The family Varanidae probably originated in Asia at least 65 million years ago, although some estimates are as early as the late Mesozoic (112 million years ago).
Monitor lizards probably expanded their geographic range into Africa between 49 and 33 million years ago, possibly via Iran, and to Australia and the Indonesian archipelago between 39 and 26 million years ago.

Varanids last shared a common ancestor with their closest living relatives, earless "monitors", during the Late Cretaceous.

During the Late Cretaceous era, monitor lizards or their close relatives are believed to have evolved into amphibious and then fully marine forms, the mosasaurs, some of which reached lengths of 12 m (39 ft) or more.

Snakes were believed to be more closely related to monitor lizards than any other type of extant reptile; however,  snakes have been more recently proposed to be the sister group of the clade of iguanians and anguimorphs.
Like snakes, monitor lizards have forked tongues, which they use to sense odors.

The genus Varanus first emerged in Laurasia.
During the late Oligocene to early Miocene, the group had dispersed to Australia, New Guinea, and the Solomon Islands three separate times.
By the late Miocene, the genus was also present in Africa, Arabia, Asia, and Eastern Europe.

By the Pleistocene epoch, giant monitor lizards lived in Southeast Asia and Australasia, the best known fossil being the megalania (Varanus priscus, a giant goanna formally known as Megalania prisca).
This species is an iconic member of the Pleistocene megafauna of Australia, thought to have survived until around 50,000 years ago.

Many of the species within the various subgenera also form species complexes with each other:
V. indicus species complex (V. indicus, V. cerambonensis, V. caerulivirens, V. colei, V. obor, V. lirugensis, V. rainerguentheri, V. zugorum) 
V. doreanus species complex (V. doreanus, V. finschi, V. semotus, V. yuwonoi)
V. gouldii species complex (V. gouldii, V. rosenbergi, V. panoptes)
V. bengalensis species complex (V. bengalensis, V. nebulosus)
V. acanthurus species complex (V. acanthurus, V. baritji, V. primordius, V. storri)
V. exanthematicus species complex (V. exanthematicus, V. albigularis, V. yemenensis)
V. timorensis species complex (V. timorensis, V. auffenbergi, V. scalaris, V. similis, V. tristis)
V. niloticus species complex (V. niloticus, V. stellatus)
V. salvator species complex (V. salvator, V. cumingi, V. nuchalis, V. togianus, V. marmoratus)
The tree monitors of the V. prasinus species complex (V. prasinus, V. beccarii, V. boehmei, V. bogerti, V. keithhornei, V. kordensis, V. macraei, V. reisingeri, V. telenesetes) were once in the subgenus Euprepriosaurus, but as of 2016, form their own subgenus Hapturosaurus.

V. jobiensis was once considered to be a member of the V. indicus species complex, but is now considered to represent its own species complex.
The family Varanidae probably originated in Asia at least 65 million years ago, although some estimates are as early as the late Mesozoic (112 million years ago). Monitor lizards probably expanded their geographic range into Africa between 49 and 33 million years ago, possibly via Iran, and to Australia and the Indonesian archipelago between 39 and 26 million years ago.
Varanids last shared a common ancestor with their closest living relatives, earless "monitors", during the Late Cretaceous.
During the Late Cretaceous era, monitor lizards or their close relatives are believed to have evolved into amphibious and then fully marine forms, the mosasaurs, some of which reached lengths of 12 m (39 ft) or more.
Snakes were believed to be more closely related to monitor lizards than any other type of extant reptile; however,  snakes have been more recently proposed to be the sister group of the clade of iguanians and anguimorphs. Like snakes, monitor lizards have forked tongues, which they use to sense odors.
The genus Varanus first emerged in Laurasia. During the late Oligocene to early Miocene, the group had dispersed to Australia, New Guinea, and the Solomon Islands three separate times. By the late Miocene, the genus was also present in Africa, Arabia, Asia, and Eastern Europe. 
By the Pleistocene epoch, giant monitor lizards lived in Southeast Asia and Australasia, the best known fossil being the megalania (Varanus priscus, a giant goanna formally known as Megalania prisca). This species is an iconic member of the Pleistocene megafauna of Australia, thought to have survived until around 50,000 years ago.
Many of the species within the various subgenera also form species complexes with each other:
V. indicus species complex (V. indicus, V. cerambonensis, V. caerulivirens, V. colei, V. obor, V. lirugensis, V. rainerguentheri, V. zugorum) 
V. doreanus species complex (V. doreanus, V. finschi, V. semotus, V. yuwonoi)
V. gouldii species complex (V. gouldii, V. rosenbergi, V. panoptes)
V. bengalensis species complex (V. bengalensis, V. nebulosus)
V. acanthurus species complex (V. acanthurus, V. baritji, V. primordius, V. storri)
V. exanthematicus species complex (V. exanthematicus, V. albigularis, V. yemenensis)
V. timorensis species complex (V. timorensis, V. auffenbergi, V. scalaris, V. similis, V. tristis)
V. niloticus species complex (V. niloticus, V. stellatus)
V. salvator species complex (V. salvator, V. cumingi, V. nuchalis, V. togianus, V. marmoratus)
The tree monitors of the V. prasinus species complex (V. prasinus, V. beccarii, V. boehmei, V. bogerti, V. keithhornei, V. kordensis, V. macraei, V. reisingeri, V. telenesetes) were once in the subgenus Euprepriosaurus, but as of 2016, form their own subgenus Hapturosaurus. 
V. jobiensis was once considered to be a member of the V. indicus species complex, but is now considered to represent its own species complex.
Monitor lizard