S269 Earth and Life
"Origins of Earth and Life"

The presence of life on Earth could be inferred from space: the coexistence of abundant free oxygen in the atmosphere and large quantities of complex organic (carbon-based) molecules at the planet's surface would suggest a condition of chemical instability. All the organic matter would be oxidized and all the oxygen would be used up, unless both were continuously regenerated. Only life is capable of such activity.

The essential charateristics of life are (i) the formation of complex ordered structures that require the extraction of energy and materials from the surrounding less-ordered environment; and (ii) the capacity for self-replication of these ordered structures. Upon death, the energy contained within the structures, along with the elements of which they are composed, are dissipated into the surroundings.

The only element known to be capable of forming ordered structures of teh complexity required for life is carbon, which can combine with the other principal 'life-forming' elements (chiefly nitrogen, oxygen and hydrogen) as well as with itself, to form three-dimensional molecules of enormous intricacy and great diversity. The only other element that can form complex (polymer) molecules in this way is silicon, but its chemistry is almost exclusively based on the Si-O bond, and nearly all naturally occurring silicon-based compounds are silicate minerals that form rocks. No self-replicating silicon compounds are known. The chemistry of carbon is commonly known as organic chemistry, and all organic compounds are carbon-based, though they are not all formed by life processes.

The four groups of organic compounds in living organisms are carbohydrates and fats, which provide the internal 'fuel' used to supply energy; proteins, the largest and most complex molecular structures known, built up from the basic building blocks of amino acids; and nucleic acids, the basic constituents of genes (the information store).

The simplest element of all is hydrogen, the starting material from which all other elements were formed. The hydrogen itself originated when the Universe was born in the Big Bang. The lightest elements are formed by nuclear fusion of hydrogen and deuterium nuclei to produce helium (with the liberation of energy according to the equation E = mc2). This process characterizes smaller stars like our own Sun, which still consists largely of unused hydrogen.

A feature of 'layered' stars is the CNO cycle, in which hydrogen nuclei interact with carbon, nitrogen and oxygen nuclei producing helium, and making other elements (outside the CNO cycle). Elements such as carbon and oxygen are made in larger and hotter stars by helium fusion. Fusion of these elements with more helium or with each other produces heavier elements (up to iron), often resulting in explosive nova and supernova events. Elements heavier than iron (up to uranium) are formed by neutron capture.

Some of the elements formed within stars are expelled into space as stellar winds, others remain in the body of the star until it eventually contracts and explodes. All eventually become available for recycling into new stars, where they participate in further fusion reactions with (mainly) hydrogen and helium, to form more new elements, which in their turn become availble for participation in further nuclear reactions. Our own solar system (Sun and planets) is part of this consmic cycle and was formed from 'secondhand' elements that were already in existence. Helium is the principal product of the Sun's own nuclear fusion. Virtually all of the Sun's other lements were formed in other stars prior to its birth.

The age of the Solar System (about 4.6 billion years) is known from measurements on meteorites. Chemical analyses of meteorites, along with spectral measurements of stellar radiation, provide information about the cosmic abundance of the elements.

The interstellar medium consists of dust and gas. This matter is not evenly distributed but tends to be concentrated into huge interstellar clouds, billions of kilometres across. Denser regions of such clouds attract matter from less dense regions (due to their gravitational field) and can evolve into solar nebulae, where dust, condensed gases and organic molecules can aggregate to form progressively larger bodies, reaching tens of kilometres across, called planetesimals.

Graviational collapse of large interstellar clouds increases both the energy of the system and the pressure at its centre, where temperatures eventually rise enough for fusion reactions to start, whereupon a star is born. Our solar system is believed to have originated from a rotating disc-like molecular cloud (nebula). The planets grew by accretion of dust and planetesimals at various points down the temperature gradient away from the centre, where temperatures were highest. The four inner (rocky) planets - along with the asteroids - formed nearest the Sun and are dominated by (refractory) silicates and metals. The outer planets, formed at lower temperatures, have much higher proportions of condensed gases and of organic compounds.

The materials of the planets were physically and/or chemically midified during gravitational collapse and accretion. Thus they no longer truly represent the primordial matter from which the Solar System was formed. Beyond the orbital limit of the planets, in the Kuiper belt and Oort cloud, are billions of orbiting bodies ranging in size from dust to planetesimals, and formed of aggregates of dust, condensed gases and organic molecules. These two regions are believed never to have been heated during Solar System formation and so probably consist of primordial matter. They are the source of comets which episodically enter the Solar System and occasionally collide with planets. Complex organic molecules in comets could have 'seeded' the Earth during and soon after its formation.

The oldest terrestrial rocks include remains of sediments similar to those forming today and provide evidence that surfact geological processes and plate tectonics were operating at the Earth's surfact 4 x 109 (4 billion) years ago; though probably on smaller space- and time-scales than today. The sediments contain carbonaceous traces which could be the remains of organisms, so there may have been life on Earth at theat time. The resemblance of these oldest rocks to modern analogues make it plausible to suggest that the Earth's layered structure (iron-rich cnore, silicate mantle, continental and oceanic crust) had been established 4 billion years ago.

Stromatolites (fossilized algal mats formed in shallow marine waters) are known from rocks 3.5 million years old, showing that life was definitely present on Earth at that time. Organic carbon in the oldest (3.8 Ga old) sediments of western Greenland may represent fossil life-forms. Carbon isotopes can be used to make inferences about ancient life. Organisms take up lighter carbon-12 in preference to (much rarer) carbon-13, so the 13C/12C ratio in fosil organic material is typically lower than in non-organic carbon (ie the δ13C values are more negative).

The Earth has a long history of bombardment by asteroids, comets, meteorites, and cosmic dust. Geological processes continually eshape the Earth's surface, so ancient impact scars have long since been obliterated - very few craters older than about 600 Ma are preserved. The Earth itself formed from aggregation and accretion of asteroid fragments, meteorites, comets and interstellar cust, and its likely bulk composition is close to that of carbonaceous chondrites.

The average density of the Moon is much less than that of the Earth, so it probably has no iron-rich core. The lunar Highlands are formed of relatively low-density rocks composed principally of plagioclase feldspar and are between 4.5 and 4.4 billion years old. The lower-lying lunar maria are made of denser basaltic rocks and have ages from 3.8 to 3.2 billion years. The favoured theory for the origin of the Moon is that it was torn from the Earth's mantle by glancing collision of another large planetary body some 4.5 billion years ago, i.e soon after formation of the Solar System, but also after the Earth had differentiated into iron-rich core and silicate mantle.

The Moon is geologically inert and its surface records a history of meteoritic impacts that spans the Hadean period of Earth's history, the first 600 Ma or so before the oldest rocks were preserved. It provides a 'witness plate' for the bombardment history of the early Earth. The larget lunar maria, some more than 100 km across, appear to have been formed between 4.0 and 3.8 billion years ago. This is also likely to have been when the Earth experienced its heaviest bombardment.

The following inferences can be drawn about the conditions on the Hadean Earth, largely from circumstantial evidence.

  • Internal convection was more vigorous than at present because of heat energy liberated by decay of large complement of radioactive elements and by impacts, and from a greater supply of primordial heat (resulting from initial accretion and core formation) than at present.

  • The surface was at times sufficiently cool for water to condense and oceans to form (especially at the end of the period, when the sediments in the oldest rocks were formed).

  • Hydrothermal circulation would be associated with any volcanic activity in ocean basins.

  • Any land areas would have been repeatedly submerged by tsunamis associated with meteorite impacts, if not oblitereated by direct hits from the largest bodies.

  • Because of the Coriolis effect, weather systems and surface current circulation would have had gyral patterns broadly similar to those observed today, but probably smaller and with greater circulation rates because of the more rapid rotation of the Earth about its axis.

  • The atmosphere had no oxygen, so there was no ozone layer to absorb ultraviolet radiation and warm the upper atmosphere. Atmospheric convection and cloud formation may have extended to considerable altitudes.

  • Outgassing from the mantl gave an atmosphere probably dominated by nitrogen, carbon dioxide and sulfur dioxide.

  • The Sun is thought to have been aobut 25-30% weaker than it is now, but surface temperatures on Earth may have been kept well above freezing by a combination of heat from the interior and the blanketing effect of a CO2-rich 'greenhouse' atmosphere.

  • The ocean(s) may have been relatively alkaline, like present-day soda lakes and some hot springs, which may explain how original limestone sediments in the oldest rocks could have been (inorganically) precipitated.

  • Rotation rates of all Solar System bodies, both axially and orbitally, were probably greater than now (the Earth day may have been as short as 12 hours), but all decreased with time on account of tidal friction arising from gravitational interactions between Sun and planets (and satellites).

  • Meteorites, comets and cosmic dust brought organic molecules to the Earth's surface throughout the period. Along with those that survived the initial accretion, they contained the raw materials for the self-replicating molecules essential for life.

The essential elements of life, carbon, hydrogen, oxygen, nitrogen and phosphorus, are among the most abundant in the cosmos. The complex molecules necessary to construct self-replicating life-forms develop under only a relatively narrow range of conditions, the chief requirement being the presence of liquid water (ie at temperatures greater than 0 deg C and less than 100 deg C).

Theories about the origin of life have included spontaneous generation and panspermia, both now discarded in favour of some kind of chemical evolution. Life is thought to have arisen abiotically (abiogenically) from organic molecules which may have been synthesized on Earth and/or in space, in the latter case being brought to Earth by meteorites and comets.

The first organisms to evolve on Earth were prokaryotes (bacteria) which are unicellular. Eukaryotes evolved much later. they include all plants and animals but some are also unicellular. Organisms are based on cellular units and the properties of genetic information sotrage and replication reside in the nucleic acid, DNA. The information store (genetic code) consists of four nucleotide bases: the purines adenine and guanine (A and G), and the pyrimidines cytosine and thymine (C and T), which are linked by sugar and phosphate groups in the double-helix structure of DNA. The bases in the two strands of the DNA double helix are linked according to the A-T G-C pairing rules.

Laboratory experiments have synthesized complex organic molecules by passing electric discharges through a variety of starting materials intended to represent the early ocean and atmosphere. The largest yields of amino acids and related compounds were obtained when reduced gases were used (methane, ammonia, etc).

Ideas about the origin of life on Earth include the proposal that hydrothermal vent environments could have provided a suitable setting. In the absence of oxygen, chemosynthesis of organic molecules would have been achieved via the reduction of carbon dioxide using hydrogen from the dissociation of hydrogen sulfide supplied in hydrothermal vent solutions with solar radiation as the energy source.

The variety of organic compounds, including amino acids, found in carbonaceous chondrites suggests that the building blocks of life may have been supplied to Earth from space. The chirality of organic compounds in meteorites (including amino acids) points unequivocally to an origin in space and excludes and excludes contamination within the Earth System. It is possible that life may have originated on other planets, especially Mars, early in the history of the Solar System.

Amino acids and other compunds would need to have become sufficiently concentrated in order to be able to combine into complex molecules (nucleic acids) capable of self-replication. Possible substrates upon which this might have happened include the surfaces of clay minerals or of sulfide particles.

The question of whether DNA or RNA was the first to appear in primitive organisms remains unresolved. Some types of RNA can replicate themselves, ie they can provide their own 'blueprint'. In the earliest Archean, primitive life may have inhabited an 'RNA world'w hich was soon supplanted by the 'DNA world' which has dominated all life-forms since.

In the oxygen-free environments of the Hadean Earth, metabolism of the earliest organisms must have been centred round (anaerobic) fermentation rather then (aerobic) respiration. The earliest autogrophs probably resembled anoxygenic photosynthesizing methanogens but the possibility of xoygenic photosynthesis cannot be ruled out. Either way, free oxygen would have been toxic to all organisms. It is not known how or when the transition to oxygenic photosynthesis occurred, but only then was it possible for atmospheric oxygen to begin to accumulate, by removal of small amounts of organic matter from the carbon cycle. Aerobic respiration then became possible, but atmospheric oxygen concentrations had to reach significant levels before eukaryotes could evolve.

The Earth System is not a closed on; the Sun provides most of the Earth's energy, gravitational and electromagnetic energy are exchanged with other parts of the Solar System, and the Earth continues to receive cometary and meteoritic material from space.

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