I totally blew the book cover last week and I've nearly blown it this week. Sorry about that.
This week we have "Protector" by Larry Niven. It turns out that Humans are actually the descendants of an alien race. They seeded the earth with their own genome and a liberal dose of radiation in hopes of preserving some form survival for their species. They also built the Ringworld.
The aliens spend most of their lives being really very cattle like. However, when they reach about forty years old they go through a transformation that makes them mentally and physically superior. These transformed aliens are called Protectors.
Humans still have a remnant of this alien life cycle - it's why people get arthritis and explains menopause, among other age related conditions.