Miocene
All flesh is grass (Hebrew: כָּל־הַבָּשָׂ֣ר חָצִ֔יר kol habbasar chatsir) is a phrase from the Old Testament book of Isaiah, chapter 40, verses 6–8. It applies from the last part of the Miocene to the present day, at any rate so far as land animals are concerned. The steppes of Asia, the mid-West of the USA and the grasslands of Africa all offer the same lesson: that when the temperature and rainfall dropped, most of the forests changed to grassland. Atmospheric carbon dioxide is the other factor which research looks at.