De-Evolution

It took time for the human colonists to realize a type of local fauna was actually a sentient race, one that had had a thriving civilization as little as 100 years ago. Then they discovered a second specie that had made great evolutionary advances in the past 100 years. The humans then realize their memories and skilled knowledge were being effected, and they might not be able to complete building their colony. Can they find out what is causing these drastic changes and nullify it before it becomes too late and they are reduced to little more than animals?