What if Marx & Engels had never seen the light of day?

I�m not so sure what you mean, communism as a political theory or communism as the political system of the Soviet-Union and other so-called communist states. As Mixcoatl rightly states, Marx and Engels weren�t by far the first ones who developed the idea of a communist, in its true sense of an egalitarian and non-authoritarian, society as the ideal state of human community, in one form or another this utopia has existed throughout history, many with religious undertones as  the English Levellers of the 17^th century, or others being the immediate predecessors like the group around  Babeuf during the French Revolution. I think the idea that all men, and women, are equal and should share the wealth of a society that they helped to create, appears immediately, where this isn�t the case, in other words, wherever people are politically and economically oppressed.

The question if this can be realized, is besides the point, but brings us to the next question. Would the Soviet-Union ever have appeared without Marx and Engels?

Instrumental in establishing the Soviet-Union and in consequence all the other communist states were Lenin and later Stalin, and if their  practical policies had anything in common with the political theories of Marx and Engels in other but the name, must surely be open to debate. I don't believe they had.

When Adam delved and Eve span, who was then the gentleman?