The topsoil of a world is rich with life, living dying and born again without ever seeing the sun. But there is no life on a dead world, and even a Tyranid cannot survive without organic matter. Or at least, no Tyrannid ever had.
But life, back in its earliest origin, came from unlife. From that which was nothing but chemical elements life came forth from nothingness. And so it was, that in the earth, nearly a mile beneath the surface of a cooling lump of metal, the small worm-like Tyrannids with no fleet to call their own found new life.
The chance was one in a trillion. That a perfect combination of pressure, radiation, and the right elements coming together would create something the galaxy had never seen. But right here, on this world, for only the second time ever witnessed, life began from nothingness again as a single cell.
And within a year it was completely consumed. But its story would not be forgotten. Because a Tyranid does not forget. When a struggle against death is found, a Tyranid recognizes its plight and accepts it into their family to be immortallized in the endless tale of evolution, the struggle of progress. To be spread among the stars that all may know its story when they too become a part of the glory of perfection.
And perfect this strange organism they did. Within a single generation the metabolic processes of the new Hive had changed. It did not need carbon, phosphorous, oxygen... It craved silicon, iron, sulfur...
The population exploded and the mind, the simple primitive mind awakened. It did not remember it's former glory. It did not recall the galaxies it traveled to reach this desolate rock. All it remembered... was the fire.