Formally, a comma is needed right after the end of the attribution ("He theorizes that across the centuries,"), but it's absolutely necessary only in written prose meant for publication. In text to be actually used for speeches and media broadcasting, sometimes it's wise to omit that comma altogether. Its presence may only force the speaker to pause longer than necessary, which could be distracting to both the speaker and the listeners. Try enunciating that statement a few times and you'll see that that comma actually forces you to pause more milliseconds than necessary.