It's old dirt, but it's old dirt that scientists can't get enough of.

Cosmic grit that survived a fiery ride from space 1.4 billion years ago has been discovered in a layer of sandstone in Finland, offering a glimpse of conditions on Earth during the earliest stages of life's formation.The micrometeorites are by far the oldest extraterrestrial debris found on Earth, according to the German and Finnish field team, which published its discovery in the journal Nature.

The particles are known as cosmic spherules. They measure a fraction of a millimeter in diameter. At least 18 of them were deposited in a layer of red sandstone 1.4 billion years old, at a time when oxygen-rich, life-giving conditions on Earth were developing.

Red sandstone beds of a similar age have been mapped in North America and Asia and could yield more such deposits, said lead author Alexander Deutsch of Germany's Institute for Planetology.

Astronomers who specialize in cosmic dust described the discovery as spectacular. They said the composition of the Finnish micrometeorites will open a window onto the environmental conditions of the early Earth.

The particles also could help explain the behavior of meteorites, comets and other features of the solar system that probably generated the bombardment.

Spherules have been found all over the world since 1876, most frequently in core samples of deep ocean sediments and polar ice. Until now, the oldest known spherules were less than 200 million years old, and their condition had been distorted by time and weathering.

"These spherules hit the Earth far earlier than any other meteoric material we have, and yet they are in an excellent state of preservation," said University of Washington astronomer Don Brownlee. "Until now, there was little hope that the historical record could be extended further back into time."

The Earth is pelted by an estimated 40,000 tons of rocky space debris every year.

Hollywood producers and, to a lesser degree, space scientists, are concerned that mountain-sized asteroids might someday slam into the planet and extinguish life.

But the vast majority of extraterrestrial material arrives in the form of microscopic grit. Much of it is believed to have been shed by passing meteors and comets, but some may be spawned by distant dust clouds that are the raw material for new solar systems.