When we reflect on the long ages of geological time and the development of life on earth, we can appreciate the unimaginably long process, spanning hundreds of thousands or even millions of years, in the development of one stage of consciousness from the prior basis. Life did not suddenly arise on earth in all its forms, but went through a process of increasing complexity and awareness. This occurred on the basis of Matter, which itself required many millennia to be prepared to harbor and hold the life-energy. The evolution of the mental consciousness also required many millennia to move from the rudiments we find in the lower animal kingdom to the more evolved forms we find eventually reaching a current peak in certain parts of homo sapiens, the current iteration of the human being.
So we can expect that the process of Nature in bringing forth a new and more concentrated and complex form of consciousness, the supramental consciousness, would require long ages to prepare the basis of Matter, Life and Mind, and then begin to influence and finally fully emerge in its own right as a new active consciousness in the world.
The difference here is that where Matter and various forms of Life were acted upon without conscious participation, the developed human individual, self-aware and aware of the process to be undertaken, is able to actively take steps to prepare the being, and to call in the action of the higher consciousness, and this process, called Yoga, can lead to the compression of the time required for this step to occur.
Sri Aurobindo observes in The Life Divine: “The spiritual evolution obeys the logic of a successive unfolding; it can take a new decisive main step only when the previous main step has been sufficiently conquered: even if certain minor stages can be swallowed up or leaped over by a rapid and brusque ascension, the consciousness has to turn back to assure itself that the ground passed over is securely annexed to the new condition. It is true that the conquest of the Spirit supposes the execution in one life or a few lives of a process that in the ordinary course of Nature would involve a slow and uncertain procedure of centuries or even of millenniums: but this is a question of the speed with which the steps are traversed; a greater or concentrated speed does not eliminate the steps themselves or the necessity of their successive surmounting. The increased rapidity is possible only because the conscious participation of the inner being is there and the power of the Supernature is already at work in the half-transformed lower nature, so that the steps which would otherwise have had to be taken tentatively in the night of Inconscience or Ignorance can now be taken in an increasing light and power of Knowledge.”
Sri Aurobindo, The Future Evolution of Man, Chapter Seven, The Ascent Towards Supermind, pp. 84-85
Sri Aurobindo Studies 