We probably have had millions of cycles, what Im going into is theoretical now, and is based on stuff Ive read. But the difference is all planets go through cycles of evolution. Before the last 75,000 years this planet had existed for approximately 2.4 billion years as a second density planet. Second density life forms are plant/animals, primitive man (dinasaurs etc) life that are striving towards 'self-awareness'. Not yet fully conscious of themselves on an individual level.
75,000 years ago approximately there was an intervening of sorts (genetic manipulation from higher life-forms?) the result being 3rd density life on this planet. Third density life is fully aware of self and is striving towards the infinite. (intelligent infinity). A planets stage in 3rd density is generally brief. It is a stage of new knowledge and of choice.(and often much misunderstanding and sorrow.)
So although there have been likely millions of cycles of destruction there have been only 3 in third density.