Rock of Ages; the Listener ; in His Theory of Evolution, Darwin Explained How Life Developed on Earth - but Not How It Came to Exist Here in the First Place. Now Many Scientists Believe We Are on the Brink of Finding That out. in This Week's Selection of the Best of BBC Radio, Experts Discuss How the Original Microbes May Have Arrived, and Thrived, on the Planet We Call Home

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THE SPEAKERS

1 PAUL DAVIES

Physicist and broadcaster

1 PROF BILL SCHOPF

Palaeontologist and director of UCLA's Center for the Study of Evolution

1 PROF MALCOLM WALTER

Dept of Earth and Planetary Sciences, Macquarie University, Australia

1 DR DUNCAN STEEL

Reader in Space Technology at Salford University

1 CHRIS MACKAY

Scientist, Nasa Ames Research Centre

1 MONICA GRADY

Head of Petrology and Meteoritics, Natural History Museum

1 PROF NORMAN SLEEP

Geophysics, Stanford University

1 PROF KARL STETTER

Institute of Microbiology, Regensburg

1 PROF JOHN PARKES

Geomicrobiology and biogeochemistry, University of Bristol

1 PROF TOM GOLD

Professor Emeritus of Astronomy at Cornell University

PAUL DAVIES: When Darwin published his theory of evolution in 1859, he explained how life on earth has emerged by natural selection. The incredibly rich and diverse biosphere could have evolved over billions of years, starting from a single microbe. But Darwin evaded the problem of how life got started in the first place. "One might as well speculate about the origin of matter," he commented.

The origin of life remains a mystery. Recently, though, many scientists have become convinced that a breakthrough is imminent. Some pin their hopes on making life in the lab, others expect we'll soon discover traces of life on Mars, a dedicated few think they may stumble across a message from an alien civilisation. But, as Bill Schopf cautions, the subject is in its infancy.

BILL SCHOPF: Virtually all our knowledge of the origin of life is very young, it's only 10, 20, 30 years old. It says we're in an exciting time, but it also says that we ought to realise that we're at the beginning of a search.

PD: A good place to begin is to ask when life began. We can get some clues by following the fossil record back in time as far as it'll go. One member of the team that found the world's oldest fossils is Malcolm Walter.

MALCOLM WALTER: The oldest convincing ones are 3,500 million years old. They're in Western Australia, near a place called Marble Bar. There is a whole range of different things: ancient reefs built by microbes; and microscopic fossils of the microbes themselves, where you can see the cellular structures; and chemical evidence in the form of carbonised and sulphurised distribution patterns.

PD: These fossilised microbes show that there's been life for at least three-quarters of the Earth's 4 billion year history. But these were advanced life-forms, they couldn't have been the first living things. So how about looking at even older rocks?

BS: The older the rock is, the greater the chance that it has been heated and pressurised to such an extent that the evidence of life it once had in it has been all but obliterated. I hope we've overlooked some rocks, because I truly would like to understand when and how life began. I've spent my scientific career going back and back in time, and there's always life.

PD: Even if conventional fossils are unlikely before about 3 billion years, earlier life-forms might still have left behind some traces. In Greenland there are rocks as old as 3.86 billion years which contain carbon, a possible biological relic.

BS: There are hints of life in as much as there are minute scraps of organic matter. Not really organic matter - it is what's known as graphite, tiny little scraps of carbon that have been heated up to 500C or 600C and subjected to pressures of 4,000 to 8,000 bars of pressure - this is a very tough environment. But if you look at the composition of these pieces, it suggests maybe they came from life.

PD: So how far back in time do we have to go to find when life began?

MW: I like to use a nice round number - 4,000 million seems reasonable. …