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?

There are no requirements. The common understanding of what capitalism actually is, is false. Capitalism is both a political ideology and a description of a set of natural laws, that together, give rise to an economic order that is emergent among any human population. The ideological version of Capitalism is more of a religion than an economic system, and I won't address that here beyond saying that it's not really capitalism at all.
Capitalism doesn't require that the laws of a given society honor the natural laws that produced capitalism, and this really has never fully happened at any point, nor in any nation, since at least the Age of Judges in old testament Israel. That is not required. If the natural order is supressed, by law or otherwise, capitalism will still express itself in onther ways; usually in black markets & contraband trade. There were capitalists in the former USSR and communist China the entire time.