Succulent plants waited for cool, dry Earth to make their mark

ScienceDaily (2011-05-06) -- Between five and 10 million years ago, the landscape on Earth changed dramatically. Biologists and colleagues have determined that cacti exploded onto the global scene then, about the same geologic time as other succulent plants and tropical grasses. The trigger: A global period marked by cooling and increased aridity, possibly with lowered atmospheric carbon dioxide levels.