In the first Krita Yuga, after the creation of the earth, Brahman created a thousand pairs of twins from his mouth, breast, thighs, and feet respectively. They lived without houses; all desires which they conceived were directly fulfilled; and the earth produced of itself delicious food for them, since animals and plants were not yet in existence. Each pair of twins brought forth at the end of their life a pair exactly like them. As everybody did his duty and nothing else, there was no distinction between good and bad acts.
After the Krita or Satya Yuga, things get progressively worse: each successive yuga sees the human race falling into increasing unhappiness and evil, until at the end of the Kali Yuga, the world is set on fire, deluged with water, and then reborn. [Joscelyn Godwin, 1996]