Person years is a bad metric to compare to time years. You should compare the 18B person years to the about 5 trillion[1] person years that have existed since these cave paintings. Now that’s truly a long time, and shows how slowly these changes really have been. To get from those cave paintings to where we are now would take trillions of years for one person.

[1]: ~100B people ever[2] * ~50 years of life span = ~5T person years.

[2]: https://en.wikipedia.org/wiki/World_population