The Doctor once struck up a conversation with this individual and came onto the subject of monsters. The Doctor noted that, although he had faced many in his travels in space, Earth had once been home to its own crop of gigantic, strange, dangerous monsters. To prove it, on a whim, the Doctor took the human on a trip in his TARDIS, darting around the long period of Earth's history populated by dinosaurs and similar creature, pointing out various creatures to the human.
After the dinosaurs, the Doctor gave the human a brief glimpse of the rise of humanity, showing them cavemen taking on a Smilodon and then a Mastodon, before dropping them off home with some parting words on the majesty of dinosaurs. The Doctor told them that if they ever fancied another trip in the TARDIS, they should stay on the lookout for blue police boxes; with them becoming less and less common in the human's era, it was a good guess that any police box they saw would be the TARDIS. (PROSE: The Doctor Who Dinosaur Book)
Behind the scenes
The text of The Doctor Who Dinosaur Book was entirely written as the Fourth Doctor's ongoing "lecture" to his companion about the various prehistoric animals they encounter. As a result, the companion is very sketchily characterised, not even being given a gender. This narrative device permitted greater reader engagement: the young readers were implicitly invited to imagine themselves as the companion the Doctor had picked for this adventure, and to feel like the Doctor was "talking to them" directly from the page. It can, thus, be said with some certainty that the authorial intent was for this companion to be a minor from the mid-1970s.