Board Thread:Warhammer 40k General Discussion/@comment-25618639-20150220210346/@comment-6078851-20150403155659

as things stand, yeah, tyranids remove all matter from a world/galaxy that could lead to life evolving again on planets (unless, as you say, bacteria is introduced to a world to restart the evolutionary cycle), and is why tyranids are such a deadly threat. but i was referring to the fact of - way, way off into the future, billions of years minimum, and trillions of years for big stars - supernova stars dispersing elements, the accretion of nebulae, the accretion of new solar systems and planets, and the cycle that leads to life possibly beginning anew, not on the same dead worlds but on newly made worlds. pretty much the same cycle that's theorised to have led to life having evolved on earth.

stars continually create elements through the course of their life-span, and when a star exceeds the chandrasakar limit (the minimum mass needed to go supernova) then whatever elements are within such a star are dispersed to someday coalesce within new planetary bodies, asteroids etc.

i'm pretty sure tyranids don't/can't consume stars :)  so theoretically life could evolve again in galaxies left barren by tyranids when/if sufficient stars go supernova and there's a re-infusion of the basic elements needed. supernova class stars, while not that common, are still abundant when taking into account the sheer volume of stars :)