Well, I can see this is addressed to a general case but I'll try a reply anyway.

When I talk about human nature I'm talking about the essential nature of who and what it means to be human.

True, this is always tied to the context and environment. Part that human essentialism is our adaptability, our plasticity.

Had Marx access to the research and theory we have today, I imagine he could have produced a more accurate and refined theory!

As a thousand men can make a thousand shapes with clay their finished product will always be clay, though with a thousand uses.