The monkey planet

The main events of the book are placed in a frame story, in which Jinn and Phyllis, a couple out on a pleasure cruise in a spaceship, find a message in a bottle floating in space. The message inside the bottle is the testimony of a man, Ulysse Mérou. Ulysse explains that he was a friend of Professor Antelle, a genius scientist on Earth, who invented a spaceship that could travel at nearly the speed of light. In 2500, Ulysse, the professor, and a physicist named Arthur Levain flew off in this ship to explore outer space. They traveled to the nearest star system that the professor theorized might be capable of life, the red sun Betelgeuse, which would take them about 350 years to reach. Because of time dilation, however, the trip seems to the travelers only to last two years.
They arrive at the distant planetary system and find that it contains an Earth-like planet, which they name Soror (Latin for sister). They land and discover that they can breathe the air, drink the water, and eat the local vegetation. They encounter other human beings on the planet, although these others act as primitively as chimpanzees and destroy the clothing of the three astronauts. They are captured by the primitive humans and stay with them for a few hours. At the end of this time, they are startled to see a hunting party in the forest, consisting of gorillas and chimpanzees using guns and machines. The apes wear human clothing identical to that of 20th-century Earth, except that they wear gloves instead of shoes on their prehensile feet. The hunting party shoots several of the humans for sport, including Levain, and capture others, including Ulysse.