And how did the first voters on Stack Overflow pull themselves up by their bootstraps?
It's been insinuated in answers about proposals on Area 51 for new Stack Exchange sites that proposals don't do well unless they somewhat overlap with a popular SE site. (Example; example.) This way the site can bring in at least 100 beta users with an association bonus, who presumably know their way around the Stack Exchange web application enough to reward good questions. This association bonus provides, among other things, the privilege to upvote questions and answers. Users earned their association bonuses by, say, having eight answers upvoted to 2 and three accepted on a different site.
But who upvoted them on that different site? People who brought association bonuses from another site could have, but that's disproven by infinite descent unless there was a base case that awarded the first association bonus. It turns out that in the beginning, there were the "trilogy" or "SOFU" sites (Stack Overflow, Server Fault, and Super User), which Jeff launched as an ExpertS-exChange clone without having to go through anything like Area 51 commitment.
On day one of the trilogy's private beta, when everybody had 1 rep, Jeff and the other moderators were doing all the voting. So someone didn't have to provide a 0-rated answer to a 0-rated question, wait for that answer to be accepted, and then use the 15 reputation from that to cast the first votes on all the 0-rated questions on the front page. But who was the first non-diamond to earn earn enough reputation to cast a vote?
(Incidentally, the oldest non-deleted questions on SO appear to be one by Jeff's unprivileged alt account about setting opacity for a form in Windows Forms and another about div widths in Internet Explorer 7's box model.)