So the oxygen-methane strategy would not have picked up signs of life on the early ("Archaean") Earth, even though the planet was teeming with organisms — just as it may result in false negatives when applied to exoplanets as well, researchers said.
But studying the microbes that thrive today in Earth's oxygen-free environments suggests a way to broaden the search for alien life. For example, Archaean life probably released some pretty stinky stuff into the ancient planet's air, such as sulfur-methyl gases, which Domagal-Goldman recalled smelling while walking past a colleague's lab.
"They don't last long in modern-day Earth's environment because they get oxidized," he said. "But if you went back to the Archaean, or any planet without oxygen, and you had life making these gases — which they clearly do; I detected them myself — then they might have built up enough for us to see with a telescope from far away."
Scientists have proposed looking for other biosignatures as well, including industrial pollutants such as cholorfluorocarbons that could be indicators of advanced alien civilizations.
Of course, even looking back to the biosignatures of ancient Earth still involves a very large assumption — that alien life will probably resemble Earth life in important ways.
That would seem to suggest that it will be tough to detect lifeforms vastly different from those of Earth — organisms with exotic and undreamed of metabolic pathways. But Lyons thinks astrobiologists shouldn't view the challenge as insurmountable.
"We have chemical principles, and we hope that those are universal," Lyons said. "Life today is about the flow of electrons amongst bacteria, simple, single-celled organisms. And so if you had a sense for the chemistry on that planet that you're inferring from an atmosphere, you could start to envision reactions that could lead to that chemistry that could be a source of energy."
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