Lucy had been officially scheduled to go on tour some time this month. But the quiet nature of her first departure from Ethiopia in more than three decades surprised locals.

"A fossil like Lucy is so rare and so fragile that it should only be moved out of the country if there is a compelling reason that would benefit the nation in a unique way," Zeray Alemseged, a well-known Ethiopian palaeontologist, told Reuters.

He discovered the remains of another 3 million-year-old girl known as "Selam" in 2000 in the remote Afar region, where Lucy was discovered in 1974 by US scientist Donald Johanson.

"The question that should be asked is whether the amount of money that will be earned from this trip is worth the risk we are taking," he said.

Lucy's fossilised skeleton is thought to be between 3 million and 4 million years old and was hailed by experts as the earliest hominid ever discovered.

Scientists say the almost complete skeleton is a landmark in the search for the origins of humanity.

But few Ethiopians have glimpsed the treasure.

"Why should Lucy be seen by Americans when Ethiopians have only seen her twice?" said 34-year-old administrator Dilu Kebede in Addis Ababa.

The "Lucy" normally on display at the Ethiopian National Museum is a replica as the real remains have been locked away.

The Smithsonian Institution and Ethiopian community groups in the United States have objected to the tour on the grounds that fragile fossils such as Lucy should not travel from their countries of origin unless for significant scientific reasons.

"I worry that it may break apart," said Yoseph Tensai, a 46-year-old shop owner. "But I think it is good for Ethiopia to show our treasure to the world."

Ethiopian government officials were unavailable to comment on Lucy's tour on Tuesday.