What did social interaction becoming more complex mean?

It means people started doing more kinds of things together, just like how your toys can have more fun when you play with more friends.

Imagine you're playing with friends in the park, at first, it's just tag or hide-and-seek. But then, some kids bring boards, others bring kites, and someone even brings snacks. Now, there are more things to do, and everyone has different ideas about how to play. That’s like social interaction becoming more complex, people started doing more kinds of activities together, and they had to talk, share, and work out who does what.

When you have a bigger group at the park, sometimes it's easier to get confused or forget who is playing with whom. But that also means there are more ways to have fun, like building forts, telling stories, or even putting on a little show. This is just like how people in bigger groups had to learn new ways to talk and work together, making their interactions more interesting and complex.