From a history of thought point of view, it is important to note that the distinction is a relatively modern one, starting in the 1930s. The term macroeconomics was first used, most likely, by Ragnar Frisch in his lectures notes in 1933-34, and first used in print by Jan Tinbergen and Eric Lindahl in 1936 and 1939, respectively (for a full discussion see Vela Velupillai, 2009; subscription required). Keynes did not use the term in his General Theory, even though the book is considered the foundational book of macroeconomics. To a great extent the work of John Hicks -- both his ISLM paper and his Value and Capital -- is central to understand the direction in which the profession developed.
Yet, Keynes book was central in the eventual split of macroeconomics and microeconomics. In particular, because Keynes defied the conventional wisdom, associated with marginalism, that the rational maximizing behavior of firms and individuals would lead to the optimal (meaning full) utilization of resources, including obviously labor resources. Full employment was not the normal equilibrium position of the economy. In that sense, Gary Mongiovi provided the most enlightened note to the SHOE debate. He tells us:
"the macro/micro distinction actually arose after Keynes's theory took root, and students had to be taught two distinct and apparently incompatible stories about how market economies work: (1) a neoclassical distribution theory in which the real wage adjusts to bring the S & D for labor into line with one another; and (2) a Keynesian story about how the economy could settle into an equilibrium in which the labor market doesn't clear."That incompatibility and incoherence is still with us. Students are taught that markets are efficient in their micro text, and some imperfectionist story is told in order to provide a foundation for the Keynesian macro story (at least in salt-water departments). Of course an alternative would be to discard the marginalist approach altogether, since it does have insurmountable problems as admitted by Paul Samuelson during the capital debates in the 1960s. But that's another story for a different post.