Gigantism and flightlessness in birds are almost exclusively correlated due to islands lacking mammalian or reptilian predators and competition.[9] However, ratites occupy environments that are mostly occupied by a diverse number of mammals.[10] It is thought that they first originated through allopatric speciation caused by breakup of the supercontinent Gondwana.[3] However, later evidence suggests this hypothesis first proposed by Joel Cracraft in 1974 is incorrect.[12] Rather ratites arrived in their respective locations via a flighted ancestor and lost the ability to fly multiple times within the lineage.