Life was tough enough for Australia's first human inhabitants during the last ice age, but they had the added danger of having to survive alongside giant killer lizards, researchers say.

A fossil unearthed in Queensland of a giant apex predator lizard has been dated to 50,000 years ago, making it the youngest record of a giant lizard ever found on the entire continent, they say.

That puts it on the ground alongside Australia's first Aboriginal inhabitants, they explain, since previous discoveries of human remains in New South Wales are evidence of human presence in Australia between 50,000 and 46,000 years ago.

University of Queensland vertebrate paleontologist Gilbert Price says the researchers' "jaws dropped" when they unearthed a tiny fossil bone belonging to a giant carnivorous lizard.

"The find is pretty significant, especially for the timeframe that it dates," he says.

"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 kg (110 lbs) and grew up to six meters (19 feet) long," he says.

The extinct Megalanias could have preyed on large mammals, reptiles and birds, which would have made them food competitors for Australia's earliest humans.

Both giant lizards and 25-foot-long crocodiles lived in Australia during the last ice age, and some have argued their disappearance was due to the arrival of the first humans on the continent, while others put it down to other factors.

"It's been long-debated whether or not humans or climate change knocked off the giant lizards, alongside the rest of the megafauna," Price explained. "Humans can only now be considered as potential drivers of their extinction."

The largest lizard found in Australia today is known as the perentie and can grow to six feet long.

The new fossil, described by the researchers in the journal Quaternary Science Reviews, is at least 30,000 years younger than any other giant lizard fossils found in Australia to date.

It was discovered with the help of citizen scientists taking part in the sieving and sorting specimens from a site known as the Capricorn Caves in northeastern Australia, the continent's most fossil-rich location, the researchers say.

Millions of bones of a wide variety of ancient species have been found there, they point out.