicate question remained: If living organisms do not arise from non-living matter, how had life come about in the first place?

In the late nineteenth century, another hypothesis was initiated by the Swedish chemist Svante Arrhenius (1859–1927). He strongly believed that the whole universe is replenished with living germs, a phenomenon that he called “panspermia.” He suggested that microorganisms and spores of cosmic origin spread from solar system to solar system, and thus they arrived on Earth. Although Arrhenius' view avoids rather than solves the problem of the origin of life, and despite the extreme unlikeliness of microorganisms surviving the interstellar effects of cold, vacuum, and radiation, a few twentieth-century members of the scientific community returned to the idea of panspermia. Among these scientists are astronomer Fred Hoyle (1915–) and molecular biologist Francis Crick (1916–), who are convinced that the time span between the origin of Earth and the appearance of first cellular organisms on this planet was too short for life to have occurred spontaneously.

Darwin's theory of “natural selection as motive power for evolution” resulted in a new view on the phenomenon of life that is still valid. Although Darwin did not commit himself on the origin of life, contemporary scientists such as Thomas Huxley (1825–1895) extended his idea, asserting that life could be generated from inorganic chemicals. Pursuing this opinion, Alexander Oparin (1894–1980) was the most influential advocate of the successive origin of cellular organisms from non-living matter. He suspected this transition was proceeded by a series of regular and progressive chemical reactions under the physical and chemical conditions on early Earth. Together with John Scott Haldane (1860–1936), Oparin recognized that the abiological production of organic molecules in the current oxidizing atmosphere of Earth is highly unlikely. Instead, both suggested that the beginning of life occurred in primordial hot waters under more reducing (i.e., hydrogen-rich) conditions. Furthermore, Oparin postulated the existence of pre-cellular coacervates—globular units with membrane-like surface structures—that may have high concentrations of certain chemical compounds. Coacervates indeed form spontaneously from colloidal aqueous solutions of two or more macro-molecular compounds.

However, many fundamental problems on the transition from non-living to living matter remained unsolved. The central question concerned the role of the second law of thermodynamics, which defines the equilibrium in an isolated system as a state of maximum entropy that appears to contradict the origin and existence of highly ordered living organisms. Erwin Schrödinger (1887–1961) gave a decisive answer to this question, stating that “living matter evades the decay to equilibrium” or death by steadily compensating for the production of entropy. In any organism, this is achieved by feeding it free energy or energy-rich matter which is used by the cellular machinery to drive essential chemical reactions. Schrödinger and others also realized that living organisms can thermodynamically be described as open systems, but they could not explain the general physical conditions for self-ordering processes. These were perceived by Ilja Prigogine (1917–) and Paul Glansdorff (1904–1999), who worked on a thermodynamic theory of irreversible processes. According to Prigogine, selection and evolution cannot occur in equilibrated or nearly equilibrated reaction systems, even if the right types of substances are present. Instead, certain combinations of autocatalytic reactions with transport processes may lead to peculiar spatial distributions of reaction partners, called “dissipative structures.” These ordered structures are of importance for the formation of functional order in the evolution of life, especially for early morphogenesis. However, the first steps of self-organization probably involved little organization in physical space but extensive functional ordering of a tremendously complex variety of chemical compounds. Manfred Eigen (1927–) explained the process of ordering among molecules by augmenting the Prigogine–Glansdorff principle with phenomenological considerations on the behavior of self-replicating molecules: A certain quantity approaches a maximal value in any open system that is replicating autocatalytically with sufficient fidelity, and thereby continually consuming energy and matter. This quantity is called “information” and is closely related to the “negative entropy” postulated by Schrödinger. In addition to setting the stage for a molecular interpretation of biological information, Eigen developed the mathematical models for describing “selection.” According to Eigen's theory, selection is the fundamental natural principle that brings order into any random arrangement of autocatalytically replicating species. With selection, information is generated successively, leading to a steady optimization of species, which can either be organisms or molecules.

The mathematical models developed by Eigen support a detailed hypothesis of the origin of life which comprises multiple, successive steps for the transition from inorganic to living matter. However, it should be mentioned that some scientists have theories on the emergence of life that differ from Eigen's theory. Among these is Stuart Kauffman (1939–), who believes that natural selection is important but not the sole ordering principle of the biological world. Instead, he considers spontaneous self-organization to represent the predominant source of natural order. Kauffman demonstrated that sets of inter-related autocatalytic reactions can undergo a transition to a newly ordered (i.e., self-organized) state as soon as their connectivity reaches a certain threshold value. Furthermore, Kauffman emphasizes that the phenomenon of autocatalysis, which plays the central role in his theory, is not limited to nucleic acids. Therefore, he concludes that even genes were not necessary for the origin of life. In contrast to Kauffman, Eigen distinguishes “random” autocatalytic or self-replicating activity which is observed for a variety of molecular species from the “inherently” self-replicating nucleic acids. Inherent capability for self-replication, in turn, represents the molecular basis for natural selection according to Eigen's theory.

Well-defined experiments were invented in order to simulate the principles that were postulated for molecular evolution. With certain experimental set-ups, replication and selection can be performed in a test tube. Similarly, the chemical conditions on primordial Earth can be mimicked in the laboratory. Several scientists attempted to verify experimentally the twentieth-century ideas on biogenesis. Their experiments are discussed in the following section.