Task10. Read and translate the text about the evolution of prokaryotes.

Prokaryotes are the ancestors of all life forms. Although scientists debate the events of early evolution, evidence suggests that the archaebacteria, the first cells on earth, evolved at least 3.5 billion years ago, about a billion years after the earth was formed, possibly in waters with very high temperatures. The environment of the archaebacteria lacked free oxygen, which did not accumulate in the atmosphere or water for another billion and a half years. Prior to and during the time archaebacteria evolved, frequent volcanic eruptions poured mixtures of hot gasses into the air, which eventually dissolved in the boiling seas, constantly changing their chemical composition. As a result, natural selection favored the evolution of diverse metabolic pathways in the archaebacteria.

The biochemical activity of the archaebacteria further altered the composition of the water and the atmosphere, paving the way for evolution of bacteria. Among the early bacteria are the cyanobacteria, formerly known as blue green algae. Fossils of cyanobacteria, found in ancient rock forms called stromatolites, indicate that the cyanobacteria evolved from 2.5 to 3.4 billion years ago. Through the process of photosynthesis, which releases oxygen, cyanobacteria introduced oxygen into the atmosphere. As the oxygen content in the atmosphere increased over the centuries, bacteria evolved that used this oxygen in the process known as aerobic respiration, an efficient method of producing the energy molecule adenosiine triphosphate (ATP). Aerobic respiration set the stage for the evolution of eukaryotic cells—larger, more complex cells that require efficient energy production to carry out their life processes.

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 the cells, 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. The bacteria-like DNA and ribosomes found in mitochondria and chloroplasts provide evidence for this theory.

Task11. Answer the questions.

1. When did the first cells on earth evolve? What were they?

2. Why did natural selection favour the evolution of archaebacteria?

3. How did the biochemical activity of the archaebacteria alter the composition of the water and the atmosphere?

4. When did cyanobacteria evolve?

5. When did bacteria begin to use aerobic respiration?

6. What set the stage for the evolution of eukaryotic cells?

Task12. Describe the theory of endosymbiosis.

Task13. Answer the question: do you know any other theories of early evolution?

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