Why would the structures of organism look so differently even though they come from common ancestors

Even though they might have a common ancestor there might be a closer common ancestor to the original ancestors than the organism you're looking at.

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Explanation:

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the relative standard reduction potential of the half-cell in which reduction occurs; more positive than the other half-cell. reduction is a chemical process in which electrons are added to an atom or an ion; it always occurs accompanied by oxidation of the reducing agent.