Invasions from outer space

Astronomers think they have found the origins of life in Sagittarius. But, writes Tom Wilkie, the explanation may be more down to Earth Darwin may still be right in the end

If the sky is clear tonight, look up and you will see Mars, the red planet, standing high in the south at nightfall. Jupiter will be just south of the Moon, against the background of the stars forming the constellations of the spring night sky - Leo, Gemini, Virgo. But there is, quite literally, more up there than meets the eye. It was revealed yesterday that American astronomers have detected one of the fundamental buildings blocks of organic life, an amino acid called glycine, in the giant molecular cloud near the centre of our galaxy, in the constellation of Sagittarius.

Complex chemistry in the inky void of interstellar space is not new - since the discovery of ammonia in space in 1968, more than 100 moderately complex molecules have been detected. At the beginning of this year, it appeared that the warm gas near the tar-forming region of the Orion Nebula acts as a cosmic cocktail shaker - astronomers from NASA Ames Research Center and from Manchester University discovered both grain and wood alcohol there. If there is not a restaurant at the end of the universe, as depicted in the Hitchhiker's Guide to the Galaxy, then it would appear that there is at least a bar at the end of the Milky Way.

The glycine was discovered yesterday and is the first sub-unit of protein, the main stuff of life, to have been detected in outer space. It raises anew one of the ultimate questions: where and how did life begin? The founder of modern evolutionary biology, Charles Darwin, speculated in one of his letters that life came about when "in some warm little pond, with all sorts of ammonia and salts, light, heat, electricity, etc, a protein compound was chemically formed ready to undergo still more complex changes". From these beginnings, Darwin thought, might come molecules that could reproduce themselves, and so exhibit the hallmark of life.

Darwin's speculations were inevitably earthbound. Could it be that the cradle of living matter was not the warm surface of the earth, but the cold darkness of outer space?

Certainly, the discoveries of the past 20 years indicate that the "void" between the stars is far from empty. Vast areas in our galaxy are seething with complex chemical reactions, forging atoms together to form complicated molecules ranging from alcohol to formaldehyde.

The giant molecular clouds where these molecules are found, according to Dr Simon Mitton of the Royal Astronomical Society, "are the birthplaces of new stars and planets and are very rich in these complex molecules. The clouds are the most massive individual entities in our galaxy". They are 150 to 250 light years across and although composed of dust and gas contain as much material as 10 million of our suns.

The molecules made in these interstellar chemical plants emit weak radiation - each one giving out its own characteristic radiation "fingerprint" or spectrum. Today's astronomers have access to telescopes which can "see" not just visible light, but into the infra-red and below that into radio wavelengths. According to Dr Mitton, all of these molecules emit radiation "at the interface between the infra-red and radio wavebands".

Because the water vapour in the Earth's atmosphere absorbs the signals from the molecules, observations have to be done using telescopes at high altitudes, such at the UK's James Clerk Maxwell Telescope on top of the extinct volcano of Mauna Kea in Hawaii. Only when one is above the clouds and most of the water vapour of the earth's atmosphere, according to Dr Mitton, is it possible to detect the whispers of the creative chemistry of the interstellar clouds.

Are these areas, where stars and planets are being born, also the cradle of life itself? There is a tempting neatness about the idea. And some respectable astronomers, most notably Fred Hoyle, have argued vehemently that life did indeed come from outer space. After the Earth condensed out of the swirl of dust and gases surrounding the Sun some 4.6 billion years ago, life began comparatively early: paleobiologists have found evidence of fossil cells in rocks that are 3.5 billion years old. Could life have been helped on its way by pre-existing building blocks, such as glycine, already formed in the interstellar medium?

Things are never quite that neat. To today's biologists, the most important molecule of life is not protein, whose glycine sub-unit has been found in space. Instead they give primacy to DNA, the chemical messenger of inheritance that carries the genetic blueprint down through the generations.

The bewildering variety of plants and animals on the face of the Earth today is composed of an equally bewildering variety of proteins. But behind the complexity and variety of the living world, the double helix of DNA provides an underlying unity. It is, in effect, the book that contains the recipes from which all of the proteins are built up.

As Francis Crick notes in his autobiography What Mad Pursuit, DNA "is a remarkable molecule. Modern man is perhaps 50,000 years old, civilisation has existed for scarcely 10,000 years and the US for just over 200 years, but DNA has been around for several billion years. All that time, the double helix has been there and active, yet we are the first creatures on Earth to become aware of its existence". All human (and animal and plant) life is there: the information on how to build proteins - from insulin through collagen to adrenaline - is all woven into the double- helix strand.

In this scheme, DNA comes first and protein (and amino acids) second. If the secret of life's origins is to be found anywhere, on Earth or in space, then it is to be found where there is DNA rather than protein.

But there is a subtlety even here. Many researchers, most notably the British-born Leslie Orgel, now working in the US, think that before the present variety of the living world developed, there may have been a shadowy "RNA world" in which all life was specified not by DNA but genetic instructions written out in the closely related chemical RNA.

RNA can assist its own replication and, unlike DNA, it does not need proteins to catalyse the process. So the first signs of life may have been a stretch of RNA, created by an accident of chemistry, which catalysed its own reproduction without the help of a protein. But RNA itself is not as stable as DNA, and no organism as complex even as a bacterium could have existed in the RNA world that preceded our own. Consequently, it was only when life switched to DNA as the molecule of inheritance that the diversity of today's living world became possible.

Modern biology, then, has reversed Darwin's speculation and put DNA and RNA rather than protein in the primary position for the origin of life. But pending the discovery of RNA or DNA in outer space, it seems that Darwin was right in putting the origins of life in some warm little pond here on Earth and not in the constellation of Sagittarius.