I think human beings have had to multitask for a long time. Our ancestors might have said “That looks like good fruit in the tree and what’s that furry thing over there?”. Attention to multiple things in our surroundings kept some of us alive. The DNA of those monotaskers did not get passed on for long.
But while our evolutionary history may have made it necessary to pay attention to more than one thing for survival, our mental health survival these days may depend on toning down all the stimulation and focusing on one thing.
Monotasking, or single-tasking, is a 21st-century term for what your high school English teacher probably called “paying attention.”