Simplify the following text for a Grade 11 native English-speaking person.
Original: The dates derived suggest the main mechanism for extinction was human burning of a landscape that was then much less fire-adapted ; analysis of oxygen and carbon isotopes from teeth of megafauna indicate sudden , drastic , non-climate-related changes in vegetation and the diet of surviving marsupial species , as well as the loss of megafaunal species .
Simplified: Scientists have studied oxygen and carbon isotopes from teeth of megafauna . These show sudden , drastic , non-climate-related changes in plants , and in the diet of surviving marsupial animals , as well as the loss of megafaunal species .