In the time of the beginning of the worlds eventually spanned by Yggdrasil the first living creature to emerge was Ymir, a Frost Giant and mother and father to them all as they first came from the sweat of his armpit. One of these Frost Giantesses married Buri, the first man, when he was freed from the ice by the licking of Audhumla, the primeval cow. Then their son Bor married a Frost Giantess called Bestla and they had three children, Odin, Vili, and Ve.
Unfortunately Ymir and many others of his children were less pleasant and eventually Odin, Vili, and Ve took up arms against him. Accounts suggest that when he was slain all the Frost Giants drowned in his blood with the exception of one called Bergelmir and his wife, who floated to safety in a hollowed out tree trunk. Whether this means the three brothers drowned all their relatives except each other and their father and father's father is unclear.
What is clear however is that the Gods and the Giants of those worlds have been intimately connected since the beginning and the descendants of Bergelmir have good reason to resent those of Odin and his ilk.