This isn't going to be a thorough historical account, but I wanted to share a couple interesting reads about the origins of the Protectors.
It is not necessary to have a Highest Yoga Tantra initiation to rely upon Protectors in general, and there are even brief sadhanas for this, but to perform the tantric rituals associated with many of their sadhanas it is. There are also two different methods for relying on Protectors which I've written about:
http://www.dorjeshugden.com/forum/index.php?topic=5443.msg59764#msg59764There is a link to Alex Berzin's site in that post but I will post it again here for immediate connect. I also came across this wonderful bit:
Choje Lama Namse Rinpoche (Kagyu Master) explained:
"The Dharmapala tradition as we know it arose in India during the 6th, 7th, and 8th centuries. The most popular Dharmapala practice that spread within the context of Buddhism was the four-armed Mahakala, which originated in a vision that Tilopa had while practicing meditation intensively and for quite a while before he accepted any pupils.
Residing in utter solitude near Somapuri, the site of one of the great Indian monastic universities, Tilopa fervently meditated the Chakrasamvara Tantra for twelve years. During this time, Chakrasamvara often manifested to him - it is said “face-to-face.” It was during the most advanced stage in Tilopa’s practice that immense hindrances arose and subtlest clouds of obscurations had to be removed. Therefore Chakrasamvara manifested from his heart the four-armed Mahakala, who taught Tilopa supplication and offering prayers, syllables, and mantras. Tilopa wrote down these instructions and transmitted them to most worthy disciples. This was the beginning of Dharmapala practice in Vajrayana. The Dharmapala tradition as we know it arose in India during the 6th, 7th, and 8th centuries. The most popular Dharmapala practice that spread within the context of Buddhism was the four-armed Mahakala, which originated in a vision that Tilopa had while practicing meditation intensively and for quite a while before he accepted any pupils.
Residing in utter solitude near Somapuri, the site of one of the great Indian monastic universities, Tilopa fervently meditated the Chakrasamvara Tantra for twelve years. During this time, Chakrasamvara often manifested to him - it is said “face-to-face.” It was during the most advanced stage in Tilopa’s practice that immense hindrances arose and subtlest clouds of obscurations had to be removed. Therefore Chakrasamvara manifested from his heart the four-armed Mahakala, who taught Tilopa supplication and offering prayers, syllables, and mantras. Tilopa wrote down these instructions and transmitted them to most worthy disciples. This was the beginning of Dharmapala practice in Vajrayana."
Alex Berzin also has a great explanation here:
http://www.berzinarchives.com/web/en/archives/advanced/tantra/level1_getting_started/what_dharma_protectors_ab.html