Did life's building blocks fall from the skies?
Life on Earth may have got off to a sweet start nourished by sugar from space.
Researchers from the American space agency, Nasa, say their study of the two space rocks has revealed a range of organic substances called polyols - the technical name for sugars.
These sugar compounds were found in the Murchison and Murray meteorites, which are believed to be fragments broken off a much larger body.
Backbone of life
The Murchison meteorite was recovered in Australia in 1969; the Murray meteorite was picked up in Kentucky in 1950.
Both are examples of carbonaceous chondrite meteorites and are believed to be fragments of much older and larger bodies that once resided in the asteroid belt between the orbits of Mars and Jupiter.
It is known that meteorites contain many carbon-based compounds - such as amino acids - that could become the building blocks of primitive life. Analysis of the Murchison meteorite found over 90 types of amino acids.
But to date, no conclusive evidence of sugars - also crucial for life - have been found in meteorites. Claims have been made, as far back as 1962, but there was always the suspicion that terrestrial contamination was the real source.
However, the Nasa researchers, writing in the journal Nature, say their detection method is "relatively definitive". They add that as well as sugars common on Earth, they have detected forms of sugar that are rare on our planet.
First steps
According to Mark Sephton, of the UK's Open University: "Sugars are important biologically because they provide the carbon skeletons for many other molecules."
Perhaps the most famous example here is DNA - sugars provide part of the backbone for "molecule of life".
The presence of amino acids implies that life on Earth might have been "seeded" by organic compounds falling on to the planet. The discovery of sugar molecules in the meteorites suggests that another essential building block of life may have come from space as well.
The sugars may have been formed before the Solar System itself was formed, by the action of starlight on molecules resting on cold grains of dust drifting in between the stars.
When our Solar System was created, the sugar could have become incorporated into small bodies, like asteroids, that formed in the system's cold outer reaches.
When the asteroids broke up, fragments would have fallen to Earth and delivered their life-forming molecules.
Mark Sephton says the findings suggest how "the first chemical steps towards sweet life" were taken.