Main character is a schoolteacher on one of the first human colonies in space. He is puzzled over and concerned with the uniform lack of intellect he is observing in his students, especially since they are the progeny of accomplished parents who were hand-picked to found this colony. None of his students are mastering any of the material, and he is becoming concerned that their generation will be unable to maintain the colony. Teenage pregnancies become the norm, and one of the students proudly shows the teacher her baby, which has distinctly simian characteristics. The teacher comes to the realization that humans aren't intrinsically intelligent -- intelligence is just one step in the species' propagation cycle (like a pretty flower) that lasts only as long as necessary for it to build up the technology required to reseed on another planet, then is discarded until the next cycle.
The Locusts by Larry Niven and Steven Barnes
A colony expedition to Tau Ceti ends up de-volving into apelike creatures.
There are no men on Tau Ceti IV
There are things in the forest. They prefer it there, but will occasionally shamble out into the grasslands and sometimes even into the town. They themselves do not understand why they go: there is no food, and they do not need building materials or other things which may be there for the scavenging. They always leave the town before nightfall arrives.