Short story with spaceship and humanity not realising they should be on it

The plot is about:

• A spaceship travelling through the universe


• Earth is a rogue planet whose inhabitants have been cut off and don't realise that their true purpose is to be part of the spaceship
  
  
  
  • The reason earth was in turmoil (wars etc.) was that they didn't realise that their true role was to fit into the ship
  
  
  
  


• The ship contained 3 or 4 beings that made the thing work


• Planets throughout the universe are populated with the various beings which are components of the space craft
  
  
  
  • I seem to remember like an eye shaped being
  
  
  
  • Each of them fulfil their role
  
  
  • Humans are part of that as well.
  
  
  
  


• There are loads of planets with humans on but earth has gotten overlooked so it never realised that's what they were designed for


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  The story concludes with the ships picking up someone from Earth

  
  
  
  
  • and with the "human" realising that this is what they should really be doing
  
  
  
  
  


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  Humans have a different name that explains their role in the ship.

  
  

I probably read it around 30 years ago in English in a sci-fi collection book.

Answer

"Specialist" by Robert Sheckley. It was the subject of this question and this one. You can read the whole story at the Internet Archive. Here is the plot summary from Wikipedia:

A galactic deep-space cargo ship is blown off course by a photon storm. When the crew recover, they don't know their location in space and one of their members, known as Pusher, is dead.

The Crew comprises varied members of vastly different intelligent races, all in close mental rapport known as the Cooperation, and serving specialized functions; they are known as Engine, Thinker, Eye etc. But without a Pusher, they cannot accelerate to ftl speeds. They manage to locate a planet known to be rich in primitive Pushers, Earth. They try to communicate with a man they find, but he is so violent and resistant that they are forced to bring him on board the ship to try to reason with him.

Gradually, the man begins to realize the nature of the Crew and the pleasure to be found as a part of the mental Cooperation. He agrees to join the crew. Hesitantly at first, he tries to Push and finally learns how it is done. The ship speeds away at eight times the speed of light and accelerating.