Kevin Drum has a graph with one of the weirdest y-axes I've ever seen. "Time till next event" -- does that mean the time until the next dot that gets plotted on the graph? Then the arbitrary decision to assign dots for some things and not for others makes a big difference. If it so happened that humans controlled fire immediately before learning agriculture, and you decided to put in a dot for fire, you'd get an big downspike in the graph. Really, there are tons of other dots you could think up -- "domestication of animals", "man on the moon", "discovery of alcohol"...
From the look of the graph, though, "next event" seems to be whatever happens (roughly) at the present. That would explain why 10^9 years before the present, you get an event that's 10^9 years until the "next event". So it's no wonder that the graph is pretty much a straight line -- the number of years before the present varies directly with, um, the number of years before the present.
Reflections on Adoption, Part 2
7 minutes ago