World of Four Dimensions

(from ebookjapan):

On a planet inhabited by those who have escaped from the Earth just before it was swallowed by the sun. Jack lives in a cave with his grandfather, where he paints on the walls.

He is weak and cannot hunt like the other men. Maya, who had been the model for his paintings, grows weary of him and leaves on a journey with Mu, a stronger man. The world of survival of the fittest has begun. (Chapter 4).