When it comes to explaining human behavior, just about every answer combines some elements from the categories conveniently labeled as "nature" and "nurture."
In this context, nature is a category that includes all biological, and more specifically genetic causes for human behavior. You might say that these are "internal" explanations for human behavior.
Nurture is a category that includes all of the environmental factors that explain human behavior. This includes the physical environment, such as the climate and topography where someone might live, as well as the social environment, such as the economic and political systems in place where a person lives.
People tend to try to get a lot of mileage out of explanations appealing to social factors. Of course, a lot of human behavior is explained by social factors. However, this cannot be the ultimate explanation of all human behavior. It's plausible to think that human beings existed before social systems existed, since social systems are human constructs.
So what were people like before the advent of society? This question is of interest to many people, including myself. A lot of people wonder if human beings are inherently good or evil, or if they're inherently selfish or altruistic. Knowing what human beings were like before they organized into social groups may be illuminating in this respect.
How do we know what they were like? Well, we'll never know for sure. If you identify as part of some religious tradition, then you probably some kind of origin story that's supposed to explain mankind's initial state.
Let's set those aside for now and explore non-religious accounts. From, the general consensus regarding the origins of human beings is that they are the products of a long evolutionary process. If we accept that, then we need to piece together a story about how human beings came to be what they are now.
We can infer that at least some behavior of human beings is explained as a result of genetic inheritance. So, we can investigate the behavior of other primates or evolutionary ancestors to get an idea of what human beings were getting passed down to them.
That's one piece of the puzzle. From here, the key is to find roughly where human beings began to abide by some,kind of social contract, i.e. when they began to set aside immediate personal wants in favor of gains had by cooperation. Around here is where we can start asking questions about the initial formation of social groups. For instance, it is conventional wisdom to think that early social groups were hunters and gatherers. How did they behave in those groups? How were responsibilities delegated, and why? For instance, it is common to think that the men were the hunters and the women were the gatherers. Was this actually the case? If yes, then why? Did it have to do with physiological differences?