Prokaryote to Eukaryote Symbiotic Jump

Proliferation of prokaryotes literally changed the face of the earth. According to the Margulis/Sagan scenario, collective teams of bacteria gathered nutrients, disposed of toxins, recycled organic matter by turning waste into food and stabilized the atmosphere. Prokaryotic bacteria produced ammonia which adjusted the acidity of oceans and lagoons and increased the earth's temperature through a "greenhouse" effect similar to that of carbon dioxide (which lets in more solar radiation than can escape). About two billion years ago purple and green photosynthetic bacteria began using water to manufacture hydrogen rich compounds, giving off oxygen which was poisonous to most ("anaerobic") prokaryotes. This "toxic waste crisis" pressured adaptations including motility systems to escape oxygen exposure, detoxification, and eventually oxygen breathing. The resultant early "aerobic" prokaryotic bacteria flourished for a few hundred million years, but as atmospheric oxygen increased, aerobic and anaerobic bacteria begat a new improved form of life, the eukaryotic or nucleated cell.

We are all eukaryotes, as are all animals and nearly all plants existing on earth today. Eukaryotic cells differ from their prokaryotic ancestors by having organized cell interiors (cytoplasm or protoplasm) including separate membrane enclosed compartments (nuclei) which contain, among other structures, chromosomes: DNA libraries and their supportive proteins. Eukaryotic cytoplasm usually contains mitochondria, chemical energy factories which utilize oxygen to generate ATP to fuel cellular activities (respiration) and, within green plants, chloroplasts which convert solar energy to chemical energy foodstuffs (photosynthesis).

Eukaryotic cells are enormously sophisticated compared to their prokaryotic predecessors. Fossil records indicate that eukaryotes appeared abruptly, with no apparent intermediate form which would indicate progressive genetic mutation from prokaryotes. This evolutionary gap, which separates bacteria and blue-green algae from all other present day cellular life forms, is a mysterious dichotomy, an evolutionary chasm. Explanations based on symbiosis-a mutually beneficial association-were advanced by Marishkowski in 1905 and Wallen in 1922 (Margulis and Sagan, 1986). They proposed that eukaryotic cells resulted from a symbiotic association of two types of prokaryotes-a primitive "monera" and a more advanced cocci-type bacteria. Ingestion of the cocci by the monera is thought to have led to a stable symbiosis in which the more evolved cocci became the nuclear material and the monera became the cytoplasm. Marishkowski proposed other examples of symbiosis such as the emergence of green plants from a union of colorless nucleated cells and minute cyanophycae which became chloroplasts specialized for photosynthesis. This proposed union is similar to the symbiosis of green algae and fungi to form lichen, and of chloroplasts in metazoa such as hydra. Wallen proposed that mitochondria originated as symbiotic bacteria which entered, and became indispensably entrenched within, animal cells.

A more complete "endosymbiotic" theory of eukaryote origin was introduced by biologist Lynn Sagan (later Lynn Margulis) in 1967. She suggested that prokaryotic cells (specifically anaerobic heterotrophic bacteria) underwent a series of three symbiotic events leading to the first eukaryotes. During the period of adaptation to oxygen breathing an aerobic heterotroph was engulfed by an anaerobic heterotroph. The aerobic bacteria became the ancestor of the mitochondria, converting oxygen to ATP and remained as an intracellular organelle. The next symbiotic event, according to Sagan-Margulis, was the ingestion of a spirochete-a motile organism which traveled by whip-like beating of its tail-like flagellum composed of cytoskeletal proteins. Ingestion of flagellae and their intracellular anchors, basal bodies, are thought to have led to cilia, centrioles, and microtubules-cytoskeletal structural and organizational elements which brought the capabilities for cell movement, cytoplasmic organization, and (apparently) information processing (Figure 3.1). Multiple cilia attached to cell membranes and extending outward enabled single cell organisms such as paramecium to swim about in their aqueous medium, greatly expanding their ability to find food, avoid predators, and increase their horizons. In other stationary organisms cilia could flow the environmental medium past the organism, achieving the same results. Within the cytoplasm, cytoskeletal structures such as centrioles, basal bodies and microtubules organized, oriented, and transported organelles and materials. The eukaryotic cytoskeleton took on functions akin to mechanical scaffolding, conveyor lattice, and the cell's own nervous system.

Basal bodies, cilia, flagella, and centrioles are assemblies of microtubules, themselves complex cylindrical assemblies of protein subunits, and are ubiquitous throughout eukaryotic biology. In these organelles, nine pairs or triplets of microtubules are arranged in a super-cylinder, which may have an additional microtubule pair in its center (9+2 or 9+0 arrangements, Figure 3.2). Involvement of these structures in nearly all instances of dynamic cell activities (mitosis, growth and differentiation, locomotion, food ingestion or phagocytosis, cytoplasmic movement etc.) greatly accelerated the capabilities of eukaryotic cells. Utilizing the chemical energy from mitochondrial ATP, these cytoskeletal elements appear to have provided not only stable structure and motility, but also a sophisticated "computer-like" information processing system.

Eukaryotic microbial technology was as different from the basic bacterium as a main frame computer to an abacus. The eukaryotes flourished, evolved and solved environmental problems by mixing and merging. Forming new collectives, they eventually found their way from water to land and air and branched into the myriad forms of plant and animal life that have since populated the biosphere. The human brain and nervous system are recent innovations; Homo sapiens apparently appeared about 50 thousand years ago.

The endosymbiotic theory can explain the nonlinear jump in evolution that occurred with the advent of eukaryotes, but does have its detractors. For example, Hyman Hartman (1975) of MIT has noted that, while mitochondria and chloroplasts are agreed to have originated as free living prokaryotic cells, there is some question as to the pedigree of basal bodies and centrioles. Sagan-Margulis (1967) had claimed that:

upon entry into a host, such a symbiot may lose from none to all of its synthetic capabilities except the ability to replicate its own DNA and synthesize complementary RNA from that DNA—the sine qua non of any organism.

Figure 3.1: Symbiotic ingestion of motile spirochetes by a primitive bacteria, resulting in the first "eukaryotic" cells. The spirochete's filamentous proteins became, according to Sagan-Margulis (1967), the centrioles and cytoskeleton providing movement and organization of cytoplasm. By Paul Jablonka

Figure 3.1: Symbiotic ingestion of motile spirochetes by a primitive bacteria, resulting in the first "eukaryotic" cells. The spirochete's filamentous proteins became, according to Sagan-Margulis (1967), the centrioles and cytoskeleton providing movement and organization of cytoplasm. By Paul Jablonka

This implies that symbiotic organelles which originated as separate organisms must retain their nucleotide synthesis capabilities. Mitochondria and chloroplasts have been shown to have their own DNA, however DNA has not been isolated with centrioles or basal bodies. These cytoskeletal organelles in fact, routinely self replicate without DNA, although Hartman has shown that RNA may exist in association with basal bodies. Perhaps centriolar DNA became lost in the evolutionary shuffle, or the cytoskeleton possesses other mechanisms of information transfer. Evidence of cytoplasmic information being transmitted over hundreds of generations of paramecium without genetic involvement (Aufderheide, Frankel and Williams, 1977) suggests that centrioles and other cytoskeletal elements may have a degree of independence (Figure 5.27). Real time information processing is in the cytoskeletal province, so DNA replication may not be the "sine qua non" of living organisms. Dynamic, collective activities of centrioles, microtubules, and other cytoskeletal proteins may manifest biological intelligence and be closer to life's essence than are genetic mechanisms. Ambiguous life forms may be particularly important in the future.

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