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Superkingdom Prokaryotae


animal-like protists, endosymbiosis, aerobic respiration, process of photosynthesis, cyanobacteria

Prokaryotes are the ancestors of all life forms. Although scientists debate the events of early evolution, fossil evidence suggests that the cyanobacteria were among the earliest organisms to evolve, an estimated 3.4 billion to 3.5 billion years ago. The environment of the early Earth lacked oxygen, and cyanobacteria probably used fermentation (a chemical process performed without the presence of oxygen) to produce the energy molecule adenosine triphosphate (ATP). Cyanobacteria introduced oxygen into the atmosphere through the process of photosynthesis. As the oxygen content in the atmosphere increased over the centuries, bacteria evolved that used this oxygen in the process known as aerobic respiration, a more efficient method than fermentation for producing ATP. Aerobic respiration set the stage for the evolution of eukaryotic cellsólarger, more complex cells that require efficient energy production to carry out their life processes.

A few molecular studies of the evolutionary development of the genes of archaebacteria suggest that archaebacteria may have evolved 3.5 billion years ago, slightly before cyanobacteria. Like cyanobacteria, archaebacteria probably relied on fermentation to synthesize ATP.

In a widely held theory known as endosymbiosis, scientists propose that simple eukaryotes evolved from prokaryotes that engulfed other prokaryotes. According to this theory, the engulfed prokaryotes, which remained active in their hosts, underwent changes over time and became the mitochondria (energy-producing organelles) of protozoa, or animal-like protists, from which animals evolved. The theory further holds that when photosynthetic bacteria were engulfed by other prokaryotes, the bacteria continued to photosynthesize within the cells that had engulfed them. The engulfed photosynthetic bacteria evolved into the chloroplasts of photosynthetic protists, the ancestors of plants.

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animal-like protists, endosymbiosis, aerobic respiration, process of photosynthesis, cyanobacteria, evolution of eukaryotic cells, protozoa, Prokaryotes, mitochondria, life processes, early Earth, fossil evidence, oxygen content, evolutionary development, ATP, fermentation, chemical process, life forms, ancestors, animals, time, hosts, atmosphere, stage, centuries, cells, environment, scientists, active, According, years

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