One of the questions that has troubled humankind ever since it began to think about its position in the universe concerns its own origin and the origin of life in general. There are two basic alternatives for answering this question. Either we believe that life has a supernatural origin (such as being created by a transcendental God), or we think that the origin of life can be explained in a scientific way. This means that we try to describe an empirical scenario in which life originated through the workings of natural mechanisms. Here we are interested in the second alternative, but we must also say some words about the first one because the origin of life still is a problem about which religious, metaphysical, and scientific positions quarrel with each other.
Regarding the first alternative, speculating on a supernatural origin of life can be traced back to prephilosophical thought that is handed down to us by many orally transmitted and ethnologically collected myths of origin. The history of religions and metaphysics prolongs this mythical quest for origins. For example, the biblical Genesis reports that the world, all living beings included, was created by a personal God who himself is not part of his creation. Because the existence of such a transcendental creator is not an empirical fact that could be verified or falsified by scientific methods, the adequate cognitive attitude toward reports of creation like the biblical one is to either believe it or reject it for subjective reasons.
Nearly the same is true for metaphysical accounts of the origin of life. Metaphysics is a functional substitute of religion. It is the attempt of the human mind to develop a complete conceptual picture of the world that is structured by logical reasoning. To be complete, any metaphysical system must answer our question about the origin of life. The speculative assumptions with which a metaphysics starts its reasoning are not of an empirical nature and impregnate all other metaphysical propositions with irrefutability because ad hoc explanations that agree with some metaphysical principle can be found anyway. Therefore, one must either believe a metaphysical system or reject it for subjective reasons that must be expressed in a coherent way.
For want of an empirical procedure of deciding which metaphysics is false, any metaphysical account of the origin of life, be it based on concepts such as entelechy (the Aristotelian tradition) or vital élan (the Bergsonian tradition), is not up to elementary standards of modern science that must be met if we want to follow the second way of answering the question about the origin of life: to describe an empirical scenario in which life originated by the workings of natural mechanisms.
Since the times of Galileo Galilei and Isaac Newton, modern science has searched for an empirically based and mathematically expressible description of all natural phenomena, its living inhabitants included. The aim is to acquire intersubjectively checkable and systematic knowledge of mathematical relations between empirical phenomena. Modern science approaches its aim by making controlled and replicable experiments with isolated natural systems. Of course, it also relies on hypotheses that are not themselves empirically verifiable (such as the supposition that nature can be described mathematically). Yet this is no drawback because, altogether, those hypotheses demand that any specific scientific proposition must be falsifiable by empirical data.
Any attempt to blur the boundary between religious belief and scientific doubt must be criticized. The result of mixing them up could only be a way of thinking that would lead to a religious ideology and a pseudoscience at the same time. It would let religious people lose the only legitimate source of their belief out of sight, and it would hinder science to follow its own dynamics of proposing and criticizing theories without ideological restrictions.
This is especially true when it comes to the origin of life. The empirical science that must find this origin is biology. Its general framework consists of evolutionary theory, which states that life is not a static phenomenon but rather a dynamic process. Life has a natural history; existing species die out, and new species are born. In the history of biology, different evolutionary theories gave diverse answers to the question of how this process of speciation happens. Today, the evolutionary theory bearing the name of Charles Darwin is the theory that is accepted by nearly the totality of active biologists.
Since its second edition, Darwin’s main work, On the Origin of Species, seems to give an answer to our question about the origin of life. This is no surprise because an evolutionary theory of life that takes into account the finite age of Earth must also explain the origin of life. But the famous last sentence of the book might be surprising because it states that the “several powers” of life have “been originally breathed by the Creator into a few forms or into one.” Yet there is a plausible explanation for this religious stance. In the first edition, published in 1859, the prepositional phrase “by the Creator” was missing. Darwin added it to the second edition, published in 1860, because his secular account of the evolution of life suggested an equally secular explanation of the origin of life, but he did not want to bother his wife, Emma, who was a devoted Christian. Darwin himself was rather agnostically minded.
Scientific research on the origin of life tries to explain how the workings of physicochemical mechanisms led, in the boundary conditions of primitive Earth, to the development of material systems that possess necessary properties of life. Modern evolutionary theory assumes a natural continuity between the dynamics of prebiotic chemical systems and the evolution of the first living systems. It does invoke neither a supernatural principle nor some unknown natural force to explain the transition from the nonliving to the living. Older ideas of vitalism also proposed that the origin of life can be scientifically explained, but only if one also assumes the existence of some mysterious force that should be effective just in organisms. Because such a force was not found and even turned out to be unnecessary for explaining the structure and function of living systems, modern theories of the origin of life are reductionistic and mechanistic; they explain the origin of life exclusively by physical and chemical mechanisms giving rise to complex natural systems that we call “living.”
One cannot get rid of our problem by proposing that life was imported onto Earth from some extraterrestrial origin, be it by chance (on an asteroid) or intentionally (by an intelligent extraterrestrial civilization). Wherever life appeared for the first time in the universe, its origin at that place must be explained. Although, according to the hypothesis of an outer space origin of life (put forward by, for example, the English astronomer Fred Hoyle), life on Earth did fall from the heavens, the first of its extraterrestrial precursors could not do so. Thus, let us concentrate on the problem of how life originated on Earth without hoping that some extraterrestrial biologist will come to solve the problem for us.
The main conceptual prerequisite for answering our question involves a clear definition of life. This is not an easy task. We must combine necessary characteristics of living organisms in an evolutionary perspective. A modern biologist will specify at least three such properties. First, an organism has a biochemical metabolism. Second, it reproduces itself by transmitting genetic information to its descendants. Third, these descendants may differ from their parent because mutations and recombinations can occur in the transmitted genetic information. From an evolutionary perspective, these properties must be interpreted as features of systems that live in ecologies where natural selection occurs. The selective struggle for existence between such systems arises whenever a population of varying natural systems such as macromolecules or organisms self-reproduces under restrictions on growth.
Therefore, do we need not a single origin of life but rather as many origins as there are necessary characteristics of life? In this way, the physicist Freeman Dyson proposed that we must accept two separated origins of life: the origin of metabolism and the origin of self-reproduction. Of course, one could add the origins of other defining characteristics of primitive organisms such as having some kind of membrane. The more we know about the necessary structural elements of early life forms, the more origins we get—until we have a sequence of many origins of life. But then we must ask what the first origin of life was. So we are back again at the very same point from which we started.
We do not need, however, to follow this line of thought. The previously mentioned characteristics of life are not standing separately next to each other. There is, for example, no simple dualism between information-based replication and biochemical metabolism. The behavior of an organism—made energetically possible by its metabolism—aims, after all, at transmitting the genetic information that is needed to construct the structural prerequisites for metabolism. Because of this causal and functional dependence of metabolism on genetic information, we must assume that the origin of genetic information is identical to the origin of life. The origin of life is nothing other than nature’s first discovery of information processing via codes, that is, the evolutionary establishment of a primeval language. We are able to integrate mutability into this overall picture as well because the replication of genetic information in real chemical systems is often incorrect.
If the origin of life is identical to the origin of genetic information, can we infer that the origin of genetic information is the origin of deoxyribonucleic acid (DNA)? It seems so because molecular genetics describes inheritance as a complicated dynamics of storing, transmitting, processing, and transforming information where DNA acts as the material carrier of genetic information from parents to offspring. But the case is not so simple. According to current knowledge, there was a ribonucleic acid (RNA) world before the DNA world of today. Nowadays, the different types of RNA play important roles in the cell (such as transferring genetic information out of the nucleus in eukaryotic cells), but they do not act as the material carrier of the genetic information flow from parents to offspring.
The assumption that there was an RNA world solves an important evolutionary problem. To reproduce genetic information, proteins that act as catalytic enzymes in this process are needed. To produce such enzymes in a cell, their structure must be encoded in genetic information. To have this information in a cell, it must be copied before in the ancestor cell. To reproduce genetic information, those enzymes are needed and so on. RNA breaks out of this circle because RNA can store information in the sequence of its building blocks and catalyze its own replication enzymatically.
What is more, the biologist must also take into account the possibility that there was a precursor, or even a sequence of precursors, of RNA. The detailed chemical structure of such primeval information carriers is not known. Therefore, experimental research on the origin of life is of utmost importance. After a theoretical prelude during the 1920s (Alexander Oparin and J. B. S. Haldane), this kind of research started during the 1950s with the famous experiments of Stanley Miller and Harold Urey, who were able to synthesize, in a laboratory simulation of the atmosphere of primitive Earth, a multitude of organic compounds such as purines and pyrimidines (the bases that are components of the fundamental building blocks of RNA and DNA) and amino acids (the building blocks of proteins). Since Miller and Urey’s experiments, great progress has been made in experimental research on the origin of life. One could especially note experiments with viral RNA, which self-replicates in a test tube without any surrounding cell so that one can start a selective evolution of information-bearing macromolecules in the laboratory (Sol Spiegelman).
In spite of this experimental progress, until now no biologist could experimentally reconstruct the historical origin of the first life form on Earth. It is even probable that we will never be able to simulate the real history of the transition from the nonliving to the living on primitive Earth. This impossibility can have two reasons.
First, one might suppose that the origin of life was a single event that happened against all odds. Because there are so many possibilities of how to combine all the compounds that are necessary for the spontaneous assembly of the first self-replicating macromolecule, it seems to be a priori very improbable that they all came together in the right concentration at the same place and at the same time (Jacques Monod). If life originated in such an extraordinary moment of absolute randomness, it is a realistic assessment that we cannot reproduce it in the laboratory because we would need to simulate successively an unforeseeable number of those possible chemical scenarios.
Yet the idea of a random origin of life underestimates the role of law-governed dynamic processes that can lead, from a diverse set of initial conditions, to the origin of life in many possible courses of natural history. But even according to this second conception, it is rather probable that the real primeval history of life will never be reconstructed in all details. We do not have enough information about the concrete circumstances of the transition from prebiotic to biotic evolution. So we can define only the class of possible histories that could have led to this transition. To do so, we must apply to the origin of life the very same theoretical idea that is used to explain the origin of species—natural selection.
How can we extend the Darwinian logic from biotic to prebiotic evolution? First, we must show how a set of chemical compounds can self-organize into a self-replicating system of macromolecules. For example, let there be three different kinds of macro-molecules: A, B, and C. Now imagine that the rate of self-replication of B is proportionally dependent on the concentration of A, the rate of self-replication of C is dependent on the concentration of B, and the rate of self-replication of A is dependent on the concentration of C. So we get a closed cycle of macromolecules that depend on each other to maximize their rate of replication. As such, a cooperative structure, called “hypercycle,” arises (Manfred Eigen and Peter Schuster).
Second, we must realize that such hypercycles are systems that self-reproduce (by reproducing their components cooperatively), have a metabolism (by importing the building blocks of their components and energy from their environment), and are mutable (by changing the structure of the components due to, as an example, the thermal movement of the involved molecules). In sum, hypercycles show all the phenomenological characteristics that are necessary for calling a system “living.”
Third, we must see hypercycles as self-replicating and varying systems from a Darwinian perspective. In a common prebiotic ecology on primitive Earth, they would act as selective units that struggle against each other for chemical and energetic resources. As such, a process of natural selection among hypercycles would start, so that we are encouraged to call them “living” even from an evolutionary point of view.
Fourth, we must connect this rather formal model with our considerations on the first material carriers of genetic information. In a hypercycle, a nucleic acid—be it RNA or some precursor—could reproduce autocatalytically. Of course, it would not store information about how to construct something other than its own structure. But if there arises some code by which the nucleic acid can control the synthesis of other macromolecules such as proteins and which helps to better replicate its chemical structure, our hypercycle would enter the next evolutionary phase of natural information processing. If this happens, our information-theoretical criterion of life would be fulfilled; it would identify the origin of life with the origin of genetic information, which is based on a code.
By elucidating even the origin of life, Darwinism shows its explanative power. Abstracted from the struggle for existence between macroscopic organisms and applied to the level of microscopic molecules, the Darwinian logic of variation, inheritance, and selection can explain the origin of prebiotic self-reproducing systems, which are the very first ancestors of what we are—macroscopic organisms.
If we accept Darwinism as an explanation of the past origin of life on Earth, we are urged to look into the future and to ask whether there could appear new forms of life—via new types of genetic information— by the same mechanism of Darwinian selection. This might occur not in the chemical ecology of our ancestors but rather in complex computer systems, where varying self-replicating programs could fight against each other for the resources of an electronic ecosystem without being controlled by some human programmer.
References:
- Dyson, F. (1999). Origins of life (rev. ed.). Cambridge, UK: Cambridge University Press.
- Eigen, M., & Winkler-Oswatitsch, R. (1992). Steps toward life: A perspective on evolution. Oxford, UK: Oxford University Press.
- Küppers, B.-O. (1990). Information and the origin of life. Cambridge, MA: MIT Press.
- Maynard Smith, J., & Szathmäry, E. (1999). The origins of life: From the birth of life to the origins of language. Oxford, UK: Oxford University Press.
- Popa, R. (2004). Between necessity and probability: Searching for the definition and origin of life. Berlin, Germany: Springer.
