The oldest record of life on the ball-spherical and rod-shaped single-celled bacterial fossils was found in the flint layer in the Baton area of South Africa and Western Australia. After isotope age determination, the two were formed before 3.8 Ga and 3.5 Ga, respectively. It is a rock formed by SiO2 colloidal deposition caused by a hydrothermal eruption on the seafloor (Awramik et al., 1983). These findings indicate that, at least in the early Archean, life has appeared on the earth, and these primitive bacteria are very similar to the primitive bacteria living in the hypoxic environment near the hydrothermal vents of the ocean floor today (Brock, 1980). Since then, in the long geological age, life has passed through the anaerobic heterotrophic prokaryotic stage → anaerobic autotrophic prokaryotic stage → eukaryotic marine life stage → eukaryotic landing → human evolution, forming a rich and colorful biosphere world. With the evolution of organisms, the metabolic mechanism that organisms rely on for material and energy exchange with the environment-biogeochemical action is constantly evolving over time. The development of the biosphere continuously changes the earth's environment, and the changes in the earth's environment affect the evolution of living things.
9.4.2.1 Early photosynthesis
The evolution of photosynthesis is one of the most important biological influences on the earth. In the early Cambrian before 2 Ga, the atmosphere was hypoxic, and life evolution was at the stage of anaerobic heterotrophic prokaryotes and subsequent anaerobic autotrophic prokaryotes. Studies have found that the following photosynthesis may exist during this period.
(1) Photosynthesis that produces methane. The most primitive photosynthesis may be a metabolic process in which primitive bacteria break down simple organic molecules (such as acetate) to produce methane (CH4). These simple organic molecules were formed by non-biological methods and existed in the ocean at that time. This kind of photosynthesis can be expressed as:
Geochemistry
Organisms that use this type of metabolism used to be scavengers of non-biosynthetic products, belong to obligate heterotrophs and can be classified as chemically alien organisms. This kind of microorganism is very similar to the fermenting bacteria in the modern Methanomycetes. After this methane-producing metabolism, photosynthesis of methane production by reducing CO2 may have appeared in early heterotrophic microorganisms:
Geochemistry
The general reaction is carried out in two steps: fungi convert organic matter into acetate, H2, and CO2, and primitive bacteria then convert them into methane (Wolin and Miller, 1987).
(2) Photosynthesis that oxidizes sulfur. The primitive bacteria that can perform photosynthesis under hypoxic conditions may be very similar to today's purple sulfur bacteria and green sulfur bacteria. They cannot photosynthesize under aerobic conditions but can make reduced H2S and S under hypoxic conditions. Oxidation to form S0 and, this metabolism can be expressed as follows:
Geochemistry
Geochemistry
Organic substances produced in photosynthesis are represented by (CH2O).
Unlike photosynthesis carried out by cyanobacteria, higher-level algae, and plants, there is no oxygen involved in this type of photosynthesis, so this process belongs to anaerobic photosynthesis. Sulfate minerals have been found in sedimentary rocks formed before 3.5 Ga, which provides the best evidence for the existence of such photosynthesis at that time (Welter, et al., 1980). Although the photolysis reaction of water once produced a small amount of oxygen, which caused the oxidation of reduced sulfides to form sulfates, some or most of the sulfates in the early earth were likely to be produced by this hypoxic photosynthesis (Butcher, et al., 1992).
9.4.2.2 oxygen-producing photosynthesis
Both photosynthesis by sulfur bacteria and oxygen-producing photosynthesis by cyanobacteria have been found to have existed in ancient oceans (Schopf, 1993). Both forms of photosynthesis can produce organic carbon, and the 13C abundance (δ13 C=-28‰) is much poorer than the 13 C abundance in dissolved bicarbonate (δ13 C≈0‰). And no other known process can produce such intense fractionation between stable isotopes of carbon (Schidlowski, 1988). Fossils of organic matter with such poor 13 C have been found in rocks with an age of 3.8 Ga in recent years (Mojzsis, 1996). However, only for oxygen-producing photosynthesis, the evidence was first seen in metamorphic rocks formed before about 3.5 Ga, in which banded Fe2O3 deposits were found in flint layers. Although it cannot be ruled out that the formation of Fe2O3 deposits by other biological processes, a large number of global banded iron deposits in the Archean and Paleoproterozoic are generally regarded as evidence of the existence of oxygen-generating photosynthesis.
Cyanobacteria can produce oxygen by replacing hydrogen sulfide with water for photosynthesis:
Geochemistry
This metabolic process is called oxygen production (oxygen) photosynthesis, to show the difference from anaerobic photosynthesis. All cyanobacteria, algae, and higher plants can carry out this type of photosynthesis.
Oxygen production, photosynthesis, removal of respiration, and decomposition of the remaining part of the oxygen is the oxygen that composes the earth's atmosphere. It has been pointed out above that oxygen-producing photosynthesis appeared in the geological record much earlier than 2.0 Ga, because a large number of band-shaped iron-built deposits have been attributed to the oxidation of Fe2+ by O2 in ocean water. The amount of O2 needed to form a strip of iron construction is huge, and it can only be explained by the oxygen produced by organisms capable of photosynthesis. The only organisms capable of such photosynthesis in the Precambrian were cyanobacteria, and fossils like cyanobacteria also appeared widely in the geological formations of this era. The red layers appearing before 2.0 Ga have more oxidized iron, which indicates that they can only be formed in an environment with more oxygen than during the deposition of banded iron. The oxygen content of the atmosphere seems to increase slowly over a period of several million years. As a result of oxygen production and photosynthesis, the earth’s atmosphere has undergone a transition from hypoxia to the oxygen content of 21% during the period from 2.0 Ga to 0.5 Ga. Change.
9.4.2.3 The effect of oxygen on the evolution of life
Most complex organisms need oxygen to grow. Therefore, it is unimaginable that the evolution of these organisms occurred before free oxygen (2.0 Ga) appeared on the earth. When free oxygen began to accumulate on the earth, the initial oxygen was toxic to all living things that existed at that time, just as O2 can kill most living things in an oxygen-deficient environment. This may be the first pollution event in biology. It caused the extinction of most anaerobic microorganisms, leaving only a very small number of such organisms living in modern local hypoxic environments. However, the emergence of free O2 has also changed the face of life on earth, and eukaryotes have gradually developed.
The metabolism of eukaryotes can proceed when the atmospheric O2 reaches 1% of the current level (Chapman and Schopf, 1983). Fossils of eukaryotes have been found in rocks formed before 1.7 Ga to 1.9 Ga (Knoll, 1992). O2 in the environment enables eukaryotes to confine their heterotrophic respiration to mitochondria, which provides an effective metabolic mechanism and promotes the rapid proliferation of higher life forms. At the same time, the chloroplasts in eukaryotic plant cells can perform more effective photosynthesis, thereby increasing O2 production and further accumulating O2 in the atmosphere.
In short, bacteria and cyanobacteria have dominated about three-quarters of the time since the emergence of life, while the colorful and diverse advanced life forms mainly appeared and evolved during the next 600 Ma. It is generally believed that the biological explosion of the Cambrian was closely related to the production and accumulation of oxygen.
9.4.2.4 Geochemical effects after biological landing
O2 in the stratosphere of the atmosphere suffers from a photochemical decomposition reaction, which leads to the appearance of the ozone layer barrier, which creates the necessary conditions for biological landing. Although there is some fossil evidence that extensive microbial communities appeared on the land during the Precambrian (Horodyski and Knauth, 1994), it is unlikely that a large number of higher-level organisms will migrate to the land before the emergence of the ozone barrier. Multicellular organisms have been found in marine sediments formed before 680 Ma, but plant landing appears to have occurred after the Silurian (Gensel and Andrews, 1987). Soon after the plants landed, they developed lignified woody tissues (Lowry et al., 1980), and effectively symbiosis with mycorrhizal fungi, which enabled plants to obtain inactive phosphorus from soil phosphorus storage (Pirozynski and Molloch, 1975; Simon et al., 1993).
Several new types of metabolism, which are critical to the global biogeochemical cycle, have also been developed under the promotion of oxygen. The main ones are:
(1) There are oxidative autotrophic effects that affect the sulfur geochemical cycle. This effect is accomplished by various thiobacteria based on sulfur or H2S (Ralph, 1979):
Geochemistry
The hydrogen ions generated in the reaction are coupled with the reaction that fixes CO2 in the organic substance to generate energy. On the primordial earth, these microorganisms can use elemental sulfur (S0) produced by hypoxic photosynthesis, but now they only appear in the local areas where elemental sulfur and H2S exist, including near some hydrothermal outlets in the deep sea.
(2) There are oxidative autotrophic effects that affect the nitrogen geochemical cycle. The nitrogen conversion reaction completed by nitrosating hair bacteria and nitrifying bacteria is also very important:
Geochemistry
These reactions constitute nitrogen nitrification, and the energy released will be coupled with the carbon fixation effect of these chemical autotrophic bacteria. The nitrate produced by these reactions is easily soluble in water, so it is the main form of inorganic nitrogen supplied by rivers to the ocean.
(3) Anoxic and heterotrophic effects affecting the nitrogen geochemical cycle. The Pseudomonas bacteria living in soil and wet sediments can perform anoxic and heterotrophic denitrification reactions (Knowles, 1982):
Geochemistry
Although the denitrification reaction requires an anaerobic environment, the denitrification bacteria are only functionally anaerobic. There are several sources of evidence that the appearance of denitrification may be later than the strict anaerobic metabolic process of methanogenesis and sulfate reduction (Betlach, 1982). Most denitrification bacteria, such as Pseudomonas, belong to fungi that are more evolved than primitive bacteria. Furthermore, denitrification can only be carried out effectively after the concentration in the ocean reaches a high level, and the early ocean water may only contain very low levels (Kasting and Walker, 1981). Therefore, the process of denitrification should not exist until the environment exists. Enough oxygen can drive the nitrification reaction of nitrogen before it occurs.
(4) The emergence of continental adsorption geochemical barriers. Biological landing gradually changes the exposed rocky landforms of the continental surface, making it continuously covered by soil vegetation, thus forming an adsorption geochemical barrier on the continental surface. Since then, a large number of soluble elements released by the weathering of continental rocks, especially trace elements, are adsorbed by organic matter and clay minerals in the soil and stored in the soil, which will inevitably reduce their input to the ocean through rivers. This may also be the Early Paleozoic. One of the main reasons why black shale is obviously richer in metal elements than later similar rocks. Although the input of metal elements from the seabed hydrothermal fluid should not be ruled out at that time, there is no definite evidence that the seabed hydrothermal activity in the Early Paleozoic was obviously stronger than that in the later period.