1.1 The Origins of Life: What Makes This Question So Important to Us?
Can’t we as human beings simply accept that what was formed a long time ago exists today without knowing exactly how and why? Well, the simple answer is no, we can’t. The development of human beings and hence the development of an abstract thinking organ, our brain, inevitably lead to questions about everything that happens in the environment of our brain. This has always been the case since a certain point in development. Questions exist that could be answered instinctively. The most obvious one is why does game recognize that a hunter is approaching shortly after they move closer? While in a similar situation under the same cover and at the same distance, it continues grazing without a care as is an easy kill. Experience provided the answer to this dilemma, which, after many repeated scenarios, demonstrated that wind direction plays the decisive role. Other questions about what causes lightning and thunder or rainbows, illness, death, and a lot more besides were unanswerable and were simply accepted as a given. They found their place in the realm of the uncontrollable, the divine, something superior to man. This represented a very successful method of reducing the burden on the psyche with regard to questions that could not be answered at that time.
The approach to answering such questions and many more besides changed with the establishment in human thinking of scientific principles. A coherent answer required proof that was reproducible and universal. A statement about the gravitational pull of the earth had to be valid on every continent or also on ships in the case of the oceans. It goes without question for us today that objects everywhere on the earth accelerate in free fall toward the center of the earth. The physical law associated with this, which Newton formulated in the second half of the seventeenth century, is known as the law of gravitation and was supplemented by Einstein’s general theory of relativity in the twentieth century. These scientific laws mean that we know that masses attract each other, everywhere, throughout the whole universe. No salvationist or conspiracy theorist, no matter how charismatic, can deceive the general public into thinking today that this is not the case on the moon or any other planet.
This scientific way of thinking meant that things that had long been accepted as a given were gradually removed away from the realm of the divine. Thanks to the natural sciences, we only have a few things left in our world of thought whose explanation is still considered by some to be the creation of God. One aspect here is the Big Bang, which established itself as an explanatory model for the origin of the universe a few decades ago and has special character. And it is only thanks to physics that this aspect found its way onto the agenda at all. It opens up a dilemma based on the following fact: the explanation model for the Big Bang is the result of a consistent physical examination of the processes that took place after the Big Bang, postulated up to the present day, without any divine influence. Some scientists even take the difficulties involved in physically describing the actual start of the Big Bang as a reason to claim it as being God-given, since there is still no clear explanation for this. In the meantime, however, alternative ideas have been developed to explain the emergence of the universe. These ideas involve an infinitely vibrating expansion and contraction without the need for a single Big Bang as the initial stage [1, 2]. This theory has not made things any easier.
The second aspect which remains unanswered is the question posed at the beginning of this book on how life on earth originated. This question has probably moved humanity since we had the ability to ask questions. Directly connected with this are the “why?” and the “where to?” and the reflection on the meaning of life in general. In a world where intelligent thinking beings exist, but where scientific principles still remain unknown, solutions to answering major questions like this have to be found in other ways. From the beginning, the solution was called religion. It gave and still gives answers to topics that no one can understand through their simple experiences of the everyday world. The truthfulness or reproducibility of the statements is of no import here. What is important is to calm your own insecurities and fears, which inevitably arise when thinking in these “incredible” spheres.
1.2 What Exactly Is Life?
In addition to providing gains in fundamental knowledge, only natural science has contributed to depicting the complexity of life and has raised a wealth of seemingly unanswerable questions. It showed that life, which is so natural in its existence for us, is surprisingly difficult to define.
Don’t we just have to take a look around us to see what life is? No, because it’s not as simple as that. In science, an all-encompassing definition still does not exist that explains life and hence the starting point of life as well. This is a big difference to chemistry and physics, where, for example, theories exist to explain matter or forces that act. That said, we can at least specify criteria or characteristics that are key features of life and are accepted by all natural science disciplines. These are inevitably the physicochemical characteristics that make up a living biological system. And here you can already see that it should be a system that corresponds to our knowledge of biology. Some of the characteristics can also certainly occur in nonbiological systems; the combination and simultaneity sharpen the definition of a description of life.
At the top of the list of criteria is the existence of at least one cell, a compartment enclosed by a cell membrane. This is where the biochemical reactions take place that prevent the cell from dying or, in other words, ensure that it stays alive. Biochemical reactions require information storage, a metabolism for absorbing energy and for exchanging molecules from the environment, and catalysts for efficient chemical reaction chains. With a precisely tuned regulation, the interaction of all the components leads to the reproduction of cell components, and to growth, and the proliferation of the cell through its division. Added to this is the ability to adapt to changing environmental conditions and to develop into more complex molecular groups.
A small note in the margin in relation: what if we put all the functioning cell components of trillions of cells (except their cell membranes) in a large vessel or an almost closed hole in the earth and supply them with energy and add and remove the necessary molecules? All the processes that would otherwise take place in a cell (but without cell membrane-related reactions) would continue in this vessel. The products multiplied thus could enter other space through flow paths and multiply overall. Would we call this system life? We could simply take the position that we do not need this to any thought, because the molecular cocktail cannot multiply, of course. But the idea is still important, because the end of this book gives us model ideas about the beginnings of organic chemistry up to the formation of vesicles and cells, which come close to such a situation and therefore need to be isolated.
As early as in 1980, the theoretical physicist Gerald Feinberg and chemist Robert Shapiro tried to make the principle of life universally applicable to other possible forms of life in space. They concluded that life originates from interactions between free energy and matter. In this way, matter is able to achieve a greater order within the common system [3].
Today, we can imagine a colony of robots that extract the raw materials from which they are made on their own, process them into components, and use them to reproduce themselves. They would be controlled by a computer, and each would have outer shell and solar cells on their body to generate energy. The metabolism would be defined by the entire colony, and artificial intelligence would ensure adaptation to changing environmental conditions. Most of the components could even consist of organic chemical components. In contrast to biological life, which developed itself on a physicochemical basis, a colony of robots would be the result of human creation. Would we attribute the facet of life to this colony?
It becomes apparent that borderline areas exist that require longer discussion. From a certain point onward, the step toward life as we know it was taken. In the period prior to this, a transition from purely physicochemical to information-driven organic molecule formation must have taken place. This important period is narrowed down further in Sect. 8.3. Two further examples should show you how difficult it is to clearly describe life in just a few words. A group of experts around the chemist Gerald Joyce coined the definition: “Life is a self-sustaining chemical system with the capacity for Darwinian evolution” [4]. The US space agency NASA also uses it as a working definition. Stuart Kauffman, a US American theoretical biologist, on the other hand, focuses on self-organization: “Life is an anticipated collective asset of catalytic polymers for self-organization.” [5].
The definitions by Joyce and Kauffman focus on chemical systems, which consequently exclude technical forms of self-organization. Kauffman’s definition, however, would allow the thought experiment involving molecular soup in a larger vessel to be considered life. The robot community, which ultimately could be created by humans, would from a biological point of view almost approach what we consider to be life.
From the point of view of astrobiology, the search for a definition is important, because, in the search for life in space, the question could arise as to what signs of life we can accept as such (see more in Chap. 9).
1.3 Who Was LUCA?
On the basis of biochemical data, we have good reason to assume that all living beings on earth descend from just one ancestor. It must have been a cell that managed to grow and divide for the first time, actually leading to surviving daughter cells. The descendants needed to survive until they themselves divided again—a process which continues to this day. This first cell is called LUCA (Last Universal Common Ancestor), the last common ancestor of all living plants, fungi, and animals, including humans. For LUCA to form, a continuous production of molecules must have taken place long before that, which provided the necessary basic building blocks for the experiment we call life. These include organic bases, such as adenine or guanine, and amino acids or the lipids required for building cell membranes.
But building blocks alone are not enough. Spaces for reaction were also required where attempts to assemble more complex connections could take place. Small caverns or pores were sufficient in which the molecules could accumulate. Their concentration must have been at least high enough for them to meet and react with one another sufficiently frequently. A very large number of tiny labs were required, all linked to one another against a background of changing conditions, material replenishment, and the disposal of unusable components. That said, however, high-molecular concentrations also pose a new problem: the variation and the number of different molecules are so large that special selection processes are required to crystallize functional connections for life. The biological cell LUCA must have formed under such conditions as the most successful system that has ever been created on earth. From then on, planet earth entered into a unique development.
The composition of the atmosphere changed significantly as a result of photosynthetic bacteria and plants. While the level of carbon dioxide (CO2) reduced, the oxygen content rose continuously. Organic acids and later on plant roots and the activities of animal contributed to increased weathering. On the one hand, this resulted in increased erosion, but, on the other hand, with the onset of soil development and the formation of plant cover, a delay in erosion processes. This changed the water balance in the rivers, and also the type of sediments and their transport. Organogenic sediments such as coal and reef limestone were formed, which again had a direct influence on the composition of the atmosphere through the carbon dioxide balance.
And finally, human beings appeared on the stage, who brought about changes over a short period of time, the scope of which can only be compared to the impact of a large meteorite. Ultimately, everything we see today on the hard surface of the earth is the result of the successful propagation of LUCA. Without LUCA, even mountains would look different today, free of biogenic limestones and oxidized iron minerals, manifesting other forms of erosion, and free of lichen and bacteria films. One small exception may exist that is still evident to us, however. These are the very young volcanic structures that protrude from the earth void of vegetation. But here, too, LUCA has had its fingers in many pies. The frequently red surfaces of the lavas, which contain ferrous minerals that have been oxidized by atmospheric oxygen, can be seen from afar. They are evidence of a change in the atmosphere that began more than 2.4 billion years ago, when the mass production of oxygen caused by cyanobacteria led to a constantly increasing concentration in the atmosphere.
1.4 The Beginnings
“How did life actually come about?” This question was posed to the small group of scientists who had gathered for the first time in the university cafeteria in Essen. Everyone looked at each other and shrugged. The opinion of everyone present was unanimous: “It’s much too long ago to find out; we can only speculate; there’s nothing really tangible; the general conditions are hidden in the fog of the past”. “But’s that the reason why we’re meeting now, so we can talk about it in the first place,” interjected one of the colleagues present. “I have a slight suspicion”, I offered tentatively and began to sketch the model of a tectonic fracture zone on a napkin with a pencil…
Haven’t we all already asked ourselves the question of how life originally came about? Certainly, those of us who see consider the laws of science to be the basis for our existence. So far the answers are vague. And it’s not just about the beginning of life. It is just as important to clarify the phase in which the planet was born from which the preconditions were created that could give rise to life. Looking back even further, the birth of the solar system is also of fundamental importance. Its development is associated with decisive influences that still determine fundamental processes on earth. The beginnings go so far back that we know only too little about the earth in its infancy or about outside influences and processes inside the earth and on the surface. Numerous hypotheses exist about life, none of which is generally accepted. The group quickly came to the conclusion that the question of the origin of life is one of the most complex questions in science. One that is unsolvable! So why don’t we just let the matter rest?
No, we certainly won’t—thus the unanimous opinion of all those present, who continued to meet me on a regular basis after the first cafeteria meeting to discuss this new idea about the origin of life—with the attraction of researching something completely unknown, with its infinite number of question marks, as the motivating factor. This unified the group without any obligation to deliver results by any specific deadline, satisfying our curiosity alone, and perhaps simply identifying a small segment in the process and taking a first step toward a possible solution—that was certainly worth it.
From a scientific point of view, the search for an introduction to the origin of life immediately made it clear that nothing else we know matches the complexity of life. Life encompasses our entire earthly world view. In its complexity, its development has gone so far that simple attempts to explain individual processes seem almost impossible. Time was needed for this development up until the present day, an expanse of time that infinitely exceeds every human horizon in terms of experience, perhaps 3.5 billion, maybe 3.8 billion years, or more. This vast period of time, which was apparently necessary for an abstract thinking being to develop, makes an introduction to understanding the infancy of the earth up to the present day so difficult. The initial phase is immersed in a thick fog that seems impenetrable. Too many unknowns exist up until the present that make the environment in which life was created so complicated. What exactly do we know about the conditions on earth in its infancy? What was the primordial atmosphere, or the waters of the primordial oceans composed of? What proportions of organic molecules came from space transported by meteorites or comets; how much land surface existed and up until when? What influence did the moon have after it was formed?
In addition to planetary and geological unknowns, those relating to physical chemistry and biochemistry all exist. Which processes contributed to such a high-molecular concentration that allowed reactions from the simplest chemical building blocks to complex molecules to take place over long periods of time? How were these building blocks linked on earth—in an environment containing water, which is defined as a general precondition for life but to an extreme extent hampers reactions? They only take place today with the help of enzymes in the aqueous environment of the cell or in the lab in an organic solvent. For the earth in its infancy, only organic solvents like alcohols or ethers in very low concentrations are conceivable. Above all, we have to answer the fundamental question of how the chemical storage of the information came about that is contained in every DNA and which carries the entire development of the biochemical processes over a period of more than 3.8 billion years. One data chip in every cell: can’t we just read it out and identify the beginnings like that?
The answer is a resounding “no”. It is not without reason that many statements on this topic by past researchers echo the resigned statement that it will probably be impossible to fully determine, and let alone explain, the processes that lead to life.
1.4.1 But How Did It All Begin?
Every research project is underscored by a history, one longer and the other shorter. The history of the research into our own origins began at the end of the 1980s in Westerwald while processing the 20 million year old volcanoes in this region in the Rhenish Massif in Germany. In the course of investigations, structures became apparent that could only be explainable through special supra-regional tectonic processes and fracture structures in the crust, but this could not be clarified. It was only later on, after the turn of the millennium, that the opportunity came about to investigate the formation of fracture structures further in the neighboring region of the Eifel. The mapping of tectonic fault zones, which provide a vertical gas-permeable connection to the earth’s mantle, delivered a surprise. Every time fracture zones and the escape of gas—mainly of carbon dioxide—were identified, the presence of hill-building forest ants was identified locally at the same time. The correlations were so obvious that, based on personal experience, predictions about forest ant sites could be made based on geological knowledge alone, an absurdity in biology! The observations gave birth to a new field of research that sparked many discussions. Initially, there was rejection from both sides representing geology and ant research, which, after a long period of intransigence, eventually led to a successful collaboration with entomologists. The search for the causes as to why the representatives of the forest ant genus Formica settle on gas-permeable fault zones led to considerations in all directions. Is it the moisture, heat, biofilms in the fissures, or substances that rise to the surface besides the gas and possibly feed the ants or bacteria in the biofilms? Does the CO2 present help to prevent the fungus on eggs and larvae or keep enemies and parasites under control at high gas concentrations which they are able to tolerate? Or does the carbon monoxide that rises in small concentrations along with the carbon dioxide have any function at all? How is it removed from the hemolymph, the circulating fluid, or “blood” of insects? Do forest ants use this to form formic acid—their chemical weapon—with a molecule of water?
The more extensive realm of the fault zones came more and more into focus with all these considerations. Weren’t all the raw materials required for forming organic molecules present here? The pressure and temperature conditions can also be found to exist there that are defined in technical-chemical processes, such as those found in the Fischer–Tropsch synthesis to form hydrocarbon compounds (see Sect. 3.1). This synthesis can, for example, be used to produce synthetic gasoline. Enough metal compounds also exist that can serve as a catalyst. Could all these things possibly be the key to the ants’ preference for building their nests on these faults?
And then suddenly it clicked! These represented the ideal conditions for the early stages of life: a protected space, available for millions of years, with all the raw materials required and an infinite number of small reaction spaces, in which different pressure and temperature conditions and pH values existed and still exist today.
The idea for developing a model for the origin of life was born. From that point onward, the evening meetings between Oliver Locker-Grütjen and his scientist colleagues came into play, and these continued to take place over a period of more than 10 years. Representatives from the fields chemistry, biology, physics, physical chemistry, bioinformatics, microbiology, and my discipline, geology, met. It was the relaxed atmosphere with food (usually cooked by Hans-Curt Flemming) and wine and beer that ultimately helped to let all those gathered discuss the question that concerned everyone. And then following the disillusionment, none of us really knew anything about the specific question being asked. Certainly, the common knowledge around the beginnings of this discussion was known, the old ideas and the initial investigations. Even the newer developments, which had been repeatedly published in the media, were familiar to us. But the actual core questions concerning cell formation, information storage, and enzyme formation, like for all our colleagues worldwide, remained completely in the dark. As a result, we began to research, lectured each other about newly learned subject matters, invited colleagues who already made a name for themselves in this field of research to colloquia, and even planned initial experiments designed to have something to do with a tectonic fault environment in the continental crust.
The suspicion arose more and more, however, that the field resembled an impenetrable fog with no beginning or end. The progress our considerations were making slowed and seemed to be approaching a limit. In a side note, I attempted to ascertain the meaning of CO2 in the development of molecules like amino acids or organic bases. While doing so, I remembered a paragraph from the book Chemical Evolution by Horst Rauchfuss [6]. At his advanced age, we even dared to invite him to a lecture in Essen, which he gladly accepted. The paragraph in his book addressed reactions in CO2 excesses which are meant to be beneficial for the formation of organic molecules. Since no one could remember the passage, I researched it on the Internet the next day. Even the first links returned by the search query contained information in their sublines that hit me like a bolt of lightning and changed everything. When the temperature rises above 31 °C (304,15 K), CO2 is present in the supercritical phase down below a crust depth of about 740 m (73,75 bar).
This fact, which was taken for granted, had simply disappeared in the enormous fog we were poking around in. I just reported my new discovery to Christian Mayer. As a physical chemist, he immediately knew that this gave us a tool that opened up a whole new world, a solvent which we could use to carry out reactions that were impossible on the earth’s surface.
Further coincidences also occurred that led to the point where we are today. One of them was the introduction of a new software system for the university’s administration. This led to the financial situation for all the faculties seemingly disappearing from sight into a kind of primordial fog for more than 2 years. During this period, considerations concerning the purchase of a high-pressure system matured. Since our traditional sources of finance persistently refused to support our research, I purchased the system we required from supposed personal financial reserves. It represented the most important action taken in all my research. Each experiment undertaken brought results that represented something completely new and were a big step toward understanding the processes in place when life began. Once the financial fog had cleared, the precise six-figure sum that the system had cost had accumulated as debt in my household account. Without this fog, I would never have dared to incur such an enormous debt, and the experiments would never have been undertaken.
Another circumstance was very fortuitous as well. Whereas time constraints forced the group to be reduced to just a few participants, a new colleague, Oliver J. Schmitz, joined from the field of analytical chemistry. His new, state-of-the-art lab and his immediate willingness to work on the project offered us the first opportunity we had to analyze the low-molecular concentrations from our high-pressure experiments carried out by Maria Davila-Garvin. The series of experiments undertaken by Christian Mayer on cyclic vesicle formation led to a special analytical challenge, which the analyist Amela Bronja ultimately took on and with success.
A multitude of lucky coincidences of a small and large nature also occurred that ultimately led to contacts and progress being made, without which many aspects would have become bogged down in the introductory phase. This includes the connection in the field of geosciences to Heidelberg to the research groups under Heinfried Schöler and Frank Keppler, who had a significant involvement in the investigation of the fluid inclusions in hydrothermal quartz (along with their employees, Ines Mulder, Tobias Sattler and Markus Greule, and Mark Schumann from my research group), and to Jonathan Williams from the Max Planck Institute for Chemistry in Mainz, who cultivated a whole network of important contacts in the USA. Gerald Dyker from Bochum, as an organic chemist, also provided several suggestions for further experiments. Last but not least, I need to mention the cafeteria principle, which made direct exchange and “further education” possible in a subject that was relatively foreign to me. The physical chemist, Christian Mayer, the biochemist, Peter Bayer, the bioinformatician, Daniel Hoffmann, and the analyst, Oliver J. Schmitz, were all targets for my barrage of questions and sometimes excessive ideas. But it brought results: the cafeteria meetings followed by coffee proved to be the most effective form of scientific exchange.
If, like us in the Essen Group, you are addressing the topic of the “origin of life” for the first time, the first thing that arises are fundamental questions concerning the state of science. What do we know so far, what considerations did past researchers have, and what experiments were undertaken that provide hint at the beginnings from which life may have developed?
A number of natural scientists have dealt intensively with the question of the origin of life over the past century. It all started with a cautious approach to the complex topic, which could only be treated theoretically at the beginning. Experiments did not begin until the middle of the last century, which gave the first indications of the possibility of a biochemically based start to life. As expected, the success of early research could only be very limited. Little was known about the general conditions, such as the planetary and geological development of the earth or the influence of astronomical quantities. The compilation of older works listed in the later chapters in this book provides insights into how every time new discoveries were made new ideas were developed. Chapters 2–4 summarize the current state of knowledge in brief, which focuses on the planetary and physicochemical preconditions. This alone results in a multitude of important fringe conditions for the development of life as we know it. The essentials of the models made known to the general public in recent decades are presented here. These take the form of explanatory approaches that offer a broader basis and contain more than just one section in an interesting series of reactions. The consideration takes place in the context of more recent planetary knowledge. We need to take previous researchers into account here who were forced to take the current state of knowledge as their starting point back then, which, of course, placed limits on the statements made in their models.
In recent years, research on the origin of life has received an increased impetus around the world. New findings from the analysis of meteorites, the ocean realm, and continental crust have led to new model ideas, which make it possible to perform laboratory experiments for the first time under robust general conditions. Furthermore, documents from the early days of the earth have been discovered that demonstrate the beginnings of organic chemistry. All the data now available means that we can see a first blurred outline in the fog of the past, which makes one thing clear: the riddle about the origin of life can be solved. However, it is already becoming apparent that it took a physicochemical process over an extremely long period of time to develop the cell, which is considered the last common ancestor of all life on earth.
In the evening discussions undertaken by our “Origin” group, one aspect existed that resonated latently in the background: if we succeed in developing a notion about the early stages of life, does that mean we can also transfer this model to other planets in the universe? Or even crazier than that: can we conclude from this that there are other planets in space with some form of life, perhaps a higher form? It soon became clear: it was not out of the question. But the chance of an intelligent developing will quite so often not be found elsewhere.