Scientists hunting for signs of alien life shouldn’t be so quick to dismiss carbon monoxide (CO), a new study suggests.

The substance is highly poisonous to people and most other animal life here on Earth because it latches firmly onto hemoglobin, preventing this blood protein from carrying vital oxygen in the required quantities.

And the gas hasn’t typically rated as a promising “biosignature” that astrobiologists should target in the search for ET. Indeed, many researchers regard CO as an anti-biosignature, because it’s a readily available source of carbon and energy that life-forms should theoretically gobble up. So, finding lots of CO in an exoplanet’s atmosphere would suggest the absence of life as we know it, according to this line of thinking.

But it may be time to revise such reasoning, the new study said. In it, researchers used computer models to better understand the atmospheric chemistry of Earth about 3 billion years ago, when our planet’s air contained very little oxygen. Microbial life was common on Earth back then, but animal life was a long way off. (The earliest fossils of multicellular organisms date to about 600 million years ago.)

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