I was inspired by this (also by the lovely long blackboard and the availability of coloured chalk) so I did my talk this afternoon as a chalk talk too. I started by pointing out that, because meiotic sex evolved once early in eukaryotes, any acceptable explanation must solve a problem experienced by diploids and haploids, uni- and multicellular organisms, and obligately and facultively sexual organisms. This constraint rules out many of the explanations and examples proposed at this meeting. Then I said that, because bacteria don't have sex (don't have any process evolved to cause recombination of chromosomal genes), any acceptable explanation of meiotic sex must also solve a problem that bacteria don't have. Then I went over the evidence that bacteria indeed don't have sex. I was in an adrenalin fog and so have no idea how much over my allocated 20 minutes I went, or whether I ever looked at the audience. But Matt Meselson thinks I'm right!