How did the increasing amounts of oxygen in earth atmosphere allow organisms to live on land

The increasing amounts of oxygen in earth atmosphere allow more growth of blue green algae.

Oxygen is present about 21 % in the earth atmosphere. Oxygen is important for the survival of living organisms because every living cell needs oxygen for respiration. In respiration, food molecules are broken down to produce energy with the help of oxygen. This energy is used by the cell in different processes.

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c) 50% there are four chromosomes. after crossing over, 2 of the four have been changed. that means any gamete has a 2 in 4 chance or 50% chance of receiving the mutated chromosome.