There are 4 yugas, Satya Yuga, Treta Yuga, Dwapar Yuga and Kaliyuga. Shri Hari Vishnu Took different avatar in different yugas to save dharma, satya.
You may be wondering, what was the height of humans in saty yuga. In this video i told you about, life span of humans in different yugas, Height of humans in different yugas. and also Environment on earth in Different Yugas.