Since childhood, I have periodically tried to reason about when the latest moment in time was that a single human could have substantially all the knowledge that humans collectively possessed.
As a kid, it was an easier thing to reason about (wrongly). As an adult, I’ve concluded that it was never possible to both have something we’d think was civilization and for one person to know substantially everything humans knew.
You don't need civilization for this to be unfeasible. People tend to underestimate how much nomadic people know about things like animal behaviors, medical use of natural substances or about geography, especially with regard to the change of seasons. Arguably, it was never possible to know the entirety of human knowledge.
As a kid, it was an easier thing to reason about (wrongly). As an adult, I’ve concluded that it was never possible to both have something we’d think was civilization and for one person to know substantially everything humans knew.