The Zan are an alien species that came to Earth and wiped out all life.
A human observer notes that while they look horribly different from humanity, they must have about the same metabolism as humans, the same type of circulatory system, and probably the same type of digestive system. The only major difference is that the Zan are a species that don't age, as their cells renew thus death is a rare phenomena to them.
The Zan had wiped out all life on Earth, though they preserved two members of each species, humanity included.
Walter Phelan is the last man on Earth and Grace Evans is the last woman. Though emotionless towards their charges, they grow disturbed when the animals under their care begin to die. Turn to Walter for advice as he explained to him he concept of death. The human suggests that maybe creatures perished from lack of affection and suggests that they pet the survivors regularly to keep them alive. However the Zan begin to die, and in fear they depart the planet. Later it is then revealed that the creature Walter advised them to pet was a poisonous snake.
The Zan are a highly advanced culture, an ark ship being the size of a mountain. Due to their longevity death was unknown to them.
- Knock, by Fredric Brown (1948)
- Thrilling Wonder Stories (1948)