Table of contents
Life is the set of characteristics of living things that manifest biological processes such as homeostasis, metabolism, reproduction and evolution, where:
- Homeostasis is a self-regulating mechanism through chemical reactions designed to maintain an internal balance when external conditions change. In this sense, a living organism can be seen as an open system.
- Metabolism is the set of chemical transformations by which the energy produced and used by the organism is managed, and is called catabolism when it breaks down organic matter to release energy (e.g., cellular respiration), and anabolism when it builds up the same matter to use the energy itself.
- Reproduction is the process by which living organisms (called parents) produce other individuals of the same species (called offspring). In cellular organisms (Cytota), when this occurs by cell division, it is called mitosis, in which the one parent cell disappears and produces two daughter cells, while when it occurs by other more complex mechanisms, typical of multicellular organisms, it is called meiosis.
- Evolution is the product of changes in characters that are transmitted hereditarily to successive generations. Several factors contribute to this change, such as genetic mutations (although they are usually individually insignificant, their slow accumulation can lead to the appearance of new characteristics), natural selection, and genetic drift. Their effect determines the evolution of traits up to the appearance of new species.
Biology, or the science of life, has led to its recognition as an emergent property of the complex system that is the living organism. The idea that it is supported by a “life force” has been the subject of philosophical debate, pitting proponents of mechanism against proponents of holism, about the existence of a metaphysical principle capable of organizing and structuring inanimate matter. The scientific community has yet to agree on a universally accepted definition of life, avoiding, for example, the qualification of systems such as viruses or viroids as living organisms.
Scientists agree, however, that every living thing has its own life cycle during which it reproduces and adapts to its environment through a process of evolution, but this does not imply life, because every characteristic that living things have can be found in other situations that are not considered living, for example, some software viruses that have a life and reproduction cycle in their computer environment but are not alive, or some crystals that grow and reproduce, and many other examples. A more basic set of characteristics of life has been advanced, such as a system composed of homochiral molecules, maintained in homeostasis, and capable of autocatalytic reactions (James Tour).
The forms of life that exist or have existed on Earth are classified as animals, chromists, plants, fungi, protists, archaea, and bacteria.
The spontaneous generation theory and creationism
Questions about the origin of life have a very ancient history, going back directly to the theory of spontaneous generation, according to which sometimes even very complex organic beings can originate from organic matter, preferably decaying matter. This theory, widespread since antiquity, was supported by Aristotle himself and was enriched with magical and fantastic elements throughout the Middle Ages. Even at the end of the 16th century, a Flemish alchemist physician, van Helmont, claimed to have subjected the theory to experiments and to have derived positive results from it, claiming to have witnessed the spontaneous generation of mice, frogs, scorpions, etc.. But in the 17th century, the Aretine physician and humanist F. Redi demonstrated experimentally, at least for insects, that new offspring are indeed derived from eggs previously laid by adults of the same species. In the 18th century, however, the resource theory and its proponents, including the English clergyman J. T. Needham and the great French naturalist Buffon himself, believed that the tiny creatures (e.g., infusoria) that could be observed with the then-invented microscope must be spontaneously produced.
It was L. Spallanzani’s famous experiments that defeated the theory of spontaneous generation for the second time, but in the nineteenth century it found new adherents in France, notably a naturalist from Rouen, F.-A. Pouchet, who, in a volume published in 1859, recounted its history and reported his favorable experimental results. During this period, the problem of spontaneous generation became more and more entangled with the more general problem of the origin of life; historically, the proponents and opponents of the theory are not clearly connoted from the point of view of a worldview, since vitalism, vitalistic materialism, and spiritualism have from time to time underpinned this doctrine. However, the debate that arose in France around 1860 developed between two precise philosophical conceptions: materialism on the one hand, and spiritualism on the other.
For French public and scientific opinion at that time, those who supported spontaneous generation supported the materialistic origin of life. Despite the brilliant experimental results obtained by Pasteur and his own way of presenting them as conclusive, giving them the value of decisive experiments (“the doctrine of spontaneous generation will never recover from the fatal blow dealt to it by this simple experiment”), his experiments did not definitively settle the issue. Today, the spontaneous generation hypothesis is still used to explain the origin of life, but no longer at the organic level, but at the chemical level.
Also historically important was the creationist theory, according to which the origin of life must be attributed to supernatural intervention. It is essentially based on a literal interpretation of the first chapter of Genesis, which describes the creation of all living things by God. This theory is fundamentally untenable from a scientific point of view, because it is incapable of providing a plausible scientific explanation of the origin of life based on experimental data, and indeed the results that science has achieved in this area of research sharply contradict this conception. The supposed final defeat of the theory of spontaneous generation and the impossibility of accepting the creationist theory have led some scientists to argue for the eternity of life. Perhaps the clearest position in this regard was that of the German physicist H. Helmholtz, who expressed himself as follows: “It seems to me a perfectly correct scientific procedure, when all our attempts to obtain the formation of organisms from inanimate matter fail, to wonder whether life ever came into existence, whether it is not as old as matter.
The cosmic origin and panspermia theories
As an alternative to the theory of the eternity of life, the cosmic origin of life has also been proposed. This theory is based on the hypothesis that the planets, due to their rapid motion, are constantly shedding solid particles which, traveling through interplanetary space, can reach the surface of other planets and bring with them viable spores of microorganisms. Meteorites have also been suggested as possible pathways between planets. A variant of the cosmic origin theory of life with claims to scientific validity was “panspermia”.
Arrhenius, for example, became a proponent of the idea that life is everywhere in the universe, and demonstrated by simple and suggestive calculations that small particles can be transported from one planet to another, especially by the effect of light pressure and upward air currents originating at the sites of large eruptions.
The primordial soup theory
The hypothesis first formulated by A. I. Oparin in 1922 and definitively developed in 1936 is the one that has long been the most widely accepted by scientists, albeit with variations in detail: according to this hypothesis, the cooling of our planet would have led to the formation of a primordial atmosphere in which carbon would already have been present. At a later stage, carbon would have come into contact with the other elements that made up the Earth, in particular heavy metals and iron in particular, and would have reacted chemically with them to form the most stable carbon compounds, namely carbides. All these chemical reactions took place, according to Oparin, under the Earth’s crust, which was not yet in contact with the atmosphere of the forming planet, an atmosphere, moreover, that initially lacked both nitrogen and oxygen in the free state, but was rich in water vapor and halogens.
The transformation of the atmosphere, the formation of primordial oceans and the chemical reactions of the crustal compounds led to the transformation of carbons into organic substances (primordial soup), from which microscopic aggregates (coacervates) of organic nature originated, direct precursors of life understood in its evolutionary aspects. A first confirmation of the hypothesis of an original reducing atmosphere of our planet came in 1952 from H. C. Hurey, who showed that in the presence of excess hydrogen, methane, ammonia and water are the stable forms of carbon, nitrogen and oxygen. On the other hand, the cosmic dust clouds from which the formation of the Earth is now thought to have originated contain excess hydrogen.
The experiment carried out by the American S. L. Miller in the early 1950s proved the possibility of the formation of chemicals typical of living organisms in conditions where no life form exists. He assumed that the primordial atmosphere was similar to that of Jupiter, i.e. highly reducing, because it was rich in methane, ammonia, hydrogen and water vapor. By subjecting these inorganic compounds to electrical discharges designed to simulate ultraviolet radiation and atmospheric lightning, he achieved the synthesis of some amino acids.
Another step towards the artificial synthesis of proteins, the constituents of the living cell, was taken by S. W. Fox and K. Harada, who succeeded in obtaining polymers composed of 18 amino acids by means of heat treatment. These polymers, also called “proteinoids”, have characteristics that make them qualitatively similar to today’s proteins and the components of the cell nucleus, nucleotides, were synthesized again under abiological conditions by Fox and Harada in 1961 and by C. Ponnamperuma in 1964. It was therefore possible to hypothesize the formation of a primordial soup rich in protein-like organic matter, which gradually coalesced to form coacervates, which in turn gave rise to the earliest forms of life.
The primordial RNA world theory
The original hypothesis of a reductive primordial atmosphere has never found much support among geologists, who believe that the primordial atmosphere was rich in carbon dioxide and molecular nitrogen, similar to today’s, but lacking in oxygen, and therefore more neutral. Moreover, in view of the current knowledge of the dynamics of the lithosphere, the endogenous and exogenous phenomena acting on it, and the action of light rays, it seems unlikely that large accumulations of organic matter could have formed: minerals remove organic matter from the water that accumulates in the oceans, just as the action of ultraviolet rays is to destroy biological molecules, not to promote their synthesis. Consequently, to justify the appearance of the first organic molecules, it is necessary to assume a different chain of events.
The theory of a primordial RNA world, according to which these molecules, or others very similar to them, were the first step in the formation of today’s living beings, has received a great deal of attention. In fact, RNA has catalytic abilities, can duplicate itself, and coordinates protein synthesis. Recently, it has been shown that modified purine bases exist in nature that react by spontaneously combining. These probably played a crucial role in giving the first RNA molecules the enzymatic properties necessary for the formation of structures with catalytic functions (ribozymes), and in particular for the formation of hydrophobic “pockets”, which are indeed characteristic of ribozymes and represent catalytic binding sites.
But how could these first small organic molecules, so rare and dilute, meet and aggregate? The most recent hypotheses take into account the rocky structure of the Earth’s crust, and in particular the clays, whose presence and abundant distribution were remarkable even in the primordial rocks. To substantiate this hypothesis, let us begin with the chemical analysis of clays: these are sedimentary clastic rocks characterized by very small fragments, no larger than 2 microns, formed by the weathering erosion of preexisting rocks.
Lithification, that is the slow transition from free fragments to coherent rocks, occurs through a series of processes known as diagenesis: the compaction of materials involves the action of the superimposed weight of successive layers, which reduce the voids by compressing those below them, while cementation is essentially due to water, which, circulating between the pores, dissolves certain substances that gradually precipitate and cement the granules together; the most common cements are calcite and silica. The latter polymerizes very easily, removing water molecules and then depolymerizing, depending on the acidity of the environment in which it is found, and in neutral pH environments it can react with metal cations, particularly those of magnesium, sodium and calcium, to form a mixed polymer.
It has been shown that in such a chemical structure, the interposition of organic molecules is possible and detectable, which can be neatly layered in the clays, if not even chemically react with them. Thus, the possibility that clay materials played a role in the selection of certain organic complexes over others and their arrangement in a specific order cannot be ruled out.
According to one school of thought, the conservative option, which assumes that organic molecules have always looked like they do today, clays would have acted as sites for collecting materials and as catalytic factors in their assembly. In fact, clay substrates would have been able to facilitate the polymerization reaction of organic molecules from their respective monomers without energy input, facilitating their proper orientation for the actualization of bonds; moreover, they would have “offered” proton donor groups placed on adjacent layers. Given the current trend that sees RNA as the first step toward the construction of complex organisms, it has been shown that the temperature and pH conditions present in sulfide-rich clay structures could have provided the ideal physical location to contain, concentrate, and facilitate the assembly of the first ribonucleotides; however, this assertion is not yet supported by significant experimental data on the primordial synthesis of these monomeric elements.
The radical option hypothesizes the existence of primordial organisms composed of clay components that were later replaced by the present ones. This more extreme hypothesis is based on the consideration that we cannot exclude the ancestral existence of genetic material, i.e. capable of storing information and transmitting it to subsequent generations through reproduction, which was completely different from what we have today: during evolution it was gradually replaced by more efficient systems such as RNA and later DNA. Such an early origin of life hypothesizes the formation of crystalline mineral genes, composed of clays with a particular distribution of charges, or provided with characteristic surface structures, or constrained by particular stacking rules; these structural features could represent “information” for growth, which could only take place according to the same rules as the parent clay.
Such a situation would have facilitated the uptake, local concentration increase, and assembly of the first organic materials, which could only occur by following the structural rules imposed by the clay itself. Thus, the first organic systems, corresponding to the phenotype, would have been organized by obeying the rules imposed by a control system of mineral origin, which in this case would represent the genotype. Once the ability to duplicate had developed, such organic materials, probably the first RNA molecules, would no longer have needed this genetic control, and later natural selection would have favored them because they were able to store and transmit genetic information more efficiently: in fact, knowledge from geology shows that environmental variations produce changes in the deposition and conformation of all rocks, and especially clays; this would correspond, in genetic terms, to a loss of information.