Study the image about geologic time. a geochronological scale measured in millions of years ago or m y a. cenozoic era: paleogene period, 66 to 23 m y a, diatrima, many types of mammals, smilodon. neogene period, 23 to 2.6 m y a, smilodon, mammoth. quaternary period, 2.6 to today, human, mammoth. during which period did humans first appear on earth? quaternary neogene paleogene cenozoic