Shock waves from comets bombarding the Earth may have helped to build proteins and set the stage for life, scientists have learned.
Comets, giant snowballs of ice and dust, are known to have carried organic chemicals and water to the early Earth.
But just what caused life to spring out of nowhere on a barren and desolate planet billions of years ago remains a mystery.
Now scientists may have part of the answer. Laboratory experiments have shown that amino acids - organic molecules that are the building blocks of proteins - would have survived violent comet impacts.
What is more, the shock of a large comet impact would have provided the energy needed to start bonding amino acids together to make proteins.
Proteins provide the raw material that allows all living things, from microbes to humans, to exist and function.
Their creation by comets may explain how life appeared so quickly at the end of a period 3.8 billion years ago called the "late heavy bombardment". During this turbulent time the Earth was showered by both comets and rocky asteroids, leaving crater scars that are still seen on the Moon.
Dr Jennifer Blank, who led the US scientists from the Nasa/Ames Research Centre in Moffett Field, California, said: "Our research shows that the building blocks of life could, indeed, have remained intact despite the tremendous shock wave and other violent conditions in a comet impact.
"Comets really would have been the ideal packages for delivering ingredients for the chemical evolution thought to have resulted in life. We like the comet delivery scenario because it includes all of the ingredients for life - amino acids, water and energy."
Dr Blank described the experiments at the annual meeting of the American Chemical Society in San Diego, California.
Her team set out to simulate conditions that existed when Earth was repeatedly struck by amino acid-carrying comets, some of which were 10 miles or more in diameter.
One study used powerful gas guns to fire supersonic blasts of gas at capsules filled with amino acids, water and other materials.
Despite the heat and shock of the simulated collisions the amino acids did not break down. Instead, they began forming the chain-like "peptide bonds" that link amino acids together to make proteins.
Intense pressure from the impact offset the intense heat and supplied the energy needed to create the peptides.
Other experiments used computer models to simulate conditions as comets ploughed into the Earth's atmosphere.
Both comets and asteroids may have brought multiple deliveries of the "seedlings of life" to Earth, said Dr Blank.