The majority of domestic rats originate from Rattus Norvegicus i.e. the brown rat. The brown rat began its domestication in the 18th an 19th centuries when rat-catchers would trap rats all throughout Europe.
These trapped rats would then be killed or sold to be used in the popular blood sport called Rat-baiting until the beginning of the 20th century. It is believed that both rat-catchers and those that participated in the blood sport began to keep certain odd coloured rats that differed from the dark murky agouti typical wild rats eventually breeding them and selling them as pets.
The two men that are believed to have formed the basis of the rat fancy are Jack Black - Rat catcher to Queen Victoria and Jimmy Shaw - Manager to one of the largest sporting public houses in London.
Rats are very social animals by nature and this is much of the reason why they are very well suited as human companions.
Many owners and breeders describe rats as small dogs - This is due to the bonds they form with their owners, their eagerness to interact with you, their love of food and the tricks they can be taught but unlike dogs they are fine to be left in their cage while you are at work, you don't have to leave the house to exercise them, there's no unpleasant surprises on the floor if you don't make it home in time to let them out (although some rat owners may beg to differ if there's anything within reach of grabbing paws)
They can also be litter trained, will generally be awake in time for coming home from school or work and a well socialised rat is very reluctant to bite. This makes them much more suitable choice for children than other small animals but that doesn't mean that they aren’t the perfect companion for adults as well.
REMEMBER: Always supervise children and all animals when together, it can take a split second for a child to hurt or scare a rat unintentionally.