Humans are Earth's dominant species. They are a race of bipedal beings. Very few humans in both the books and franchise have dragon companions. The majority of humans that appear in How to Train Your Dragon are Vikings, though not all. Some other groups include the Northern Wanderers, the Inuit, and the Sami.
According to Viking legend, a Viking known as Great Hairybottom lost his boot in the bogs of the Barbaric Archipelago and declared that as long his bog remained there, there would always be Vikings in the Archipelago.
Later (it is unknown how much later) the human race was at war with dragonkind, who were being led by a great Seadragonus Giganticus Maximus known as Merciless. The war was ended by Hiccup Horrendous Haddock I, who befriended the dragon Wodensfang and became the first King of the Wilderwest. Hiccup the First was told the secret of the Dragon Jewel in exchange for putting a mark upon his forehead. The Kingdom of the Wilderwest was a place of equality between humans and dragons even though each new king treated dragons worse than the last. This culminated in the reign of Grimbeard the Ghastly who enslaved both humans and dragons to build the city on Tomorrow.  
Around 300 years prior to the events of How to Train Your Dragon, a Hairy Hooligan Tribe arrived on the Isle of Berk. The Vikings were almost immediately bedeviled by dragons. The Vikings decided to fight back, and a full out war started that lasted for generations.