The key was a rare metal element known as hafnium, Mojzsis said. It is found with the zircons.

"This is one of the few Earth materials that we know of that is capable of surviving recycling of the crust," Mojzsis said in a telephone interview.

Mojzsis and his colleagues found their clues in sedimentary rocks from the Jack Hills in Western Australia, which date to almost 4.4 billion years ago.

"First we determined how old these little grains were," Mojzsis said. Their composition suggested they were formed at the right temperatures for making crust, and in the presence of water, the researchers report in Friday's issue of the journal Science.

"It was thought that the (early) world looked more like a lunar blasted landscape than a water-filled world with a continental landscape," Mojzsis said.

But he said the finding now showed that within a few hundred million years of its creation, the planet had the three necessary elements for life -- water, energy and organic compounds.

"Now we see that was probably true back to 4.5 billion years ago," Mojzsis said.

"This helps us explain how the Earth became the habitable world it is."

It also might mean that life can occur easily -- if the conditions can be right so soon after a planet forms, he said.

"We don't know how life got started. But if it happens so quickly, that might mean it's common -- that it's easy," he said. "And that looks well for finding habitable planets outside Earth."

The earliest signs of life on Earth are 3.5 billion-year-old filaments and tubes in lava deposits in South Africa that appear to have been left by bacteria.

In 2001 Mojzsis and colleagues published a study showing evidence of water on Earth's surface 4.3 billion years ago. "The view we are taking now is that Earth's crust, oceans and atmosphere were in place very early on, and that a habitable planet was established rapidly," said Mojzsis.