Final answer:
The organism with the fewest amino acid differences compared to organism A would be the closest relative, reflecting a more recent common ancestor. Examples such as the identical protein sequences in cytochrome c of humans and chimpanzees indicate a very close relationship, and varying degrees of differences suggest different evolutionary distances.
Step-by-step explanation:
To determine which organism is most closely related to organism A, you would look at the number of amino acid differences between organism A and the other organisms. The organism with the fewest differences in the amino acid sequence of a shared protein is considered to be more closely related. This concept is grounded on the principle that the amino acid sequences of proteins become more divergent as species evolve separately over time. Hence, the fewer differences in these sequences, the more recent the common ancestor shared between the organisms.
For instance, if we consider the protein sequence of cytochrome c, which is shared among various species, we can infer evolutionary relationships. Humans and chimpanzees show an identical protein sequence in cytochrome c, suggesting a very close evolutionary relationship. When comparing the protein sequences of humans and rhesus monkeys, only one amino acid difference is found, indicating they also share a relatively close common ancestor. In contrast, the human-to-yeast comparison reveals 44 differences in amino acids, pointing to a more distant common ancestor.
Therefore, within your provided data, the organism with the fewest amino acid differences when compared to organism A would be deemed the most closely related. The specific data points are crucial to provide this answer, and without them, we cannot determine which organism is closest to organism A among organism B, C, or D. However, the principle that DNA sequences and protein sequences are more similar in more closely related organisms is evidence of common ancestry.