In plain words
In the Kṛta and Tretā Yugas, Dharmamūlam Idaṃ Jagat
Avatāras of this kind have been taking place from time without beginning, from age to age. The children of today may be very surprised. When they hear of the mighty power of the Kṛta Yuga, anyone at all is filled with wonder. There is room for the doubt whether it can be believed. Man in the Kṛta Yuga lived for hundreds of years. Besides, the physical body was not as small as this. They were men whose arms reached to their knees. How did that man of the Kṛta Yuga go on living? On what support did he go on living? In the Kṛta Yuga, even when he had become a skeleton, life stayed as long as there were bones. Reduced to a skeleton, with no muscle and no blood, life still stayed in the bones. In the Tretā Yuga the height kept growing less. The span of life kept growing less. Life stayed as long as there was muscle and flesh in the body. The Dvāpara Yuga came. In the Dvāpara Yuga life lasted while there was blood. It is well known from the Bhārata war! Bhīṣma fell to the ground. He lay on a bed of arrows. He stayed alive until all the blood in his body had flowed out. For 56 days he remained alive. What was the cause of his life? Blood was his very life. Struck by the arrows, the blood went on slowly flowing out. It went on for 56 days. As long as there was blood, there was life. The Kali Yuga began. In the Kali Yuga man lives only as long as there is food. Only when he eats food does he live; if there is no food, he wastes away. So it has come to be said, ‘annagata prāṇa.’ In the Kali Yuga, if men have no food, they waste away day by day. In those days food was not the main thing. In those days they kept their link with God.
In the Dvāpara Yuga, Dhanamūlam Idaṃ Jagat
By the time the Dvāpara Yuga came, they had built up a link with the head. In the Kṛta and Tretā Yugas the link was with God. Understand this well.