There are no marsupials native to Europe, Asia or Africa. Since this occurred in the Tertiary period, before the rise of placental mammals, the Australian marsupials never had to compete with placentals and flourished while their relatives in other areas of the world became extinct or, at best, existed in restricted forms and numbers. Marsupials in Australia expanded to fill all the ecological niches occupied by placental land animals elsewhere. There are grazers, browsers, carnivores and insectivores. The koalas remind us of bears and other marsupials are totally unique.
Some forms are arboreal, some semi-aquatic and some are terrestrial.
Many Australian species have become extinct or endangered since the beginning of settlement of that continent by Europeans, through both hunting by the settlers and competition with placental mammals these people introduced. Marsupials have much less complex brains than other mammals and their brain is smaller in proportion to their body. Therefore, they are not considered very intelligent and this is probably one of the reasons they have not competed well with placental mammals. Associated with the simpler brain of marsupials is their limited vocal ability. Though not completely silent, they do not make noises as often nor in the variety exhibited by placentals.