Some of Australia's earliest human inhabitants may have had to deal with giant predator lizards that weighed up to half a tonne, Queensland researchers have found.
The University of Queensland researchers have discovered evidence of "giant killer lizards" that walked the earth in Australia about 50,000 years ago. 
UQ vertebrate palaeoecologist Gilbert Price said researchers in central Queensland were shocked to discover evidence that Australia's early inhabitants shared the countryside with the "giant apex predator lizards".
"Our jaws dropped when we found a tiny fossil from a giant lizard during a two metre deep excavation in one of the Capricorn Caves, near Rockhampton," Dr Price said. 
"The one-centimetre bone, an osteoderm, came from under the lizard's skin and is the youngest record of a giant lizard on the entire continent."
Radiocarbon and uranium thorium testing showed the bone was about 50,000 years old, which coincided with the arrival of Australia's first Aboriginal inhabitants.
"We can't tell if the bone is from a Komodo dragon ??? which once roamed Australia ??? or an even bigger species like the extinct Megalania monitor lizard, which weighed about 500 kilograms and grew up to six metres long," Dr Price said.
"The find is pretty significant, especially for the timeframe that it dates."
The largest living lizard in Australia today was the perentie, which could grow up to two metres long. 
Dr Price, from UQ's School of Earth Sciences, said massive lizards and nine-metre long inland crocodiles roamed Australia during the last Ice Age.
"It's been long-debated whether or not humans or climate change knocked off the giant lizards, alongside the rest of the megafauna," he said.  
"Humans can only now be considered as potential drivers of their extinction."
The Capricorn Caves were considered one of Australia's most fossil-rich sites, with millions of bones, many of which were the remains of rodents taken there by owls.
Capricorn Caves manager Ann   Augusteyn said her team had a "huge responsibility" to care for the caves.
"This study also begs the question ??? what else is entombed in our caves and what else can we learn?" she said.