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PROKARYOTES

1) THE ARCHEOZOIC EON OF THE PRECAMBRIAN

   It is estimated that the earth formed 4.6 billion years ago (billion years ago).  The oldest meteorites date to about this time but the earth would have been semi-molten for quite some time. Components of sedimentary rocks have been dated at almost 4.4 billion years old indicating that the earth cooled enough to allow the formation of oceans early in its history (Nemchin, 2006).

Carbon isotope readings from ancient rocks strata support the conclusion that life existed on earth by 3.8 billion years ago and perhaps by 3.85 billion years ago (Mojzsis, 1996). In rocks that are 3.8 billion years ago, there are carbon isotope ratios that suggest the presence of living organisms.  There are different forms of carbon called isotopes.  Living things use a certain carbon isotope (12C) and exclude a second isotope (13C) while neither is discriminated against in the formation of minerals.  After more than 10,000 analyses of carbon ratios in rock sediments, there is only one period in which a qualitative difference is observed.  At around 3.8 billion years ago, the ratios of carbon isotopes in certain sediments changed from those made in the absence of life to those made in association with living things.  This analysis suggests that living things were present on earth 3.85 billion years ago. This change in carbon isotopes occurred shortly after the appearance of liquid water on the planet (Brocks, 1999; Schidlowski, 1988; Holland, 1997). Rocks dated at 3.5 billion years of age also seem to indicate the presence of life (Akai, 2006).

     In the rock strata of Isua, Greenland, there are “fossils” of small organic material which are 3.8 billion years old.  Although there is organic matter inside them and there are examples of them splitting (budding in the way that yeast do), it is not clear whether they are living cells or nonliving microspheres (Pflug, 1979; Roedder, 1981; Bridgwater, 1981; Dunlop, 1978). 

MICROFOSSILS

     There have been “nanofossils” found on Martian rocks which contain organic material.  Some feel that these tiny structures have were formed by primitive organisms on Mars (although many disagree with this interpretation) (Trevors, 2003b).  The biomineralization of ancient microbes which resulted in the formation of microfossils often is the result of the incorporation of metal ions on the cell surface.  Nanobacteria’ may be portions of microbes which were mineralized such as stalks and vesicles (Southam, 1999). Nanobacteria, which range in size from 0.1 to 0.5 microns and replicate every one to three days, are implicated in some diseases such as polycystic kidney disease in humans (Ciftcioglu, 2002).

     Most fossils are found in a type of rock known as sedimentary rock. The oldest known sedimentary rock deposit is the 3.8 billion years ago Isua formation of Western Greenland.  Given that the oldest known rocks are only 4.0 billion years old, it is unlikely that sedimentary rocks which are significantly older than 3.8 billion years ago will be found.  Although there are no fossils known in Isua, carbon ratios in this rock are similar to those found in later rocks which contain fossils of photosynthetic microbes (McClendon, 1999). 

     Very small carbonaceous microstructures are known in 3.8 billion year old rocks in Isua which are probably abiotic.  The oldest microfossils from Australia are 3.5 billion years old  (Pflug, 2001; McClendon, 1999).  Microstructures in meteorites resemble microfossils of primitive microbes.  Chemical analysis of the size of organic structures and mass spectra of positive ions from meteorites can be similar to the analyses of rock strata containing Precambrian microfossils.  The Murchison meteorite possessed small structures surrounded by a bilayer lipid membrane.  These small structures can be in the form of helical and spherical carbon microstructures between 10 nm and 1 micron and filaments composed of units which are 2-20 nm wide and 1-2 nm long.  How should these small organic structures be interpreted?  Although some feel they were either alive or the precursors of life, at the very least they demonstrate that organic microstructures with some resemblance to fossils of cells can form in abiotic conditions on earth and in space  (Pflug, 2001).

Given that bacteria can be found deep within the ocean, underground, and even within rock, these environments must be considered when pondering the origin of life. Bacteria can exist within the channels of certain rock minerals, such as feldspar (Smith, 1999).There are an estimated 4-6 x 10 30 prokaryotic cells on earth whose total carbon mass represents about as much as the total carbon mass of plants on earth (350-550 Pg; 1 Pg = 10 12 kg). Although the exact values are unknown, there seem to be an enormous number of prokaryotes deep in the soil and under the ocean floor. Deep aquifers and the water associated with petroleum can contain between a thousand and a million microbes per milliliter. Underground, more than 400,000 microbes/cubic centimeter are found at depths of 2000 meters and more than 300,000 microbes/cubic centimeter are found at depths of 3000 meters (Whitman, 1998). Enormous quantities of microbes live in the rock underneath the ocean floor. Microbes have been found more than 1.6 km under the seafloor in sediments which are more than 100 million years old. Some of these microbes were metabolically active and dividing. Among these are archea which survive on the anaerobic oxidation of methane. Some have suggested that 2/3 of earth’s prokaryotes live in sub-seafloor rocks (Roussel, 2008). Many of the most primitive modern bacteria inhabit deep sea hydrothermal vents. Prehistoric hydrothermal vents are known from the fossil record including Precambrian vents associated with a diverse bacterial fauna (Li, 2006; Campbell, 2006).

     There are two different types of cells on earth today.  Prokaryotic cells are represented by modern bacteria and are the simplest type of cell.  Prokaryotic cells are small (typically 10,000 times smaller than eukaryotic cells) and lack membrane-bound internal compartments called organelles.  There are 2 major divisions of modern bacteria: the eubacteria and the archebacteria (or archaea).  The eubacteria make up most of the bacteria humans commonly encounter while the archaea  inhabit many of earth’s harshest environments.   In recent years there is a great developing interest in the archaea because of similarities between them and the more complex eukaryotic cells (such as those which make up the human body).

 

Archebacteria

ARCHEBACTERIA
eubacteria
eubacteria eubacteria
eubacteria
Eukaryote (human intestinal cells)
eukaryote

 

    The first fossils of prokaryotic (bacterial) cells are known from 3.5 and 3.4 billion years ago.  These bacteria were photosynthetic (although non-oxygen producing) so it is likely that simpler non-photosynthetic bacteria evolved prior to this (Schopf, 1987; Beukes, 2004). Prehistoric photosynthetic bacteria formed large mounds called stromatolites which could reach 30 ft in height.  Today, bacterial stromatolites exist in Australia and the fossil record shows that they were once much more common worldwide.   Stromatolites are even known from the Archaeozoic Eon from 3 continents, including some which are 3.5 billion years old.  Younger stromatolites contain bacterial fossils were produced by bacteria (Lowe, 1980; Walter, 1980).   Older stromatolites were also probably formed by bacteria (their microscopic structure, their subtidal position, their similarity to biotic fossil stromatolites indicates this) but it is possible that some formed have abiotically (without bacteria as a result of geologic forces) (Grotzinger, 1996).  A Precambrian stromatolite is pictured below.

Geochemical evidence indicates that methane-producing bacteria were present on earth 3.46 billion years ago. The methane they produced could have played an important role in warming the planet (given that methane is a potent greenhouse gas) at a time when the sun would have produced energy (Ueno, 2006). Hydrocarbons dating from the Late Archeozoic indicate that both bacteria and archaea contributed to the early biosphere. The methane produced by archaea is likely to have contributed to the greenhouse gases which kept water liquid while the young sun’s output of energy was less than its modern amount ( Ventura, 2007).

    Bacteria are the only known inhabitants of earth for at least the first 1.5 billion years in the history of life.  Genetic comparisons suggest that the oldest, most primitive bacterial lineages were anaerobic and the ability to survive in the presence of oxygen evolved separately in different lineages.  Photosynthesis seems to be an ancient process, present in half of the major eubacterial lineages (including a number in which photosynthetic ancestors seem to have given rise to nonphotosynthetic descendants (Fox, 1980). Hydrothermal vents are known to have supported microbial communities in the Precambrian ( Campbell, 2006). Many of the most primitive modern bacteria inhabit deep sea hydrothermal vents. Prehistoric hydrothermal vents are known from the fossil record including Precambrian vents associated with a diverse bacterial fauna (Li, 2006; 2007).

Seawater contains an average of more than 100,000 microbes per milliliter and for an estimated total of 3.6 x 10E29 microbes in the ocean. This mass of microbes (including bacteria, archaea, protists, and fungi) composes most of the ocean’s biomass, an estimated 3 x 10 E17 g. The diversity of these microbes is proving to be far greater than previously estimated (Sogin, 2006).

OXYGEN IN THE ATMOSPHERE

     Three billion years ago, cyanobacteria (a type of bacteria better known by the somewhat incorrect name “blue green algae”) evolved which release oxygen during photosynthesis. The first bacteria probably released hydrogen and hydrogen sulfide in photosynthesis, instead of oxygen.  By 2.7 billion years ago, stromatolite-forming microbes seem to have used oxygenic photosynthesis (Buick, 1992).  The amount of oxygen in the atmosphere increased from significantly between 2.4 and 2.2 billion years ago (Rye, 1998).  Molecular evidence for the existence of cyanobacteria dates to 2.7 billion year old rocks and specialized cells are known from 2.1 billion year old rocks (Tomitani, 2006). By 2.2 billion years ago, chains of cyanobacteria existed with enlarged cells (that were probably specialized for tasks such as nitrogen metabolism as in modern species)–this is the first known example of cell specialization (Nagy, 1974; Schopf).

     The earliest atmosphere did not have much oxygen gas.      Before photosynthesis, so much oxygen would have been absorbed by reduced geothermal outputs and by weathering rocks that very little would have been present in the atmosphere.  The only source of oxygen gas would have been the dissociation of water molecules by sunlight (Des Marais, 2000).  This is evident in observing the sediments formed at that time—metals didn’t rust and minerals which form in the presence of oxygen are absent.    Iron deposits of the Archaeozoic indicative of anoxic environment (Lascelles, 2007). By 2.1 billion years ago there was enough oxygen in the atmosphere to rust iron for the first time and by about 1.8 billion years ago oxygen was oxidizing significant amounts of iron.  Other signs of oxygen also appear about this time such as sterane biomarkers from 1.7 billion years ago and oxidized sulfur deposits from about 1.6 billion years ago. Atmospheric oxygen levels increase towards the end of the Precambrian, just prior to the diversification of the Ediacaran fauna (Kaufman, 2007;Canfield, 2007).

      Since the first bacteria would have probably lacked enzymes (such as peroxidase) which deal with destructively reactive forms of oxygen, oxygen would have represented the first air pollution and would have caused many extinctions among bacteria.  There are still many bacteria today that cannot live in the presence of oxygen. 

cyanobacteria

     The fossil record of photosynthetic microorganisms is fairly good; that of cyanobacteria is robust from around 2.5 billion years ago through to the Cambrian Period.  The organic molecules left in rocks support the fossil chronology.  (For example, 2-methyl bacteriohopanepolyols are only known from cyanobacteria and its presence in rocks indicates that cyanobacteria were present when the rock was formed.) (Marais, 1992; Logan, 1999; de Duve, 1996).   

 

     How many types of cell are there?  Until  recently, it was simply assumed that all cells which were not eukaryotes were similar and should be grouped together as bacteria.  We now know that the two groups of prokaryotes (archaea and eubacteria) are as distinct from each other as they are from eukaryotic cells (Lopez-Garcia, 1999).  Which group is more closely related (and perhaps ancestral to) the eukaryotes?  There are a number of features which make archaea distinct from eubacteria.  One of the most surprising developments of recent years is the long list of similarities between archaea and eukaryotes.  There are many features which were once thought to be eukaryotic traits which are now known to exist in eukaryotes and archaea, especially processes involving DNA and the synthesis of proteins.  Thus, a good deal of evidence suggests that eukaryotes are more closely related to archaea.   However, there are some features, such as a number of metabolic pathways, in which eukaryotes are more similar to eubacteria.  Thus, there is data which supports each of the two phylogenetic trees depicted below.

There is conflicting evidence regarding the question of the relationships between these three domains of life.  Another unanswered question is the origin of the eukaryotic nucleus.  In now seems that these two questions are related and the answer to each of them may be the same.  Eukaryotes seem to be related to different prokaryotic lineages because the eukaryotic cell evolved from a collaboration of cells from different prokaryotic lineages, as is discussed in the next chapter.