House above the abyss. Outcome

Aliens flew to the ground. It was not a scientific expedition, but a mission of relocation. Several million people from someone else"s galaxy found a new house on Earth, fleeing the cataclysms on the old planet. The traces of the aliens remained in earthly geography, cosmogony, engineering, medicine, architecture, marine business, account. And most importantly, these traces remained in earthly DNA - over the millennia, two humanities merged into a single family.
On the beginning of the resettlement and is told in the first part of the trilogy of Vyacheslav Sukhnev “House over the Abyss”. Why did the high civilization of aliens, capable of intergalactic flights, degenerated, assimilated and did not leave at least significant fossil artifacts on Earth? About this - in the following books