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Which group of mammals has the more ‘primitive’ reproductive strategy — marsupials, with their short gestation periods, or humans and other placental mammals, which have long gestation periods? For decades, biologists viewed marsupial reproduction as ‘more primitive.’ But scientists have discovered that a third group of mammals, the long-extinct multituberculates, had a long gestation period like placental mammals. Since multituberculates split off from the rest of the mammalian lineage before placentals and marsupials had even evolved, these findings question the view that marsupials were ‘less advanced’ than their placental cousins.