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.
But scientists have calculated that random evolution of RNA molecules could take an extremely long time to reach their modern sophistication. Last month, Charles Carter of the University of North Carolina published the results of a study suggesting enzymes could have accelerated the process.
Another origins-of-life chemist, Steven Benner of the US-basedWestheimer Institute of Science and Technology has suggested that assembling RNA would have been very difficult on the ancient earth. Speaking at a conference in August, he presented results indicating that boron and molybdenum minerals can significantly help the process. But three billion years ago, the earth was too wet and the atmosphere had too little oxygen for these minerals to have been sufficiently common and in the right forms.
Instead, Benner suggested that Mars had the right chemistry, and life may have come to earth on a Martian meteorite. "The evidence seems to be building that we are actually all Martians; that life started on Mars and came to Earth on a rock," he remarked.
Within days of Benner's presentation, a paper was published that had reached similar conclusions regarding the "phosphate problem" - concerning the phosphorus compounds crucial for RNA, DNA and proteins. There would have been few such compounds in the water on early earth, but Martian phosphates were both more soluble and plentiful, making the "red planet" appear a better nursery for life.
Just possibly, fossil microbes have been found in meteorites from Mars. The most famous of these meteorites made world news in 1996, when Nasa scientist David McKay argued that tiny structures were fossils of life forms smaller than any known cellular life.
Meteorites from asteroids are another otherworldly source of ingredients for life. They can be rich in organic material such as amino acids, which may form as comets crash into moons or planets. After a meteor fell to earth over California in April last year, researchers at the University of Arizona gathered fragments for study. At first, they were disappointed to find few organic molecules. Then, they took insoluble material and baked it for six days in conditions akin to those around hydrothermal vents.
"And lo and behold, this meteorite left behind something we have never seen," team leader Sandra Pizzarello told New Scientist. There were long molecular chains that can form scaffold-like structures. Rather than soapy bags, it might have been this scaffolding that trapped organic molecules which led to life.
Speculation about the origins of life look set to continue. But there is agreement that the earliest known life dates from around 3.7 billion years ago. And last month, British graduate student Andrew Rushby announced that as the sun ages, earth will become too hot and dry to support life within 1.75 billion to 3.25 billion years. So if any earthlings are to survive, they will have to find a way to colonise other parts of the universe.
Martin Williams, a Hong Kong-based writer, holds a PhD in physical chemistry from Cambridge University