Sugar compounds similar to some in chewing gum and toothpaste have been found inside two meteorites, showing that complex chemical compounds that are the building blocks of life readily exist in space.
The discovery by a NASA team, announced yesterday in the British science weekly Nature, is the most recent in a series.
The findings fuel some scientists' belief that life on Earth was jump-started by extra-terrestrial contributions from falling asteroids.
But other scientists suggest that the latest announcement shows only that whatever happened when life first bloomed on Earth can occur elsewhere in the universe.
The latest find was made by a team led by George Cooper of the U.S. National Aeronautics and Space Administration Ames Research Center.
"Organic compounds important in contemporary biochemistry are thought to have been delivered to the early Earth by asteroids and comets, where they may have played a role in the origin of life," the team's article says.
The team extracted from two meteorites so-called polyhydroxylated compounds (polyols), also known as sugar alcohols, of the same family as sweetening agents such as sorbitol or xylitol, low-calorie sweeteners commonly found in gum and mouthwash.
Sugars are vital to life as a source of energy and as a skeletal carbon component for other, more complex, organic molecules.
"We conclude from this that polyols were present on the early Earth and therefore at least available for incorporation into the first forms of life," the research team says.
The two meteorites that yielded the sugar compounds were recovered in Murray, Ky., in 1950, and Murchison, Australia, in 1969.
The Murray and Murchison rocks had been found to contain amino acids, the basic elements of proteins and of nucleic acids, the molecules that can carry genetic information. Other amino acids of a simpler form were discovered this year inside a meteorite that fell in May, 1864, in Orgueil, France.
Also last year, astronomers using a telescope in Tucson, Ariz., detected sugar's signature radio emission from within a massive cloud of gas and dust 26,000 light-years from Earth, in the Sagittarius B2 (North) region, near the centre of the Milky Way.
More spectacular claims, such as those made in 1996 that a Martian meteorite contains fossilized bits of bacteria, now are in doubt.
"There are reasons to believe that some of the building blocks needed to start life on Earth may have come from outer space," says an article in Nature accompanying the Ames announcement.
"Meteorites can be particularly useful vehicles in this regard," especially carbon-rich ones such as the Murray and Murchison samples, says the article by Mark Sephton, a lecturer at the Planetary and Space Sciences Research Institute of the Open University, in Milton Keynes, United Kingdom.
Other scientists, however, treated the discovery of the sugar compounds with more caution.
"This is a very nice illustration of the kind of chemistry that's possible under natural conditions. But whether these compounds coming from space had any important role in the origin of life, I think, is a stretch," said Jeffrey Bada, director of NASA Specialized Center of Research and Training in Exobiology at the Scripps Institution of Oceanography in San Diego. Prof. Bada led the team that found amino acids in the Orgueil meteorite.
Whatever meteorite-borne organic compounds can be found in space would have been calcinated by the time they landed on our planet, because of the friction from the air as the meteorite slashed through the atmosphere, he said.
The compound could have been released if a meteorite landed at sea and its organic chemicals dissolved into the primordial soup, but even then, sugar compounds are fragile and decay within years. The exception could have been if the compounds were encased inside a hunk of unburned meteorite. But if that were the case, the compounds would have remained inaccessible until scientists pried the rock open.
There's a little ET in all of us
A recent study supports the theory that all life comes from space. Scientists found that, along with 70 different amino acids already discovered, the Murchison and Murray meteorites contain sugar-like substances, which are part of the chemical building-blocks of life. The meteors contain polyhydroxylated compounds, or polyois. Examples of this compound include the sugar you buy in a store, sugar alcohols and sugar acids that are vital to all known life forms. The compounds act as energy sources for all organisms. They help build proteins and cell membranes and they are components of RNA and DNA, the software of life. The study by a team lead by George Cooper of NASA's Ames Research Center focuses on two carbonaceous chrondite meteorites found in Murray, Ky., in 1950 and Murchison, Australia, in 1969. Almost 3 per cent of all meteorites are carbonaceous chondrites, organically rich meteorites that represent the most primordial samples known from the solar system.
Tu Thanh Ha