The detection was made in material that came off Comet 81P/Wild-2
Scientists have identified one of the fundamental chemical buildings blocks of life in a comet for the first time, the US space agency (Nasa) reports.
Glycine is an amino acid found in proteins, the sophisticated molecules that organisms use to build and maintain their systems.
It was detected in the material ejected from Comet Wild-2 in 2004 and grabbed by Nasa's Stardust probe.
The idea that life was "seeded" on the early Earth by comets is a popular one.
Many scientists hold to the theory that billions of years ago, a bombardment by these mountainous balls of ice and rock brought important chemical precursors for life to our planet.
Somehow evolution was kick-started from this primordial "soup", they believe.
"The discovery of glycine in a comet supports the idea that the fundamental building blocks of life are prevalent in space, and strengthens the argument that life in the Universe may be common rather than rare," commented Dr Carl Pilcher, who leads Nasa's Astrobiology Institute.
See how Stardust grabbed material from Comet Wild-2
Glycine has been detected in meteorites before and there are also observations in interstellar gas clouds claimed for telescopes, but the Stardust find is described as a first in cometary material.
The Nasa spacecraft flew past the 5km-wide icy Comet 81P/Wild-2 in January 2004.
The probe swept up particles fizzing off the object's surface as it passed some 240km (149 miles) from the comet's core, or nucleus. These tiny grains, just a few thousandths or a millimetre in size, were then returned to Earth in 2006 in a sealed capsule.
Distributed among the world's leading astro-labs, the specimens have since been giving researchers a remarkable insight into the conditions that must have existed in the earliest phases of the Solar System when planets and comets were forming.
Nasa said in a statement that it took sometime for the investigating team, led by Dr Jamie Elsila, to convince itself that the glycine signature found in Stardust's sample bay was genuine and not just Earthly contamination.
Planets being 'seeded' with extraterrestrial material is a popular theory
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