"It was never really intended as a place for people.
No ape ever walked upright in Australia's thickets and savannas, no tarsier or lemur chattered in its trees, no half-man in the first light of humanity's awakening crouched watchful in his middens. The great land slept on its southern seas, while on all the other contients the human race developed and spread and made its artifacts and began its cermonies. Bone had been carved and needles made and the cave walls of lascaux and altamira magically painted before any man ever trod the earth of the southern continent."
"Roughly three-quarters of the surface of the Australian contient is comprised of outcrops of Pre-Cambrian rock, most of the worn down by wind, dust,frost, rain and time into a vast, low-lying plateau. Some scientists believe this is the largest and most intact great fragment of the ancient primal continent of Gondwnanaland, which once linked what are now India, Africa, South America, Australia and Anarctica. This residue of the lost time from the earth's birth labours has been reasonbly stable for about 500 million years. Parts of it - the 280,000 square miles of the barren wasteland in Western Australia, which geologists have named Yilgarn, the eroded domes which form the Musgrave Ranges and the bizarre monoliths of Ayers Rock and the Olgas in the Northern Territory - go back perhaps 1,000 or 2,000 million years. It is enough to say that few parts of the earth's surface are known to be older than this."
Goodman and Johnson, The Australians, The Griffin Press (1966)
Note: The Cambrian period is the earliest period in which there was an expectation of life on the planet and goes back about 600 million years. Anything before that is the Pre-Cambrian period.
Without primates or other major mamallian predators, the land was ripe for the development of reptiles. The reptiles proved amazingly adaptable, and prospered. Once you get inland from the coast, you are in the outback. Since lizards generally prove to be more adaptable to hot, hostile envrionments than other reptiles, it is not surprising to find the vast majority of reptile species in Oz to be lizards.