Complex sentence: A hypothesis on the evolutionary pathway of eubacterial flagellum argues that a secretory system arose first, based around the SMC rod-and pore-forming complex, which was the common ancestor of the type-III secretory system and the flagellar system.
Split into simpler sentences: A hypothesis on the evolutionary pathway of eubacterial flagellum argues that, a secretory system evolved first, based around the SMC rod-and pore-forming complex. This is presumed to be the common ancestor of the type-III secretory system and the flagellar system.