The Golgafrinchans realised that were three types of beings on the planet of Golgafrincham: the leaders (or thinkers), the workers (or doers), and the middlemen.
The leaders contained the artists and "achievers". The workers were the people who "did all the actual work", and who made and did things. The middle management was comprised of hairdressers, telephone sanitisers, and other such "worthless jobs."[1]
The three classes of Golgafrinchans, as seen in Episode 6 of the TV series.
The group of leaders built a ship and convinced the middlemen to leave Golgafrincham by telling them several different reasons, including: that the planet was going to crash into the sun (or perhaps the moon was going to crash into the planet), that the planet was being invaded by a gigantic swarm of twelve foot piranha bees, and that "the entire planet was in imminent danger of being eaten by an enormous mutant star goat."[1]
The middlemen were sent off, told that the other Golgafrinchans would follow soon, however they remained on the planet with no intention of leaving. The middlemen stayed in space for a long period of time, with many on board in suspended animation for the majority of the journey, with the exception of the Captain and his Number One and Number Two. This third class eventually crashed onto Earth, while the other two-thirds of their society on Golgafrincham lived full, rich and happy lived until they were all suddenly killed off by a raging disease contracted from a dirty telephone.
Eventually, while sustaining major losses, and settling down in a cave-dwelling lifestyle, becoming 'cavemen', the Golgafrinchan middlemen wiped out the hut-dwelling original humans of Earth, and became the ancestors of present day humanity, "mucking up the program to determine the Ultimate Question."
https://hitchhikers.fandom.com/wiki/Golgafrinchans