For the sake of argument, let's assume that human beings are like programmable Lemmings that don't really know anything. They've recently climbed down from the trees and are faced with resource scarcity for the first time ever. Thus, Capitalism does not already exist, it needs to be somehow created... If people want it to exist, that is.
I guess "resource scarcity" might be one prerequisite. If everything is abundant and easily available, there's no real need for trade, is there?
And I guess the
idea of Capitalism and
wanting Capitalism to exist might be other requirements. The humans might proclaim in Modern English: "hey, let's
trade stuff!"
What other requirements are there, if any? And why might they be needed?

You are misinformed about what capitalism is.
Pure capitalism is absolute freedom coupled with respect for rational property ownership. Consequently those are also the only two prerequisites you need for capitalism.
(definition of rational property ownership: ownership based in first principles supported with reason and not some illusion supported with violence)