Scientists increasingly think life on earth originated from another planet

In 1953, PhD student Stanley Miller conducted an experiment that seemed so outlandish that he had to persuade his supervisor Harold Urey to let him try it. Working in a laboratory at the University of Chicago, he injected steam into a mixture of methane, ammonia and hydrogen to simulate the early atmosphere on earth. He then applied an electric discharge, mimicking lightning, and left the mixture to react for a week.

Analysing the resulting liquid within his flask, Miller found five amino acids - carbon-based chemicals that also include elements such as oxygen and nitrogen and are the building blocks of proteins, which in turn are essential to life as we know it.

This landmark experiment fitted a notion of Charles Darwin's, described in a letter to a friend: "But if (and Oh! What a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity etc., present that a protein compound was chemically formed ready to undergo still more complex changes."

Yet while there was great appeal in the idea that the warm little pond - often known as "primordial soup" - could support a progression from amino acids to proteins, and eventually to replicating cells containing DNA, there soon came a realisation that a huge gulf separated the Miller-Urey experiment from any plausible pathway to even the simplest life forms. The origin of life remains a mystery. Even so, there have been several announcements of research results providing tantalising clues as to ways life could have arisen - with indications that some important components may have extraterrestrial origins.

One popular theory for how life emerged from the soup is that it began with RNA - ribonucleic acid, a family of very long molecules with a similar structure to DNA. In 2009, a team at the University of Manchester announced that they had created one of the building blocks of RNA in an experiment involving simple chemicals and conditions similar to those that may have existed on ancient earth. "What we have ended up with is molecular choreography," said lead author John Sutherland, quoted in a Nature article.

As seems typical of work on the origins of life, some scientists are critical of the experiment, which echoed the one conducted by Miller but was more complicated, involving steps like heating molecules in water, evaporating the water, and irradiating the molecules with ultraviolet light. Undeterred, Sutherland said: "My ultimate goal is to get a living system (RNA) emerging from a one-pot experiment."

Adding support to the idea that self-replicating RNA was a precursor to life on earth, in August this year a team of University of Washington scientists reported that they had found ways primitive cells could have formed. Their experiments involved combining organic chemicals called fatty acids with building blocks of RNA. In some cases, the fatty acids formed tiny soapy bags containing the building blocks.