Why I...

The recent spate of biographies of astrobiologist Fred Hoyle has allowed us to ponder some profound questions. Strangely, the question left out of many discussions is the idea, developed by Hoyle and me, that life did not originate on Earth but arrived on a comet. Hoyle's most acclaimed work on the chemical elements in stars marked the first step in unravelling our cosmic ancestry. The carbon, nitrogen, oxygen and other elements from which we are made all had their origin in the deep interiors of stars. So much, at least, is certain. But how much further does our cosmic line of descent extend - from atoms to complex organic molecules to life itself? Could these steps have been accomplished before the Earth was even formed?

These were the questions that Hoyle and I sought to explore in a 40-year collaboration. In the mid-1970s, we began arguing that dust grains in interstellar space had an organic composition. At the time, such ideas were intensely controversial, but nowadays everyone accepts that cosmic dust does include complex organic material and that such material may have a relevance to life. Most scientists in the 1970s held tenaciously to their pre-Copernican point of view that Earth life must be Earth-centred. As the quality of astronomical observations improved over the years, our thesis that cosmic dust could have a biological connotation began to take a more definite shape. The biochemistry of life on Earth provided more clues. The staggering complexity of life at the molecular level militated strongly against a purely terrestrial origin.

Moreover, with no success in the many attempts to create life from non-living matter in the laboratory, the cosmic option seemed increasingly attractive. Despite the many space missions to search for extraterrestrial life that were already under way in the 1980s, projects connecting astronomy and biology were not, at the time, properly recognised. This situation changed dramatically in the autumn of 1996, after the publication of a paper purporting to show evidence of microbial fossils in a meteorite from Mars.

As it subsequently turned out, the microfossil identification remains questionable and might even be wrong. The salutary outcome, however, was a surge of interest in astrobiology.

Disappointingly, the trend is to look for independent de novo origins of life on the many extraterrestrial locations where it could survive. Since this merely multiplies the already insuperable odds against an origin in a single location, such an approach is likely to prove counterproductive.

Microbial life appears abruptly in the Earth's geological record at the first moment it can survive, and this happens during an epoch of intense cometary bombardment, which is suggestive of a cometary origin that is, unfortunately, being ignored. But there is hope. Comet dust in the Earth's stratosphere is being inspected for signs of life that may perhaps be discovered in the not-too-distant future. Studies of the behaviour of microbes in extreme environments on Earth are providing renewed confidence in searches for life on planets and satellites in our solar system and beyond. The quest for life on Mars continues unabated. The hunt for Earth-like planets outside the solar system that can support life is also moving into a new phase with the launch of Nasa's Kepler mission (2007) and the European Space Agency's Darwin mission (2015), which will look for spectral biomarkers (ozone, water, methane and so on) in such planets'

atmospheres. I am confident that sooner or later the interconnectedness of all life in the cosmos will become established beyond doubt. Hoyle's work in this area would then be reckoned to be his most important contribution to science.

A Journey with Fred Hoyle: The Search for Cosmic Life by Chandra Wickramasinghe was published by World Scientific Publishing earlier this year, Pounds 20.00.

Chandra Wickramasinghe is director of the Cardiff Centre for Astrobiology at Cardiff University