Early Life on Earth

Evolution, Diversification, and Interactions

Kenichiro Sugitani

• Publisher: CRC Press
• Year: 2022
• ISBN:
  9781000534467
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Microbiology
• Index: Biological Sciences

This book comprehensively explores the early evolution of life and the Archean environment. Topics include the differences between prokaryotes and eukaryotes, variations in metabolisms, concepts of ecosystems and biogeochemical cycles (nitrogen, sulfur, phosphorous), Archean geology and environments, and the widely accepted early evolutionary history of life. The text addresses controversies regarding early life and its environment, particularly the unusual microfossil assemblages from the 3.4 Ga Strelley Pool Formation and the 3.0 Ga Farrel Quartzite of Western Australia. Readers will get a fuller picture of the Archean world, and an appreciation of many still unresolved questions.

Key Features

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• Illustrated with figures visualizing ecosystems, biogeochemical cycles etc which are indispensable for understanding the Archean Earth.

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• Includes tables arranging key words, definitions, and interpretations.

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• Documents the Archean environment with photographic evidence and detailed descriptions the rocks, minerals and microfossils.

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• Summarizes the latest field research.

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• Details exciting unresolved questions for future study.

1 Space, Solar System, and the Earth

DOI: 10.1201/9780367855208-1

1.1 Introduction

In this first chapter, the origins of elements in the universe, evolution of our solar system and the Earth, and the origins of the oceans and the atmosphere are reviewed. Based on “the Big Bang theory”, our universe began 13.8 Ga ago, whereas our solar system had evolved around 4.6 Ga ago, which is assumed to be nearly identical to the age of the Earth (Figure 1.1). Our (not only human beings but also the other organisms) existence is inseparable from the universe, where elements have been generated through dynamic processes. Formation of our habitable planet is closely related to the evolution of the solar system. Habitability for “the Earth-type life” prerequisites the prolonged presence of water (H₂O) and the preceding supply of building blocks of life. These topics are mentioned here.

FIGURE 1.1 Time scale from the Big Bang to the present. This book mainly deals with the deep time (Hadean to Archean) of the Earth.

1.2 Elements in the Universe and Their Origins

To date, 118 elements including synthetic ones have been identified, from ₁H to synthetic ₁₁₈Og (oganesson). Despite such diversity of elements, their abundance in the universe deduced from that of our solar system is extremely biased to elements with small atomic numbers (Figure 1.2). The elemental abundance of our solar system has been deduced based on spectroscopic analyses of the solar sphere for volatile elements and chemical compositions of chondrite for nonvolatile elements: chondrite is a sort of meteorite and is thought to have preserved primitive solid phase compositions of the solar system (see Column).

FIGURE 1.2 Relative abundances of elements in the solar system (Si = 10⁷), from H to Mo. White-circled elements are essential elements for plants and animals (Cuss et al., 2020, and references therein).

Hydrogen is the most abundant element in our solar system and the universe, and He is the next one. These two dominate more than 98% of all the elements. Elemental abundances exponentially decrease to elements with larger atomic numbers, with some irregularities represented by relative depletion of Li, Be, and B and by relative enrichment of Fe (Figure 1.2). The most important essential elements for life such as H, C, N, O, S, and P, comprising carbonhydrates, lipids, nucleic acids, and polyphosphates are major constituents.

Hydrogen, He, Li, and Be were produced soon after the beginning of the universe. In other words, the other elements were not present at that time. Some of the other elements have been produced by stepwise nuclear fusions inside fixed stars composed dominantly of H. The heaviest element produced by this process is Ti, and the size of a star (fixed star) gives constrains on what element is finally generated in its core. In the core of the Sun, four atoms of ¹H are fused to produce one atom of ⁴He. Inside fixed stars larger than the Sun, heavier elements could be produced by further nuclear fusion. This process is followed by a process called neutron capture that could increase the mass number of an element, with an increase of atomic number to a lesser extent, eventually producing ⁵⁶Fe. Elements heavier than ⁵⁶Fe are produced also by neutron capture. However, this process occurs only during supernova explosion, which would occur at the end of fixed stars more than ca. 10 times heavier the Sun. This neutron capture process is called “rapid process”, whereas the process of neutron capture inside fixed stars is called “slow process”.

Associated with nuclear fissions, energy is produced and emitted to the space as the electromagnetic radiation from fixed stars. The Sun has solar radiation estimated to be 3.6 × 10²⁶W. The Earth receives only 4.55 × 10⁻⁸ % of this radiation energy. The solar radiation is dominated by visible light (47%) (380–700 nm), ultraviolet (7%) (10–380 nm), and infrared (46%) (700 nm–1 mm) {note that these ranges are somewhat ambiguous (https://photosyn.jp/pwiki/)}. Most photoautotrophic organisms (see Chapter 3) almost exclusively utilize visible light to produce organic matter (OM), and some algae can utilize near infrared (700–750 nm) (e.g., Nürnberg et al., 2018). The terrestrial and the marine surface ecosystems are founded on this solar radiation energy.

1.3 Evolution of Our Solar System

Our solar system had evolved from molecular clouds composed of interstellar dusts and gases (interstellar matter). The interstellar matter was originated largely from previously existed stars and their planetary systems, scattered by for example supernova explosion. These wrecks were raw materials for our solar system and life on the Earth and if present, others. Elements produced directly by the Big Bang (Big Bang nucleosynthesis) were limited to light ones up to Be. Namely, our bodies and those of other organisms are composed largely of elements produced by nucleus syntheses within fixed stars and during their supernova explosions. Elements produced by the rapid process include Cu, Zn, and Mo, which are essential to many of the organisms on the Earth. These elements comprise reaction centers of several important enzymes such as superoxide dismutase, laccase, and nitrogenase.

Interstellar medium refers to interstellar matter and cosmic rays present between solar systems. The region with higher density of interstellar medium, compared with the surroundings, is called “interstellar cloud”. Molecular cloud refers to the region with a density of 10⁴–10⁶ H₂ per cm⁻³; the major component of molecular cloud is H₂, although many of other molecules, including e.g., carbon monoxide (CO), H₂O, ammonia (NH₃), hydrogen cyanide (HCN), and formaldehyde (HCHO), have been detected. Recently, methylamine (CH₃NH₂), which is a precursor of glycine (C₂H₅NO₂), the simplest amino acid, was recently detected (Bøgelund et al., 2019). Within such a molecular cloud, fixed star forms. The fixed star formation is thought to be triggered by fragmentation of high-density area of the cloud (molecular cloud core) or by shock wave generated by, for example, supernova explosion. In the following, the assumed formation process of our solar system is briefly described.

Destruction of gravitational stability resulted in contraction and rotation of the molecular cloud, eventually forming the primordial sun and the surrounding disk (the primitive solar sytem nebula) composed of gases (mainly H₂) and dusts (mainly H₂O ice), with trace amounts of minerals and metals. The formation of this disk was associated with precipitation of dusts to the rotating surface, which released their potential energies. Released potential energy was transformed to thermal energy, which was higher in the region closer to the primitive sun. Obviously, the solar radiation was stronger in the region closer to the primitive sun. Temperature gradient in the primitive solar system expected from these two factors (potential energy of dusts and solar radiation) indicates that within the inner region closer to the primitive sun (~3 astronomical unit: AU), dusts were once entirely evaporated, except for materials with very high boiling points. Once vaporized, materials within the inner zone would subsequently condense to form minerals and metals.

The growth of planets is thought to have been a stepwise dynamic process. Namely, particles (dusts) once formed planetesimals, which had grown through collision coalescence with each other to protoplanets (100–1,000 km in diameter). Compositions of planetesimals were different depending on distance from the primodal sun (Figure 1.3). Planetesimals formed within the inner zone had rocky composition, whereas those in the outer zone had icy H₂O composition, and protoplanets as well. Through further collision coalescence, protoplanets had grown to the primitive planets, direct precursors of the present planets. The planets had grown in the primary solar atmosphere mostly composed of H₂ at least in their early stage of growth. As H₂O was much more abundant compared with rocky materials in the solar system, icy primitive planets could have grown much larger than the rocky planets and captured the primary H₂ atmosphere, resulting in the formation of gas giants such as Jupiter and Saturn. Uranus and Neptune are also gaseous and icy planets (ice giants), although their masses are much smaller than Jupiter and Saturn. One explanation for this is that their primitive planets had formed after the blow-off of the primary atmosphere by strong solar wind. This wind had completely removed the primary atmosphere surrounding the rocky planets, which was closely related to the origins of atmosphere and oceans of the Earth and Mars (e.g., Wurm, 2019 and reference therein).

FIGURE 1.3 The evolution of the solar system. Modified from Dig. 14.2 of Genda (2019).

1.4 Evolution of the Earth's Inner Structure

The Earth is characterized by a three-layered concentric structure composed of core, mantle, and crust. The core is composed dominantly of metallic Fe, with some light elements required. It is also divided into the inner solid core and the outer liquid core, which is predicted by analyses of seismic waves and the presence of dipole magnetic field of the Earth. Candidates for light elements contained in the core have been thought to be C, S, O, H, and Si. Recent studies emphasize H. One approach was made by estimation of temperature just above the core (the bottom of the solid mantle) (Nomura et al., 2014). The estimated temperature was much lower than that required for melting of pure metallic Fe. In order to explain this inconsistency, a large amount of H (0.6 weight %) should be contained in the outer core. The authors suggested the possibility that dissolution of H into metallic Fe had occurred within m...