By _The Manifesto of the Communist Party_ , every Marxist knows the A,B,C's of historical materialism or the materialist conception of history. The history of all human society, since the breaking up of the ancient communes, is a history of class struggles between oppressor and oppressed. Classes are groups that associate in a division of labor to produce their material means of existence.
In _The German Ideology_, Marx and Engels asserted an elementary anthropological or "human nature" rationale for this conception. In a section titled "History: Fundamental Condtions" they say: ... life involves before everything else eating and drinking, a habitation , clothing and many other things. The first historical act is thus the production of material life itself. And indeed this is an historical act a fundamental condition of all history, which today, as thousands of years ago, must daily and hourly be fulfilled merely in order to sustain human life."
Production and economic classes are the starting point of Marxist analysis of human society, including in the Manifesto, because human life, like all plant and animal life must fulfill biological needs to exist as life at all. It is an appeal to biologic ( which I support, all anti-vulgar materialist critiques to the contrary notwithstanding, but that's another essay). Whatever, humans do that is "higher" than plants and animals, we cannot do if we do not first fulfill our plant/animal like needs. Therefore, the "higher" (cultural, semiotic etc.) human activities are limited by the productive activities. This means that historical materialism starts with human nature, our natural species qualities.
Yet, it is fundamental in biology that the basic life sustaining processes of a species are twofold. There is obtaining the material means of life and subsistence or success of survival of the liviing generation, for existence ("production"). But just as fundamentally there is reproduction or success in creating a next generation of the species that is fertile, and survives until it too reproduces viable and fertile offspring. Whoever heard of a one generation species ? In fact, one test of two individual animals being of the same species (at least class mammalia or phylum chordata) is their ability to mate and produce viable and fertile offspring. We can imagine a group of living beings with the ultimate success in eating and drinking, a habitation, clothing and many other things. But if they do not reproduce, they are either not a species or they are an extinct species (unless the individuals are immortal). Thus, having premised their theory in part on human biology, our "species-being", Marx and Engels were obligated to develop historical materialism, the theory of the Manifesto, based not only on the logic of subsistence production, but also on the logic of next generation reproduction.