表紙は，さまざまな金属タンパク質の誕生プロセスを模式的に描いたものである．岩石が水と反応すると，岩石に含まれる多種多様な鉱物からさまざまな元素が溶解する．この過程は原始生命にとって重要な無機栄養塩を供給し，多様な元素が大気起源の無機生命構成主要元素と反応し，さらに高次の有機物が合成されていったと考えられる．冥王代の地球表層に露出していた典型的な岩石は，KREEP玄武岩，アノーソサイト，およびコマチアイトの3種類で，マグマオーシャンから結晶化したシュライバサイト（Fe3P）などの無水の還元的な鉱物を含んでいた．約43.7億年前にABEL爆撃（Part I）がはじまると酸化的大気（CO2–H2O–N2）が生まれ，その時に岩石–水相互作用がはじまった．KREEP玄武岩は，Fe2+, S, P, Ti, Zn, Moなどを供給し，アノーソサイトは，Ca, Cr, Mg, Fe2+, Mn3+など，またコマチアイトはFe2+, S, Ni, Mgなどをおのおの供給した．それぞれの岩石表面には岩石–水相互作用を通して供給された元素を利用した多種多様な金属タンパクが合成され，自然原子炉が駆動するエネルギー・物質循環系のなかで高次の生命構成単位を生みだした．そのプロセスのなかで第一次生命体がバイオフィルム様のコロニーを上記の3種類の岩石表面でつくり，さらに複雑に進化していったと考えられる（詳細は吉屋ほか，2019参照）．
To understand the influence of high CO2 contents in a hydrothermal fluid on ultramafic rock-hosted seafloor hydrothermal systems on the early Earth, the reaction between tremolite-bearing serpentinite exposed in the Hakuba-Happo area and a CO2-rich NaCl fluid at 200°C and 100 bars was monitored. The H2 concentration in the fluid reached a constant value of approximately 0.4 mmol/kg within 6312 hours. This concentration is lower than the H2 concentration in a natural hot spring in the Hakuba-Happo area. During the experiments, the total carbonic acid concentration (ΣCO2) in the fluid decreased, and magnesite was formed through the water–rock interaction. This is the first experimental report of magnesite formed from Ca-bearing ultramafic rocks, which is possibly attributed to the lower temperature condition compared to previous studies. Incorporation of ferrous iron into the magnesite probably suppressed iron oxidation in fluids and resultant H2 generation during serpentinization and carbonation at 200°C. Compilation of existing experimental data indicates that precipitated carbonate species depend on reaction temperature, initial fluid composition, and composition of reacted rock. The compilation further suggests that the amount of precipitated carbonate decreases and iron-poor calcite becomes predominant at higher temperatures. Therefore, the influence of the fluid CO2 on H2 generation at high temperatures becomes small even on the early Earth. The description of carbonate species formed in komatiites of Archean greenstone belts tells us that H2 generation in low-temperature hydrothermal fluids circulating peridotitic komatiites was possibly limited by the precipitation of Mg-rich carbonate and iron incorporated therein. On the other hand, experimental hydrothermal fluids co-existing with Mg-bearing carbonate during serpentinization of ultramafic rocks clearly have high Mg contents or become more magnesian, compared to modern basalt-hosted hydrothermal fluids. This implies that carbonation of ultramafic rocks would be one of the main sources of oceanic Mg on the early Earth.
Several models of the birthplace of life have long been proposed and discussed. However, those discussions are chaotic. To test the birthplace models, we introduce nine requirements that must be met for the emergence of life. These requirements are: (1) energy source (ionizing radiation and thermal energy), (2) supplies of nutrients such as phosphorus and potassium, (3) supplies of main life constituent elements (C, H, O, and N), (4) condensed reducing gas, (5) dry wet cycle, (6) Na poor water, (7) clean lacustrine environment, (8) diversified surface environment, and (9) cyclic nature. Based on these nine requirements, most proposed birthplaces, such as the mid-oceanic ridge hydrothermal system, do not meet the requirements for life to emerge. The only possible site is a nuclear geyser system. Under the Hadean surface environment, sites satisfying the nine requirements are extremely limited because it is significantly difficult to meet the nine requirements, including extrinsic conditions. This difficulty to fulfill all the requirements indicates that only one site is the birthplace of life on Hadean Earth.
The evolution of eukaryotes is one of the most important issues in the history of life. Paleontological studies discovered the oldest eukaryotic fossil from the Paleoproterozoic Francevillian Group in Gabon. To clarify specific features of ancient sedimentary basins in Gabon, geologic evidence for the Francevillian Group is summarized and a new geotectonic model is proposed. The model gives much weight to the upwelling of mantle plume, which can explain why the Francevillian basin only hosted natural reactors. The Great Oxidation Event in the Paleoproterozoic played an important role in not only the evolution of eukaryotes but also in the formation of natural reactors. Reductive weathering of the continental crust, which was affected by plume-related volcanisms, transported Uranium-rich minerals into sediments of the Francevillian Group without dissolution of Uranium. The subsequent oxidation event enabled uranyl ions to accumulate within an oxic failed rift basin, and settled large quantities of organic matter on the seafloor. Hydrothermal circulation within the Francevillian Group precipitated highly Uranium-rich ores, which became natural reactors at approximately 2.0 Ga, and might have influenced the evolution of eukaryotes in this basin. In a large sense, the degree of oxidation of the ocean-atmosphere system has been linked to the amount of sedimentary rocks. In that way, mantle overturn in 2.7 Ga, which created a large continental crust, was one of the crucial events that affected the evolution of eukaryotes.
Phylogenetic analysis is one of the useful tools available for revealing the evolution of life on the Earth; however, it has difficulty in principle distinguishing old and new genomes just by comparing phylogenomic trees. To overcome this difficulty, a new method is introduced which utilizes the Earth's history derived from geologic information to trace genomic evolution. This idea is inspired by Darwin's natural selection, and explains how living organisms change with the environment. In other words, life's genome does not change if the environment remains the same. A key is the birthplace of life on Hadean Earth, which is thought to be an ultra-reducing environment with H2 produced in abundance through serpentinization. OD1 is a potential microbe that has survived on the Earth since the Hadean. Its habitat, Hakuba-Happo in Japan, is a unique serpentinite-hosted hydrothermal system on land, and it has avoided evolution by remaining in a super-reducing environment from the Hadean to the present. OD1 is regarded as a “living fossil” of the Hadean microbe. Ultra-reducing environments have disappeared over the Earth's history. How has OD1 survived since the Hadean to the present? A possible scenario is proposed based on Plate Tectonics. OD1 habitats have gone through the following transitions: (1) super-reducing environment in a natural nuclear geyser on a primordial continent in the Hadean; (2) serpentinite-hosted hydrothermal system along a mid-oceanic ridge transform fault during the Archean-Proterozoic; (3) subduction-accretion and escape from oxygenated Phanerozoic ocean floor; and, (4) jacked up by growth of accretionary complexes and taking refuge in a hydrothermal system above a volcanic front. OD1 habitats have been reduced with geological age as free oxygen has increased in the surface environment. OD1 may be a “living microfossil” of the Hadean, making its way continuously through ultra-reducing environments on a tightrope.
The Earth is the only planet where liquid water and life have existed through geologic time. Therefore, it is important to investigate the surface environments of the early Earth to understand its early life. However, little is known about the environments of the early Earth because of a lack of geologic evidence. Recently, > 3.95 Ga supracrustal rocks including carbonate rocks, named Nulliak supracrustal rocks, were found in the Saglek Block, northern Labrador. The seawater composition of the early Earth and vestiges of life from the oldest supracrustal rocks are investigated. The carbonate rocks occur in four areas of the Saglek Block, and are accompanied by meta-chert and meta-pelitic rocks or metamorphosed banded iron formation and meta-basalt. They all are regionally metamorphosed under the amphibolite or the granulite facies conditions (> 500°C) to yield metamorphic minerals such as amphibole and pyroxene. The whole-rock compositions of major, trace, and rare earth elements of the carbonate rocks are assessed. The carbonate rocks are composed of CaO, MgO, FeO, and SiO2 with trace amounts of Al2O3, Zr, and Ba. Carbonate rocks with low Al2O3 (< 0.2 wt.%), Zr (< 2 ppm), and Ba (< 5 ppm) contents are selected to avoid the influences of detrital and volcanic materials on their compositions. Post-Archean Australian Average Shale (PAAS)-normalized Rare Earth Element (REE) patterns of detritus-free samples show flat to slightly light REE-depleted patterns with positive La, Eu, and Y anomalies and without a Ce anomaly. The positive La and Y anomalies indicate that the carbonate rocks originated from chemical sediments precipitated from seawater. The lack of the Ce anomaly indicates that Eoarchean seawater was anoxic. Because the carbonate rocks accompanied by the pelitic rocks have obvious Eu anomaly, even seawater near a continent had the Eu anomaly and was widely dominated by hydrothermal influx. Pelitic rocks, conglomerate, carbonate rocks, and chert nodules within the carbonate rocks in St. John's Harbour South, St. John's Harbour East, Shuldham Island, and Big Island contain graphite, whereas no graphite is found in the sedimentary rocks in Pangertok Inlet. Some of the pelitic rocks and conglomerate have low δ13Corg values with a nadir of −28.2‰, which is comparable to the minimum value of δ13C values of graphite in the Isua supracrustal belt. The δ13Corg values of graphite in the politic rocks show a weak negative correlation with total organic carbon contents. On the other hand, carbon isotope values (δ13Ccarb) of carbonate in the carbonate rocks range from −2.6‰ to −3.8‰. The δ13Corg values positively correlate with the metamorphic grade and negatively with total organic carbon contents. The correlation indicates that the variation in δ13Corg values is possibly due to carbon loss during metamorphism. It is concluded that the large fractionation between the δ13Ccarb and δ13Corg values, up to 25‰, provides the oldest evidence for organisms over 3.95 Ga.
The Hadean surface was mainly covered by three kinds of rock: komatiite, KREEP basalt, and anorthosite, which were remarkably different from those on the modern Earth. Water–rock interactions between these rocks and water provided a highly reducing environment and formed secondary minerals on the rock surface that are important for producing metallo-enzymes for the emergence of primordial life. Previous studies suggest a correlation with active sites of metallo-enzymes and sulfide minerals based on an affinity with their structure, but they do not discuss the origins of metallic elements contained in these minerals, which are critical to understand where primordial life was born. Secondary minerals formed through water–rock interactions of komatiite in a nuclear geyser system are investigated, followed by a discussion of the relationship between active sites of metallo-enzymes and secondary minerals. Instead of komatiite, we used serpentinite collected from Hakuba Happo area, Nagano Prefecture in central-north Japan, which is thought to be one of the Hadean modern analogues for the birthplace of life. Several minor minerals were found, including magnetite, chromite, pyrite, and pentlandite, in addition to the major serpentine minerals. Pentlandite is not been mentioned in previous studies as a candidate for supplying important metallic elements to form metallo-enzymes in previous studies. It also acts as a catalyst for hydrogen generation, because it closely resembles the structural features of an active site of hydrogenases. Nickel-iron sulfide, pentlandite, is considered to be one of the important minerals for the origin of life. In addition, what kinds of minor mineral would be obtained from water–rock interactions of these rocks is estimated using a thermo-dynamic calculation. KREEP basalt contains large amounts of iron, and it could be useful for producing metallo-enzymes, especially for ferredoxins, an electron transfer enzymes associated with the emergence of primordial life.
Natural ionizing radiation, which potentially affects biota inhabiting the Earth, can be broadly divided into two types according to origin: cosmic radiation and subsurface radiation. Cosmic radiation contains galactic cosmic rays and solar energetic particles. Subsurface radiation is derived from radionuclides such as uranium, thorium, and radon. The levels of these forms of natural radiation were not constant temporally and spatially, and underwent a lot of changes in the early Earth environment. However, the ground level radiation dose rate of secondary muons derived from a supernova event that causes the most severe biological effects among forms of cosmic radiation is estimated to be 1 sievert (Sv) per year at most, which is too low to have lethal and mutagenic effects on terrestrial microbes. On the other hand, a nuclear fission chain reaction occurred in Oklo uranium ore deposit in Gabon about 2 billion years ago and continued intermittently for 105-106 years. The average total radiation dose rate of a typical natural fission reactor in Oklo is estimated to be 47.4 Sv per hour. This value is high enough to serve as a physical mutagen for subsurface microbes inhabiting areas near the reactor, and a million years is long enough to generate a new species of microbes. The observed growth-inhibitory critical dose rate for Escherichia coli is estimated to be 36 to 67 Gy per hour. On the other hand, the radioresistant bacterium Deinococcus radiodurans is shown to be cultivated without any growth delay at up to 126 to 180 Gy per hour of gamma rays. Recent EXAFS and isotopic analyses indicate that biogenic processes are more important for uranium ore genesis than previously understood. D. radiodurans and its closely related species Thermus thermophilus are shown to have the ability to reduce U(VI) to U(IV) under anaerobic conditions. These lines of evidence suggest that a common ancestor of Deinococcus and Thermus might be involved in the formation process of Oklo uranium ore deposit. Therefore, the radiation dose rate at Oklo-type natural nuclear reactors would be suitable for affecting the growth of microbes and generating genome evolution through accumulated mutations.
It is important to know the influx of extraterrestrial material on old Earth in order to understand global environmental changes. Helium is suitable for detecting extraterrestrial material in marine sediments, as well as platinum group elements, because there is more helium in extraterrestrial matter than on the Earth's surface. Extraterrestrial material is detected in old sedimentary rocks collected from a Permian/Triassic (P/T) boundary section in the Mino Belt, central Japan, which accumulated in a deep seafloor environment in the ancient Pacific Ocean. Much higher 3He/4He ratios (up to 150 Ra; 1 Ra = the atmospheric ratio) are observed in the samples. These high 3He/4He ratios may infer the existence of extraterrestrial helium carried in fullerenes or interplanetary dust particles. Moreover, the distribution of 3He concentrations indicates a significant increase in the influx of extraterrestrial material before the P/T boundary, which is equivalent to the influx at the Cretaceous/Paleogene boundary, because of the long duration. This increase may have caused global cooling leading to mass extinction.