In the Hypothetical Biochemical Pathway, P $\rightarrow$ Q + R $\rightarrow$ S $\rightarrow$

In the hypothetical biochemical pathway, P $\rightarrow$ Q + R $\rightarrow$ S $\rightarrow$ T, which step is likely to have evolved first?

A) P $\rightarrow$ Q
B) Q + R
C) R $\rightarrow$ S
D) S $\rightarrow$ T