He was forced to live in the sewers, where he made friends with the rats. One day, a small child was returning from the pet store with a fish bowl of baby turtles, when he tripped and fell, causing the bowl to crash on the street and the turtles to fall into a drain. Yoshi made four new friends. The turtles became human, because of their recent contact with Yoshi in his effort to clean them up, and Yoshi was transformed into a rat because of his recent contact with the rats. April now realizes that Splinter is Yoshi.
Splinter continues by relating how they got their names.
From the TMNT home page. Discuss the nature of the original mutagenic event. List no fewer than 10 mutations that had to have occurred in each of the animals, and discuss the probability that each of these could be expected to occur simultaneously in four individuals. Include in your discussion the population or species with which they could be expected to successfully mate.