"2600 Under 2600" reference?

[GrandizerII]

Hey, does anyone know what the hell "2,600 Under 2,600" list means?

I'm reading a post from Wait But Why that had this statement, and I did use Google to try to find out but to no avail. Anyone get the reference?

For context:

http://waitbutwhy.com/2016/01/horizontal-history.html

For my first crack, I present to you a big pile of famous names, organized by birth decade—kind of a “2,600 Under 2,600” list. The purpose is to help orient ourselves on when people lived, especially in relation to each other.