3. The Evolution of Cells

3.2 The Eukaryotes

3.2.1 The origin of the eukaryotes

The way eukaryotes could emerge and evolve from a more primitive organism is one of the great questions debated, and a number of explanations have been put forward. In 1977, Woese and Fox presented 11 compelling evidence that the bacteria were composed of two well distinct groups, which they named the Bacteria and the Archaea. These two prokaryotic domains are clearly separated from each other. Woese advocated the creation of a new taxon, the archaea, separate from the bacteria. The Eucarya constitutes the third domain of Life, according to Woese. Cavalier-Smith vigorously condemned this creation of a new taxon. According to him, there is no reason not to consider the archaea as bacteria. They would thus be the Archaebacteria and the other branch of the bacteria would be the Eubacteria (A revised six-kingdom system of life; Biol. Rev. 1998, 73: 203-266).

However, there is a fundamental unity of life: all extant organisms are cellular, all have their genetic information stored in DNA, transcribed in RNA and translated in proteins. The genetic code is the same for all and the translation and transcription machinery are very similar. The enzymes used are all homologous and they all use the same energy-rich metabolites. The last common ancestor of these three lines postulated by Woese was a prokaryote that was not primitive but endowed with all the attributes (DNA, RNA, enzymes) pertaining to the evolved prokaryotes. It must have been able to adapt to various niches, including extreme environments. Gogarten-Boekels et al. postulate 12 that a gigantic meteoric hit the planet about 3.6 to 3.7 billion years ago, sterilizing the earth by completely vaporizing the oceans. Such large impacts were frequent in former times. This impact would have eliminated all the living forms except the most thermophiles. Therewith is resolved the enigma that life could have eclosed only in a mesophilic environment, because a hot environment would have been more destructive than constructive since biomolecules are unstable at high temperatures, together with the observation that the deepest branches of the tree of life are occupied by extreme thermophiles. According to this hypothesis, the thermophiles appear as late products of evolution, that were the only survivors after the cataclysm; only those prokaryotes that had adapted to hot environments were able to survive the impact. After the postulated impact, the eukaryotes separated from the archaebacteria, about 1750 million years ago.

How they emerged is another big controversy. The essential characteristic of the eukaryotic protozoa that differentiates them from the bacteria is the existence of a nucleus that encloses the genetic material. However, they possess other distinctive features, namely chloroplasts and mitochondria, a cytoskeleton and a variety of organelles such as the Golgi apparatus, the endoplasmic reticulum and lysosomes. Some primitive eukaryotes, as Gigiardia lamblia, have no mitochondria and a reduced Golgi apparatus. G. lamblia is a parasite and the question is if this absence is due to a loss due to a parasitic way of life or if it is the primitive character that forced parasitism. If the mitochondrion is a primary character preceding the nucleus, as proposed by Margulis in 1981, we may follow the arguments of Vellai et al.13 who claim that the distinctive character of eukaryotes is not the nucleus but the mitochondrion. In their view, the evolution of the prokaryotes is limited by the maximal size their genome may acquire. Poor energy supplies limit this size. With the acquisition of the mitochondrion, much larger amounts of energy may be delivered at selected moments and favor the synthesis of much larger amounts of genetic material.

In this view, an anucleate archaebacterium cell engulfed a respiring eubacterium. The two merged bacteria coevolved in a demanding environment that forced their communality: they could not separate, under penalty of death for both of them. The respiring engulfed eubacterium transformed into a mitochondrion. The therewith energy-laden prokaryote could thereafter increase its genome size with impunity and acquire a nucleus. The diploïdy and large genome size of eukaryotes are then a secondary character.

If, however, the primitive eukaryotes were devoid of a mitochondrion, then various hypotheses hold that a thermo-acidophilic sulfur-metabolizing archaebacterium and a Gram-negative eubacterium fused their genomes to form the genome of the eukaryot. The two sets of genes were thereafter resolved into a single functioning cell. The engulfement of the eubacterium by the archaebacterium also may explain the formation of the nucleus and of the endoplasmic reticulum but it fails to explain the origin of the other organelles, as the mitochondrion.

3.2.2 The oxidant challenge

Oxygen is a corrosive and poisonous gas. The release of oxygen as a result of photosynthesis by blue-green algae resulted for some bacteria in a retreat to the oxygen-free corners of the planet where their descendants are still found today. The presence of oxygen and its utilization produces peroxide and also the extremely oxidant radical O2-. This superoxide anion has such a chemical reactivity that it precludes diffusion out of the cells as a means of disposal. A mechanism of neutralization of this oxidant is imperatively required within the cell. For this achievement, complex biochemical adaptive measures had to take place. Bacteria such as the Clostridia are unable to survive in the presence of oxygen. Aeration is fatal for them. These anaerobic bacteria lack the system necessary for aerobic respiration. These would be primitive bacterial species. Another group of anaerobic bacteria, such as the Streptococci, is not killed by exposure to oxygen. The vast majority of these aero-tolerant anaerobes are able to eliminate the superoxide O2-. This is thus a case of preadaptation to oxygen utilization. The third bacterial group is aerobic (Escherichia coli, Salmonella, Pseudomonas) and all bacterial species of this aerobic group can efficiently eliminate peroxide as well as the superoxide.

3.2.3. Respiration

Algae that developed ways to neutralize the oxygen by combining it with their own waste products took up the oxidant challenge successfully. This was beneficial as it allowed further building of living matter with the energy released by the combustion of the waste products. This process is respiration or oxidization.

The activity of photosynthesizing organisms depends on the presence of light. On earth, this activity is shut down every day by the daily disappearance of the sun. Photosynthesizing organisms lie idle about half of the time. The utilization of a chemical source that may be tapped during night hours has a tremendous selective advantage. It increased the supply of energy available, which permitted in turn the specialization of cells and the appearance of multicelled plants and animals.

In bacteria, alcohol is the final product of catabolism of glucose. In eukaryotic cells, alcohol is either further degraded or else is the starting point of fatty acids and lipid synthesis. The fatty acids and lipids provided the foundation needed for the formation of sophisticated membranes and turned out to be also effective storage means of energy. The catabolism of lipids proceeds through the intermediary of molecular oxygen.

Lipidic membranes are fragile and vulnerable, be it only to alcohol that dissolves them. The lipidic membranes of the eukaryotic cell could evolve only, provided a mechanism was devised for the destruction of alcohol and other lipophilic substances that could contaminate and impair the function of the membranes. Again, molecular oxygen plays a prime role in the destruction of lipophilic molecules. Lipids, which arrange themselves into a double layer, form a lipidic membrane. The solidity of the layer is assured by a ligand, which is usually cholesterol. Bacteria have no cholesterol. In their case, the rigidity of the membrane is acquired by the use of hopanoids. These hopanoid molecules have a structure quite similar to cholesterol, ending up at one end with a hydrophilic group. Hopanoids are found in fossil bacteria, in living species and in algae. They are synthesized in the absence of oxygen, from squalene. In the presence of oxygen, squalene can be transformed into lanosterol, which is a precursor of cholesterol. This synthesis occurs in methylcoccus, which is then the first evolved bacterium, known to exist

3.2.4. The green algae

The physical aspect of green algae has improved drastically over that of bacteria and prokaryotic blue algae (cyanobacteria). Green algae have a nucleus completely surrounded by a membrane (fig. 3.7). This nucleus contains the genetic material. The rest of the cell functions are accomplished outside the nucleus, in the cytoplasm.

Figure 3.7. The size of the alga Chlamydomonas" is 0.01 mm. It has a nucleus, an eye, a flagella, mitochondria and its chlorophyll is enclosed in chloroplasts.

Figure 3.8. In green algae, the cytoplasm contains chloroplasts and mitochondria.

The mitochondria are the energetic centrals of the cell: the internal machinery of the mitochondrion is basically composed of a series of enzymes – the cytochromes – whose duty it is to transport electrons. During this transport, glucose is oxidized. The oxidization does not, however, stop at the level of alcohol with a poor yield of 3% but proceeds right down to the state of 6 molecules of CO2 and 6 molecules of H2O. The oxygen produced during the synthesis of glucose is used again in the degradation of this glucose into water and CO2. By this means, 55% of the energy contained in the glucose molecule is now stored in the cell under the form of adenosine-triphosphate (ATP).

The earth’s history is divided into three great periods: the Archaean world was followed about 2 to 2.5 billion years ago by the Proterozoic (2,500 million years ago to 544 million years ago), which yields little altered sedimentary rocks sometimes replete with morphological and chemical remnants of microscopic familiar organisms. Conventional wisdom holds that the search of biomarkers in formations older than about 2 billion years ago is vain because these formations have presumably heated in later times to temperatures that would have destroyed them. Some lipid biomarkers are, however, stable. Brocks drilled a 700 meter deep hole in a formation about 2600 million years old, which had been subsequently heated to only 200° to 300°C, which is not high enough to destroy the lipid biomarkers it contains14. Their find confirms that cyanobacteria lived in Archaean environments. The late Archaean biomarkers found also included molecules derived from sterols. No prokaryotes are known to form the elaborate sterols extracted by Brocks et al. The overall biology of the organism that synthesized the sterols 2,700 million years ago is still not known but the early appearance of eukaryotic attributes directs attention to the immense time interval between the divergence of the Eukarya and their rise to prominence about 1,200 million years ago. It is postulated that oxygen concentrations grew from low to contemporary levels about 2,200 million years ago but molecular oxygen is required for the synthesis of sterols. Also, some methane-trophic bacteria that depend on oxygen are supposed present in the late Archaean. This indicates that aerobic respiration by single cells was possible at that time.

Eukaryotic green algae existed with certainty 1.3 billion years ago. Their appearance was dependent on several conditions. Firstly, there is no eukaryotic organism able to live at temperatures higher than 62°C. Eukaryotes appeared only after the temperature of the earth’s surface cooled below the 62°C mark. On the contrary, many prokaryotic organisms live at temperatures far above 62°C (e.g. 90°C to boiling water and even 250°C, in marine volcanoes), to the point that the 62°C temperature is a minimum at which they will grow. The reduction in temperature originated perhaps through the release of oxygen in the atmosphere by blue green algae. The critical component of the eukaryotic cell, which is functional only at lower temperatures, is the nuclear membrane and/or the membrane of the mitochondrion. These membranes differ greatly from the cellular membrane, in that they must allow the selective passage of large macromolecules. They are also organizationally sophisticated and possess large leaky pores, which may not be compatible with thermostability.

Secondly, even with lower temperatures in the environment, blue green algae dominate over green algae as long as the amount of available CO2 is small. Indeed blue-green algae are very efficient extractors of CO2 from low environmental concentrations and will usually predominate over green algae. This predominance is lost whenever large supplies of CO2 are at hand.

Thirdly, the photosynthesizing apparatus of the blue-green algae is made of a series of cytoplasmic membrane systems. In eukaryotic algae, the system is much more refined and the delicate photosynthesizing system is located in chloroplasts. This protection may be the reason why green algae can proliferate in acidic environments. Blue-green algae are barred from any environment where the acidity is high (pH below 4.0). Two hundred and seventy million years ago, the lakes of North America, covering about 200,000 km2, were extremely acid (pH about 1), and acid lakes (pH about 3) are nowadays existing in Southern Australia. The newly evolved chloroplast of the green algae provided these algae with an immediate selective advantage in that they could invade acidic environments absolutely free of competition from the more primitive algae. From there, other environments could be reinvaded in accordance with the availability of CO2.

These acidic environments may have originated after 2 billion years through the appearance of ridges in the earth’s crust. These ridges allow the extrusion of magma. Also, these ridges are sometimes filled with water that percolated through the earth’s crust, where this water was heated up and able to dissolve several salts. The initial composition of seawater was very high in magnesium and CO2 which made this water quite acidic. It is only during later times that the large reservoir of magnesium and free CO2 became depleted and that sodium dominance took place. This dominance was achieved about 500 million years ago at the end of the Precambrian period. Pure water solutions of sodium chloride are unable to durably sustain the growth of algae and bacteria but algae may grow in water solutions of potassium chloride and other salts. This lack of durable growth in pure sodium chloride solutions is fortunate, otherwise, the oceans would be replete with algae and bacteria and these would have hindered the development of higher living forms, appearing during the Cambrian period.

The genetic material of green algae is organized within the nucleus. Photosynthesis is conducted in cytoplasmic organelles called chloroplasts. Respiration is carried out in the mitochondria. This fundamental division of labor has been retained in all higher plants. All animals soon abandoned the needless chloroplast or never had any, but retained or actually were the first with the mitochondrion.

The mitochondria have an independent life of their own, right within any eukaryotic cell. They possess a single-stranded circular nucleic acid of the DNA type and they divide as they see fit, even if the cell does not. All mitochondria of a pluricellular organism derive from such division from the initial mitochondria included within the ovule, before fecundation with a sperm. Since the mitochondria carry a small genetic message, we somehow are all a little more the children of our mother than of our father. This is because the mitochondria located in the tail of a sperm are cast out after penetration of the head within the ovule. Or else, even if the tail and mitochondria from the male do penetrate, they remain inactive and are eliminated. This is the orthodox teaching. It is important to be sure that mitochondrial DNA is exclusively of female origin because all research made to define the common human ancestor, thought to be an Hottentot woman, is based on this assumption. If not true, the conclusions drawn on the false premises may be false. It well seems that it is not always true.

3.2.5 Cellular differentiation

From about 2 billion to 1 billion years ago, the cyanobacteria reigned supreme, changing little from eon to eon. During that long hegemony, the eukaryotes remained evolutionarily stagnant. D. Canfield, of Odense University in Denmark, who advanced that the world ocean remained oxygen-free, with the exception of the near-surface, after oxygen appeared in the atmosphere 2.2 billion years ago, explained the reason for this long twilight in 1998. According to him, the atmospheric oxygen weathered large amounts of sulfur off the land and into the ocean. There, they formed insoluble compounds with iron as well as other metals, including molybdenum, copper and zinc, which were removed from the sea. The eukaryotic algae would have missed these metals sorely because they rely on enzymes built around an iron and around a molybdenum atom to extract nitrogen from nitrate. Contrary to the cyanobacteriae, they have no way to fix nitrogen gas (N2) into nitrogen usable in biological processes. The Mesoproterozoic meant nutritionally hard times for ocean life. This could explain the long low diversity of relatively simple eukaryotic algae through the Mesoproterozoic until 2 billion years after their first appearance.

3.2.5.1 The protozoans

The alga Chlamydomonas is an animalcule made of a single cell. This cell contains a system of chloroplasts wherein molecules of chlorophyll utilize the energy provided by the sun. This is the source of energy utilized by Chlamydomonas when in the sunshine. However, once in darkness, Chlamydomonas uses other sources of energy. Chlamydomonas is not a static animalcule but can move around by the action of two flagellae. It possesses a sophisticated system for capturing prey and digesting nutrients of living origin; it is provided with a means of locomotion, a photo-sensitive organelle, a kind of mouth, a system of digestion and excretion and one or two nuclei. Some unicellular organisms developed from there into predatory organisms of monstrous proportions such as Paramecium (fig.3.9) that attains a size of 0.24 mm, and into sophisticated animalcules endowed with a distinct bilateral symmetry (fig.3.10).

Figure 3.9. Paramecium caudatum is a eukaryotic unicellular organism that survives on organic matter. It is a sophisticated predator.

Figure 3.10. Stylonychia is a unicellular predator endowed with bilateral symmetry.

These animalcules still multiply mainly by cellular division into two.

Another tendency towards the complex is observed when several cells of the same prototype remain hooked onto each other. They form in this way a hugger total mass of protoplasm. This association is possible because the cellular membrane of eukaryotes is composed of large amounts of lipids. Such membranes are sufficiently flexible and permeable to permit an association with other cells on a permanent basis. This association tends to efficaciously counteract the presence of voracious giants as Paramecium and presents an immediate selective advantage (fig. 3.11).

Figure 3.11. Epistylis is a predatory protozoan colony.

3.2.5.2 The mesozoans

Salinella is an elaborate pluricellular protozoan (fig.3.12).

Figure 3.12. Salinella is not at the origin of the metazoans because it is separated from the external world by a single layer of cells.

This animalcule has a mouth and an anus and is endowed with bilateral symmetry. Yet, it is not through such a structure that evolution gained a higher level because Salinella separates the external world from its own interior environment with the help of a single layer of cells. It is however assumed that Metazoans have evolved from a living form that has at least a bilayer of cells: the cells in contact with the exterior should be different from those lining the inner environment. They are said to be diploblastic. Triploblastic structures consist of a triple layer of cells, wherein two epithelia, one in contact with the exterior environment and the other with the interior environment, enclose a median collagenous connective tissue. Since sponges are the first organisms recognized as animals and are fundamentally triploblastic, the diploblastic condition is a derived simplification. Another possible origin for the metazoans is a form where several different cells are packed together without any interior environment at all. Some exemplars of this intermediary form, the mesozoans, still exist.

These primitive multicellular organisms are presently restricted for the main part to a very peculiar habitat, namely the urinary tract of cephalopods, where they apparently find the nutrients needed for their survival. These mesozoans are composed of between 20 and 30 cells. The free form is able to wander from one cephalopod to the other and is a ciliated little mass of protoplasm (fig. 3.13) that has a radial symmetry just like the primitive forms of metazoans, the jellyfishes.

Figure 3.13. Mesozoans as this Dicyemid are ciliated masses of protoplasm composed of about 20 to 30 well-differentiated cells. They may be at the origin of the coelenterates.

The external cells of this mass have assumed a protective and locomotion role. In order to fulfill their role even better, these cells should be able to play an informative role that would coordinate the movements of cilia of the various cells and command avoidance as well as perception of harmful chemical substances and physical conditions of the environment. Such a role of information would be fulfilled by nerve cells. Mesozoans are devoid of such cells and it is possible that they do not represent the missing link. It is very possible that mesozoans are animalia that have lost nervous system and gut due to parasitism, and that the kingdom Animalia evolved by a transition between a colonial flagellate and sponges, during which connective tissue located between two epithelia first evolved.

3.2.5.3 Cyclic AMP

Communication is based not only on electricity, i.e. a nervous system but also on chemicals, i.e. hormones. Slime molds are primitive eukaryotes whose appearance is that of human white cells. These slime molds have the ability to secrete large amounts of cyclic AMP. Sometimes this cyclic AMP is used for predatory purposes: whenever a bacterium is present that also secretes cyclic AMP, this cyclic AMP serves as a lure to the mold and leads it to its prey. Cyclic AMP is also used by the mold to attract individual amoebae to each other. This process of cellular assembly occurs only when a critical high concentration in individual amoebae is reached. In this way is formed a slug (fig. 3.14).

Figure 3.14. Unicellular molds multiplying to high concentrations (A) assemble (B) into a slug (C) that is the seat of formation of reproductive spores (D). The assembly of the unicellular molds into a pluricellular organism where identical cells assume different functions is thus only transient.

The slug is the seat of formation of spores that will be disseminated for reproductive purposes.

The cyclic AMP used by bacteria is thus further exploited by eukaryotes. It has been retained in animals and man as a hormonal messenger that prompts target cells, motivated by a specific hormone, into action.

3.2.5.4 Prostaglandins

The ability of bacteria to use their membrane as an initiation point of synthesis for nucleic acids was apparently also retained by primitive eukaryotes and is thus also a part of the bacterial legacy. The cellular membrane of eukaryotes has the ability to synthesize new nucleic acids, proteins and lipids. Some lipids such as steroids are found in bacteria. Other lipids, the polyunsaturated fatty acids, have an absolute requirement for oxygen. Bacteria do not synthesize them probably because these evolved before oxygen became available in its free form. The prostaglandins (fig. 3.15) are polyunsaturated fat endowed with remarkable properties.

Figure 3.15. Prostaglandin A1. Prostaglandins cannot be synthesized without oxygen.

These prostaglandins are ubiquitous throughout the animal reign, down to corals and are even found in blue-green algae. These prostaglandins are synthesized only according to needs. Since no storage location is known, only tiny amounts of them are available at any one time.

The prostaglandins are the basic primary hormones of the eukaryotic cell, with the possible exception of higher plants. They put in connection and synchronization the lives of formerly independent entities. Eukaryotic cells work in synchrony thanks to cyclic AMP and prostaglandins. These substances modulate and coordinate the life of the colony on its road to pluricellularity. Soon, specialized cells will be able to evolve from the initially undifferentiated mass and, here again, the prostaglandins will play a major role in the modulation of activity of the various organs. They now regulate the activity of smooth muscles, they raise or lower the pressure of blood, they stimulate the contraction of the uterus, play a role in parturition, induce menstruation, insure the nidification of the ovum, inhibit gastric secretions, etc.

3.2.5.5 Hormones

The mechanism of cellular interrelations is beginning to become unraveled. The prostaglandins control the hormonal secretions of various organs. The hormones secreted are usually effective only after the cyclic AMP present in the target cells has been synthesized. Taking the hormone norepinephrine as an example (fig. 3.16), one knows that this hormone is specifically bound to the surface of target cells with its cyclic moiety. After an interaction took place between this part of the molecule and the cellular receptor, the other moiety of the hormone triggers within the cellular membrane the synthesis of cyclic AMP. This c-AMP in turn generates other systems before being destroyed. The state of sleeplessness obtained in many people by the absorption of coffee, tea or Coca-Cola is due to the fact that caffeine and theophylline block the destruction of the cyclic AMP generated by norepinephrine in the central nervous system.

Figure 3.16. Nor-epinephrine, also called nor-adrenaline, is a neuro-transmitter. One moiety of the molecule recognizes receptors located on the membrane of target cells and binds to them. The other moiety activates the production of cyclic-adenosine monophosphate (cAMP) by the cell.

With prostaglandins at hand and with cellular differentiation under way, we are on the long road that leads ultimately to the human brain. For this, we need the evolvement of a nervous system and of neurotransmitters. Simple neurotransmitters such as dopamine require oxygen for their synthesis. The simplest neurotransmitter known is ATP.

At the lowest organizational levels of the metazoa, corals, flatworms and annelids, a mutilated organism is still able to rebuild itself. At higher levels, this regenerative ability is lost. The human brain is a product of the skin. Specialized cells such as the nervous cells, which compose the brain, are unable to multiply by cell division. The nerve cells of the periphery are endowed with regenerative properties: a nerve cut in two is able to rejoin again. This is due to the presence of Schwann cells that secrete a stimulatory substance -the laminine-, which promotes nerve cell growth. In the brain, there are no Schwann cells. The nerve cells of the central organ, although possessing the capacity to regenerate, cannot do it due to the lack of an adequate stimulatory factor. Stem cells may perhaps resolve this problem.

References

11. Proc. Natl. Acad. Sc. USA 74, 5088

12. Origin of Life 25, 251, 1995

13. J. Mol. Evol. 46, 499, 1998

14. J. Brocks et al. Archaean molecular fossils and the early rise of eukaryotes. Science 285, 1999: pages 1033-1036

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