The following is an excerpt from the teachings of Sri Ramana Maharshi.
D.: How shall I reach the Self?
M.: There is no reaching the Self. If the Self were to be reached, it would mean that the Self is not now and here, but that it should be got anew. What is got afresh, will also be lost. So it will be impermanent. What is not permanent is not worth striving for. So I say, the Self is not reached. You are the Self. You are already That.
The fact is that you are ignorant of your blissful state. Ignorance supervenes and draws a veil over the pure Bliss. Attempts are directed only to remove this ignorance. This ignorance consists in wrong knowledge. The wrong knowledge consists in the false identification of the Self with the body, the mind, etc. This false identity must go and there remains the Self.
A woman, with her necklace round her neck, imagines that it has been lost and goes about searching for it, until she is reminded of it by a friend; she has created her own sense of loss, her own anxiety of search and then her own pleasure of recovery.
Similarly the Self is all along there, whether you search for it or not. Again just as the woman feels as if the lost necklace has been regained, so also the removal of ignorance and the cessation of false identification reveal the Self which is always present - here and now.
This is called realisation. It is not new. It amounts to elimination of ignorance and nothing more.
Rig Veda X.72.3 says as follows:
Existence, in the earliest age of Gods, from Non-existence sprang. Thereafter were the regions born. This sprang from the Productive Power.
Translation of H.H. Wilson (p.130) is also on similar lines.
We have to remember that even the Rig Veda, which is called apaUrushEya and composed by Sages, is silent on how the Existence, came into being.
Then how can we can to know the root of mysterious thing called Body consciousness?
In my humble opinion, it is next to impossible to know this.