History
When Odin and his brothers killed the giant Ymir, his blue blood rushed forth and formed the seas, flooding the land of the Giants. Odin and the Aesir believed the race of Giants to be extinct, but the giant Bergelmir and his wife escaped in a small boat. Thanks to this, the giant species lived on.[1]
Notes
In mythology, Bergelmir was a direct descendant of Ymir's, being his grandson. Their exact relationship in Marvel Comics was not confirmed.