We are stardust

Cosmic cradle: image of deep space captured by the Hubble telescope

FOR THE PAST half century or so, scientists speculating about the origins of life on Earth have fallen, roughly speaking, into two camps: either life began on Earth itself out of some fortuitous mixing of the primordial soup, or it arrived here, in the form of bacteria or proto-genetic material, from outer space.

In the way of disputes that may ultimately never be resolved, scientific consensus has been slow to build. The weight of evidence seems, however, to be falling increasingly on one side of the question. The glory days of Earthborn-life theories date back to 1953 – when Stanley Miller and Harold Urey famously made amino acids by passing electrical charges through a 'young Earth' fog of water, methane and ammonia; in 2008, the smart money is all with the extraterrestrials.

A paper published, with little fanfare, in Earth and Planetary Letters in June of this year may prove key to building a coherent theory of life's extraterrestrial origins. An international team, including researchers from Imperial College London, have confirmed that an important component of early genetic material, found in meteorite fragments from Australia, had cosmic origins. The finding suggests that raw materials to make the first molecules of DNA and RNA – the genetic building blocks of life – may indeed have come from the stars.

Professor Mark Sephton, the leader of the Imperial research team, continued: "At the compound-class level, you have all the basic components needed to make the buildings blocks of DNA in a single meteorite. It's not the compete jigsaw to explain the origin of life, but it is the partial jigsaw. This discovery lends weight to the idea that the building blocks of life came from space."

Between 3.8 to 4.5 billion years ago large numbers of rocks similar to the Murchison meteorite are known to have rained down on Earth; the earliest life has been dated to around 3.5 billion years. For the extraterrestrial school of thought, this neat coincidence in the geological record supports the notion of a cosmic 'trigger' for the origin of life, with the meteorite-borne nucleobases providing a strong candidate for the trigger itself.

If terrestrial life had its origins in space, the same phenomenon may, of course, have happened elsewhere. Sephton continues: "The analysis of meteorites for signs of life is mired in controversy. However, it is clear that organic molecules of surprising complexity seem to be ubiquitous in space, either on asteroids orbiting the solar system or on meteorites analysed on Earth."

"Because meteorites represent left-over materials from the formation of the solar system, the key components for life – including nucleobases – could be widespread in the cosmos. As more and more of life's raw materials are discovered in objects from space, the possibility of life springing forth wherever the right chemistry is present becomes more likely."

‘Extraterrestrial nucleobases in the Murchison meteorite’, Earth and Planetary Science Letters, Volume 270, Issues 1-2, 15 June 2008