Introduction

The search for origin of ‘life’ is complicated by the fact that “there is no broadly accepted definition of life” (Cleland and Chyba 2002). This is because the concept of life is “too vague and general, and loaded with a number of historical, traditional, religious values” (Luisi 2006). The essential issue is that although life is “a useful word in practice”, it is not a scientific concept (Gayon 2010) and thus a workable concept. Any definition of life is subjective and arbitrary as is the boundary between living and non-living systems or pinpointing the moment when non-living systems would have become living. This means that the statement that any such boundary or moment exists is not falsifiable because no experiment can be performed to prove that it is wrong (Tessera 2012). Saying for instance that virus or prions are living systems (or not) adds nothing more than the definition of life one would propose. A working definition of life “that has become increasingly accepted within the origins-of-life community” could certainly be “Life is a self-sustained chemical system capable of undergoing Darwinian evolution” (Cleland and Chyba 2002). However even this definition of life can be questioned, arguing for instance that “early cellular life on Earth or some other world passed through a period of reproduction without replication, during which Darwinian evolution was not yet established” (Cleland and Chyba 2002).

By contrast the distinction between systems with evolvable capacity and systems without is not so problematic. In Biology the term “evolution” refers to the changes in heritable information within populations over time and the dynamics of population origins and extinctions. However the more general use of the term “evolution” employed by physicists and applied to what are generally regarded as non-living systems connects directly to the narrow biological meaning. Physical evolution is best understood as a thermodynamic phenomenon, and this perspective comfortably includes all of biological evolution (Tessera and Hoelzer 2013). Four dynamical factors may be considered to build on each other in a hierarchical fashion and set the stage for the Darwinian evolution of biological systems: (1) the entropic erosion of structure; (2) the construction of dissipative systems; (3) the reproduction of growing systems and (4) the historical memory accrued to populations of reproductive agents by the acquisition of hereditary mechanisms (Tessera and Hoelzer 2013). Within this approach a level-4 evolution can be described showing three conditions requested to allow natural selection to apply to populations of system lineages, i.e., (1) Local conditions allowing the emergence of open non-equilibrium systems organized on a macroscopic level, generated by a continuously supplied flow of matter and energy. These open far-from-equilibrium systems can maintain themselves far-from-equilibrium because they are able to use the matter and energy supplied by the favourable local environment; (2) The systems should be able to reproduce; and (3) They should show heritability (Tessera 2011).

Thus we find it crucial to test the hypothesis that populations of such systems could have emerged in the prebiotic early-Earth environments.

Proposed vesicle-based model

The above issue is addressed by a vesicle-based model that will be briefly presented now as it has been presented more deeply in a previous paper (Tessera 2011). Studies on vesicles from potentially prebiotic amphiphiles have so far been limited to systems containing one or two types of amphiphiles. This in contrast to the output of simulated prebiotic chemical reactions, which typically produce very heterogeneous mixtures of compounds (Chen and Walde 2010). Moreover, artificial vesicles can be formed from mixtures of amphiphiles in such a way that the vesicle membranes become molecularly, compositionally and organizationally highly complex, similarly to the lipidic matrix of biological membranes (Walde et al. 2014). Within the hypothesis of a bi-layer membrane composed of a mixture of various distinct amphiphilic compounds there is the opportunity of a huge number of theoretically possible combinations in the arrangements of these amphiphiles in the membrane. Among all these potential combinations, a specific local arrangement of a group of amphiphiles (Am) would have appeared in the inner surface of the membrane able to catalyze larger products from carbon-based small molecules. Actually small molecules “can easily penetrate the membrane, and the products of adding them will tend to be retained because of difficulty of the larger produced molecules crossing back out of the membrane” (Woolf 2015). Thus, the combination of two classes of compounds (Aj, Bj), among classes of carbon-based small molecules (Aa, Ba) that penetrate the membrane would lead to the synthesis of a new class of compounds (Cj) trapped into the vesicle (Fig. 1).

Fig. 1
figure 1

Schematic representation of the catalyzing effect of a local arrangement (Am) of an heterogeneous membrane (Aa, Ba: classes of carbon-based small molecules that can penetrate the vesicle membrane); Among Aa and Ba molecules there are Aj and Bj that can be transformed into Cj compounds by the catalytic effect of a specific arrangement of the membrane, Am; The chemical reaction is reversible; Cj compounds cannot cross the membrane and thus are trapped

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Among the class of Cj compounds there are specific Ck compounds able to catalyze the transformation of Am into a stabilized site (S) by the irreversible formation of covalent bonds (instead of intermolecular H-bonds) most likely between the hydrophilic poles of adjacent amphiphiles (Fig. 2). This kind of catalysis mediated solely by small organic molecules (here Ck compounds) is comparable to organocatalysis. Organocatalysis or “small-molecule catalysis” is a field recently rediscovered in chemistry that in the past decade has become “a thriving area of general concepts and widely applicable asymmetric reactions” (Mitchinson and Finkelstein 2008). Carlos Barbas believed “that this chemistry not only provides for fascinating and efficient syntheses of chiral molecules but may serve to explain the emergence of homochirality in the prebiotic world” (Barbas 2008). This is one of our main hypotheses already developed in a previous paper (Tessera 2011).

Fig. 2
figure 2

Schematic representation of the catalyzing effect of Ck compounds: stabilization of local arrangements (Am) into stabilized sites (S)

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Let us consider the kinetics of the process once a Ck has been synthesized:

  • Ck has the property to stabilize Am into S;

  • when Ck is released from S, S is free to catalyze the synthesis of new Ck, provided there is a supply of Ak and Bk;

  • as Ck is trapped and encapsulated in the vesicle its intra-vesicular concentration increases (Fig. 3);

Fig. 3
figure 3

S catalyzes the synthesis of Ck ligands from Ak and Bk and thus Ck ligands accumulate into the vesicle

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the probability for Ck to binding to new emergent Am increases with Ck concentration and new S are formed, more or less randomly distributed over the inner surface of the vesicle membrane. However the maximum number of S is limited by the number of possible emergences of Am (Fig. 4);

  • the result of the process is the constitution of an hypercycle, according to the terminology by Manfred Eigen (Eigen 1977). It is actually a positive feed-back composed of mutually catalytic Ck/S pairs;

  • the high concentration of encapsulated Ck leads to a significant internal osmotic pressure which leads to vesicle swelling and thus membrane tension (Szostak 2011). According to experiments by Szostak et al., when osmotically swollen vesicles are mixed with isotonic vesicles, the swollen vesicles start to grow while the relaxed vesicles begin to shrink (Szostak 2011) (Fig. 5);

  • As the filamentous vesicles are extremely fragile, even very mild shear forces caused by pressure-driven fluid disturbances are sufficient to trigger division of the filamentous vesicles into multiple smaller spherical daughter vesicles (Szostak 2011). Accordingly, both Ck and S are likely to be present in the daughter vesicles after such divisions as S are more or less randomly distributed over the inner surface of the vesicle membrane and the intra-vesicular Ck concentration is at its steady state, depending of the kinetic parameters of the chemical reaction Ak + Bk ↔ Ck (Fig. 6)

Fig. 4
figure 4

Wherever appears an Am on the inner part of the membrane the Ck ligand stabilizes it into a S site

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Fig. 5
figure 5

The vesicle shrinks into a filamentous vesicle

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Fig. 6
figure 6

Triggered by mild shear forces, the filament divides into several vesicles, each containing Ck ligands or not and with or without S sites onto the inner part of the membrane; Then the cycle can repeat for each new vesicle which has at least either Ck ligands or S sites or both

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Thus this kind of division would have allowed S and Ck to be transmissible to the daughter vesicles. When Ck molecules are chiral it is possible to say that “once a single chirality is developed, a proto-organism could grow that way, and divide, so propagating the chirality” (Woolf 2015).

Proposed experiments

Hydrothermal fields on the prebiotic Earth are candidate environments for biogenesis. Some authors hypothesize that the origin of life emerged in hydrothermal pools of early Earth volcanic regions (Mulkidjanian et al. 2012; Damer and Deamer 2015). In particular Damer and Deamer propose a model in which molecular systems driven by cycles of hydration and dehydration in such sites undergo chemical evolution in dehydrated films on mineral surfaces followed by encapsulation and combinatorial selection in a hydrated bulk phase of a hydrothermal pool (Damer and Deamer 2015). However, there are issues with this hypothesis. First, one may ask whether life spans of hydrothermal pools located in the neighbourhood of volcanos are not too short to allow the emergence, development, maintenance and above all Darwinian evolution of the first organisms. Second, according to our approach of the origin of Darwinian evolution, the first prerequisite is to have local conditions providing a continuously supplied flow of matter and energy allowing both the emergence of open non-equilibrium systems organized on a macroscopic level and their maintenance from far-from-equilibrium. This is possible because the systems are able to permanently use the matter and energy supplied by the favourable local environment similarly to the way organisms function today or in the known past. While hydrothermal pools may provide matter and energy Damer and Deamer’s scenario is based on cycles with alternative phases of active production of populations of functional vesicles (i.e., hydration phases) and inanimate phases (i.e., dehydrated phases consisting of concentrated eutectic mixtures or multilamellar liquid crystalline matrices) during which vesicles are reduced to crystalline matter. Such a situation is very far away from the way organisms function today and in the known past. Finally, the major problem with this scenario is that from one cycle to the next one it is not clear why populations of vesicles would grow as there is no vesicle replication process and evolve as there is no inheritance mechanism.

Another scenario may be envisaged with other kinds of hydrothermal fields. Thus, it has already been recognized that vents systems were chemically reactive environments that constituted suitable conditions for sustained prebiotic syntheses (Baross and Hoffman 1985). It should be noticed that alkaline hydrothermal vents like Lost City can remain active for up to 100,000 years (Martin et al. 2014). At high temperatures, lipid compounds can be produced by aqueous Fischer-Tropsch-type (FTT) synthesis (Rushdi and Simoneit 2001). Such abiogenic production of short-chain hydrocarbons has been recently found at the Lost City Hydrothermal Field, LCHF (Proskurowski et al. 2008). The millimolar concentrations of abiogenic CH4 present in the LCHF effluent could be at the origin of the carbon reduction in such hydrothermal systems (Martin et al. 2008). Actually, fluids collected from the Rainbow and the Lost City hydrothermal fields were clearly enriched in organic compounds with a dominance of aliphatic hydrocarbons (C9-C14), aromatic compounds (C6-C16) and carboxylic acids (C8-C18) even though a mixed origin, i.e., both biogenic and abiogenic, is probable (Konn et al. 2009). Thermodynamic calculations demonstrate that biomass synthesis is most favourable at moderate temperatures such as at LCHF, where the energy contribution from HCO3 and H+ in seawater coupled to the reducing power in hydrothermal fluid are optimized (Amend and McCollom, 2009). Moreover simulations of molecular transport in elongated hydrothermal pore systems showed extreme accumulation of molecules in a wide variety of plugged pores (Baaske et al. 2007). New experiments demonstrated that thermal gradients across narrow channels can provide the energy necessary to concentrate dilute molecular solutions and thus allow the self-assembly of lipidic vesicles from an initially dilute solution (Budin et al. 2009). Vesicles with membranes composed of bi-layers from mixtures of amphiphilic and hydrophobic molecules could have formed from the organic compounds present locally at high concentrations. The stability of bilayer lipidic membranes at high pressure and temperature is nevertheless still debated. Experiments have shown that bilayers formed of simple amphiphiles are extremely fragile: high pH, ionic strength, high temperatures (even 45 °C) will destroy them (Deamer et al. 2002; Maurer et al. 2009). However, as already specified, primitive membranes would have been composed of a diverse mixture of amphiphiles. This mixt character may have imparted essential stability to primitive membranes (Maurer et al. 2009; Namani and Deamer 2008). Furthermore, polycyclic aromatic hydrocarbon (PAH) may have contributed to stabilizing them as cholesterol stabilizes cell membranes of extant organisms today (Deamer et al. 2002; Konn et al. 2009). Recent experiments show that monoglycerides are synthesized under hydrothermal conditions by simple condensation reactions which represent a plausible step in the self-assembly of protocellular structures toward boundary membranes that would be stable over a range of pH values in the salty seas of the prebiotic environment. Actually, in these experiments, no salts were present either during the synthesis or during the formation of bilayers, which occurs at pH 8.5. The authors justified their conclusions by the fact that monoglycerides are virtually immune to the effect of pH and divalent cations, because they do not have ionic head groups that can interact with cations in solution (Simoneit et al. 2007). Nevertheless, this membrane stability problem would have been a strong selection factor among all the possible sorts of vesicles with heterogeneous membranes.

Finally, experiments are yet to be developed that prove that lipid vesicles with heterogeneous membrane can form, grow and divide in the conditions of hydrothermal locations akin to LCHF (Kelley et al. 2005) and that their heterogeneous membrane may catalyse the synthesis of compounds able to stabilize membrane local arrangements into stable sites.

Thus a program of experiments is proposed to test the following issues:

  1. 1.

    Confirmation that the synthesis of a large variety of amphiphiles is possible from basic chemicals such as H2, CH4, H2O and possibly hydrogen cyanide (HCN) in the conditions of hydrothermal locations akin to LCHF:

  • Pressure: around 80 bars, corresponding to a depth of about 750-850 m below the surface of the sea);

  • Temperature: moderately high, i.e. 40°-90 °C;

  • Salinity and other ions: higher Na concentrations than that of modern seawater, i.e. 464–607 mmol kg−1 (Knauth, 1998; Amend and McCollom 2009). For other ion concentrations see Amend and McCollom (2009).

  • pH: 9–11;

  • Nitrogen: nitrogen species at Lost City were not previously reported (Kelley et al. 2001; Martin and Russell 2007). However, more recently, Lang et al. specified that there were relatively low concentrations of nitrate and ammonia (<6 μM) in the pure endmember fluids of the Lost City, while those of N2 were indistinguishable from local sea water (Lang and GL, 2013). Now, whether such nitrate and ammonia could lead to the production of HCN which is central to most of the reaction pathways leading to abiotic formation of simple organic compounds containing nitrogen is still debatable. However HCN is likely to have been present in prebiotic hydrothermal environments because it is formed by a variety of processes driven by thermal energy (Holm and Neubeck 2009). Hence, it seems plausible to envisage the possible abiotic synthesis of organic compounds containing nitrogen in hydrothermal locations akin to LCHF.

This first objective is crucial for the next stages of the program.

  1. 2.

    Whether this mixture of amphiphiles may lead to the formation of vesicles with heterogeneous membranes composed of bi-layers of such mixture.

  2. 3.

    Permeability of these heterogeneous membranes to various small carbon-based molecules.

  3. 4.

    Whether such heterogeneous membranes may have catalytic properties.

  4. 5.

    Whether specific ligands (that cannot pass through the vesicle membrane and thus are trapped in the vesicle) may stabilize specific arrangements of the amphiphiles in the membrane into stable sites.

  5. 6.

    Whether hypercycles, based on ligand/site pairs, may be found out.

Then the program plans the following stages (see algorithm):

  1. 1.

    Using a ‘hydrothermal’ (continuous high-pressure flow) reactor such as recently described and used by NASA Russell’s team (Mielke et al. 2010), to study the spontaneous synthesis of a large variety of amphiphiles from basic chemicals such as H2, CH4, H2O and possibly HCN. Identification of the synthesized amphiphiles, in particular by mass spectrometry.

  2. 2.

    From mixtures of the amphiphiles identified at stage 1 using thermal gradients across narrow artificial channels as did Szostak ‘s team (Budin et al. 2009) and studying the emergence of vesicles with heterogeneous membranes composed of bi-layers of mixtures of amphiphiles.

  3. 3.

    Selecting the most resistant vesicles in the course of time (by natural selection). Roughly identifying the different populations of vesicles by optic methods; extraction of samples of vesicles from different populations and identification of the composition of their membranes in order to identify different kinds of vesicles.

  4. 4.

    From the selection of the most resistant vesicles at stage 3 (actually the vesicles with the relevant heterogeneous membranes) investigation of the permeability of the membranes to a large number of various small carbon-based molecules, Ax and Bx. The permeability could most easily be done in a chamber that is separated in 2 parts by a wall with a tiny aperture. The aperture would be covered by the different kinds of heterogeneous membranes. Then, by adding radiolabeled small carbon-based molecules at one side, the permeability could be tracked. Selection of the Aa and Ba small carbon-based molecules that can pass through the different kinds of heterogeneous membranes.

  5. 5.

    Among the small carbon-based molecules (Aa & Ba) selected at stage 4 investigation of their potential for yielding reactions of the type: Ai + Bi ↔ Ci which are possibly easily catalyzed by surface catalysts (Sc). Identification of the relevant compounds Ai, Bi and the Ci products of the reactions.

  6. 6.

    Investigation of the capability of different membranes from the vesicles selected at stage 3 (i.e. of specific Am arrangements of Membranes) to catalyze some of the reactions Ai + Bi ↔ Ci. Identification of the catalyzed reactions (Aj + Bj ↔ Cj) via the identification of the Aj, Bj and Cj compounds. Identification of the Am arrangements.

  7. 7.

    Following the identification of the Cj compounds at stage 6 selecting the specific Ck ligands able to stabilize specific Am arrangements into S sites by the formation of covalent bonds between amphiphiles. Identification of the Ck ligands. Identification of the amphiphiles covalently bonded by the Ck ligands, i.e., identification of the S sites.

  8. 8.

    Study of the molecular interactions between S sites and Ck ligands to better understand the binding of Ck ligands onto S sites.

  9. 9.

    Study of the interactions between the various Ck ligands and the membrane in order to select the specific Cs ligands that have beneficial effects on vesicle survival. Study of the parasitic/selfish element issue (Konnyu et al. 2008):

  • parasitic elements would be natural compounds CP from the environment that can block S sites;

  • selfish elements would be, among the Ck ligands, Csf ligands without any Darwinian advantage for the involved vesicle population lineage, competing with Cs ligands in binding onto S sites.

Algorithm of the search: from the continuous production of H2, CH4 and possibly HCN in the conditions of hydrothermal locations akin to LCHF to the formation of simple hypercycles

figure a

The proposed mutually catalytic model predicts the synthesis of enantiomers of small organic molecules (Tessera 2011). Assuming that the first amino acids were synthesized through the catalyzing membrane sites and that the chiral centre carbon atom of the homochiral amino acids belonged to the catalysing domain, it would explain the emergence of the first L-enantiomeric amino acids. Actually, at the beginning, there would have been two kinds of membrane sites, those that synthesized carbon-based D-enantiomeric peptides compounds and those that synthesized carbon-based L-enantiomeric compounds. As the model has the ability to combine sites there is the possibility for the synthesis of polymers. If the synthesized carbon-based compounds were amino acids their polymerization would have led to different kinds of peptides. Actually only pure D-enantiomeric peptides or pure L-enantiomeric peptides could be functional, e.g. able to catalyze other compounds and thus, only vesicles with homogeneous kinds of combined sites leading to the synthesis of one kind of pure enantiomeric peptides would have survived. Thus, there would have been two kinds of populations of vesicles, those that synthesized D-enantiomeric peptides and those that synthesized L-enantiomeric peptides. Later on the predominance of the population of vesicles that synthesized L-enantiomeric peptides would have occurred only by chance.

Thus, I propose the search for specific signatures such as a selective pattern of simple enantiomeric carbon-based compounds in the oldest presumed fossils either on Earth, Mars or any other Solar System bodies that can be considered candidates for evolution emergence (e.g. Europa, Titan, Enceladus etc.) as described by McKay (2011) even if these are not necessarily L or D-enantiomeric amino acids.

Conclusions

The search for the origin of ‘life’ is hampered because “there is no broadly accepted definition of life”. By contrast there is a feasible and practical track for the search for the origin of Darwinian evolution as the distinction between systems with evolvable capacity and systems without is not an issue. Darwinian evolution has a relatively strict definition referring to the changes in inheritable information within populations over time and the dynamic of population origins and extinctions. Darwin’s theory of evolution states that complex creatures evolve from more simplistic ancestors naturally over time. As random genetic mutations occur within an organism’s genetic code, the beneficial mutations are preserved because they aid survival, i.e., a process known as “natural selection”. These beneficial mutations are passed on to the next generation. Over time, beneficial mutations accumulate and the result is an entirely different organism, i.e., not just a variation of the original, but an entirely different creature.

Actually it is possible to consider the so-called level-4 evolution that shows only three conditions requested to allow natural selection to apply to populations of different system lineages, i.e., (1) Local conditions allowing the emergence of open non-equilibrium systems organized on a macroscopic level, generated by a continuously supplied flow of matter and energy. These open far-from-equilibrium systems can maintain themselves far-from-equilibrium because they are able to use the matter and energy supplied by the favourable local environment; (2) The systems should be able to reproduce; and (3) They should show heritability. Following this approach a vesicle-based model of the origin of Darwinian evolution and the research program to test its relevance are proposed.