5.1 From Ancient Times to Modern Science
The information provided in the chapters above is the minimum required for a further discussion on the origin of life. This makes it clear that only a group of scientists is capable of covering all the areas of the scientific disciplines involved in depth to some degree based on latest findings. The most important thing here is that the members of the group are only able to assess the basics of neighboring disciplines to the extent that they contribute impulses and related considerations concerning the overall question. Understandably, these favorable conditions as I have described them for the Essen Group could not have existed in earlier times. The scientific foundations alone were missing. As in science as a whole, a development took place concerning the question of the origin of life, which led from nonscientific ideas to careful attempts at explanations by individuals to laboratory experiments by smaller research groups.
In ancient times and in the Middle Ages, the scholars had the idea that life could arise spontaneously from earth or mud. The person who established this idea was the Greek philosopher and naturalist Aristotle, who propagated spontaneous generation of life from inanimate matter. But this was a fallacy given rise to by observations that were incapable of looking into the matter deep enough owing to the lack of technical aids. The development of worms and larvae in muddy environments or mold on food was known to everyone at the time. The knowledge available at that time prevented access to proof of the propagation pathways because bacteria or spores could not be perceived because of their minute size. The situation in this respect did not change until modern times. Thus, for centuries there was no reason to doubt the interpretation of one of the best known scholars in antiquity. The idea even still existed in the nineteenth century that, in addition to the known ways of reproduction (biogenesis), life could be propagated at any time, through a variant known as primordial production (abiogenesis). During this time, however, the discussion on the primordial generation of life was no longer so much about the fundamental question of the origin of life but about the additional possibility of the spontaneous origination of the new in parallel to existing life. The research carried out by the French chemist, Luis Pasteur, in the second half of the nineteenth century put an end to this discussion. He was able to demonstrate the influence of microorganisms on fermentation and mold growth through experiments on fermentation and sterilization processes. The main focus had been on these during the late phase of the dispute on spontaneous generation.
5.2 Modern Beginnings
Scientific consideration of the question of the origin of life started only hesitantly in the last third of the nineteenth century, at the time when the British naturalist Charles Darwin formulated his thoughts on this. He assumed a warm little pond as the location for the place of origin of life, in which the development of more complex molecules began with the presence of sufficient inorganic compounds and energy. Life was meant to have developed from these ultimately. This process is no longer applicable today, however, since biological activity would immediately destroy all attempts to start over. However, it was not until the twentieth century that Soviet biochemist, Alexander Ivanovich Oparin, born in 1894, laid the actual foundations for research into prebiotic evolution. He formulated a hypothesis in which he made it clear that the initial conditions in infant earth differed significantly from those of today. He made statements about the composition of a primordial atmosphere; postulated lightning discharges, sunlight, and volcanism as a source of energy; and coined the term primordial soup for the collection of all the resulting molecules in a primordial ocean [1]. His considerations are now out of date since a different composition is assumed for the primordial atmosphere. In addition, due to a lack of knowledge, the planetary conditions could not be taken into account, and the molecular concentrations in the ocean were far too low for a reactive primordial soup. However, he did lay the theoretical foundations for one of the most well-known experiments in science, which went down in scientific history as the “primordial soup” experiment. This was an experiment performed by the American chemist, Harold Clayton Urey, and his doctoral student Stanley L. Miller, in the 1950s. He built on the hypothesis from Oparin which led to the formation of amino acids directly from simple inorganic components in the laboratory.
5.3 The Experiment by Harold C. Urey and Stanley L. Miller
Experimental setup by Urey and Miller for the abiotic synthesis of organic molecules (© Springer-Verlag GmbH [3], modified)
But criticism was quick to follow. The first thing that became clear is that the concentrations of the molecules in this type of formation were so low that they would have decayed before they could have collided with another molecule in the ocean. In addition, the idea about the composition of the atmosphere also changed. Miller and Urey assumed a reduced atmosphere, visible from the CO and NH3 in the experiment. However, the outgassing of the earth probably mainly produced CO2 and N2. There was also the problem with chirality. In the experiment, the left-handed and right-handed amino acids formed in equal proportions. In nature, only left-handed amino acids occur with a few exceptions. Connecting the results of the experiment to cell formation or even to an RNA was not possible with this experiment. Even though much of the criticism was justified, it ultimately represented the breakthrough everyone was waiting for in research into the origin of life. The experiment showed that a way existed to tackle this seemingly unsolvable question with a series of experimental steps for the first time.
5.4 The Dam Was Broken
A large number of experiments were carried out subsequently to tackle the problems of linking amino acids to peptides or connecting nucleotides to RNA strands. Studies of clay minerals showed that their mainly negatively charged surfaces offer effective contact with positively charged organic molecules and can serve as catalysts in the connection [4]. This also happens on iron disulfide pyrite (FeS2) which is reduced in contact with hydrogen. This means that a sulfur atom is removed from the connection with the iron and connects with the hydrogen. This process also provides energy that can be used to link the amino acids together, for instance. Pyrite is a mineral that was extremely common on infant earth. The research on this is linked to the name of Günter Wächtershäuser, a patent attorney from Munich who developed an alternative scenario in the 1980s for the early evolution of life, but not without extensive criticism [5, 6]. His hypothesis on biogenesis differed significantly from all models discussed so far. While the hypothesis for the RNA world (cf. Chap. 6), for example, requires a system for the initial phase of life that maintains itself by continuously copying chemically stored information, for Wächtershäuser, the metabolism was the most important factor. He assumed that the formation of organic molecules with the carbon from CO2 or CO required a mineral surface that was available as an electron supplier. The focus of his discussion revolved around the formation of iron-sulfur minerals, in which the mineral pyrite (FeS2) occurs. In this way, FeS2 can be oxidized from an iron-sulfur compound (FeS) in contact with hydrogen sulfide (H2S). This process releases enough electrons and H+ ions, which are said to have been directly available to adherent CO2 or CO molecules for building more complex molecules. His considerations cannot be dismissed entirely out of hand. Some of the enzymes today do have so-called iron-sulfur clusters. There, they adopt important catalytic functions, whereby they transform the substances that were available at the very beginning of development, such as H2, N2, or CO. But Wächtershäuser did not get much further with his considerations. The link to enzyme development and the formation of RNA was missing.
5.5 Black Smoker: A Parallel World
Black smoker—summit region of the active North Su volcano in the eastern Manus basin at an approx. water depth of 1200 m (© MARUM, Center for Marine Environmental Sciences, with kind permission)
A highly specialized ecosystem has developed in direct connection with black smokers, whose food base is one of the very rare exceptions that is not sunlight. The entire system is based on the chemical energy provided for chemolithotrophic bacteria and archaea by the escaping metal sulfides. The microorganisms use electrons from redox processes to control chemical reactions for their metabolism. They acquire the carbon necessary for their own cell components from the inorganic carbon from CO2 and CO. Building on this, a food chain has developed that includes worms, crayfish, and other higher animals. The discovery of these highly specialized unicellular organisms quickly led to the consideration that black smokers could be a model for the origin of life [7]. The thermophilic microorganisms seemed to form an ideal link to those that later, in the course of evolution, probably specialized in cooler environments. They are the simplest organisms we know. This also made it clear that the depths of the ocean provided protection from solar wind and UV radiation. Even large meteorites would have only partially evaporated the water, meaning that the continued existence of the developing world would not have been endangered. However, objections ensued because the chimneys only have a short lifespan of a few years and as a result do not provide long-term conditions for the requisite periods of time. Furthermore, it became clear that the solution load along with the metals and the temperatures in the ascent paths is too high, so that hardly any organic molecules such as nucleotides can be formed. At high temperatures, such molecules disintegrate faster than they are formed. If under certain fringe conditions they still form with the help of mineral surfaces, any further connection to the development into the first biological cell cannot be discerned. Linking amino acids to peptides in water is also very problematic. The connection of two amino acids takes place by splitting off a water molecule, which is formed from an OH molecule from one amino acid and a hydrogen atom from the other. If the amino acids are in the water, they are shielded from one another by water molecules so the reaction cannot take place. The connection only works in the aqueous environment of cells because enzymes provide the appropriate help. In the end, approval for the black smoker model waned more and more, and it was no longer pursued.
5.6 A New Discovery: The White Smoker
Special smokers in the summit region of the active North Su volcano in the eastern Manus basin off Papua New Guinea at an approx. water depth of 1200 m. These are “smokers” from which liquid sulfur and bubbles of CO2 escape. With a pH of 1.4, the water is very acidic (© MARUM—Center for Marine Environmental Sciences, with kind permission)
Geological profile of the hydrothermal field lost city with “white smoker.” The underlying limestone layer sequence (breccia and limestone) and debris from a coral reef (Talus) are clear. It overlaps a tectonic shear zone in the upper area of the mantle rock. mbsl meters below sea level. Figure from Kelley et al. [10], public domain
5.7 The Search Continues: Warm Ponds
Charles Darwin was the first to come up with the idea that life may have started in a warm pond. In a letter to botanist Joseph Hooker in February 1871, he speculated: “But if (and oh what a big if) we could conceive in some warm little pond with all sorts of ammonia and phosphoric salts, light, heat, electricity etcetera present, that a protein compound was chemically formed, ready to undergo still more complex changes” [13].
This idea has been taken up recently since it apparently helps to solve one of the main problems. Wet-dry cycles on land make it possible to link amino acids to peptides that do not function in water without the existence of enzymes. The proponents of this model are supported by the discovery of organic molecules in meteorite rocks, in which a variety of different amino acids have been found, including those that are not found in biological cells. This is an important indication that the finds do not represent contaminations that can take place very quickly when contact is made with the atmosphere and ground.
Example of a mud pool connected to a hydrothermal system (Iceland)
The ponds drying up created the moment when the amino acids could react to form peptides. The continuing steps are complex and have not yet been fully formulated. Cells which formed the first bacterial colonies (stromatolites) as they continued to flow into the sea are said to have developed within a very short period of time. The time span for this appears to be extremely short, considering the millions of years required to obtain decisive changes in the development of later cells. In later times, however, a functioning system of processes that ensured the cells’ self-preservation already existed. It is very likely that the initial phase up until the first cell required a great deal of time for the infinite number of trials using the trial-and-error principle. No consensus on the time frame exists, however, because the steps on the path to life lie too far back in time. Only the time period for individual steps in the reaction can be specified. Only one thing can be determined from the many attempts required to exploit the range of possibilities available: a great deal of time was needed. And it cannot even be taken into account here that a development that was on the verge of a breakthrough was wiped out by a global event. After this event, everything started over again.
The geological processes that took place on the first volcanoes to appear above sea level were anything but slow. Acid rain strongly affected the unprotected rock since neither soil nor plant coverage protected the rock surface. The result was intense weathering, which led to high erosion rates caused by heavy rainfall. This resulted in a high sediment load in streams and rivers, which filled every pool and small pond in a very short period of time. The pools with a low absorption capacity that dried up were filled after a few hundred years. If arid zones, similar to Australia, existed on the first continents, pools that dried up must have been quickly filled with salts, carbonates precipitated out of solutions, and volcanic ashes from the ubiquitous volcanoes. Corresponding examples are still widespread in arid regions. If new pools were formed, they were not linked to the processes that took place in their predecessors in most cases, so it was like a new start. However, the space required for the development of life needed to have been available for millions of years.
Other arguments exist against the pool model. As the moon is so close to the earth, the ocean tides were extremely strong shortly after the earth formed. The water masses that smashed against the island mountains in the ocean resulted in much faster erosion than is the case today. Not infrequent impact events also took place, which statistically speaking predominantly took place in the sea and presumably washed over the islands completely. The situation was different for larger complexes like Iceland or the first small continents, which were spared the major floods. In this case, the factors mentioned at the beginning need to be discussed in relation a model based on the unprotected surface of the earth as the point of origin. During the initial phase, the sun radiated at around 30% less, but UV radiation was stronger by several orders of magnitude. The reason for this was the initially high rate of rotation by the sun, which produced a strong dynamo effect with increased activity on the sun’s surface [15]. Since there was a lack of oxygen in the atmosphere to the development of photosynthesis, no ozone layer existed to block the UV radiation. As a result, much stronger radiation hit the earth’s unprotected surface during the period of the first billion years. Possible formations of long-chain connections were therefore exposed to an extreme selection factor right from the start. The same was true for the strong particle stream, which still has a weaker influence on the earth’s atmosphere on some days as a solar wind even today. With absolutely no or a very low magnetic field, the particle flow from the sun had unhindered access to everything that developed on the earth’s surface. As long as it is not clear when an effective magnetic field existed, the possible particle flow from the sun needs to be discussed as an important parameter.
Another problem is the concentration of molecules supplied by the cosmic particles in the package. Only on a small fraction of the meteorites was it possible for the organic substances to survive on their way through space and the atmosphere that existed back then to the surface of the earth. Smaller particles do not protect the molecules from the UV radiation in space that destroys everything. The next largest units in centimeter and decimeter size are heated up so much when crossing a possible atmosphere that the organic molecules are destroyed. It is no different with really large bodies, which convert so much kinetic energy into heat when they collide with the earth that everything evaporates. Only a small fraction in between is large enough to offer the molecules inside sufficient protection against the radiation in space and not to burn up when they come into contact with the atmosphere or the earth’s surface.
The first question that follows this relates to the frequency of events in a catchment area for an assumed pool that dries up. Are the extremely low molecular concentrations (in the range from 1 to 1012 per meteorite material) sufficient to start a biochemical evolution with an assumed meteorite event of 1 per year (or per 10 years)? It needs to be taken into account here that the molecules required for the reactions were not all available at the same time. The release took place in the course of the surface weathering and from the smallest cracks, starting from deeper parts, micrometer for micrometer, over a period of hundreds of years. This means that the first organic compounds released were flushed into the flow systems or embedded in the sediment long before the last molecules from the meteorites were available. Only many billions of meteors of the same size weathering at the same time would have been able to provide a resource for the chemical-organic processes required. I have already described above that these molecules were exposed to a large number of destructive mechanisms during their release and subsequent transport. All of the reasoning above is invalid if the surface of the earth was completely iced over before an atmosphere was present (cf. Sect. 2.4).
5.8 Panspermia: Space Seeds
“Space is pulsating with life, unimaginable creatures fight for supremacy in the last galaxy to be defended, spaceships career through space….” Our heads are full of images from all the science fiction films and novels which have thrilled countless people for decades who consider the confines of earth too narrow for expanding our imaginations. The narratives shaped the ideas of entire generations about intelligent, extraterrestrial life shaped by the same weaknesses in character demonstrated by life on earth. Scientists are not free from these thoughts either, although they can assess more precisely the universal laws of physics which science fiction novels appear to override. However, they are not so much concerned with battles fought in space, but much more fundamental things, namely, that of life itself. In the early days of modern science, it seemed very questionable to scientists that life on earth could have originated. In their view, the cosmos offered many more opportunities for the development of life, so that the likelihood of the earth being “impregnated” from the outside was seen as a more plausible alternative. Surprising is the fact that these considerations were discussed more than 100 years ago, at a time when little knowledge of the planets outside our solar system or the laws of space existed. The ideas involved distant planets, on which more favorable conditions existed for the origin of life than on earth. The earth was subsequently infected by a transfer of germs, a concept now known as panspermia. Not only was the transport mechanism unclear, but also their chance of survival in space and once they had landed on earth was speculative.
This early thought, which reminds us of science fiction very much, was even expressed in the 1970s by well-known scientists including Francis Crick and Leslie Organ. Crick, along with James Watson and Maurice Wilkins, received the Nobel Prize in Medicine for discovering the molecular structure of DNA (cf. Sect. 4.2). The chemist Leslie Orgel conducted research in the field of chemical evolution. The two scientists even discussed the possibility of targeted panspermia. According to their ideas, civilizations at risk of extinction on distant planets sent grains containing bacteria into space in an attempt to “infect” distant planets with the germs of life. Doing so would then make colonization possible [16]. This approach can be quickly challenged because the time ranges between vaccinating a planet with biospores and its possible habitability by subsequent intelligent life forms diverge completely. It took over a billion years to produce oxygen on earth alone; it took this amount of time until consumption by oxidative reactions with iron and sulfur had decreased to such an extent that excess oxygen could be enriched in the atmosphere. The civilizations under threat on distant planets certainly did not have this much time available.
Ignoring the reasons given by Crick and Organ, the fascinating idea of vaccinating the earth from the outside raises a lot more questions than it does answer. First of all, in no way does it help to clarify how life came about. It outsources the problem to an unknown region where we have no knowledge of all the general conditions present. Furthermore, there is a lack of information about the conditions in the region from where this launch took place and how the transport could have survived over such very long periods of time under constant bombardment by cosmic rays. Ultimately, the small grains would have any protective cover. The main problem here would be the particle density. From an assumed starting point in space, a huge number of particles would have to be catapulted in a direction that would demonstrate such large scatter after a few light years that actually encountering a planet would be an enormous coincidence. The image of a shower head on the moon sending out a spray of water toward the earth offers a small idea of this. Let us leave out the gravitational pull of the moon and the earth’s atmosphere and position exactly one person on the earth. If a jet of water hit the earth at all, the likelihood that precisely this person would be hit would be extremely slim. Or let us take a supernova explosion in a neighboring galaxy that really throws out a lot of material into space: how many particles would arrive on earth? If a particle loaded with bacteria actually managed to strike a planet in the habitable zone and its contents survive the landing approach, the microbes would be confronted with very mundane things such being embedded in sediment or dissolving in aggressive water. The chance of life blossoming would be almost zero.
However, the exchange of matter between two neighboring planets does not seem quite so unrealistic. Billions of tons of rock material have arrived on earth from Mars through the impact of large meteorites. The proportion was much smaller in the opposite direction. However, the amounts may have been sufficient to transport resistant unicellular organisms with the rocks in both directions [17]. The chance is given in theory if the unicellular organisms in the porous rock are covered by a layer of rock at least one meter thick. Even after an expected flight time of more than a million years (on orbits that are slowly approaching the earth), UV radiation is not expected to cause any effect. And that unicellular organisms can survive such a long period of time has been proven, at least in the case of bacterial spores. In this way, it has been possible to obtain at least 25-million-year-old specimens from bees enclosed in amber over this time [18]. The early phase of Mars, which had sufficient water at the beginning, offered quite similar conditions for the development of life as on earth. An interesting consideration is whether life actually came into being twice, perhaps on Mars and earth at the same time. It is difficult to imagine an exchange, however, when the two forms meet. With every genetic code that develops independently, a kind of independent language arises. A biological communication between these two separately developed lines of life would not be possible.
It is well documented in the meantime that organic compounds are formed in space and on other planets. Analyses of the Murchison meteorite that struck Victoria, Australia, in 1969 revealed an astonishing variety of organic molecules which had survived the impact [19]. The presence of 70 amino acids, most of which do not originate from biological processes, and the high percentage of the D configuration instead of the L configuration make contamination unlikely. Even organic bases, which comprise the basic building blocks for RNA and DNA, have been found in carbonaceous meteorites. In some cases, these included bases that do not or only occur very rarely on earth. This is seen as a clear indication of an extraterrestrial origin [20].
All in all, it has been confirmed that the formation of organic chemical molecules is not limited to earth but is also widely possible in space. Explaining the formation of amino acids, lipids, and organic bases in general does not seem to be the problem. The problem is to bring them together in one place so that they can interact in high concentration over a very long period of time and that an engine exists to push these interactions. Only under such conditions is it possible to develop a complex machinery as that of the biological cell.
5.9 Additional Considerations
Numerous other ideas and models exist that deal with individual aspects and provide information about possible detailed special steps in development. A full description of all research methods would quickly go beyond the scope of this book. However, two other methods are worth mentioning briefly. One model also presented in several popular scientific publications includes the process of freezing seawater as the basis for the development of life. The core of the model is made up of a concentration of organic molecules by freezing seawater. On freezing, ice crystals are first formed from freshwater, which causes salts and organic molecules to accumulate in the remaining pores, which in turn are surrounded by ice membranes. This should create favorable conditions for linking the molecules. As has been described for other environments, water represents a major hurdle for linking molecules to peptides or RNA strands. Freezing removes the pores to a sufficient extent, which means that the reactions can take place. This highly controversial model was developed by the physicist Hauke Trinks from the University of Hamburg-Harburg, who died in 2016 [21]. He assumed that phases of icing existed on infant earth, which cannot be excluded. His model does not explain the origin of the molecules or how the vanishingly small proportions in the primordial sea could bring about the high concentrations needed for reactions in the ice pores.
Another, much-discussed model originates from Manfred Eigen, 1967 Chemistry Nobel Laureate, from the Max Planck Institute for Biophysical Chemistry in Göttingen, Germany. He passed away shortly after my unsuccessful attempt to share my new knowledge and this book with him. Eigen developed mathematical models based on Darwinian evolution, which aim at the self-organization of larger molecules. Self-organization should lead to the formation of self-reproducing units and, at the same time, develop large functional molecules. The terms “quasi-species” and “hypercycle” originate from him [22].