Ecology: Biology of Interaction. II-14. (Supplement) Some Stages in the History of Life on Earth
The development of life has led to a fundamental transformation of the Earth's surface. What do we see when we look around beyond human settlements? One or another landscape covered with vegetation characteristic of each region. In the vast majority of places, rocks are covered with a layer of soil. Here and there the earth's surface is cut by watercourses — streams and rivers...
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II-14. (Supplement) Some Stages in the History of Life on Earth
The first appearance of life at the time of biosphere creation could not have taken the form of a single organism appearing, but must have taken the form of an assemblage of organisms corresponding to the geochemical functions of life.
V. I. Vernadsky
There is a widespread view that at a certain stage in Earth's history a "first" organism arose, whose descendants consumed all the reserves of organic matter (the "primordial soup") and gave rise to all forms of life. Of course, such views are extremely naive. The emergence of an individual organism would have required an improbable coincidence. Life arose as the result of the gradual complexification of geochemical cycles through the selection of autocatalytic reactions that supported their individual stages. Even before living organisms arose, these cycles involved both the creation and the destruction of organic matter. This meant that life did not arise in the form of individual organisms, but in the form of ecosystems that ensured the cycling of matter.
Biochemical "innovations" that arose at one stage of the geochemical cycle could be transferred to other stages as well. This is how the means of energy storage, matrix synthesis of biopolymers, and finally the cellular organisation of living systems must have spread.
In modern ecosystems, cycling takes place on the basis of the formation of organic matter by autotrophs and its destruction by heterotrophs. The metabolic waste of each of these groups of organisms serves as a resource for the other group. Their remarkable complementarity is a consequence of the fact that such cycling is older than life itself.
One of the oldest sedimentary rocks known to modern science was found in the Isua Formation in Greenland. The age of these rocks is 3.7–3.8 billion years. These rocks were formed at depth, near the equivalent of a black smoker; carbonaceous inclusions have been found within them. Isotopic analysis of these carbonaceous inclusions indicates that they are remnants of living organisms. The oldest remains of organisms with complex structure were found in Australia in the Warrawoona Formation, dating to 3.5 billion years ago, and in South Africa (Onverwacht Formation, 3.4 billion years ago). These are cyanobacteria very similar to modern forms. This similarity extended even to their biochemistry. For example, in rocks formed 3.1 billion years ago, degradation products of chlorophyll and phycobilins — pigments exclusive to cyanobacteria — have been found.
Characteristic of the Proterozoic (and to some extent the Archaean) are sedimentary rocks called stromatolites (literally "stone carpets"). They have a layered structure and were often deposited as separate mounds. The origin of stromatolites long remained unclear. Their formation was elucidated by the discovery of modern stromatolite-builders in Shark Bay in Australia. This is a lagoon isolated from the ocean with very saline water. In the shallows lie sedimentary mounds whose surfaces are covered with a cyano-bacterial mat. Cyanobacteria occupy the surface of the mat, and beneath their layer live a very diverse array of bacteria from other groups, as well as archaebacteria. To convey this duality, the mat is called a cyano-bacterial mat, with a hyphen indicating that it is composed of cyanobacteria and other bacteria.
Mineral substances that settle on the surface of the mat and are produced during its vital activity are deposited in layers (approximately 0.3 mm per year) onto its base.
A second type of bacterial ecosystem is known on the territory of Ukraine. On the Arabat Spit, a sandspit in Syvash Bay, there are estuaries that are periodically flooded and then dry out. On the soil surface there lies a cyano-bacterial mat up to several centimetres thick.
Living organisms not only depend on their environment, but also influence it themselves. The Earth initially possessed an atmosphere of reducing character, in which stable oxidised gases (carbon dioxide CO2, water vapour H2O, sulphur dioxide SO2) and reducing gases (carbon monoxide CO, hydrogen H2, hydrogen sulphide H2S, ammonia NH3, methane CH4, hydrogen cyanide HCN, hydrogen chloride HCl, etc.) were present. Throughout a prolonged period of Earth's history, easily oxidised minerals could form on its surface, such as graphite (C), lazurite (Na2S), pyrite (FeS2), and others.
The first organisms on Earth, both autotrophs and heterotrophs, were anaerobes (adapted to life in oxygen-free conditions). In the course of photosynthesis by autotrophs, free oxygen (O2) was released, which was toxic to anaerobic organisms. At first it was rapidly oxidised by reducing agents present in abundance in the environment. Once oxygen had oxidised the main stock of reducing agents in the environment, relatively neutral conditions were established. As a result, bacterial ecosystems adapted to existence in conditions of oxygen surplus, and aerobes (organisms that live in oxygen-rich conditions) became widespread. Since photosynthesis took place in water, oxygen could oxidise substances dissolved in it, promoting their precipitation. Most importantly, this involved the oxidation of divalent iron (readily soluble) to trivalent iron, which precipitates. In this way, jasper iron ores (banded iron formations) were formed — the most important source of this metal for modern humanity. They consist predominantly of haematite (Fe2O3) and magnetite (FeO×Fe2O3).
With the spread of photosynthesising aerobes, the accumulation of oxygen in the atmosphere continued. About 2 billion years ago, the gravitational differentiation of the Earth led to virtually all iron not bound in sedimentary rocks migrating to the planet's core. The cessation of iron supply to the Earth's surface meant that living organisms could oxidise virtually the entire biosphere and accumulate surplus oxygen in the atmosphere. This turning point (which occurred 2.5–2 billion years ago) is called the oxygen revolution. One should not, however, regard this revolution as a momentary change. It occurred through prolonged oscillation, during which oxidising conditions prevailed in some parts of the biosphere and reducing conditions in others.
The oxygen revolution was the most important turning point in Earth's history. Not only did the composition of the atmosphere change, but so did the composition of rocks forming on the Earth's surface. A consequence of the oxygen atmosphere was the formation of the ozone layer in the atmosphere — a prerequisite for the colonisation of the land.
Both prokaryotes and eukaryotes are characterised by the formation of complex systems. In prokaryotes these are bacterial ecosystems of closely associated individuals of different species and even kingdoms (both eubacteria and archaebacteria), such as cyano-bacterial mats. The morphofunctional differentiation of cells in such systems is the result of their independent evolution. In eukaryotes these are multicellular organisms — clones descended from a single cell, whose differences are conditioned by the realisation of different variants of the same hereditary programme. The development of eukaryotic multicellularity (more precisely, multitissularity) requires a far greater complexity of regulatory cellular systems.
In recent decades it has become possible to provide convincing proof of the symbiogenetic theory of the origin of the eukaryotic cell. Every eukaryotic cell contains genomes of different origins: in the cells of animals and fungi these are the genomes of the nucleus and the mitochondria, while in plant cells they also include those of the plastids.
The molecular clock method (accounting for neutral, from the standpoint of natural selection, and non-directional changes in DNA sequences) indicates that eukaryotes arose at the same time as prokaryotes. Notwithstanding this, it is evident that for a considerable part of Earth's history prokaryotes were dominant. The first cells that correspond in size to eukaryotes (the so-called acritarchs) are 3 billion years old, but their nature remains unclear. Nearly certain remains of eukaryotes are approximately 2 billion years old. Only after the oxygen revolution did conditions favourable to eukaryotes prevail over most of the planet's surface. The era of their dominance began approximately 1 billion years ago.
In all likelihood, archaebacteria that transitioned to feeding by engulfing food particles became the "principal" ancestor of eukaryotes. The change in cell shape required for such engulfment was provided by a cytoskeleton composed of actin and myosin. The hereditary apparatus of such a cell moved inward from its variable surface, while retaining its connection with the membrane. This gave rise to the nuclear envelope with nuclear pores (connected via the endoplasmic reticulum to the outer membrane of the cell). Bacteria engulfed by the host cell were able to continue their existence within it. Thus, the ancestors of mitochondria were a group of photosynthesising bacteria adapted to life in oxygen-rich conditions — the purple alphaproteobacteria. Within the host cell they lost their capacity for photosynthesis and took over the oxidation of organic matter. Thanks to them, eukaryotic cells became aerobic. Symbioses with other photosynthesising cells led to plant cells acquiring plastids.
Modern fauna provides numerous examples of engulfed organelles and cells persisting within the cytoplasm of predator cells. Some ciliates, radiolarians, cnidarians, flatworms, molluscs, and representatives of other groups "cultivate" engulfed algae within themselves. Flagellates inhabiting the gut of termites enter into close symbiosis with spirochaetes that attach to their surfaces. In some of these symbiogenic complexes, even a reduction of the genetic material in the symbiont and its falling into dependence on substances synthesised by the host cell has been recorded. Similar processes also took place during the origin of eukaryotes.
Originally, the hereditary apparatus of eukaryotic cells was organised in much the same way as in prokaryotes (dinoflagellates, a group of unicellular flagellate algae, remain at this stage to this day). Subsequently, owing to the necessity of managing a larger and more complex cell, the organisation of chromosomes changed, and DNA became associated with histone proteins. The prokaryotic organisation was preserved in the genomes of intracellular symbionts, but part of their functions was transferred to the nuclear genome.
Different groups of eukaryotic organisms arose as a consequence of different acts of symbiogenesis. Red algae arose as a result of the symbiogenesis of a eukaryotic cell with cyanobacteria. Green algae arose through symbiosis with prochlorophyte bacteria. This recently discovered group contains only a few extant species, but is closely related to the chloroplasts of green algae and higher plants. Finally, the chloroplasts of golden, diatom, brown, and cryptomonad algae arose through two successive symbioses, as evidenced by the presence of four membranes in them. The endosymbionts of their ancestors were eukaryotes within which symbiotic golden bacteria resided.
The development of life has led to a fundamental transformation of the Earth's surface. What do we see when we look around beyond human settlements? One or another landscape covered with vegetation characteristic of each region. In the vast majority of places, rocks are covered with a layer of soil. Here and there the earth's surface is cut by watercourses — streams and rivers. Animals are far less conspicuous than plants, but looking closely at the vegetation, we will almost certainly see insects, and raising our eyes, we will notice birds against the blue sky. What in this picture is a consequence of our planet being inhabited? Everything! In colonising the land, life substantially transformed it. Even the blue colour of the sky is a consequence of the accumulation of oxygen in the atmosphere. Soil is a product of the activity of living organisms. It retains water and biogenic elements on the land surface in a form optimal for their uptake by organisms.
Overland runoff from the continents, during which water moved through many ephemeral channels, was replaced by channelled flow. Before the colonisation of the land, rainwater collected into streams that eroded and carried away fragments of bedrock. Loose rocks were rapidly transported to the ocean, where the current sharply decelerated and deposition of sediment began. In the modern world, similar conditions arise where rivers carrying large quantities of particles discharge into the sea. This leads to the formation of river deltas — areas projecting into the sea — "neither land nor sea". An example of such landscapes is the Danube Delta. Today such areas are covered with lush vegetation that stabilises temporary channels within them. Prior to the appearance of vascular plants this effect did not exist, and tidal waves continually created such transitional environments that eased the exit from water onto land. The land itself consisted of remnants of hard rock subjected to intensive weathering. The mass migration onto land was a consequence of mutualism (see Section IV-7) between plants and fungi — mycorrhiza was detected even in rhyniophytes. Passing through the transitional environment, vegetation colonised the continents. The remains of plant tissues served as the basis for the formation of soil. The soil cover protected bedrock from weathering, and plants protected the soil from erosion. Runoff from the surface of the continents became channelled. Soil retains rainwater, and the leaf surface increases the area for its evaporation (approximately doubling the surface area of the land). Water exchange on the continents proved to be altered, in a direction favourable to organisms.
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Column: Pre-life