Shiva means 'the comfort giver' - and there is no greater 'comfort giver' to prehistoric man in the wilds than fire. The hearth that nurtured the fire became associated with the female 'Yoni' (vagina) of Shiva's consort - 'Shakti'. The combination of the male-female union 'Shiv Shakti' is a primordial symbol of creation within Indian culture and tradition.

The term and concept of 'Shiv-Shakti' is well established in Sikh thought. For example, Akali Guru Nanak explains that a man caught in illusion “makes his home in the house of Shiv-Shakti [the world].”
(“Raag Maru, M1”, Adi Guru Durbar, Pa.1026.)

Also see 'Mahakal', 'Shiva'.