While we're yet to find any, scientists in the US say new research suggests life elsewhere in the universe is probably quite common.
Australian microorganisms nearly 3.5 billion years old already had the ability to photosynthesise and produce methane gas, researchers at the University of California have discovered.
"This tells us life had to have begun substantially earlier, and it confirms that it was not difficult for primitive life to form and to evolve into more advanced microorganisms," says study lead author J William Schopf, a professor of paleobiology at the UCLA College.
The Earth was formed only about a billion years earlier - but these microorganisms are the oldest ever found.
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Prof Schopf said they still don't know how far back life might have begun, but that it was this advanced this early on bodes well.
"If the conditions are right, it looks like life in the universe should be widespread," he said.
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But don't expect ET to show up soon. It took another 3.5 billion years for intelligent life to show up on Earth, and it's still a mystery whether humanity was a fluke or an inevitable result of evolution.
Prof Schopf said it was "very possible but less certain" alien life would be intelligent, rather than microbial.
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His latest study was published in Proceedings of the National Academy of Sciences.