The Creatures That Time Forgot by Ray Bradbury

During the night, Sim was born. He lay wailing upon the cold cave stones. His blood beat through him a thousand pulses each minute. He grew, steadily.

Into his mouth his mother with feverish hands put the food. The nightmare of living was begun. Almost instantly at birth his eyes grew alert, and then, without half understanding why, filled with bright, insistent terror. He gagged upon the food, choked and wailed. He looked about, blindly.

There was a thick fog. It cleared. The outlines of the cave appeared. And a man loomed up, insane and wild and terrible. A man with a dying face. Old, withered by winds, baked like adobe in the heat. The man was crouched in a far corner of the cave, his eyes whitening to one side of his face, listening to the far wind trumpeting up above on the frozen night planet.

Sim’s mother, trembling, now and again, staring at the man, fed Sim pebble-fruits, valley-grasses and ice-nipples broken from the cavern entrances, and eating, eliminating, eating again, he grew larger, larger.

The man in the corner of the cave was his father! The man’s eyes were all that was alive in his face. He held a crude stone dagger in his withered hands and his jaw hung loose and senseless.

Then, with a widening focus, Sim saw the old people sitting in the tunnel beyond this living quarter. And as he watched, they began to die.