Since I'm in a diagrammatical mood (see Ceres thread):
It's the goldilocks syndrome all over again. The question is, where are we on this line? The fact that we aren't yet overrun with aliens would indicate that we are on the steep bit of the climb between A and B, but who knows? Maybe we're somewhere nearer C and everyone else is just extinct already.
Actually, the line between B anc C probably ought to be far shallower and straighter, since the hyrogen->heavy element process will be quicker when the universe is young, because there's more hydrogen about to clump up and form stars.