Okay, strap in, everyone - today we're answering another reader-submitted question, and it's a meaty one! Our question reads: I love it when you talked about Hinduism's Kali Yuga on your old blog. Any chance of getting some background details on the other three eras and how they affect the pantheon?
I think we're going to start at the beginning for this one, and talk about it in really basic and simple terms, because this question is really looking for all the massive hugeness of Hindu cosmology, which is a subject you could write many books about without exhausting its possibilities. But we'll give it a quick shot!
In very basic terms, according to Hinduism, universal time proceeds forward in a cycle of four stages, which repeat multiple times over and over until the the world is reborn. Brahma, the creator god, accomplishes this new creation repeatedly, at a length of time variable depending on the source but which is always on the order of several billion years; this vast span of time is a single day and night for Brahma, who being the eternal creator does not need to operate on the same timescale as mere mortals. In some versions of Hindu cosmology, Brahma is eternally self-creating; in other versions he must first emanate from Vishnu, and thereafter create everything else.
However the ages are created, the billions of years that occur from beginning of the world to ending contain myriad repetitions of the yuga cycle, in which there are four parts: the Krita or Satya Yuga (the Age of Truth), the Treta Yuga (the Age of Three), the Dvapara Yuga (the Age of Two) and the Kali Yuga (the Age of Strife). The Treta and Dvapara Yuga look kind of like they're out of order to those of us who don't speak Sanskrit, but three and two respectively refer here to specific results in the ancient traditional dice game often played in the Veda, not to their order in the list of ages, and also refer to the legs of the great bull representing Dharma, which it slowly loses as the ages progress.
The Krita Yuga, more commonly referred to as the Satya Yuga, is the first of all the ages of the yuga cycle and lasts for the longest, although the exact period of its length (and all the yuga lengths) depends on the source. During the Satya Yuga, all humans live to be staggeringly old (depending on the source, an average of 400, 4000 or 10,000 years old), and disease, poverty, hatred and evil do not exist; everyone gets whatever they need simply by desiring it, and people are mostly concerned with sacred knowledge, meditation and bettering themselves through spirituality, leaving them free of lies and confusion. There is no need for the violent and dangerous incarnations of the gods, and various art forms, especially dance, do not exist because everyone is already sublimely happy and in no need of entertainment or education through such things. Food is grown effortlessly and in all seasons, rulers are responsible and protective of their people, all traders are fair and charitable, and humanity always takes care of one another as needed.
As far as the gods go, it is said that even they lay aside their differences, living peacefully with one another and the asura alike. The oldest Hindu myths in which the asura and deva live alongside one another, are worshiped equally by humanity and cooperate with one another (including their famous cooperative effort to churn the Ocean of Milk together) are often said to have taken place during the Satya Yuga, before the advent of the Treta Yuga and the beginning of widespread discord in the universe.
The Treta Yuga is ushered in by the beginning of evil; some mortals are born evil now, and begin to take wicked actions and threaten the peace. Where originally all people everywhere were devout and dedicated to proper worship, now only about three-quarters of them are, and the asura have begun to wreak evil upon the world and directly oppose the gods. Human lifespans shrink (to 300, 3000 or 10,000) and humans also shrink in physical size from their earlier larger stature, becoming smaller and less glorious. The emergence of all these negative and dangerous possibilities make life more difficult, and humanity begins to be forced to create settlements and farm in earnest in order to ensure their survival. Because religion is no longer instinctive and respected by all, ritual becomes important, and the religious conventions of ritual flame, dance and sacrifice are instituted to help humanity on their path toward enlightenment. The increased danger of the world in this age requires protectors, so Brahma also creates the Kshatriya, the warrior caste, to defend the Brahmins and keep order.
The Treta Yuga is sometimes called the Golden Age instead of the Satya Yuga thanks to Vishnu's activities on the earth; it's during this time that he appears as the Avatars Vamana, the dwarf who prevents the asura from overcoming Indra, Parashurama, the sage who cleared the world of its excess warriors that were causing so much damage, and Rama, the mighty warrior king and major character of the famous Ramayana.
Third comes the Dvapara Yuga, in which the world has begun to turn more greatly toward evil; where once there was no strife or wickedness in the Satya Yuga, now there is an even half-and-half split between the powers of good and the new and growing powers of evil and unrighteousness. Humanity's lifespan and size shrink again, and their new inability to tell right from wrong or truth from untruth causes disease and misfortune to become widespread. In response, they begin to work harder at religion, categorizing the Veda and performing purifying rituals to try to achieve their spiritual goals, but this is harder for them to accomplish than in the previous yuga, and only about half of humanity actively practices religion as they should anymore. The other castes are also created by Brahma at this time so that they can give more order to an increasingly chaotic society, and success at the duties prescribed for a person by their caste becomes important to humans, who seek comfort, pleasure and contentment by fulfilling them.
It is during the Dvapara Yuga that Vishnu's other most famous incarnation, Krishna, was active, and the age ended when he was shot and killed by the hunter Jara and subsequently left the earth and returned to the abode of the gods, thus finally depriving the world of one of its last great champions. His departure marks the beginning of the Kali Yuga, which is the beginning of the end.
The Kali Yuga is the final age of the world and the shortest, when the world is sinking into irredeemable sin and confusion and must be destroyed to begin the cycle anew. In this final age, evil far outweighs the presence of virtue and righteousness, and humanity becomes short-lived, short-statured and short-tempered. Rulers become despotic and abuse their people rather than performing their responsibilities to them, murder, rage and terrible crimes become commonplace as people forget their morals, and food becomes rare, forcing people to become refugees traveling in search of somewhere to live. The castes cease to perform their functions and will refuse to be virtuous, while all over the world religion becomes mocked and disregarded and indulgence in excessive pleasure in the form of drugs and wanton sexuality becomes commonplace. By the end of the Kali Yuga, it is said that all observance of sacrifice and reverence to the gods will finally cease, which will be the moment that Vishnu's final avatar, Kalki, will appear to restart the cycle; Kalki will reorganize the castes, bring the veda back into prominence and destroy the evil that plagues the world, after which he will be able to usher in the beginning of a new Satya Yuga (although depending upon your branch of Hinduism, some believe that instead the world must swing back through Dvapara Yuga and Treta Yuga before it can reach Satya Yuga again).
The "Kali" in the Kali Yuga does not refer to the goddess, but rather to an asura by the same name, the personification of the decay of time, who is Kalki's mortal enemy and who works ceaselessly in an attempt to cause the world to fall into such utter ruin that it can't be returned to the cycle of the yuga and instead simply dies. Although Kali is always defeated by Kalki in each Kali Yuga, he never dies, and instead simply rises again the next time that age comes around to continue his work.
Although there is some philosophical disagreement, for the most part interpreters of the Hindu scriptures agree that the world is currently in the Kali Yuga, although exactly where in it is up for debate and how soon Kalki might return is a subject of constant theorization and mathematical calculation. The general moral decay of humanity in comparison to the golden age of the Satya Yuga and the decline of traditional caste roles and religious practices are seen as general proof that the world is in the final slide toward the inevitable end of the age, but dedication to the veda and the gods still remains, so the end cannot be too near yet.
For the gods, the different yuga require different things from them in order to attempt to guide the world and perform their sacred functions, and affect the reverence and sacrifice due them from humanity, but they have presumably seen all this before; it is after all only a single day of Brahma's life, and a pageant that the gods have played out more than once in their eternal past.
Those heroes on the ground, however, may find the challenges of the yuga a lot to handle!