"All living things have much in common, in their chemical composition, their germinal vesicles, their cellular structure and their laws of growth and reproduction....therefore I should infer....that probably all the organic beings which have ever lived on this earth have descended from some one primoridal form, into which life was first breathed."
-- C. Darwin
Once planetary formation was complete, the Earth embarked on a 4.5 billion year journey to the present. Along the way there have been remarkable changes from the early state of a thick molten layer of surface rocks with no oceans or atmosphere to today's blue planet. The long journey will continue well into the future, with changes both natural to the system and prompted by Human (are we just agents of nature?) activity. Ultimately, the planet will evolve to a static planet, devoid of internal motions driven by thermal convection, and with a slow decay of the surface topography by erosion, unreplenished by mountain building. That state will resemble more the planet Mars, or perhaps the Moon, which have seen their internal engines run down. Of course, by then the Sun itself will be exhausting its fusion materials, and begin the core collapse that bring on the Red Giant phase. The gaseous planetary nebula cast off from the Sun will consume the Earth in one final fiery catastrophe.
The evolution of our Planet's surface conditions raises a few remarkable coincidences about the particular Earth system that we enjoy. In many ways, it is remarkable that the Earth has the narrow range of conditions for some essential consequences.
The surface of Venus is hot enough to melt lead, so there is never any ocean on the planet, and hence any life would have to evolve in the thick atmosphere. The hot surface is unable to cool and sink into the interior of Venus, unlike on the Earth, so plate tectonics does not happen there, or at least not in the same way as on Earth. Mars is cold enough that its water is permanently frozen into its ice caps. In the past, Mars may have been warmer, but was it ever enough so to allow free water? At one time it appears to have had oceans, rainfall and flowing rivers. In early Mars history it is possible that conditions were favorable for evolution of life similar to early Earth life.
These facts sometimes add up to such a Cosmic Coincidence that one may question whether there has been a grand design of some sort. Or maybe it is just our good luck. Whatever your philosophical perspective, the fact remains that Earth is precariously poised in a narrow range of conditions that are suitable for our existence. If we upset this situation by human activity we could destroy our species.
Moving on to the Dynamic Earth System, it should be recognized that all aspects of this system are studied in the discipline of Earth Sciences. This is a field of inquiry which brings physics, chemistry, mathematics and biology into the study of the Earth system. For example, the major fields are:
Geochemistry: Study chemical processes, melting, rock formation, erosion, cosmochemistryGeophysics: Study physical processes of the solid Earth, liquid core, atmosphere and oceans. Convection, heat transport, magnetic field generation, gravity.
Geology: Study of rocks and surface processes using analytic tools and field observations.
Paleontology: Study of biological processes through time, evolution, extinction.
Results of Earth Sciences research will be presented throughout the rest of the lectures, to provide an understanding of the fundamental processes underlying catastrophic phenomena.
The Earth is, to first order, best described as a chemically differentiated, radially stratified planet. The original soup of atoms of the solar nebula from which the planet accreted have been processed through gravitational, chemical, and melting processes to produce the present Earth. In particular, once the primary mass of the planet was accreted, melting has played a key role in chemically separating (differentiating) the components of the Earth. The result is that a thin (6-70 km) thick crustal layer of very light rocky materials overlies a thick (2885-2821 km) layer of mantle materials that are mainly Mg, Si, O rocky silicate minerals. Below the Mantle lies the iron and nickel rich Core, which has minor components of sulfur, silicon, oxygen and carbon. The core is itself stratified, in that there is an inner core which is solid iron, and the inner core is slowly growing as the core cools. The outer core is molten, with the temperature in this depth range being high enough to melt the metal alloy there. Melting during the magma ocean phase, accretion, and the core formation process produced the primary chemical layering of the Earth. Key to melting differentiation is that in a melt the light material rises and the heavy sinks very efficiently, which allowed the crustal materials to separate as well as the iron in the core. Blended all back together, the bulk composition of the Earth is thought to be very similar to that of primitive meteorites that fall to Earth.
This layered planet is not static, and everywhere in the interior there is motion. These motions are driven by the internal heat that was locked into the planet. Heat from accretion (collisions imparting their energy), heat from sinking of iron into the core (this causes a release of potential energy as the dense iron moves to a more stable position), heat from impacts (moon formation, and many subsequent smaller impacts), and heat from radioactive decay of unstable elements that got incorporated into the interior (large Uranium atoms spontaneously lose protons, neutrons and electrons, eventually transforming to Lead atoms; this liberates much atomic energy that keeps the interior warm). The motions occur in regions that are 'solid' (i.e. below the melting temperature) and in regions that are 'liquid' (i.e. above the melting temperature). Solid regions can deform and flow plastically when subjected to steady stresses for long periods of time. This is called solid-state convection. The Earth can be viewed as a massive dynamic system; a large heat engine, churning away as it cools. The Earth has evolved due to the dynamic motions, and is only about half way through its life cycle. In the past 35 years or so, Humans have attained a pretty good understanding of the dynamic system.
The shallow Earth is of most concern in this class, both the upper mantle and the surface, for this is where the dynamic systems affect Humans as catastrophic processes. An important element of the shallow Earth is the upper 100 km which comprise the Lithosphere. This is a relatively cool, stiff region of the Earth that behaves as a rigid plate when the surface moves laterally. The plates of Plate Tectonics are a large mosaic of lithospheric chunks which move relative to one another. The lithosphere is a region of heat transport by thermal conduction, and the temperatures are below the melting temperature of rocks near the surface. Beneath the lithosphere, the temperature increases and the next 100-200 km are more ductile, comprising the asthenosphere. This region is partially molten in some regions, and does not move coherently, but shears readily, accommodating the motions of the lithosphere. It is the increase of temperature with depth that controls the transition in material properties (rheological properties, the mechanical response to stresses).
If we look at the surface of the dynamic system, the crust is the shallowest part of the lithosphere, and it differs between oceans and continents. A plot of the proportion of surface area at different elevations shows that the Earth's surface elevation is very bimodal, with most of the 30% of continental surface area being low elevation platforms only a few hundred m high, and most of the oceanic regions being about 4 km deep. There are localized regions of very high mountains on continents and localized regions of very deep trenches in oceans, each comprising only a small portion of the surface. To sustain even these small regions requires ongoing dynamic processes, as erosion quickly reduces mountains and deep trenches quickly fill with sediments. To understand the extremes of elevation we must consider the dynamic processes that are responsible.
What happens is that hot rock rises to the surface, as crust pulls apart, with the rapid decrease in pressure allowing it to melt. This molten rock cools to form new crust, releasing the heat energy to the surface to radiate into space. The main regions were upwellings occur is under the central mountain ranges in oceans, which are in fact large volcanic edifices. The new ocean crust that is formed spreads laterally away from the mid-ocean rises in the process called Sea Floor Spreading. So, the age of the ocean crust is youngest at the rise and increases with distance laterally away from the rise. How is the creation of new sea floor balanced on a planet with constant surface area? What happens is that once oceanic crust and lithosphere age to 100-200 million years, they cool and become so dense that they sink back into the interior. This occurs near deep ocean trenches, as the surface is pulled down by the sinking oceanic plate. The margins where ocean plate sinks produce large volcanic arcs, either as islands or on continents. This is the site of the most surface volcanoes (those at the rises are more extensive, but covered by water), as well as most large earthquakes. The cycling of oceanic crust/lithosphere occurs on a time scale much shorter than the age of the Earth, and therefore 70% of the planet surface is very young.
To find older rocks, we look to the continents, which are concentrations of lighter components which cannot sink back into the interior. The continents all have central nuclei of very old rocks, for example in North America the Superior Province rocks of Eastern Canada are 2-3 billion years old, and surrounding rocks are progressively younger overlapping layers. The continental rocks provide the geological record of all processes older than 200 million years; the oceanic rocks have all been recycled for early times. Thus, all old fossils, all reconstructions of past motions of the continents, and all inferences about mountain building events more than 200 million years old are based on continental rock records.
A key to looking at the past is provided by the fact that continental rocks preserve markers of where those rocks formed in space and time. We can date the age of the rocks by various means of radioactive decay, and we can tell the latitude at which they formed by the magnetic record preserved in the rocks. This allows us to fairly reliably reconstruct the history of motions of the continents over the past 600 million years. This time interval has seen great reshaping of the surface map, with the process of Plate Tectonics and the creation and destruction of oceanic plates moving the continents into great aggregations such as Gondwanaland and Pangea, or dispersing them as in the present state, with oceans spreading the continents apart. This is the surface manifestation of the mantle convection process by which the Earth is cooling. We'll learn more about Plate Tectonics later.
We also know that the Earth's core is an active dynamic system by a somewhat different method, which is that the Earth has a magnetic field. In general, the present field is like that of a large magnetic dipole, with axial symmetry close to the spin axis of the Earth, giving rise to north and south magnetic poles. The magnetic field is not constant, but changes with time, and is known to undergo intermittent reversals of polarity. The origin of the magnetic pole is convection of the molten iron outer core. The movement of the liquid iron core in cylindrical columns surrounding the inner core, paralleling the spin axis, results in generation of the magnetic field. This is by dynamo processes, which involve the motion of the electrical conducting iron in the presence of a magnetic field, which in turn generates a magnetic field. The core convection regime is very turbulent and complex, but the net magnetic field at the surface has simple symmetry.
The atmosphere is a very complex dynamic system as well, and one that has changed composition dramatically through time. From an initial state in which the atmosphere was predominantly nitrogen with some carbon dioxide and methane, and no free oxygen, the Earth has progressively evolved to have a present oxygen rich environment (20% of the atmosphere). This transition has been critical to the presence of complex organisms, and in fact has proceeded in parallel with the evolution of life on the planet. The earliest cells did not use free oxygen, but produced it as a waste product, a mechanism that was incorporated into plant photosynthesis. Only in the last 600 million years has the oxygen level been sufficient for the evolution of complex shelled invertebrates and the higher forms of life enjoyed by Earth.
At every stage, the atmosphere has been a complex dynamic system, heated from the Sun and influenced by gravity and rotation of the planet. We will consider some of the complex systems, such as the water cycle, involving evaporation, transport, precipitation, ground water storage, etc. Understanding the solid Earth and fluid Earth dynamic systems is essential for our understanding of consequent catastrophic phenomena accompanying these systems, as well as to our ability to project the consequences of human activities in these dynamic systems.
While the notion of motions in the core and mantle and in the atmosphere has become ingrained in human perspectives about our planet, we must not lose track of how long this system has been going. Think a bit about time scales: let's assume the Big Bang happened 15 billion years ago:
15 billion years = 15,000,000,000 years
15 billion seconds = 475 years
If we transform 15 billion years into a human time scale by considering 1 second to correspond to 1 year:
-Big Bang would have happened in 1526 a.d.
-Earth Formation would have happened in 1858 a.d.
-Dinosaurs would have lived from 1993-1999
-Humans would have appeared 2 weeks ago
-Ice Ages would have happened every day for the last 2 weeks. The last one would have ended 5 hours ago
-Julius Ceasar would have lived 44 minutes ago
-World War II would have ended 56 seconds ago
Keeping this vast time scale in mind; where a single hour out of 475 years defines the span of all of western civilization, let us consider some of the early moments in the Earth.
The early Earth was inhospitable, with intense meteorite and cometary impacts persisting up until at least 3.9 billion years ago. The evidence for this is provided by the Moon, on which the scars of this period of intense bombardment persist, with no healing processes such as occur on the Earth. The primary layered structure of the Earth was established early on, during the core formation process, but the steady circulation of the rocky silicate mantle has resulted in melting of mantle rocks, from which the crust has been progressively extracted along with the bulk of the oceans and atmosphere.
In the first half billion years of Earth's history the planet was hotter throughout. This means that convection in the core and mantle was more vigorous and material cycled to the surface and back into the interior more intensely. The exposure of melted rocks from the interior to the surface allowed volatiles in the melts to escape; this contributed H20, CO2, CH4, and other gases to the early atmosphere. The magma ocean crusted over, cooling to the point where liquid water could accumulate on it. Likely, within ponds and ocean basins underlain by volcanic spreading centers, organic materials began to organize into the structures needed for life (amino acids, proteins, etc.). But throughout this time there were continued large impacts that would burn off the atmosphere and oceans, disrupting any progress life was making. This is called Impact Frustration. The probability of impacts decreased with time as space cleared out in the inner solar system, and eventually the atmosphere and ocean built up to a degree that was never again obliterated by an impact.
The most interesting aspect of the Earth system is that life developed in it, and together the animate and inanimate elements of the system have journeyed through time. The course of life has been dramatic, with great diversity of lifeforms through time, massive catastrophic death events, and even dramatic changes of the atmospheric chemistry. There is great interest in trying to understand the origin and history of life. The data that we have for this undertaking is the record of fossils in rocks found in continental regions.
The oldest rocks thus far found on Earth are 3.9 billion years old, from Northern Canada. Presumably, the vigorous convection, volcanism and impacts of the earlier Hadean precluded preservation of any rocks from that time for us to find today. The old Canadian rocks show no evidence for life in them, but that may be a result of lack of preservation; reheating and deformation of rocks can destroy fossils, and it is in any case difficult to find the likely earliest fossils, which are for single-cell organisms such as bacteria.
Rocks from Isua, Greenland, dated at 3.8 billion years old, have been found to have graphite with light Carbon (12C) in them. Light carbon is typical of photosynthetic processes, so this may be an indication of early life, but it is indirect.
The first clear evidence of life is filamentous microfossils found in 3.5 Bya rocks from Australia and Africa. These resemble cyanobacteria (blue-green algae), and appear to have used photosynthesis to generate energy for life. Stromatolites are also found in 3.5 Bya rocks in Australia and Africa. These are mounds of laminated calcium carbonate (CaCO3) and chert (SiO2). Present day microbial mats form in intertidal zones with cyanobacteria in then. Photosynthesis takes place at the top of the mat and there are anoxic baterial in the deeper layers. The modern stromatolites are complex ecosystems.
At the time that these organisms lived, the environment was more hospitable than in the Hadean, but there were major differences still:
These earliest fossils appear to be prokaryotes; cells with no internal nucleus. Fossils for Eukaryotes (cells with a nucleus) do not appear until about 2Bya, but there is some evidence for Sterols in rocks 2.7Bya, which involve lipids like those found in the larger Eukaryotic cells.
So, life has existed on Earth for at least 3.5 Billion years. Undoubtedly, there was a long precursory phase of evolution that we have not yet found fossil evidence for; indeed there may have been many initial starts in the Hadean that were terminated by impacts. But, from 3.5 Billion years ago, it appears that there has always been life on Earth, with single-celled bacteria being the dominant lifeform overall.
Once again; 3.5 billion years is a long time - 3,500 million years. There are 3600 seconds in an hour. If each second of the hour is one million years long; life has been on Earth for the full hour; humans for only a few seconds.
As we delve into the Earth Catastrophes affecting life, it is important to first come to some working definitions and understandings of life itself.
For the scientific perspective of this course, we view life as a PROCESS, involving constant change within an organized structure. Yet, how to distinguish this from the process of thermal convection in the mantle, which has constantly changed the Earth's surface, or the atmospheric circulation that leads to storms and tornadoes? The biologists define some specific activities of the process which distinguish living systems from inanimate ones:
Life Activities involve:
While mantle convection uses up heat energy, it ultimately will freeze up and die, there is no perspective of it reproducing, or passing on its essence to a successor. The life activities can be defined on the macro-scale of the entire organism, or on the micro-scale of the smallest life unit, the CELL. Cells are, for the most part, bounded by membranes (comprised of lipid bilayers with embedded proteins) that contain cytoplasm as a matrix, with DNA, RNA and Ribosomes contained with the cell (in a nucleus if it is a eukaryote). The membrane controls diffusion and chemical transport of materials in and out of the cell. The need for diffusional transport of all material in the cell to the membrane limits the size attainable for single-cell organisms.
On Earth, all lifeforms are based on carbon compounds, such as
Carbon bonds into complex molecules, and allows for very complex structures. There was fairly abundant carbon in the materials incorporated into the early Earth, and exhumed from the rocks in volcanic emissions. As the magma ocean phase ended, and water could reside in liquid phase on the surface of the Earth this material could concentrate in water. In the 1930's Alexander Oparin postulated that chemical reactions in the atmosphere produced small organic molecules, which rained out into the ocean, concentraing a primodial soup. This soup involved C, N, H, O, P, S and other elements that had been produced inside stars long before. C began to react in the soupy mixture to build up larger and larger carbon compounds.
We recognize the following building blocks of even the simplest cell:
The Nucleic Acids, built up of large agglomerations of ordered carbon compounds are distinctive in having the ability to split and self reproduce. This function opened up the chemical process of renewal and reproduction.
If Amino Acids are the fundamental building blocks, are they the rate-limiting factor in the development of life? Is it hard to make amino acids? These questions were addressed in the 1950s, and in 1953 Stanley Miller and Harold Urey conducted an experiment that showed that amino acids are readily produced. This experiment was to mix a primordial cocktail of steam, ammonia, methane, hydrogen and water in a large vat, and jolt the mixture with electrical discharges that input energy into the system. The input of energy catalyzed reactions, that lead carbon reactions to produce amino acids that precipitated out of the fluids as red precipitates. Having constructed the cocktail to replicate early Earth chemical conditions, and recognizing that lightning, impacts, eruptions, etc., provided ample energy for the environment, it became clear that amino acids would have come into existence quite readily. This is supported by more recent discoveries of amino acids in 4.5 billion year old meteorites, which suggest that some of the materials may have come in from outer space as well, and that the building blocks of life are not isolated to Earth.
But, it is a long way from amino acids to the Cell. Allowing for a soupy concentration of carbon compounds under stable conditions, how could the Cell come into existence? In the 1930s de Jong found that rich amino acid cocktails do in fact produce spontaneous agglomerations in the form of spherules. These may involve vast numbers of nucleotide bases and complex amino acid systems.
In the 1950's it was found that comparable complex protenoid spherules could spontaneously form. These spherules are not yet cells, but are essentially semi-organisms that may chemically work toward more complex functions. Sidney Fox found that dehydration of amino acids could give proteins; heating or concentration in phosphoric acid allows 18 common amino acids to make protenoids.
Amino acids, sugars and fatty acids could have formed on Earth, or come in on interplanetary dust particles; but synthesizing larger biomolecules is tricky. It does not appear to spontaneously occur in water. It may be possible to catalyze the synthesis on clay mineral surfaces, with dehydration on the surface of iron sulfide minerals assisting. Dehydration in foam or bubbles could also concetrate building blocks and allow them to react.
ATP (Adenosine tri-phosphate; a key energy source for life) and nucleic acids can build up spontaneously, and protenoids develop ordered structure such as plasma membranes; but getting this protocell to develop into a prokaryotic cell is a challenge. In the 1990s the view was that cells may evolve from symbiotic populations of such semi-organisms, as energetically favorable chemical pathways are found in a particular agglomeration. Enzymatic RNA may provide a catalyst (Ribozymes).
While this relatively agnostic take on the origin of life still lacks a complete understanding of how the process came into being, it is agreed that scientifically life is a general chemical process prompted by the many reaction opportunities of carbon molecules.
Once a cell forms, it needs energy to sustain its existence. Energy for life comes from high energy electrons, passed from molecule to molecule, releasing energy. Energy released is used to charge up ADP to ATP by adding a phophate ion; this stores the energy for processes later. Various solutions for charging up ATP have been found:
Autotrophs - capture energy from light of chemical reactions. Chemo-autotrophs use inorganic reactions; for example Methanogens, unicellular Archaea, react CO2+H to make Methane, Water, and ATP inside cow guts. This makes 500 liters/day/cow of methane!
Heterotrophs - obtain energy by consuming organic compounds. Fermenting heterotrophs reduce glucose to ethanol, carbon dioxide and 2 ATP. Eubacteria do this anaerobically.
Both of these energy mechanisms were likely used by early Earth life, but these processes use up hydrogen and prebiotic organic materials. New methods had to evolve as the environment changed.
Photo-Autotrophs - absorb light energy using molecules with organic-metal complexes like Chlorophyll. Energy charges up ATP, then makes sugar. Photosynthesis takes H2O+CO2+light to get CH2O (sugar)+2O2. This results in almost limitless sources of food. The oxygen waste product would be toxic to earlier anoxic Archaea and Eubacteria; forcing them to isolated anoxic environments. Photosynthetic organisms had to develop antioxidants to protect themselves. The increasing oxygen availability allowed the evolution of a much more efficient process.
Aerobic Respiration - Combine organic material with free oxygen. Produce carbon dioxide waste6(CH2O)+6O2 gives 6CO2+6H2O + 36 ATP. This is much more efficient than fermentation, and leaves no energy rich waste like ethanol.
A combined ecosystem of Photo-Autotrophs and Aerobic Respirators can be self-sustaining.
The early Earth environment was highly reducing, and corrosive, and no free oxygen was around for use by lifeforms. How could life have formed in this hostile environment? Part of the answer has actually been provided by Earth Sciences. This came with the discovery in 1977 of deep sea hot springs, in regions where upwelling mantle rock comes close to the surface at mid-ocean ridges. Water circulates down into the crust and is heated by the magma, and rises out of the rock in hot springs. The water is enriched in rock materials and gases leached from the rock. This environment is actually very reducing, as there is no free oxygen coming from the rocks, thus it is an analog to the early Earth (such leaching has gone on through the ages). What was most remarkable was that many strange organisms were found near the hot springs, and these had evolved to use the materials in the reducing environment. Rather than photosynthesis, they chemo-autotrophs, extracting energy for life from H2S (hydrogen sulfide) carried in the rising vents. These very distinctive forms of life, with completely different pathways to existence than most Earth life may have strong parallels with early lifeforms. Indeed, such deep hot vents may have been the main source of energy and environmental stability for life to evolve, given the heat from the geothermal engine and the sheltered status deep in the ocean. There is evidence in the DNA of most organisms that have evolved to survive in the current oxygenated environment that suggests we all have common links to chemosynthetic predecessors.
The Stromatolites of the archean were using photosynthesis as a means for releasing chemical energy, and this led to progressive conversion of CO2 to O2, and slowly the percentage of oxygen in the atmosphere increased. Transformation of the atmosphere from a reducing environment to an oxygenating environment was a critical precursor to the development of aerobic respiration and the development of multi-cellular organisms that could exploit the greater energy harvest that provides.
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