Answer
Evidence states that there was a common ancestor cell. There are too many similarities between the cells of seemingly unrelated organisms for there to be multiple paths of ancestry. For example, many organisms share similar metabolic pathways between the cells. It could also be indicated that all living cells share the same combination of compounds like proteins and acids in their cell make up, despite there being other combinations available. Being that every cell is a daughter cell of another through cell division, it is safe to say that there is a common ancestor.
Work Step by Step
With the idea that there is a common ancestor, it cannot also be said that it was just one cell that survived and dominated the world. Rather, it was more likely to be multiple cells going through various environments and trials and most dying off until one landed in a stable and safe environment. With a safe environment and enough food, the cell could evolve and replicate.
