How are Yugas related to Earth and humanity?

Are Yugas based on time frames in relation to Earth and humanity? Or are they reflective of spiritual phases?

[Mr. Talin Rowe asked: Blessed Datta Swami, please help with clarification on the concept of Yugas. Are Yugas a historical time keeping tool, or is there a greater meaning behind them that is not apparent? Is it important to know the different Yugas other than as exemplary materials, and as a means to keep Your devotees in our collective memories? Thank You, and salutations on You, Talin Rowe]

Swami replied:- From the angle of an individual soul, the Yugas are based on the progress and fall of spiritual knowledge of the soul. From the angle of the collective society, the Yugas are based on time frames in relation to the earth. Both fixation and flexibility run side by side in this concept.