in beginning only aquatics

The highest perfection of the evolutionary process, as described by the Vaisnava poet Jayadeva Gosvami, is given herein. Pralaya-payodhi jale dhrtavan asi vedam. The evolutionary process begins from the point of devastation (pralaya) when the whole universe is filled with water. At that time there are many fishes and other aquatics, and from these aquatics evolve creepers, trees, etc. From these, insects and reptiles evolve, and from them birds, beasts, and then human beings. Finally civilized human beings evolve, and at present these civilized human beings are at a junction where they can make further evolutionary progress in spiritual life. Here it is stated (sva-dharma-nisthah) that when a living entity comes to a civilized form of life, there must be sva-dharma, social divisions according to one's work and qualifications. This is indicated in Bhagavad-gita: (More...)

Regarding the question you have asked, you can give a very straight answer. Bhagavad-gita says, "dehantara praptih [Bg 2.13]''. The soul transmigrates, but there is no regular process. In the beginning of creation, when all the world was water, all living entities were aquatics. Then gradually from aquatics the soul passes into plant bodies, then insect bodies, then bird bodies, then animal bodies, then human beings. In the human form the soul can decide whether to go back to home, back to Godhead or return to birth and death, accepting material bodies according to his karma. Therefore, it is advised to take advantage of the human form of body to go back home, back to Godhead. Our movement is for this purpose, to offer all human beings a chance to go back home, back to Godhead. (More...)