There are over 700 species of Eucalyptus, in this image you can see the gum nuts and contained fruit, the leaves are waxy or glossy green. An essential oil extracted from Eucalyptus leaves contains compounds that are powerful natural disinfectants and can be toxic in large quantities. Several marsupial herbivores, notably koalas and some possums, are relatively tolerant of it. On warm days vapourised Eucalyptus oil rises above the bush to create the characteristic distant blue haze of the Australian landscape. 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. With the arrival of the first humans about 50 thousand years ago, fires became much more frequent and the fire-loving eucalypts soon came to account for roughly 70% of Australian forest.