Have Spacesuit—Will Travel

Have Spacesuit—Will Travel is a 1958 children's science fiction novel written by Robert Heinlein, featuring a small amount of prehistoric content.

Plot
After winning a space suit in a contest, the teenage boy Kip is kidnapped by an alien, taking him on an adventure that goes to the Moon, Pluto, the star Vega, and beyond the galaxy.

Prehistoric Content
When Kip is put on trial as a representative of humanity by the Landorians of the Small Megellanic Cloud, one of the other representatives is a Neanderthal taken from his native time by the Landorians. Prior to the trial, Kip observes the Neanderthal being kept in a terrarium like room, and he runs away in fear after being observed. Kip is initially disgusted by the primitive habits of the Neanderthal, but quickly comes to pity his captivity in a place he cannot possibly understand. During the trial, the judge is able to communicate with the Neanderthal, and determines that his lineage became extinct, and he cannot be considered a representative of humanity, sending him back to his original time period.