ULTIMATE COMPUTING - Quantum Consciousness Studies

50 Origin and Evolution of Life
3.2 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