Riversleigh and Naracoorte were chosen for having many fossils of many different species of extinct animals.They are listed among the ten richest fossil deposits in the world.
Homo erectus was known to be in what is now Beijing (close to Japan) 850,000 years ago so potentially it could be hundreds of thousands of years.
The delay could be attributed to the difficulties in adapting Chinese rice varieties to Japanese conditions.
(Rice was not suited to the northern island of Hokkaido.
As a result, the people of Hokkaido remained hunter gatherers until the early 20th century.) Once agriculture developed, the social structure of Japan changed.
Instead of being egalitarian with tribal elders, it became hierarchical with warlords and emperors.