Eucalypts began between 35 and 50 million years ago , not long after Australia and New Guinea separated from Gondwana . Their coming coincides with an increase in fossil charcoal deposits ( which might mean that fire was important to them even then ) , but they remained a minor component of the rainforest until about 20 million years ago , when the gradual drying of the continent and lessening of soil richness led to the growth of a more open forest type , with mainly Casuarina and Acacia trees . Eucalypts originated between 35 and 50 million years ago , not long after Australia-New Guinea separated from Gondwana , their rise coinciding with an increase in fossil charcoal deposits ( suggesting that fire was a factor even then ) , but they remained a minor component of the Tertiary rainforest until about 20 million years ago , when the gradual drying of the continent and depletion of soil nutrients led to the development of a more open forest type , predominantly Casuarina and Acacia species .