To residents of Hobart, the capital of Australia’s island state of Tasmania, a grassy flood plain bordering the Jordan River, was nothing special until archaeologists uncovered a treasure-trove: an estimated 3 million artifacts dating back 40,000 years.

The spot had been an important meeting place for Aboriginal tribes, and the tools, stones, and spear tips found there represent the oldest evidence of human habitation in the Southern Hemisphere.

But the Tasmanian government is pressing ahead with plans to build a bridge – part of a new four-lane highway – over the river. The government claims the construction will not threaten the riverbank’s hidden riches. But an Aboriginal leader, Michael Mansell, has labeled the project “cultural vandalism,” saying the white colonial sites like Port Arthur, the former convict settlement, would never be disturbed....