Imagine a planet dominated by a life form that maintains the soul of all past generations, allowing one day for each past life to control both body and mind. Then since thousands of generations have lived on this planet, each would only regain control—sentient life—for a single day in hundreds and thousands of years. Now imagine a dominant life-form that arose, not from apes, but from cats. Oh, they look basically human but with strange eyes and in perpetual heat. Also, unlike cats who only have nine lives, these civilized felines have thousands of lives.
Let’s make it tougher. Imagine a world dominated by a species where the male and the female forms are very different. The females are shaped like large pyramids with many short legs and move around all day eating the grass through their underside mouths and awaiting the return of the shiftless males each night. The males are shaped like huge dirigibles (zeppelins) and spend the day floating and cavorting in the air, propelled by powerful flatulence. At night the males moor themselves to the top of the pyramid-like females where they find not only a convenient mooring post but also a feeding tube (what! cud again tonight?) and even the appropriate female organ to enjoy some sexy-time with as they eat and recuperate from a tough day chasing clouds.