In the early 1960s, the independent scientist James Lovelock worked as a consultant for NASA developing ways to analyse extraterrestrial atmospheres. This work led him to the dramatic conclusion that life would leave an indelible stamp on the chemical make up of any planet.
Over billions of years, he said, the processes of life would create a fog of chemicals unlike anything that could form in an ordinary chemical equilibrium.
He even went further to suggest that this atmosphere and the life it supported would form a kind of self-regulating system that could itself be thought of as a living organism--the Gaia hypothesis. Lovelock famously says that as soon as he saw the first analysis of the chemical make up of the Martian atmosphere, which is almost entirely carbon dioxide and nitrogen, he knew the planet could not support life.
Since then, the search for biosignatures has become an important problem for astrobiologists. We know, for example, that large amounts of oxygen and small amounts of methane are generated by life on Earth. And that carboxylic acids in terrestrial life forms tend to have even rather odd numbers of carbon atoms, a fact that is used to identify contamination in meteorite samples.
The problem, of course, is that we have only one example of life to study. So the biosignatures of life on Earth may be of little use for identifying ET lifeforms.
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