There are roughly seventy-one mahayugas in a manvantara. A mahayuga lasts for twelve thousand years of the gods and consists of four different eras or yugas – satya yuga or krita yuga, treta yuga, dvapara yuga, and kali yuga. Satya yuga lasts for four thousand years of the gods treta yuga, dvapara yuga, and kali yuga. Satya yuga lasts for four thousand years of the gods; treta yuga for three thousand; dvapara yuga for two thousand; and kali yuga for one thousand. This adds up to ten thousand years in a mahayuga. Another two thousand years are to be added for intervening periods (sandhymasha) between two yugas. Hence the total of twelve thousand years in a mahayuga.
In satya yuga, winter, summer and monsoon were unknown. All individuals were equally handsome, equally prosperous and equally happy. There was no concept of dharma (righteousness) or adharma (evil) since people were naturally righteous. There were no prescribed places where people lived, there were no cities or villages. People lived freely on the shores of the oceans and in the mountains. Roots, fruits or flowers did not grow. People lived on juice that came out of the bowels of the earth. This juice was so miraculous that old age and disease were unknown. Hatred and envy did not exit. There was nothing to be envious about, since all individuals were equal. Moreover, people had the mental power to summon up whatever object they desired. There were no shortages.
As one moved from satya yuga to treta yuga, these mental powers that people possessed disappeared. The primary obsession of individuals in satya yuga was meditation; in treta yuga it became knowledge. Thick clouds appeared in the sky and it began to rain. This rain fostered the growth of various trees. People started to live on the fruits of these trees. The trees also provided barks for clothing and honey. These trees were so wonderful that they were known as kalpavrikshas. That is, they provided whatever it was that one desired. But as treta yuga progressed, people became evil and started to fight over the possession of these trees. Consequently, the trees began to wither away. It was then that habitations started to be built, earlier there had been no fixed dwelling-places for humans. Such habitations were built on mountain and near rivers. Villages and cities were constructed. It also became necessary to have units of measurement.
The smallest unit of measurement was an anguli (a finger). Ten angulis made one pradesha. This signified the length from the thumb to the extended index finger. The distance from the thumb to the extended middle finger was tata; from the thumb to the extended ring gokarna; and fromt eh thumb to the extended little finger vitasti. Thus, twelve angulis were equivalent to one vitasti. Twenty-one angulis made one ratni and twenty-four angulis made one hasta (hand). Forty-two angulis were called one kishku. Four hastas made one dhanu (bow-length); two thousand dhanus were one gavyuti; and four gavyutis constituted one yojana. Thus, one yojana was made up of eight thousand dhanus. The yojana was the basic unit for measuring the sizes of human habitations.
The houses that men built were known as shalas. This is because they were modelled on trees. The branches of a tree (shakha) spread out in all directions. Since the houses spread out in this fashion, they were called shalas. The palaces were called prasada, the word signifying that these dwellings pleased the mind. The kalpavrikshas having disappeared, people had to look for means of sustenance. This they found in trees and herbs. In fact, the discovery of herbs goes back to treta yuga.
But people became evil. They fought over the possessions of rivers, land, mountains, trees and herbs. Might became right and those who were strong managed to establish property rights. The weak suffered. The result of all this fighting was that the trees and the herbs disappeared and foodgrains grew no longer. The entire earth was swallowed up by thick forests. Suffering from hunger and thirst, people went to Brahma in search of a solution.
Brahma milked the earth so that trees, herbs and foodgrains might grow afresh. To make sure that people did not fight again, Brahma laid down the precepts of righteous conduct. This was encapsulated in the principle of varnashrama dharma – the system of four varnas (classes) and four ashramas (stages of life). The principle of varnashrama dharma thus dates back to treta yuga and was enuciated by Brahma himself.
People are divided into four varnas. The first class is that of brahmanas. The brahmanas are those who are knowledgeable in the true nature of the brahman. Their primary occupations are performing sacrifices, studying and receiving alms. The second class consists of kshatriyas. Their primary duties are to bear arms so that they can protect the good and punish the evil. They must also rule and fight. The third class consists of vaishyas. The primary duties of vaishyas are agriculture, animal husbandry and trade. The fourth and final class is that of shudras. The shudras are to serve the other three classes and act as artisans. They are not permitted to study, perform sacrifices or donate alms. These are only permitted for the first three classes. A brahmana who performs his duties well gets to live Brahma; such a kshatriya gets to live with Indra; such a vaishya with the gods, the maruts; and such a shudra with the gandharvas (singers of heaven).
Brahma also laid down the system of the four ashramas. The first one is brahmacharya (celibate studenthood); the second garhasthya (householder stage); the third vanaprastha (forest-dwelling stage; and the fourth sannyasa (hermithood). In brahmacharya stage, one has to study and serve one‘s teacher (guru) well. After this period is over, one is married and enters the householder stage. A householder has to perform sacrifices and serve his guests.
Garhasthya is superior to the other three ashramas, since it is on the alms provided by householders that individuals in the other three stages of life subsist. When the householder stage is over and one has had sons, one enters vanaprastha. Such a person retires to the forest and lives on fruits and roots. It is only when the mind is completely detached from material pursuits that one moves on the the final stage of sannyasa.