system
Given the following post, respond as if you're commenting on Reddit:
human
[40K] Does the Tyranid superorganism actually seed galaxies with life?

The full Tyranid superorganism arrives and envelopes the galaxy. After a few million years of feeding and growing, it ponderously begins its next intergalactic voyage towards its decided feeding ground. It leaves a galaxy mostly stripped bare of original life with the exception of some isolated pockets of micro-organisms and some deep core lifeforms in a few gas giants it was not worth the effort of retrieving. 

Eventually life will arise again on its own as it has many times before, but there would be a large amount of Tyranid biomatter left throughout the Milky Way. Some of this would likely survive indefinitely in certain conditions. Could this eventually form the basis of next order of life to dominate the galaxy?

I also hear reports of worlds discovered to contain self sustaining Tyranid based ecosystems. 

In theory, could the Hivemind have cleansed this galaxy billions of years ago, leaving behind traces of life or even Tyranid based living worlds from which the current dominant forms of life have since evolved?
gpt
Part of the problem with the idea that the Tyranids actually seed galaxies for re-development is... They consume EVERYTHING. Planets are totally stripped bare of organic matter, then water and certain minerals (coal, for instance, would be metabolized by the Hive Fleets) and then, finally, the Hive Ships consume the atmosphere. 

When the Tyranids are done, they leave a barren ball of rock with no atmosphere, making the re-development of any life rather slim.