THE CREATURES THAT TIME FORGOT (PT 1) by RAY BRADBURY

Our protagonist is a young man whose family and ancestors have been marooned on a nearly uninhabitable planet with no hope of escaping. Here, because of harmful radiation, they are born they age, and die after 8 days They have the advantage of having inherited their intelligence from past generations, so even in his first day of life the boy can think intelligently- and he soon comes up with a way he believes will save their cave- dwelling people and the girl he has fallen in love with.