April, 2011

In Celebration of Psalm Nineteen:
God's handiwork in Creation

Chapter 7: A Fit Place For Life:
Preparation of the Earth for Advanced Life
(3.9-1.8 Ga)

NOTE: Ba (Ma) = Billions (Millions) of years before the present time. By (My) = years of lapsed time.

Introduction. Chapters 5 and 6 describe the state of the Earth around 3.9 Ba, when it first became able to support a primitive sort of life. Very quickly, bacterial life appeared in all of its essential complexity. The earliest actual fossils that have survived undamaged come from some 400 million years later, about 3.5 Ba.

For the next 2 billion years (to about 1.8 Ba), bacterial life dominated the world and caused major changes. These changes are the subject of this Chapter. They prepared the earth for the next great innovation: the "proper" nucleated (eukaryotic) cell, which is the basic building block for all advanced life -- the plants and animals.

The preparation involved these things:

The gradual change from a reducing to an oxidizing atmosphere. At the start, the atmosphere had very little free oxygen; at the end, the atmosphere had about 20-25% oxygen content, and has remained at that level since.
Essentially all of the oxygen in the atmosphere originated as a "waste" product of the primordial bacteria.

The corresponding change from an ocean and outer surface crust of reduced to oxidized minerals and salts. At the start, "raw" or partially oxidized metals and minerals
dominated (iron, for example), but at the end fully oxidized minerals dominated (iron and uranium oxides, for example).

Advanced life, particularly multicellular life, requires the abundant availability of oxygen: there is no alternative to this. In practice this meant that the reducing environment of the early earth had to be transformed to an oxidizing environment. This took a protracted period of time because the oxygen produced as a waste product of early bacteria was immediately used to oxidize nearby minerals.  The early life was limited to single-celled bacteria which could thrive in a non-oxygen atmosphere, and this condition continued until the dissolved minerals in the ocean and on the surface of the earth's crust were largely oxidized, after which oxygen could finally build up in the atmosphere. At this point oxidizing bacteria and advanced eukaryotic life could (and did) form and flourish.

The global distribution of biological material and wastes. At the start, no organic material existed (of course); at the end, biological material abounded throughout the earth's oceans and outer surface crust. In particular this material included "fixed" (biologically available) nitrogen, usually in the form of organic molecules. Although fixed nitrogen may be inorganic when in the form of nitrates or ammonia, the early Earth had very little of it, and so virtually all of the fixed nitrogen was produced from atmospheric nitrogen by specialized anoxic bacteria in a slow and arduous process of biological nitrogen fixing. Once fixed and incorporated into biological molecules, it became available as food for subsequent generations of life. The protracted preparation of abundant fixed nitrogen worldwide, required most of the long timespan of this two billion year era.

The creation of permanent dry land.
The early earth was covered with a global ocean -- hundreds of feet deep -- so the first life lived in a water medium, and since the ocean was global, life and its products diffused throughout the oceans worldwide. The tidal effects of a nearby moon and the continued cooling of the earth's crust resulted in many large volcanoes whose debris often penetrated the ocean surface to form volcanic cones, but these quickly eroded due to violent tides and weather so that for many millions of years there was nothing resembling permanent dry land. The volcanic activity resulted in extensive, reasonably stable, shallow-water tidal zones -- areas that were washed by the ocean and tides but reliably remained within reach of sunlight. These shallow tidal zones became the home of photosynthetic life which derived its metabolic energy from the Sun.

Eventually, dry land (the continents) arose out of the global ocean as a result of tectonic plate movements. T
idal forces of the nearby moon provided the energy for these tectonic movements.

The Earliest Fossil Species.
Fossils, of course, exist in rocks that must be at least as old as the fossils, and that must have been preserved intact without being deformed or subjected to extreme heat or pressure. There are very few locations on the earth where rock older than 3.5 Ga exists, and even fewer locations where such ancient rock has been preserved in a way that might preserve fossils. A small amount of such rock exists in Western Australia, Eastern South Africa near Swaziland, and in the western margin of Greenland.

Without actual fossil evidence it is (probably) not possible to identify the very earliest bacterial life. However, living matter leaves evidence in rocks that show a greater than normal amount of the carbon isotope C-12 relative to the isotope C-13. This preference for C-12 is traced to the selective bias of the molecule RuBisCO (perhaps the most abundant protein on earth), which fixes atmospheric CO2 as part of the sugar production cycle in living matter
02. This indirect evidence for life goes back as far as 3.9 Ba.

The earliest actual fossils are closely dated to 3.465 Ba ± 5 Ma, discovered by J. William Schopf
03. This close dating is possible because the fossils are sandwiched between lava flows containing zircon crystals that can be precisely dated.

These early fossils appear in chains as depicted in Figure 1

Figure 1
Sketch of earliest fossil (3.465 Ba)

They appear to be chains of dessicated cyanobacteria, also called blue-green algae. A typical modern example is Anabaena, see Figure 205.

Modern Cyanobacteria
Figure 2a
Photograph of Anabaena,
      a modern Cyanobacteria

Modern Cyanobacteria
Figure 2b
Sketch of a Cyanobacteria chain

The First Fossils.
Preservation of the earliest fossils through almost 3.5 billion years of chaotic upheaval of the earth's crust is practically a miracle. Almost all of the rock on earth has been melted, compressed, distorted or otherwise changed in ways that would destroy fragile fossil evidence. Schopf's book gives a vivid description of what has to happen for these ancient fossils to survive to the present day. The result is that such fossils are found in only a few small location worldwide: small areas in South Africa (Swaziland formation) and in Western Australia. Indeed it is remarkable that there are any fossils remaining from these ancient times. In the example of Schopf's fossils they had to avoid being "cooked" by lava flows both below and above the actual fossils -- a rare event indeed -- but without this lava and the risk of overheating, the fossils could not be dated.

Cyanobacteria are moss-like species that live in oxygen-poor environments bathed in light, such as in shallow bodies of water. They are the only bacteria that produce oxygen as a waste product07 -- which is an important task of this early life. They are exceedingly complex, far from what one would think to call primitive. They grow in long chains because when the cells reproduce they divide in half and tend to remain attached (Figure 2b). They secrete a kind of mucilage or slime which solidifies to form characteristic multi-layered dome-like structures called stromatolytes that grow in highly saline tidal basins -- shallow water between high and low tide. Living stromatolytes exist today in only a few locations worldwide, one being Hamelin Pool in Western Australia (Figure 3).

Location of stromatolytes

Stromatolites-HamelinPool.jpg Martin Peters Landmarksolutions.com.au
Figure 3
Stromatolytes at Hamelin Pool
Western Australia
Photo by Martin W. Peters,
Landmark Solutions
used by permission

If these fossils are cyanobacteria (or closely related ancestors), then it immediately poses a problem because -- as we will see -- cyanobacteia are advanced bacteria, not what one would assume to be representative of the earliest living species08.

Why bacteria and not archaea?
Some paleo-biologists insist that the earliest life was from the kingdom Archaea (indeed the name implies that they are the most ancient bacteria), based on the ability of archaea to manage in very hostile environments (which the early earth certainly was), and the claimed advantages of survival near deep water thermal vents.

It is not the purpose here to confirm or deny this possibility, but there are some good reasons to doubt that archaea could "be fruitful and multiply and fill the earth" [Gen. 1:22]
to the degree required at this point in the earth's history: Archaea are too limited and specialized to fill that role. In addition, the genetic make-up of the archaea appears to be more advanced than that of bacteria, more akin to eukaryotes, and therefore (one would assume) a later development.

In the final analysis, though, it does not really matter whether the first living species were archaea; the first practical living species had to be bacteria -- oxygen-producing cyanobacteria (or close ancestors) -- and as a matter of fact, these were the first fossils preserved in the fossil record.

From the point of view that the main task of early life was to form a fit place for later life, it is significant that no known archaea species conduct photosynthesis or have oxygen as a waste product, and so they would be unable to convert the initial reducing environment to an oxidizing environment, required for advanced life.

Regarding the appearance of the first life, Alexandre Meinesz, How Life Began: Evolution's Three Geneses refers to "the strange fact that the ancestral bacteria were already highly diversified" when the first fossil evidence was found. He then continues, "The currently popular idea that life probably arose in warm subsurface waters along a mid-ocean ridge, the kind of environment where a great variety of heat-resisting bacteria thrive today, is a hypothesis without any scientific basis." p.30. Meinesz also cites the syposium Size Limits of Very Small Microorganisms mentioned in Chapter 6.

Why does animal life require oxygen?
Plants don't require oxygen???

the mitochondria in (most?) animals uses oxygen to form ATP, the major energy molecule in all cells. Anoxic cyanobacteria (as well as green plants) derive energy from the Sun by photosynthesis to form ATP (producing oxygen as a waste product).

"Without O2 life was doomed to remain quite simple; each cell was forced to be chemically self-sufficient. Only when O2 became available was there a means of transporting chemical energy to specialized cells. O2 and organic molecules travel together through blood vessels to the site where energy is needed. Here an enzyme triggers combustion. Given an atmosphere with O2, animal life took off, evolving everything from mosquitoes to dinosaurs!"
Broecker, p. 234.

Preparation for advanced life. The rapid multiplication of the early species of life was needed to prepare the earth for more advanced species. Almost two billion years separate the first bacterial fossils and the first eukaryotic fossils -- the first step towards complex, multicellular life.

Looking ahead, the main tasks for the early bacteria were:

• Distribute abundant amounts of organic food worldwide.
- This task is needed because advanced animal life cannot take the time or energy to be self-sufficient (autotrophic).
- In particular, this food provides fixed nitrogen, which is essential for all of life -- including so-called "autotrophic" plants. Its manufacture from atmospheric nitrogen is a difficult, energy-consuming and slow process. No eukaryotic species (plant or animal) is able to manufacture nitrogen (except that some plants -- the legumes for example -- have a symbiotic relationship with nitrogen-fixing bacteria). In fact, very few bacteria species are able to manufacture all of its own requirements for nitrogen, and even these require cell specialization. The nitrogen may be either organic or inorganic (in the form of nitrates or ammonia gas) but the sources of inorganic nitrogen (prior to the Haber process, first used by Germany
during WWI to produce ammonia on an industrial scale) are not sufficient to support abundant life.

• Convert the earth's atmosphere and the oceans from reducing to oxidizing. The atmosphere must have around 20-25% oxygen content. Complex life requires at least the lower limit of abundance, and the upper bound is needed to avoid spontaneous combustion08.1.

These tasks took almost two billion years to achieve, with the aid of oxygen-producing and nitrogen-fix
ing bacteria. The formation of vast mineral deposits that are so essential to the modern technological age were by-products of this push to develop the oxygen supply.
Complexity of the first living species. Cyanobacteria are the only known bacteria that produce oxygen by photosynthesis. They were apparently the bacteria of choice in the task of oxidizing the earth
. One problem is that cyanobacteria are complex -- in Margulis' classification they are phylum B-6, about half-way up the ladder of bacterial complexity. This complexity in such ancient species is something that evolutionary theory would not have predicted.

Photosynthesis. Photosynthesis -- using solar energy to energize life processes -- involves interactions between many individually complex molecules, some of which are not fully understood today.

 photosynthesis Overview
Figure 4
Overview of Photosynthesis
Shown in a plant chloroplast. In cyanobacteria the same process
is embedded in the thylakoid membrane.

Photosynthesis requires the use of a membrane that encloses an acidic interior (excess H+) to drive the production of ATP -- the universal energy storage "battery" in all living species. The chlorophyll and ATP synthase molecules are embedded in this membrane09.

The similarities between all photosynthetic systems and its complexity is such that evolutionists such as Stephen Jay Gould assert that it could only have evolved once, which means (in his lingo) that it is a very low probability expected result of random processes

The Creation of Photosynthesis

According to an analysis of the cyanobacterial genome (Haselkorn and Johnston (PNAS)) the earliest cyanobacteria already had the light & Calvin processes for photosynthesis in place. These are two very complex and subtly linked processes and involve many specialized molecules working together. These are such complex biological processes, that the complexity and early appearance on earth seems to indicate planning and design09a.

Biologists universally (as far as I am aware) point to the complexity and the similarity of photosynthesis among all species to imply that the process evolved only once in earth's history (see footnote 4) -- the chance events that had to occur for photosynthesis to arise even once by natural processes are vanishingly low probability, so that assuming the same system would arise more than once defies even an evolutionist's credulity.

Photosynthesis divides into two parts: the light process and the dark process. In the light process, chlorophyll uses the energy of sunlight to produce ATP and NADPH. Each of these processes involves complex molecules: ATPsynthase (a molecular motor described in Chapter 6) and NADP reductase.  Protons are fed into a closed membrane by splitting a water molecule (using a light photon for energy) into two protons and oxygen, which is a waste product. This is called Photosynthesis-II (PS-II). A separate process, involving two molecules, plastoquinone and cytochrome, pumps protons from the membrane exterior to its interior (see Figure 3a). The ATP Synthase motor molecule is embedded in the membrane and a proton flow from the interior rotates the molecule, producing ATP from ADP. A second photon-energized process called Photosynthesis-I (PS-I) uses an enzyme ferredoxin-NADP Reductase to form NADPH which carries the excess H to the dark process.

The  dark process, also called the Calvin cycle then uses both ATP and NADPH to form triose sugar (C3H6O3) with the help of another complex molecule (RuBisCO). The triose sugar is used to form starch, amino acids and sugars.

Wiki-lightProcess.gif Wikipedia Photosynthesis Wiki-CalvinCycle.gif Wikipedia Photosynthesis
Figure 3A
Photosynthesis: Light Process
ATP provides energy to the dark process and to the cell generally NADP is an H carrieer enzyme used in the dark process. In cyanobacteria photosynthesis occurs in a thylokoid membrane. See the Light Process Animation.10
Figure 3B
Photosynthesis: Dark Process (Calvin Cycle)
5 of 6 triose sugar-phosphates are re-used in the cycle. All cyanobacteria use the Calvin cycle.

Chlorophyll. The chlorophyll molecule of all photosynthetic species captures light energy using a special ring structure that has a magnesium atom suspended between four nitrogen atoms. When light hits this structure, it emits a high energy electron that initiates the photosynthetic activities11. The photosystems PSII and PSI have slightly different chlorophylls, P680 and P700 which are "tuned" to peak response at slightly different light wavelengths -- 680nm (yellow) and 700nm (orange) respectively. Both chlorophylls have the same ring structure centered around a Magnesium atom. The structural difference between these two chlorophylls is in the tail, producing (when stimulated by light) the strongest biological oxidizer (P680) and strongest biological reducer (P700) known10.1.

photo-chlor.gif NASA/JPL-CalTech
Figure 4
Chlorophyll "Antenna"
The hydrophobic tail is embedded in a membrane.
The magnesium complex captures light.

The Myth and the Reality of "Horizontal Gene Transfer"
The concept of "horizontal gene transfer" (HGT) is frequently invoked in explaining the appearance of highly conserved genes and gene packages in widely diverse bacterial (and even eukaryotic) species.

HGT is a well-known phenomenon and is a common way in which genes are transferred between bacteria of widely diverse species, and indeed on occasion between bacteria and eukaryotes. This mechanism has been observed in the laboratory and appears to be regularly used, for example, to propagate immunity of various types between bacteria. The specific types of HGT include the incorporation of genetic material from ingested food, transfer through a viral intermediate, and transfer by bacterial conjugation.

Bacterial Conjugation
Figure 5
Horizontal Gene Transfer by Conjugation

This is a way to explain why the same genes show up in widely diverse species. To a person who accepts the reality of a divine Creator, this mechanism may be one way that God re-uses previously created gene packages in the creation of new species. Or, God may simply repeat a successful gene package by fiat. There is probably no easy way to rule one way or the other.

One reason to postulate lateral gene transfer is to note the low probability that the genes could have arisen independently by chance. The lower this probability, the more likely that gene transfer of some sort occurred, according to evolutionary thinking.
So, invoking the mechanism is a way to get around the low probability of the genes arising repeatedly by chance. This natural mechanism does not explain how the gene packages came about in the first place, a vanishingly low probability chance event.

Carbon and Nitrogen Fixing. One of the first problems that had to be solved by life was carbon and nitrogen fixation. This means that the carbon atom, C, and the nitrogen atom, N, had to be converted from the inorganic gases carbon dioxide and nitrogen, and incorporated into an organic molecule. In the case of nitrogen, this conversion usually involves the formation of ammonia (NH3) gas or a nitrate such as sodium or potassium nitrate, which are not found in adequate or reliable amounts in the early reduced  environment.  Once reduced and placed into an organic molecule, these atoms can be passed on as food for future use. The problem is to fix the carbon and nitrogen in the first instance.

The solution to the problem of carbon and nitrogen fixation is two complex molecules. Carbon is fixed using RuBisCO, a large and complex catalyst -- incorporated into the sugar-formation Calvin Cycle of photosynthesis -- that is the most common protein on earth: it is estimated that RuBisCO accounts for about 50% of all the protein on earth12.

Nitrogen is fixed using Nitrogenase, also a large and complex catalyst -- a molecular motor -- that is perhaps the rarest of the essential molecules for life: one author estimates that "The entire world's supply of nitrogenase could fit into a single large beaker or bucket."13

Both carbon and nitrogen fixation are very slow processes, in comparison with the rate of most life processes. RuBisCO converts only 2-3 carbon atoms per second, and Nitrogenase is even slower: 1.2 seconds per fixed nitrogen molecule. Because of its abundance, the speed of RuBisCO is not a critical factor in the survival of life, but the rarity of nitrogenase combined with its slow speed is another matter: the yearly production of fixed nitrogen accounts for no more than 10-20% of the annual requirement (op. cit. p84) -- hence the need for a large reserve of organic food to supply the deficit.

Carbon Fixing with RuBisCO. The RuBisCO molecule is a large and complex molecule that catalyzes carbon fixation -- the extraction of C from atmospheric CO2   -- and adds it to enlarge a carbon chain as part of the formation of a sugar-phosphate (trios-phosphate - G3P) molecule. The enzyme aldolase  then combines two G3P molecules to form fructose sugar - C6H12O6. Sugar molecules are the basic "food" for the formation of many carbon chains in  numerous cell processes.

Figure 6
The RuBisCO molecule

The RuBisCO enzyme is a protein consisting of  two subunits: the smaller is made up of 123 amino acids, and the larger has 475 amino acids. The precise mechanism by which the enzyme works is not fully understood. RuBisCO is certainly one of the oldest enzymes, and it is the most abundant. It is responsible for the preferrence of living material for C-12 over C-14. This relative abundance marker is the earliest evidence of life on earth -- appearing around 3.9 Ga.

Is RuBisCO a "Design Flaw" of Evolution?
In 1999 a classroom handout titled Improving RuBisCO in Photosynthesis argued that RuBisCO is "the most inefficient enzyme known to man" with the implication that this is another example of a suboptimal product of evolution. The alleged problem is that RuBisCO is slow and indiscriminately wastes energy: "the reaction leaves a great deal of free energy," concluding that
"RuBisCO has the potential of becoming a more efficient enzyme in the process of photosynthesis.  If researchers are able to find ways to improve the abilities of the enzyme, then plants can grow faster and increase the amount of food available.  It will only be a matter of time before RuBisCO is engineered to be more efficient and the whole world will reap its benefits."

The commercial value of a more efficient RuBisCO is evident: if its efficiency could be increased, crops would be much more productive. What has happened in the 12 years since 1999? very little. Subsequent papers remain hopeful, but always project improvements into the future, and cite little in the way of actual results. In 2010, Biofuels Digest stated:
Slow, dim-witted RuBisCO. Though abundant, it is a slow, dim-witted enzyme if ever there was one. So slow that it fixes just three carbon molecules per second, and so dim-witted that it has trouble distinguishing between oxygen and CO2. Under many conditions, it will fix oxygen instead of CO2, in a process called plant respiration which causes carbon loss and robs the plant of growth opportunity."

One of the "defects" of RuBisCO is that it will process O2 as readily as it processes
CO2 in a process called photorespiration13.1. It detracts from the carbon fixing task because it uses energy unproductively and leads to a net loss of carbon and nitrogen. It would seem that reducing the tendency of RuBisCO to fix oxygen would improve plant efficiency, but this appears not to be the case (see Wiki article). "[P]hotorespiration drains away as much as 50% of the carbon fixed by the Calvin cycle. If photorespiration could be reduced in certain plant species, without affecting photosynthetic productivity, crop yields and food supplies would increase." [ibid. - emphasis added]

But evidently no solution has been found after over three decades of intense research -- and not for lack of biofuels and other research funding. As a Creationist, I am tempted to predict that no solution will ever be found, because RuBisCO is already close to maximally efficient. I won't make that prediction, however, because I don't know God's reasons for having things as they are.

Nitrogen Fixing with Nitrogenase. Microbes that fix nitrogen are called diazotrophs14. Although nitrogen made up about 80% of the ancient earth's atmosphere, it was not "available" to living species. As one author remarked, "No animal, plant, fungus, or protist has mastered the chemical art of converting the abundant gaseous form of nitrogen into a biologically useful one."15 Ancient life had to fix nitrogen; that is, convert atmospheric nitrogen to amonia; otherwise life could not flourish. There was no other effective way to get the nitrogen needed.

Only one way to fix nitrogen exists in nature, and that is with the use of the complex nitrogenase motor molecule. Nitrogen fixing is a very slow process. To convert a single molecule of nitrogen gas to ammonia, the nitrogenase molecule, which is made up of two giant proteins, must physically separate and rejoin eight times, and this takes about 1.2 seconds. Today, nitrogen fixing worldwide only supplies 10-20% of life's annual consumption (Wolfe, op. cit., p. 84). The rest must come from recycled organic food (or, in the past century, from commercial inorganic nitrogen).

Nitrogenase molecule, illustrated in Figure 7  has 24,190 atoms and is composed of two proteins involving a molybdenum and magnesium atoms, called MoFe (dinitrogenase) and FeMo-co (MoFe cofactor). MoFe is produced by the genes nifD and nifK. All told some 22 genes are involved in producing and regulating the molecule. A full explanation of how nitrogen fixation works is still unresolved.

[Figure 7: Nitrogenase Molecule]

The nominal formula for nitrogen fixing by the nitrogenase molecule is:

N2 + 8 H+ + 8 e + 16 ATP → 2 NH3 + H2 + 16 ADP + 16 Pi,

where Pi denotes inorganic phosphorous. This is a formal equation: other than with the use of the nitrogenase molecule as a catalyst, there is no known way to execute the equation at normal ambient temperature and pressure in any chemistry laboratory. The formula indicates that 16 ATP molecules are reduced to ADP to supply the energy to produce 2 ammonia atoms. This is very expensive energy-wise, as well as very slow. The only commercial way to fix nitrogen is by using the Haber process, which operates at high temperature and pressure, and so cannot be duplicated in the biological world.

Nitrogen Fixing and Nitrogenase

A method to fix nitrogen was absolutely critical for the early species to fluorish on the early earth; otherwise life at best could only falter along using the scarce fixed nitrogen found naturally. A major task of this early life was to spread fixed nitrogen as food worldwide so that it could be used by more advanced life, and so it had to have an abundant supply.

There appears to be only one way to fix nitrogen naturally, and that is with the use of the complex nitrogenase molecule. The nitrogenase molecule is so complex that to date (2011) the procedure that it uses is not fully understood. In any case the process is very slow (taking 1.2 seconds to fix a single nitrogen molecule), and requires not only a very complex molecular process, but it also requires a specialized cell in which oxygen is excluded.

How is such a molecule to be developed by purely natural, undirected processes? As with photosynthesis, the molecule is so complex and unique that it is inconceivable that the molecule could have arisen naturally more than one time in the history of life -- and I would argue that it stretches credulity to think that it could have arisen even one time without a creator's hand.

The nitrogenase molecule is poisoned by the presence of oxygen. Thus special care must be made to isolate nitrogen fixing from oxygen. In cyanobacteria, this is done by the use of specialized cells called heterocysts to fix nitrogen. These heterocysts have thick walls and cannot perform some of the normal tasks of the regular cyanobacteria cell; in particular they do not have PSII photosynthesis. It is dependent on adjacent cells for a supply of food and energy (ATP), which it needs in abundance. In a typical low-nitrogen medium, about one in 15 cells in a (modern) cyanobacteria chain is a heterocyst (Figure 8). Frequently the immediate neighbor to a heterocyst is another another specialized cell called an akinete, which can survive under harsh conditions -- freezing, starvation and dehydration -- for long periods of time. Since the early earth was constantly changing with no permanent dry land or shorelines, the ability to survive and resume growth in another locality or time was important. In addition the ability to go into a kind of suspended existence also allowed the cyanobacteria to drift with the ocean currents and distribute life and nutrients worldwide.

Figure 8
Cyanobacteria showing Heterocysts and Akinetes.
By permission of David Webb



The record of the next two billion years is literally written in the rocks. It is evident from ancient rocks that the earth had a radical change at around 1.9 Ba. This was the time that the oxygen content of the atmosphere reached a stable level (20-25%). It also marks the start of the eukaryotic life (the Third Genesis, the subject of Chapter 8).

Prior to 1.9 Ba the oceans cycled between times with large amounts of reduced iron in solution (which implies that the oceans were acidic) and times in which the ocean acidity dropped, resulting in the precipitation of this iron in the form of iron oxides. These cycles resulted in the banded iron formations in which iron oxides alternate with silicates. The concentration of iron oxides in these bands can reach as high as 40-50%. After this time, about 1.9 Ba, these cycles gradually ended and the ocean acidity reached a steady level. For the next billion years, the iron "red beds" formed from precipitation of the remaining iron solutes, but without the characteristic banded formation, and the concentration of iron oxides in the sediment dropped significantly: after 1.9 Ba no further high concentration iron ores appear in the geological record

 Cumulative O2
Figure 9
Cumulative production of Oxygen

At 1.9 Ba the oceans and the top portions of the Earth's crust became oxidized.  Even today, the oxidizing conditions are confined to a thin outer skin of the crust; at no place is this skin more than a kilometer or so thick, and most commonly it is much less than that. Below this thin skin, the crust consists of reduced minerals. Overall, except for the atmosphere, oceans and this thin skin, the earth is highly reduced, reflecting the overwhelming abundance of hydrogen since primordial times17.

The Banded Iron Formations
The Banded Iron Formations are a good example of the Silent Speech. Through them it is possible to reconstruct the gradual build-up of oxygen in the early atmosphere, and the change of the earth surface from a reduced to an oxidized condition.

Life's Early Boom and Bust Cycles: The Uranium and Banded Iron Ore Deposits. Because the early earth was starved for free oxygen, the ocean held large amounts of reduced salts in solution, particularly iron. The "waste" oxygen produced by the early cyanobacteria combined with these reduced salts. If the product was (relatively) insoluble, it precipitated out, forming over time vast ore deposits.

As oxygen content increases, the ocean becomes less acidic. The solubility of many minerals, including iron, silicon and uranium oxides varies with the ocean acidity. As the acidity decreases (oxygen increases) the oxides become insoluble and precipitate out of the water. When the acidity increases (oxygen decreases) some of these precipitates again become soluble. Since the solubility of the oxides are known, it is therefore possible to infer the acidity of the water (hence the oxygen level) at the time that they precipitated out. Thus the successive layers of mineral deposits provide a chronological record  of the rise and fall of oxygen content of the water (and by inference the atmosphere).

The early oceans experienced
(local) changes in acidity that reflected the changing fortunes of the oxygen-producing cyanobacteria. When the bacteria thrived they produced an over-supply of oxygen which poisoned the environment and caused the bacteria to falter. This over-supply of oxygen oxidized the reduced minerals in the local ocean. If the oxidized minerals were insoluble, they precipitated out onto the ocean floor and removed the dissolved oxygen waste. If not interrupted, this process would gradually lead to increased acidity, which also affected the ocean's solubility. Eventually the reduction of oxygen ended the bust cycle and led to a recovery of the bacteria. They again thrived, leading to a  new boom cycle.

These repeated boom and bust cycles raised and lowered the oxygen content (acidity) of the oceans and amosphere until most of the reduced salts and minerals in the oceans and crust surface had fully oxidized. At that point the oxygen content of the atmosphere reached  a steady level of 20-25%.

Uranium salts were among the first to precipitate.
Under reduced conditions uranium is insoluble and stable as uraninite (UO2). As the cyanobacteria build up oxygen waste, acidity decreases. Under this condition, soluble uranium (U6+) from volcanic activity, ocean vents and surface runoff enters the ocean water. When the cyanobacteria become poisoned by excess oxygen waste, the acidity increases and uranium oxide precipitates out to form uranium deposits, reaching pitchblende (U3O8) concentrations as high as 20% to 50% 18. This continued for about a billion years, until most of the uranium salts were fully oxidized.

For the next billion years, silicon and iron soaked up the excess oxygen, forming sand and the great banded iron iron ore deposits (Figure 10).  The precipitation of silicon and iron is sensitive to the acidity of the environment. As cyanobacterial activity produces waste oxygen,the ocean acidity lowers. Iron oxide precipitates first, and when acidity lowers further, the silicates precipitate out. Eventually the growth of oxygen  in the ocean poisons the cyanobacteria, and then the oxygen level decreases, with a corresponding increase in the ocean acidity. These boom and bust cycles can be seen in the banded iron formations. Immediately a potential problem arose: oxygen is generally poisonous to the bacteria that are the only life on earth. As long as there were minerals to draw off the excess oxygen, things could go on. But now, the earth's crust is fully oxidized. What is going to keep the oxygen from growing to the point where life hits a stagnant and unfruitful plateau?

Figure 10
Banded Iron Formation
iron oxide (Fe2O3) = dark
and silica (SiO2) = light.

About the same time that the The banded iron formations ended and most of the reduced elements in the ocean and exposed crust are oxidized, the next great biological invention -- eukaryotes -- appeared on the scene (the subject of the next chapter). The eukaryotes (and some oxygen metabolizing bacteria) took over the role previously held by the reduced elements, and used the excess oxygen that was being generated. A stable equilibrium occurred at about this time, and the oxygen level rose to a fairly stable 20-25% level in the atmosphere, where it has remained ever since. The stability is the result of an ecological balance between oxygen-consuming and carbon dioxide-consuming species of life.

The Appearance of Dry Land. For the first two billion years, the earth had no permanent dry land. Frequent volcanoes caused ashes and debris to form volcanic cones that would penetrate the ocean surface, but these cones were not permanent because storms and tidal activity eroded them over time. The final result of these temporary penetrations of the ocean surface was the formation of extensive shallow tidal areas that became homes for extensive shallow-water species including the cyanobacteria and other bacteria that formed extensive beds containing stromatolytes.

When the earth cooled from a molten state, its interior stratified (Figure 11). Gravity tended to place the heaviest material to the core and ligher materials in concentric shells with the lightest material on top. The core is heavy nickel-iron core mixed (and kept hot) with some heavy radioactive metals and their daughter products. Layers below the crust are in a plastic or semi-liquid state, maintained by pressure and radioactive heat19. The layered structure of the earth has been confirmed by analysis of global acoustic sound transmission (seismology) conducted over many years. The most recent analyses use a form of tomography (similar to the techniques used in medical imaging) to reconstruct the form of the interior.

Figure 11
Earth Layers
Source: Wikipedia

The interior heat in combination with the tidal forces of a nearby moon resulted in convection currents in the Earth's Mantle, energized by the heat emitted by the (semi-solid) core (Figure 12a). These currents carry along the earth's crust, which fractures at collision and separation points.

MantleConvection.gif tectonic-currents
Figure 12A
Mantle Convection Currents
Figure 12B
Present Day Currents

The earth's crust broke up into a number of large plates that were carried along by the convection currents in the mantle (Figure 13), which collided, causing one plate to pass under an adjacent plate. Most of the world's volcanoes lie along the edges of these plates.

When plates collide, one plate rides over its neighbor and the neighbor is forced down into the mantle, a process called subduction. The edges of the subducted plates are carried along by the mantle currents deep into the mantle itself. As this happens, the leading edges of the subducted plates melt from the heat. Material with a lower melting point melts first. This also has a lower density, and as it melts, it rises through cracks, leaving heavier, denser matter behind. Over time the lighter material forms the continents which, because of their lower density, literally float atop the mantle and crust.

Figure 13
Present Day Tectonic Plates
Showing Active Volcanic Zones
As the continental mass builds up, it rises above the ocean surface and the result is permanent dry land. This process is called plate tectonics20. Volcanic activity tends to follow the plate boundaries.

An example of subduction is along the western coast of South America (Figure 14), forming over time the South American continent and the Andes mountains.

Figure 14
Source: USGS

Where mantle currents diverge, the crust separates, causing newly formed crust to form under the oceans. The mid-Atlantic ridge is an example of such a divergence. The newly formed material is basalt. Recently formed material (within the past 600 Ma)
  underlays most of the ocean floor, both Atlantic and Pacific, and is denser than continental rock.


CREATION DAY THREE: Dry Land and Land Plants
Creation Day 3
Genesis 1:9-13 (ESV)


9 And God said, “Let the waters under the heavens be gathered together into one place, and let the dry land appear.” And it was so. 10 God called the dry land Earth, and the waters that were gathered together he called Seas. And God saw that it was good.

11 And God said, “Let the earth sprout vegetation, plants yielding seed, and fruit trees bearing fruit in which is their seed, each according to its kind, on the earth.” And it was so. 12 The earth brought forth vegetation, plants yielding seed according to their own kinds, and trees bearing fruit in which is their seed, each according to its kind. And God saw that it was good. 13 And there was evening and there was morning, the third day.

The geological record shows that the creation of dry land began around 2 Ga. with the formation of what would become the continents. The geological record agrees completely with the Genesis account in the fact that the earth was initially covered with water and that the dry land was made to appear out of the waters. This happened by forming the continents of less dense granites and other materials by the process of fractionation that is described above. Both the less dense continental rocks and the denser magma that forms ocean floor and underlays the continents float together on the fluid mantle with the result that the continents rise above the ocean surface much as icebergs float on the oceans. This is a physically stable and permanent arrangement. The opposing tendencies of weather and water erosion and dry land formation achieved an equilibrium by about 600 Ma, and then the  tectonic plate movements gradually moved the continents to the present configurations forming the seas, all of which connect with each other into a single watermass.

The tectonic forces that create the continents also lead naturally to the formation of mountain ranges along the collision lines of the plates (and abyssal depressions where they separate). The mountain ranges have a beneficial effect in climate control since the prevailing westerly winds precipitate rain as they rise over the mountains.

The "earth sprouting vegetation" -- creation of seed plants, fruit trees, etc., was a long process that started with microscopic life but the full-fledged creation of air-breathing plants: grasses (a more literal translation of "vegetation"), and eventually fruit trees, required one major innovation, namely the ozone layer in the high atmosphere, to shield exposed plants from damaging cosmic rays. This layer began to form once the oxygen content of the atmosphere stabilized at around 20%, but it took until about 300 Ma to develop fully. Thus the second half of the creation recorded in Day Three really took off by around 300 Ma, and overlaps with the creation of sea animals in Day Five.



Energies for Biological Reactions (at 300°K)
~7.3 Kc/Mole
Removal of the third phosphate group by hydrolysis: ATP + H2O → ADP + Pi
This is the standard source of energy for most bioloical activity. Pi denotes an inorganic phosphate (-PO4). The exact energy depends on the particular reaction. See Alberts et al. Essential Cell Biology, Ch. 3 p.96ff.

H2 -> 2 H+

O2 -> 2 O+
Most reactions that break down Oxygen take less energy than indicated here because they exchange the oxygen bond for other bonds.

N2 -> 2 N+ 225.94
Nitrogen has one of the highest bond strengths. As a result, nitrogen fixing is a very energy-intensive process. The usual end product of nitrogen fixing is ammonia (NH3).
N2 -> 2NH3 420
(Source: WIKI)

room temp
10 μm (mid-infrared) Thermal energy (300°K)

Space UV
100 nm
visible light

Source: Wikipedia Article,  Bond dissociation energy (Bond Strength); Handbook of Physics and Chemistry,  (60th Ed 1980) Bond Strength of diatomic molecules, table 1, pg F-220ff.
* Morowitz, Table 12.
Note: 23.065 Kc/Mole = 1.000 ev.

Archaeobacteria are more advanced than Bacteria.

One argument against the view that archaeobacteria were the first form of life is that archaeobacteria appear to be more advanced than other bacteria. For example, the ribosomes of archaeobacteria look like eukaryotic ribosomes and they differ considerably from bacterial ribosomes, as shown in the following sketch[FOOTNOTE: Source: Margulis, Kingdoms and Domains, p59].

RibosomeShape.gif NASA/JPL-CalTech

TODO: Compare the ribosome construction and function.



Nitrogen Cycle
Figure 15
Natural Nitrogen Cycle
Note: The contribution of lightning and volcanic activity to the
natural nitrogen cycle is not shown in Figure 15 because it contributes very little to the cycle.
Source: Wiki

Sharp Point          The First Living Species -- Fixed Nitrogen

The Miller-Urey experiment (1952) showed that some amino acids could be produced in a strongly reducing atmosphere containing hydrogen (H2), methane (CH4), ammonia (NH3) and water with lightning providing the energy source. Since that time, the consensus in science is that the primordial environment included only traces of ammonia.

Nitrogen is an essential component of all life molecules -- indeed all nucleotides and amino acids contain nitrogen atoms, so life can't even build its most basic parts -- genes and proteins -- without an abundance of available nitrogen.  Nitrogen gas (N2) is not available for use by living cells because the bond that holds together the nitrogen atoms into the molecule cannot be broken by any normal cellular processes.

From the very start, living cells had to include specialized cells that could fix nitrogen, because a reliable supply of available nitrogen was simply not present in the environment. In archeal times, lightening did not produce nitrates (one of the major nitrogen products of lightening today) because of the lack of oxygen in the early atmosphere.

Nitrogen fixing is the name of the process used to break up the nitrogen gas molecules into ammonia which then is available for use by living cells. Nitrogen fixing is a very energy-intense process, and there is only one way to do it in a living cell: using a process that involves a very complex molecule, nitrogenase. This process is so intensive that nitrogen-fixing cells are specialized to do just this one task, and must receive food produced by other cells in order to do the work. These cells also have to be isolated from other cellular processes because the nitrogen fixing process can be poisoned by the waste products of other cellular activity -- particularly by the presence of oxygen. At the heart of the nitrogenase molecule is a molybdenum atom, which is an example of a rare element heavier than iron (Atomic number 42) that is essential to life.

Sharp Point          The Inadequacy of the Early Earth Environment

1. Very little free oxygen. The first living species were anoxic. Oxygen is needed by all higher organisms.

2. Very little available nitrogen. The first living species had to fix nitrogen from atmospheric nitrogen gas because there was no reliable supply of ammonia (NH3) or nitrogen compounds.
3. No organic food. The first living species were autotrophs. The vast majority of species require parts of their diet to be organic -- these are the molecules that the species cannot prepare themselves. A major component of this organic food includes amino acids, sugars, and other compounds that contain free nitrogen. All advanced species require this food because their energy budget is not extensive enough to prepare all of their needs from scratch in a timely manner. True autotrophs -- species capable of living on purely inorganic matter -- necessarily use excessive energy in making food, and they do this slowly and laboriously, leaving nothing over for more advanced tasks.

4. No stable dry land.

Why I Cannot Accept Undirected Natural Evolution
Evolutionists in the trandition of Charles Darwin nearly universally claim that evolution of all living species came about by purely natural causes. Some Christian scholars accept a (slightly) modified version of this, usually called Theistic Evolution, which accepts the general Darwinian Thesis but adds God as a sort of prime-mover who fixed the parameters of nature in the beginning, but then let things evolve naturally from that point on. These "Theistic" evolutionists join the secular evolutionists in loud condemnations of "Intelligent Design" or any other means by which God may inject himself into the unfolding of natural Evolution.
I find it difficult to accept this concept of undirected natural evolution, for the following reasons, both theological and scientific:

1. The Bible clearly portrays a God who constantly "interferes" with his creation. Jesus constantly asserts that God actively cares for his creatures, and the very essence of his Salvation plan involves deliberate and directed arrangement of human affairs to bring about the culmination -- the death and resurrection of Jesus Christ as our redeamer and Messiah. It is impossible to read the Bible in any way that would imply that God is a "hands off" creator.

2. I question whether it is possible for some of the essential steps in the creation of modern complex life to be done by natural means. What evolutionists accept as "proof" of evolution is to present some "just so" stories about how things occur. What is lacking is laboratory demonstration.



^n01  The lecture A Fit Place to Live relates to this chapter. See also the excellent and highly readable book by J. William Schopf,  Cradle of Life (1999).

The biological classifications used in this and following chapters follow the nomenclature established by Lynn Margulis in Kingdoms and Domains (2009). Some biological systematists use the term "domain" where Margulis uses the term "kingdom." Furthermore, we use the term "bacteria" for the more formal term "prokaryotes" (kingdom prokaryota or eubacteria), meaning single-celled species that lack a nucleus. Nucleated species are eukaryotes, kingdom eukaryota. I call eukaryotes "proper cells" which will be the subject of the next chapter. Kingdom Archaea consists of bacteria-like species that have a number of special features and are further discussed here.

^n02 Even the earliest living species had to manufacture sugars, because they are part of the DNA spiral backbone. RuBisCO is involved in virtually (??) all known sugar production in cells.  Q: Does the early evidence for RuBisCO imply that photosynthesis was equally early?

^n03 J. Willliam Schopf, Cradle of Life: The Discovery of Earth's Earliest Fossils (1999), Fig. 3.4 p.77; "The Apex fossils are preserved in a chert bed sandwiched between two massive lavas of the Pilbara sequence." p. 88.

^n04  See Ibid., Fig. 3.4 for photographic images.

^n05   Robert Haselkorn states, based on genome sequencing of cyanobacteria, that "We propose that the first phototrophs were anaerobic ancestors of cyanobacteria (“procyanobacteria”) that conducted anoxygenic photosynthesis using a photosystem I-like reaction center, somewhat similar to the heterocysts of modern filamentous cyanobacteria. From procyanobacteria, photosynthesis spread to other phyla by way of lateral gene transfer." -- Abstract to Robert Haselkorn, et al.The Cyanobacterial genome core and the origin of photosynthesis (PNAS, 2006). This tends to support the identity of cyanobacteria as the earliest photosynthetic bacteria. The remark on "lateral gene transfer" implies that the genes for photosynthesis were likely created only one time, at the very earliest stages of life on earth, and then were re-used by other photosynthetic bacteria. A further remark in the body of this paper states, "Cyanobacteria are usually not considered explicitly as a lineage in which photosynthesis could have emerged because of the far greater complexity of their photosynthetic machinery. This fact, however, can be interpreted both ways. Indeed, the total number of genes involved in photosynthesis in cyanobacteria is much greater than that in any of the other prokaryotic phototrophs (Table 1). Only cyanobacteria possess photosynthetic reaction centers of both types, RC1 and RC2, and, in addition to chlorophyll- and phycobilin-containing light-harvesting systems, have chlorophyll-binding proteins whose function is believed to be dissipation of light energy to prevent photodamage (HLIPs; see Table 1). Thus, the majority of photosynthetic genes must have first appeared in the cyanobacterial lineage anyway [emphasis added -- dcb]."

^n06  See http://www.biol.tsukuba.ac.jp/~inouye/ino/cy/36.gif. Used by permission. For discussion of Anabaena Cyanobacteria see Lynn Margulis and Kathlene V. Schwartz, Five Kingdoms: An Illustrated Guide to the Phyla of Earth,Third Edition, W.H. Freeman, 1999, p79. The most recent edition of this work has been renamed Kingdoms and Domains (2009).

^n07  Haselkorn, op. cit. "Cyanobacteria are one of the earliest branching groups of organisms on this planet. They are the only known prokaryotes to carry out oxygenic photosynthesis, and there is little doubt that they played a key role in the formation of atmospheric oxygen ≈2.3 Gyr ago."

^n08  Schopf, ibid, p.78 "It seems to me likely that several of the Apex species are cyanobacteria, a fairly advanced group of microorganisms that until this find was not guessed to be present so early in Earth history."

^n08.1 William H. Schlesinger, Biogeochemistry: An Analysis of Global Change 2nd Ed, (1997), p. 36, "[I]t is interesting to note the significance of an atmosphere with 21% O2. Lovelock (1979) points out that with <15% O2 fires would not burn, and at >25% O2 even wet organic matter would burn freely (Watson et al. 1978)."  [MAYBE add chart of Evolution of the Atmosphere: Cumulative history of O2 by photosynthesis over geologic time. Cf http://www.globalchange.umich.edu/globalchange1/current/lectures/Perry_Samson_lectures/evolution_atm/index.html. chart is very small!]

Manfred Schidlowski, A 3,800-million-year isotopic record of life from carbon in sedimentary rocks, Nature 333:313-318 (1988). Abstract: "An increased ratio of 12C to 13C, an indicator of the principal carbon-fixing reaction of photosynthesis, is found in sedimentary organic matter dating back to almost four thousand million years ago—a sign of prolific microbial life not long after the Earth's formation. Partial biological control of the terrestrial carbon cycle must have been established very early and was in full operation when the oldest sediments were formed."

^n09 See the box on Motor Molecules (Chapter 6) for an illustration and discussion of ATP Synthase. A simplified cartoon of the molecule is shown in Figure ??. View an animation of ATP Synthase by Donald Nicholson (Leeds University) here, and the John Walker (Cambridge) animations here.

ATP Synthase
Figure ??
ATP Synthase Molecule

^n09a Proceedings of the National Academy of Scientists: "According to an analysis of the cyanobacterial genome (Haselkorn and Johnston (PNAS)) the earliest cyanobacteria already had the light & Calvin processes for photosynthesis in place. These are two very complex and subtly linked processes and involve many specialized molecules working together. These are such complex biological processes, that the complexity and early appearance on earth seems to indicate planning and design." Cyanobacterial genome core and the Origin of Photosynthesis (2006).

^n10 From Virtual Cell Animation Collection which has a number of animations of cellular processes.

^n10.1 Oxidized P680 (P680+) is "the strongest biological oxidizing agent known", which makes it possible to oxidize water in photosynthesis (Wiki). P700 with a photon-excited electron is "the strongest biological reducing agent"(Wiki).

^n11 For further information about photosynthesis in cyanobacteria, Haselkorn, op. cit, and the Arizona State University photoweb.

^n12  Wikipedia article on RuBisCo. See also the Sharepoint article, Improving RuBisCO in Photosynthesis, abstract: "RuBisCO is the most abundant protein on Earth that triggers reactions to make carbohydrates, proteins and fats used to sustain all forms of life. ...RuBisCO is the most inefficient enzyme known to man because it has an extremely slow reaction rate [2-3 reactions per second]."

^n13   David W. Wolfe, Tales from the Underground: A Natural History of Subterranean Life, Perseus, 2001, p.78. "Nitrogenase is composed of two giant proteins that physically separate and come back together eight times over the course of 1.2 seconds, to convert one molecule of N2 to one [sic.] molecule of ammonium."

^n13.1  In Origin of the Organic Soup, photorespiration is called a "design flaw in photosynthesis." "Since plants first moved onto land about 425 million years ago, they have been adapting to the problems of terrestrial life, particularly the problem of dehydration. The solutions often involve tradeoffs. An important example is the compromise between photosynthesis and the prevention of excessive water loss from the plant. The CO2 needed for photosynthesis enters a leaf via microscopic pores called stomata. However, the stomata are also the main avenues of transpiration, the evaporation of water from leaves. On hot, dry days, most plants close their stomata in order to conserve water. This response limits access to CO2, thereby reducing photosynthetic yield. Under these conditions, CO2 concentrations in the air spaces within the leaf begin to decrease and the concentration of oxygen released from photosynthesis begins to increase. This favors what appears to be a wasteful process within the leaf called photorespiration."

^n14  The Wiki definition of a diazotroph appears to be technically wrong: "A diazotroph is an organism that is able to grow without external sources of fixed nitrogen." In fact no organism can grow without external sources of fixed nitrogen. In my understanding, a diazotroph (for example a cyanobacterial heterocyst) cannot manufacture enough nitrogen to meet its own needs. In fact, it expels fixed nitrogen as a waste product (for use by other cells) and gets its food (including fixed nitrogen) from other cells. See Wolfe, op. cit. Chapter 4, "Out of Thin Air" ( p. 75ff.) is a fascinating discussion of nitrogen-fixing microbes.

^n15  David W. Wolfe, Out of Thin Air - nitrogen fixers, Natural History, Sept. 2001. Note that he suggested a source of nitrogen from lightning forming Nitrate, but this is not possible because the early atmosphere was almost entirely oxygen-free. See Schopf, p. 153: "Today, large amounts of nitrate are made when oxygen and nitrogen combine during lightening storms, but this could not happen in the early oxygen-deficient atmosphere.... The scarcity of ammonia and nitrate posed a major problem to life."

^n16  Discussions of this period can be found at: Peter D. Ward and Donald Brownlee, Rare Earth: Why Complex life is Uncommon in the Universe. (2000), Wallace S. Broecker, How to Build a Habitable Planet. (1985) p. 233ff., William H. Schlesinger Biogeochemistry: An Analysis of Global Change (2nd Ed. 1997). Schlesinger has a figure similar to Figure 9 on p. 37 and states "The release of O2 by photosynthesis is perhaps the single most significant effect of life on the geochemistry of the Earth's surface." (p.36).

^n17 Broecker, p. 233ff.

^n18 See:  World Nuclear Organization, The Cosmic Origins of Uranium (2006), and Geology of Uranium Deposits (2010). Uraninite, is relatively readily leached under low-temperature oxidising conditions. See also Michel Cuney, Evolution of Uranium Fractionation Processes through Time (2010).

^n19 Without radioactive heating, the Earth's interior would have cooled because of radiation to space, over a time on the order of a hundred million years. This realization was a great puzzle to scientists until the discovery of the heating potential radioactive decay in the early 1900s. Rutherford suggested in 1906 that radioactivity had a potential for geological time-keeping. See the excellent review of this in the Wikipedia article "Invention of radiometric dating."

^n20 For an interesting account of plate tectonics, see "Plate Tectonics in a Nutshell" by the U. S Geological Survey.]

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Bruce Alberts, et al. Essential Cell Biology: An Introduction to the Molecular Biology of the Cell (1998),
David C. Bossard, A Fit Place to Live. (2003)
Wallace S. Broecker, How to Build a Habitable Planet (1985)
Guillermo Gonzalez & Jay W. Richards, The Privileged Planet (2004)
Robert Haselkorn, et al.The Cyanobacterial genome core and the origin of photosynthesis (Proceedings of the National Academy of Sciences, 2006)
D. T. Johnston et al, Anoxygenic photosynthesis modulated Proterozoic oxygen and sustained Earth's middle age (Proceedings of the National Academy of Sciences, 2006)
Lynn Margulis and Kathlene V. Schwartz, Five Kingdoms: An Illustrated Guide to the Phyla of Earth,Third Edition, W.H. Freeman, 1999, p79. The Fourth Edition of this work has been renamed Kingdoms and Domains (2009) by Lynn Margulis and Michael J. Chapman.
Harold J. Morowitz, Beginnings of cellular Life, Yale University Press, (1992).
J. Willliam Schopf, Cradle of Life: The Discovery of Earth's Earliest Fossils (1999).
Peter D. Ward &  Donald Brownlee, Rare Earth: Why Complex Life is Uncommon in the Universe. (2000)

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