Nasa astrobiologist Dr Felisa
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The Drake Equation and the search for new lifeforms

15 December 2010 Last updated at 12:12 GMT

In 1960, Dr Frank Drake came up with the Drake Equation - a formula calculating the likelihood of of extraterrestrial intelligence from seven key elements.

The figures, approximate as they were, suggested the number of alien civilisations able to communicate with Earth was at least 10,000.

But with the recent discovery of microbes which thrive in an arsenic-rich environment, scientists are now contending with the idea that life on Earth arose not once, but twice.

Such a discovery would have profound implications for the Drake Equation and the search for alien life, as Dallas Campbell learned when he met NASA astrobiologist Dr Felisa Wolfe-Simon.

The Search for Life: The Drake Equation is on BBC Four at 20:00 on Tue 14 Sept and then afterwards on iPlayer.