You'll have a hard time convincing me that a world with several orders of magnitude more humans is somehow less diverse than the times with fewer humans unless you somehow remove the ability to process arithmetic from my brain.
Wait, how does more humans = more diversity? I would think any abstract measure of diversity would be based on average differences, or standard deviations away from the mean. Obviously, the absolute extremes are greater with more people, but it seems entirely logical to say diversity has decreased as the number of people increase.
Easy. The more populated world is the one that has a global telecommunications network, whose homogenizing influence far outweighs higher-order effects like population.