While the title of this makes me want to sing a ’50s doo-wop, the information I am referencing comes from the same research that I have talked about in my last two posts, Evolution of English and Evolution of English (again).
Though the arguments on the verb shifts they make seemed to me to be “old hat,” I had no idea what their point on the use of do changing was exactly. I’m glad to have found an IFLScience article that summarizes that point.
As another example, look at the word “do”. By our current use of the word, “do” did not exist in the same way 800 years ago. Instead of saying “Do they say?” or “They do not say”, one would say: “Say they?” or “They say not.”
The use of the periphrastic “do” emerged in two stages, first in questions (“Don’t they say?”) around 1500 CE, then in imperative and declarative statements around 200 years later. Plotkin explained: “It seems that, once ‘do’ was introduced in interrogative phrases, it randomly drifted to higher and higher frequency over time. Then, once it became dominant in the question context, it was selected for in other contexts, the imperative and declarative, probably for reasons of grammatical consistency or cognitive ease.”
How they know the cause/reason for its selection in those contexts isn’t clear. Perhaps that is just their supposition.