For decades, scientists hunting for signs of alien life have basically looked for two things: chemicals that living things leave behind, or radio waves from advanced civilizations. But what about everything in between?
That gap is exactly what astrobiologist Julia DeMarines is trying to fill. In a new paper called "Signs and Signatures of Intelligence," published on the arXiv preprint server, she proposes a brand-new category of evidence: the noosignature.
Think of it this way. It took Earth about 3.5 billion years to go from the first microbe to a civilization that could send radio waves into space. If aliens had pointed a telescope at our planet 10,000 years ago, they would have seen neither microbes nor radios—but they would have seen something. Cities. Farmland. Stone carvings. Signs of thinking minds at work, even without the technology to broadcast signals across the galaxy.
A noosignature is exactly that: a structured trace that a mind leaves on the world around it. It could be physical, like a stone axe or an ancient temple. It could be chemical, like the way farming changed Earth's nitrogen cycle about 8,000 years ago—long before anyone invented a radio dish. The key is that these traces look different from anything nature could produce by random chance.
To tell the difference, DeMarines suggests using something called Assembly Theory. The idea is simple: count how many steps it takes to build an object from basic parts. If it requires more steps than random chance would allow, a mind probably made it. By that measure, Earth has had detectable tools for at least 3.3 million years—the age of the Lomekwian stone tools found in Kenya.
Some noosignatures might never be understandable. Nobody knows what the Indus Valley script says, but we know someone intelligent wrote it. That counts.
The exciting part? This framework could catch civilizations that had thinking minds but never solved the trickier problem of building a global, cooperative society. They might have lasted millions of years and left nothing but traces of their intelligence—no radio waves, no satellites, just the marks of minds that came and went.
Right now, this idea is still brand new. At this year's Astrobiology Science Conference, there were 23 sessions dedicated to biosignatures, just one for technosignatures, and not a single session focused on intelligence in between. DeMarines hopes that changes. If scientists start treating these signs as a continuum rather than two isolated categories, they might start finding things they've been missing all along.