What does the term high point mean?
Social insects represent the high point of invertebrate evolution.
High point means it's at it's peak, or it's best point in time.
I was at my high point during the summer golf season
Means that I was doing my best, or achieving the highest during the summer time.
In your example, it means that in the evolution cycle, it was the best it can be or the best to date. Meaning social insects represent the best/pinnacle/maximum/gretest point of invertebrate evolution.