Explain how homologous structures are evidence of a common ancestor.

When two or more organisms have the same type of physical features in them, it shows that the organisms might have evolved from a common ancestor. Such structures are known as homologous structures.

For example, the forelimbs of humans, birds, whales and dogs have the same kind of structural anatomy.  Although, their functions might be different but their structures are similar. This similarity depicts that the organisms might have a common ancestor in the past.

a- all cells come from pre-existing cells

not sure if this is what you'r looking for eubacteria and archaebacteria