David Rains Wallace said this quote

“ Compared to forest or aquatic ecosystems, grassland is unstable. It requires rather precise geological and climatic conditions, and if these conditions are not maintained--if too much rain falls, or too little--it quickly turns into forest or desert, both of which are dominated by woody plants. This instability is reflected in the spectacular but brief careers of various grassland faunas. Humanity, with its dazzling symbioses, preadaptations, and neoteny, is the most spectacular of these--and may well be the briefest. ”