A comet full of soup

The chemical mix from which life emerged could well have come from outer space.

Space scientists are now ready to abandon the long-held theory that life on Earth was generated entirely by the action of lightning on an organic "soup" of chemicals. Though this theory has held sway for decades, a new group is assembling evidence which strongly suggests that the culprits are comets that crashed into our planet about four billion years ago.

"We have realised that many of the simpler organic molecules required to lead to life are present in abundance in the nuclei of comets," according to Michael Mumma, chief scientist in extraterrestrial physics at the US space agency, Nasa. He is picking his words carefully, for while it might seem that this theory confirms the ideas propounded by the astronomers Fred Hoyle and Chandra Wickramsinge - that the comets already contained living molecules such as bacteria, which seeded the waiting "soup" - in fact, it stops some way short of that.

Comets don't contain living material, says the latest thinking. They contain the essential precursors of life, including complex organic molecules such as amino acids and proteins, which are the components of all living things. The important step of going from "pre-biotic" material - able to sustain life, but not independently capable of it - to self-replicating molecules such as RNA and DNA is firmly Earth-based, scientists say.

J Mayo Greenberg, emeritus professor of astrophysics at the University of Leiden in Germany, has been propounding this theory, with refinements, for roughly 25 years. "It has been an uphill battle," he says. "People didn't believe in the photochemistry of interstellar dust. Now, everybody does. Well, almost," he adds.

His theory contains a number of stages. First, he says, the conditions in interstellar space - with microscopic grains of dust exposed to energetic ultraviolet light from the stars - are the right conditions to form larger, more complex molecules. "We've done simulations in the laboratory and produced glycine, alanine, glycerol - several amino acids in the mixture," he says. "And when we compare the absorption spectrum of interstellar dust we find evidence that molecules similar to that are out there."

Of course, laboratory tests also simulated the creation of amino acids by lightning, like the organic "soup" theory. But Professor Greenberg has gone further: an experiment on the Eureka space mission produced complex hydrocarbons from simple molecules when exposed to solar radiation.

That completes the first stage of the process: forming the molecules. Then, they would have to coalesce into the nuclei of comets, and then crash into planets - which, if they were young enough, might be just the place for the molecules to lead to better things. The nucleus of a comet would typically be about a kilometre wide and be a "fluffy" mixture of ice coating a combination of microscopic silicate particles and carbon. As it turns out, being "fluffy" (having many particles suspended in the mixture, rather than compressed into a hard solid) improves the chances of creating life.

"If a comet hit the Earth about four billion years ago, the atmosphere would have been much denser," he says. "That would have slowed it down as it fell, so the chemicals inside it could have survived the impact. Some pieces could land in the oceans - which, incidentally, are almost certainly all composed of water from comets.

"The new area of our research is that we've shown that these particles would be about three microns (millionths of a metre) across, and each contain about 100 molecules. Now, when they fell into the sea, our research suggests that they would be held together, and could let small molecules such as oxygen or whatever in - but the large molecules, such as the amino acids, couldn't get out." This key step, reducing the entropy (or disorder) of that system, is essential: "That means it's going to get more complex - which is the first step towards life."

Isn't that the same as the theory of Hoyle and Wickramsinge? "No. I think they're mistaken. Bacteria couldn't survive in the conditions of space. Ultraviolet would destroy them. I think the idea of interstellar 'spores' is, well, nonsensical."

But simpler organic molecules can, and could survive striking the Earth. Each strike would produce many "seed" particles, says Professor Greenberg: a comet is typically about a kilometre in diameter, and would contain 10 million million million million groups of such particles. "The chances of things going right are pretty high," he says.

His theories are backed by observations, including recent ones of Comet Hyukatake, which passed close to the Earth earlier this year. Dr Mumma says: "There is ethane and methane in Hyukatake, and what is significant is that their relative abundance means that they didn't come from the solar nebula." In other words, it came from outer space.

What scientists like about the "cometary seed" idea is that it offers a simpler explanation of life's origins than the "lightning and soup" version. "It's simpler, and it would deliver this material to any planet," says Dr Mumma. His opinions of Professor Greenberg's work? "I would say that many of his ideas have been confirmed. But that's how it should work: we move forward by testing theories with models and observation."

Scientists are still stumped, however, on exactly how those "pre-biotic" particles could make the vital step from complexity to self-replication - the essential element of life.