Evolution. Many very complex structures have been shown to have arisen directly from simpler ones. The same appeal to basic reasonability works the other way too. Which seems more likely, that complex things arose very slowly over a long time from other, simpler things? Or that this whole universe, our bodies made up of billions of cells that all perform myriad different complex functions, quasars and stars and nebulae and comets, fusion and fission and gravity, everything we can see and hear and touch, all just popped into existence fully formed in an instant 'cause someone flipped a switch? I know which seems more likely to me.