Meteorites Brought DNA "Ancestors" to Earth, Study Says

Ker Than
for National Geographic News
June 18, 2008
Some of life's raw ingredients may have originated in space, says a new analysis of an Australian meteorite that adds more fuel to the controversial theory.

Researchers discovered the organic molecules uracil and xanthine in the meteorite and confirmed they could not have formed on Earth.

These molecules, called nucleobases, are precursors to DNA, a set of genetic instructions for organisms on Earth. (Get the facts on DNA.)

Uracil and xanthine may also have been stepping-stones to RNA, which builds proteins in organisms.

"Emergent life systems may have adopted nucleobases from meteoritic fragments for use in an early and primitive genetic material, enabling them to pass on their successful features to the next generations," said study leader Zita Martins of Imperial College London.

The finding supports an idea first proposed by astronomer Carl Sagan and a colleague in 1992. Some of life's crucial building blocks, they said, were forged in the hearts of roving comets and asteroids, which seeded them throughout the cosmos.

(Related: "Did Comets Make Life on Earth Possible?" [October 2, 2003].)

"It could be that these molecules landed in just the right place at the right time and helped to make us what we are," said Max Bernstein, an astrochemist at NASA Ames Research Center in California who was not involved in the study.

Cosmic Hitchhikers

The team dissolved and analyzed samples of the Murchison meteorite. The asteroid fragment had crashed into Australia in 1969 and is thought to be 4.5 billion years old—as old as our solar system.

The results also showed that the nucleobases contained a large ratio of a heavy form of carbon rarely found on Earth.

The nucleobases could have formed in space or in the interiors of asteroids, where the molecules would have been shielded from harsh ultraviolet light and other radiation, study leader Martins said.

Some of these cosmic hitchhikers could have landed on Earth between 3.8 to 4.5 billion years ago, when the solar system was a tumultuous place and Earth was frequently pelted by violent flurries of comets and asteroids.

Alternatively, the nucleobases could have formed independently on Earth—but recent studies suggest the harsh conditions on early Earth might have made it difficult.

The research is detailed in the June 15 issue of the journal Earth and Planetary Science Letters.

Extraterrestrial Life

The new finding confirms what many scientists have long suspected, Berstein said.

"Knowing that [the molecules] form in space and are delivered to planets is clearly of great significance, because it suggests that some of the makings of DNA were drifting down on early Earth before, while, and after life was getting started and evolving," Berstein said.

The finding also has implications for extraterrestrial life, study leader Martins said.

"Meteorites may have delivered these building blocks not only to Earth, but also to other planets," she said.

"The meteorites were formed shortly after the birth of our solar system, so the material from the meteorites could be widespread through the universe."

© 1996-2008 National Geographic Society. All rights reserved.