The word capital in relation to the means of production is not ancient at all, coming into general use in this sense only in the 18th century. You can see that capital is not just a thing, but a relationship and a process. Wealth only takes on the attribute of capital when it is used to hire workers to create more wealth and then robs them of part of the value of their labor. Under Socialism, there will be means of production, factories, machines, etc., but they will not represent capital because they will belong to the whole of society and we the useful producers will share the fruits of our labor. One of the hallmarks of capitalism is its commodification of the world.
Every conceivable thing is weighed in terms of money.
If it is not salable, it is valueless - human beings and their labor power included. Even our language reflects the capitalist' value system - as the reference to other countries as undeveloped and backward implying some sort of innate necessity for their development. In such a system the labor of a human being has become an article of merchandise. The evidence is overwhelming. The very existence of the expression, labor market, is proof in itself.