Sugar compounds, an indispensable ingredient for life today, have been found in meteorites, bolstering the theory that chunks of rock from outer space delivered the materials that gave rise to life in Earth.
Another key ingredient, amino acids, has already been found in meteorites.
George Cooper of NASA's Ames Research Center at Moffett Field, Calif., said that while it has not been proved that meteorites delivered the materials that led to life, the discovery means meteorites containing the building blocks were at least present on the planet early in its history.
Cooper's findings appear in Thursday's issue of the journal Nature.
``One conclusion from our work is that polyols were present on the early Earth and, at the least, available for incorporation into the first forms of life,'' Cooper, the study's lead author, told the Houston Chronicle in Thursday's editions.
In biology, sugar compounds are important because they are a source of energy and provide the carbon skeletons for many other molecules.
Sandra Pizzarello, a meteorite researcher at Arizona State University, said the discovery adds another piece to the puzzle.
``This sort of completes the gamut of biological molecules in meteorites. It was kind of a missing part,'' Pizzarello said. ``Now, you can reasonably say in meteorites there are all the ingredients of life. We don't know how to make life, but the ingredients are there.''
The sugar compounds were discovered in small amounts in two carbon-rich meteorites in which amino acids had already been found.
Sugars had been found in one of the meteorites as far back as 1962, but the possibility the meteorite was contaminated after it landed on Earth could not be ruled out at that time.
Jeffrey Bada, a professor at the University of California at San Diego and director of a NASA laboratory that studies how life began on Earth, said that while the finding is tantalizing, the sugar compounds found in the meteorites are not like those found in life today.
``So I think it's a stretch to say these molecules are what was needed for the origin of life,'' Bada said.
In fact, one theory holds that the first life on Earth did not have a sugar component because sugars tend to break down quickly and thus may not have been available, he said.
Scientists since the early 1960s have thought carbon-heavy meteorites, and possibly comets, kick-started life on Earth by providing raw organic materials.
``Until now, though, the organic mixtures in (meteorites) were missing one obvious ingredient from the recipe of life: sugars,'' astronomer Mark Sephton of the British Planetary and Space Sciences Research Institute wrote in a Nature article analyzing the discovery.