Jhanas and past lives

I always thought that the remembrance of the past lives and the vision of the mechanism of the kamma were the fruits of the jhanas. BUT, it is said that the Tathagata learnt the (8) jhanas from two teachers. it is also said that he remembered his past lives during the famous seven days, after he decided to gain what he would call nibbana. Do we know whether the Tathagata had access to his previous lives or to the karmic mechanism before the week of accessing nibbana ? if no, does this mean that only the dhamma --- instead of the practice from hinduism, jainism and perhaps others doctrines --- permits to see the past lives, or the karmic mechanism ?