I understand the general Marxist theory about why class society arose, there was surplus production and that lead to the institution of private property and the rest follows.

All of that makes sense on a high level but what exactly happened on an individual human level? Conceivably those early civilizations could have elected to go for a more equitable and democratic distribution of the surplus value even if the result would probably not be 100% equitable and would result in some poverty due to material constraints at the time. How do you explain the first appearance of private property and class society on an individual level without delving into some evopsych nonsense about evolutionary instincts or whatever?