The sciences propagated by the Vedic rishis were given in the form of shrotras in Satyug ...
The sciences propagated by the Vedic rishis were given in the form of shrotras in Satyug, in the form of smritis (memories) in Tretayug, in the form of puranas in Dwaparyug, and finally as tantra and yog in Kalyug. The above presentation of the same knowledge in different forms is indicative of the constantly changing aspect of the manifested creation. As we are witnessing the last phase of Kalyug, so we are experiencing things which have never ever happened before in manifested creation...to a sadhak my point should have been clear by now. For the normal householder I shall elaborate further.