Y is for Young Hegelians
Marxism was born of a synthesis of the most advanced aspects of bourgeois social theory: English political economy, French socialism and German classical philosophy.
In retrospect the first two elements of this seem obvious enough.
Among the political economists, Adam Smith had shown that labour was the essence of value, while David Ricardo, despite being on the opposite side of the barricades, had pointed to the rationality of working class struggle.
Meanwhile, the socialist workers who Karl Marx met in Paris were living proof of an alternative to the egoistic individualism assumed to be natural by the economists.