Australia was first settled by between 1,000 and 3,000 humans around 50,000 years ago, but the population crashed during the Ice Age before recovering to a peak of some 1.2 million people around five centuries ago, a study said on Wednesday.

Estimating the early population of Australia is a source of debate in anthropology, partly because it touches on how European colonisation affected the country’s indigenous people.

The newcomers crossed via a land bridge from Papua New Guinea, across a continental shelf that is now flooded, according to a common theory.

The numbers grew but then fell by as much as 60 percent between 21,000-18,000 years ago during the peak of the last Ice Age, when Australia became cooler and far drier than it is today, Williams believes.