The earliest living forms: Prokaryotes and Stromatolites
We have seen in some previous articles how life on earth could have begun from primordial precursors. Now let's take a leap forward, beyond the protobionts, and let's analyze those primitive organisms that can be considered living forms in all respects, at least as we understand the concept of life.
Prokaryotes
The prokaryote is a poorly differentiated protocellular organism, with no internal compartments delimited by a membrane.
Nuclear DNA is naked and consists of a single strand curled up on itself. Reproduction is agamic, by simple splitting.
Appearance: about 4 billion years (beginning of life).
Examples: bacteria and calcimicrobes.
Prokaryotic organisms do not possess mitochondria, nor chloroplasts, nor other organelles. They only possess ribosomes, which are necessary for protein synthesis.
They can have various types of metabolism:
- autotrophs are able to independently produce energy substances (glucose);
- the heterotrophs, on the other hand, take energy substances from the external environment.
Prokaryotes are classified in the Monera Kingdom, and are divided into bacteria and cyanobacteria (including calcimicrobes). Both are photosynthetic and possess chlorophyll A, as plants.
Absolutely, Prokaryotes not have to be confused with Protists. Protists (or protoctists) are a heterogeneous and polyphyletic group of organisms, which include those eukaryotes that are considered neither animals nor plants or fungi.
Among the currently living bacteria, archaea, particularly primordial, are believed to be the most similar form to the first organisms that appeared on Earth.
Archaea are very different, from a biochemical point of view, both from other prokaryotes (eubacteria) and from eukaryotes. In particular, the composition of the cell membrane is different, in addition to the characteristics of ribosomal RNA. They have a particular metabolism, according to which they produce methane, starting from carbon dioxide and hydrogen. Carbon dioxide (CO2) and hydrogen (H) were very abundant in the primitive atmosphere.
They live in environmental conditions similar to those that are supposed to exist at the origin of the Earth.
We can distinguish:
- metanobacteria, living in the seabed and in rotting mud;
- halobacteria, living in highly concentrated salty waters;
- thermoacidophiles, living near thermal, acid and hot springs.
What could be the metabolism of the first organisms?
They were certainly anaerobes, that is, they did not use oxygen, which was absent in the early days of the earth's atmosphere.
This was the reaction that took place:
6 CO2 + 12 H2S ---> (C6H12O6) + 6 H2O + 12 S
which means: 6 molecules of carbon dioxide (carbon dioxide) and 12 of hydrogen sulphide are transformed into one of glucose, 6 of water and 12 atoms of sulfur.
The appearance of photosynthesis certainly occurred at a very early stage, as suggested by the findings of stromatolites dating back to 3.5 billion years.
In this case the reaction is as follows:
6 CO2 + 12 H2O ---> (C6H12O6) + 6 H2O + 6 O2
that is, 6 molecules of carbon dioxide (carbon dioxide) and 12 of water are transformed into one of glucose, 6 of water and 6 of molecular oxygen.
Stromatolites and microbialites
The term carbonate platform means, in sedimentology and biology, an area located in a marine or lake environment, characterized by a more or less accentuated topographical relief and by a high production of autochthonous carbonate material of mainly biogenic origin, derived from the accumulation of hard parts of limestone skeletal organisms or from the precipitation of carbonate induced by the activity of living organisms.
Carbonate platforms are known since the lower Proterozoic (about two billion years ago), and have developed throughout the subsequent biological history, with periods of great diffusion and periods of crisis with extreme reduction or even disappearance of the relative communities, which however they have always recovered and differentiated again, testifying to the richness and vitality of these environments.
In the geological past there are many examples of carbonate platforms composed totally or almost entirely of carbonate mud (micrite). The genesis of these bio-constructions has long been hotly debated by geologists. It is now widely accepted by researchers that they are bacterial in origin. The micrite that makes up these platforms derives from lithified sediment during or just after formation rather than after burial, due to precipitation biologically induced by bacterial activity. This type of facies should not be confused with the stromatolytic facies, characterized by alternating laminae of algal origin, even if the two facies can coexist on the same platform. The facies of the micritic platform are characterized by the absence of structures of sedimentary or biological origin (although there may be concentric or laminar structures derived from the abiotic precipitation of carbonate cements). These bio-constructions are substantially different from those derived from gregarious or colonial organisms. The biological communities that give rise to these platforms appear to grow in less oxygenated waters with a higher nutrient content than the other types of platforms. Micritic platforms are known since the Paleozoic and frequent up to the whole Mesozoic: they are common above all in the Upper Paleozoic (in particular in the Carboniferous and in the Permian) and throughout the Triassic.
Stromatolites (from the Greek stroma, carpet, blanket and lithos, stone) are sedimentary rocky structures, with a streaked appearance due to alternating dark and light alternating bands, belonging to the group of bioconstructed non-particle limestones, finely laminated due to the presence of deposited materials from the metabolic activity of benthic photosynthetic microorganisms such as prokaryotes (for example calcimicrobes similar to current cyanobacteria) and also microscopic eukaryotic algae. They have been among the most responsible for the presence of oxygen in the earth's atmosphere and have been the organism that has dominated the earth for nearly three billion years.
With the appearance of photosynthesis, oxygen began to accumulate in the earth's atmosphere and this process had at least two important consequences:
- the birth of aerobic metabolism, that is cellular respiration
- the formation of the ozone layer, which protects living forms from ultraviolet radiation from the Sun
Currently oxygen constitutes 21% of the atmosphere and it is believed that about 2 billion years ago it reached a concentration equal to 1%, preceding the appearance of the first eukaryotes and allowing them to develop eukaryotic metabolism.
The oldest known carbonate platforms, from the Paleoproterozoic, are formed by stromatolytic encrustations produced by cyanobacteria. These were probably bacterial biofilms that could "absorb" the calcium carbonate and trap the fine sediment produced by the erosive action of atmospheric agents on the platform itself. Typical stromatolites are composed of thin sheets (from millimeter to centimeter) of light carbonate mud (transformed into micrite in fossil stromatolites), alternated with dark sheets rich in organic matter (derived from the activity of algal carpets). Other types of structures, defined thrombolytes (microbialites of bacterial origin), are outwardly quite similar to stromatolites but have a very irregular internal non-laminated, cellular structure. These are always accretionary structures derived from the activity of algae and bacteria that tend to fix the sediment through biofilms and mucilages. Thromboliths more frequently give rise to bioconstructions developed in height (pinnacle), and are less related to contexts of shallow water. Some of these structures were of considerable size (up to a few hundred meters thick and kilometers in length). There are no metazoans preserved in these sediments. The ecology of these "factories" was probably very simple.
In late Neoproterozoic carbonate platform sediments of Namibia, structures of biological origin have recently been found, interpreted as metazoa incertae sedis (porifera or coelenterates?), and referred to the genus Namapoikea. These are probably colonial organisms: masses of irregular shape, encrusting, with an internal tubular structure, certainly mineralized (calcareous), which colonized mainly natural cracks within the body of the platform. The presence of these structures in the Late Proterozoic platforms seems to indicate an ecologically more complex habitat, in which already differentiated soft-bodied fauna (perhaps comparable, in another environment, to the contemporary Ediacarian one) could probably find refuge.
With the adaptive radiation of metazoans occurred in the Lower Cambrian, a new type of platform appears: the stromatolytic facies are invaded by organisms outwardly similar to sponges, the archaociates. These organisms, endowed with a conical limestone skeleton, formed small bio-constructions "patches" and carpets on the top of the stromatolite platforms.
With the end of the Middle Cambrian and the extinction of the archeociates there were no more animal bio-constructions (at least from the fossil material currently available) but only stromatolytic platforms for about 60 million years. From the Middle Ordovician, the appearance and rapid differentiation of stromatoporoids (probably calcareous sponges), tetrachorals and red algae led to the development of bio-constructions and carbonate platforms much more differentiated and with features more similar to the current ones, with real reef complexes. In the period between the Upper Ordovician and the Devonian the shelf communities differ considerably, attracting many other fauna-floristic groups: trilobites and other arthropods, brachiopods, molluscs, echinoderms (especially crinoids), bryozoans, non-bio-constructing algae.
However, these platforms suffer heavily from the late Devonian biological crisis (Frasnian-Famennian transition), and the subsequent Carboniferous and Permian platforms are still mainly micritic and stromatolytic: corals and stromatoporoids are still widespread but do not play an important role in their construction. Shelf communities, however, are still well differentiated, with brachiopods as the dominant non-builder animal organisms.
The great biological crisis at the transition between the Permian and the Triassic causes the extinction of most of the Paleozoic taxa (in particular the tetrachorals) and the disappearance of the carbonate platform facies, which resume only in the Middle Triassic and more decisively in the Upper Triassic. However, the Triassic platforms are still predominantly of bacterial-algal origin, and the bio-built coral facies (in the meantime the hexacorals have appeared, replacing the extinct tetracorals) are still underdeveloped. A characteristic feature of the platforms of this period are large bivalves, (Megalodontacea) with a characteristic wedge-shaped shell, which populated the shallow waters of the internal lagoons.
Carbonate platforms with corals and algae resume flowering in the Jurassic and Cretaceous periods. In the latter period very peculiar bio-constructed buildings also developed, characterized by bivalves with a tendentially conical shape due to the development of only one of the valves: the rudist ones, which died out in the biological crisis between the Cretaceous and the Tertiary.
In the Tertiary sector, overall there is a slow decline of coral reefs, now much more restricted in area than in the past.
Both a progressive cooling of the climate and a gradual variation in the distribution of emerged lands have probably played a part in this reduction, with the fragmentation and disappearance of the Tethys ocean (mainly east-west oriented in tropic-equatorial latitudes), replaced by oceanic domains mainly north-south facing. Calcareous algae (both green and red) tend to become one of the most important bioconstructive organisms, especially from the Miocene. Vermetids appear and become significant bioconstructors especially in the middle and high latitudes.
