Intelligence is not rare, but radio-broadcasting civilizations might be.
For decades, the search for extraterrestrial intelligence has been anchored at two extremes — the chemical whispers of microbial life and the loud declarations of radio-broadcasting civilizations — leaving an enormous middle silence unexamined. Astrobiologist Julia DeMarines now proposes a framework for detecting 'noosignatures,' the structured marks that any thinking mind leaves upon its world, from stone tools to reshaped nitrogen cycles, using Assembly Theory to distinguish intention from accident. The framework asks us to consider that intelligence may have risen and fallen on countless worlds without ever reaching for the stars, and that our failure to find them has been, in part, a failure of imagination. In naming what we have not looked for, DeMarines begins the work of looking.
- A 3.5-billion-year gap in astrobiology's search window means that entire epochs of extraterrestrial intelligence — cultures, technologies, collapsed civilizations — could exist without triggering a single detection under current methods.
- The field's own conference calendar reveals the imbalance starkly: 23 sessions on biosignatures, one on technosignatures, and zero dedicated to the vast cognitive middle ground DeMarines is trying to map.
- Assembly Theory offers a potential measuring stick — calculating how many construction steps an object requires to exist — allowing researchers to flag artifacts too complex to be accidental, from ancient tools to agriculturally transformed landscapes.
- The framework is fragile at its edges: noosignatures decay, natural processes can mimic complexity, and detecting them across light-years demands physical substrates durable enough to survive geological time.
- If the approach gains traction, worlds that once appeared lifeless could be reclassified as sites of extinct or non-technological intelligence, fundamentally widening the aperture of the search for life beyond Earth.
Astrobiology has long operated at two poles: the hunt for microbial chemical fingerprints and the listen for radio signals from advanced civilizations. Between them lies a gap of 3.5 billion years — the time it took Earth to travel from single-celled life to a species capable of broadcasting into space. In that interval, intelligence could have emerged, flourished, and disappeared on distant worlds without leaving a single trace detectable by current methods.
Astrobiologist Julia DeMarines has begun to chart this missing territory. In a new paper, she proposes detecting what she calls noosignatures — the structured marks a mind leaves on its environment. These can be physical artifacts like tools or architecture, or environmental transformations like agriculture, which altered Earth's nitrogen cycle thousands of years before the first radio telescope. The Indus Valley script serves as a useful illustration: undeciphered, yet unmistakably the product of thought rather than geological accident.
The measurement problem is addressed through Assembly Theory, which calculates how many construction steps an object requires to exist. Objects too complex to arise by chance alone must have been made — a threshold that Earth's oldest known tools, dating to 3.3 million years ago, would clear. The framework opens the possibility of detecting worlds where intelligence rose and fell without ever achieving planetary-scale coordination or radio communication — civilizations that collapsed, fragmented, or simply chose a different path, yet left noosignatures across their surfaces for geological timescales.
The idea carries real limitations. Noosignatures erode without maintenance, natural processes can produce complexity that mimics intention, and the field has barely begun to engage with the concept — this year's major astrobiology conference dedicated not a single session to intermediate intelligence research. But DeMarines's paper may shift that. If the search for extraterrestrial life is reconceived as a continuum rather than a binary, extinct and non-technological intelligences become detectable, and the universe begins to look considerably less empty.
Astrobiology has a blind spot, and it's been there the whole time. Scientists have spent decades looking for two things: the chemical fingerprints of microbial life—oxygen in an atmosphere, methane in the air—and the unmistakable signatures of advanced civilizations broadcasting into space. But between those two poles lies a vast, largely unexplored territory. It took Earth 3.5 billion years to travel from the first single-celled organisms to a species capable of sending radio waves across the cosmos. In all that intervening time, countless forms of intelligence might have emerged on distant worlds, developed culture and technology, and then vanished—leaving no radio signals behind, and therefore no trace in our current search methods.
Astrobiologist Julia DeMarines has begun to map this missing middle. In a new paper titled "Signs and Signatures of Intelligence," available on the arXiv preprint server, she proposes a framework for detecting what she calls noosignatures—structured traces that a mind leaves on its environment. The concept sounds abstract until you ground it in examples. A noosignature can be physical: stone tools, buildings, engineered landscapes. It can be signal-based: complex patterns of animal communication. The key requirement is that it must remain recognizable as the product of intelligence, even if we cannot decode its meaning. The Indus Valley script, which no one has yet deciphered, is a perfect example. We know it was made by someone thinking. We know it is not the product of random geological or biological processes.
The challenge, of course, is measurement. How do you distinguish the work of a mind from the work of nature? DeMarines points to Assembly Theory, a framework that calculates the "assembly index" of an object—essentially, how many steps of construction it would take to build that object from raw elements. If an object requires too many joining operations to have arisen by chance alone, it must have been made. Earth's oldest known tools, the Lomekwian assemblage from 3.3 million years ago, would pass this test. But noosignatures need not be tools at all. Agriculture, which transformed Earth's nitrogen cycle around 8,000 years ago, left a detectable mark of intelligence thousands of years before anyone invented a radio telescope.
This reframing opens a door to a category of worlds that current astrobiology cannot see. Imagine a planet where intelligent life emerged, developed technology and culture, even reshaped its environment—but then failed to solve the coordination problems necessary to sustain a planetary civilization. Perhaps it collapsed. Perhaps it never unified. Perhaps it simply chose a different path. Such a world might have harbored intelligence for geological timescales, leaving noosignatures scattered across its surface, yet never once broadcasting a radio signal. Under current search methods, that world would appear dead. Under DeMarines's framework, it would be visible.
The idea is young and rough around the edges. Noosignatures decay without maintenance; they require a physical substrate to survive long enough for our telescopes to detect them from light-years away. Natural processes can also create complex structures that mimic the signatures of intelligence—certain crystals form in patterns that might fool Assembly Theory. And there is a deeper problem: the field itself has barely begun to engage with the concept. At this year's Astrobiology Science Conference, researchers organized 23 dedicated sessions on biosignatures and one on technosignatures. For intelligence research between those two poles, there were zero dedicated sessions and a single abstract.
But the publication of DeMarines's paper may change that calculus. If astrobiologists begin to think of the search for extraterrestrial life not as a binary choice between microbes and radio signals, but as a continuum, the implications shift. Entire categories of worlds become searchable. Extinct intelligences become detectable. The universe suddenly looks less empty. That possibility alone is enough to reshape how the field approaches its oldest question: Are we alone?
Notable Quotes
A noosignature is a structured trace that a mind leaves on a medium—physical like stone tools, or signal-based like complex animal communication.— Julia DeMarines, astrobiologist
The Hearth Conversation Another angle on the story
So we've been looking for life in two very different ways—simple chemistry, or obvious technology. Why has nobody thought about the middle ground before?
Because it's hard to define. A microbe leaves chemical traces. A radio signal is unmistakable. But what does a civilization that never got to radio look like? How do you even know you're looking at intelligence and not just an interesting rock formation?
That's where Assembly Theory comes in, right? The idea that complexity itself is a signature?
Exactly. If you have to perform a thousand precise steps to build something, it didn't happen by accident. Nature can make complex things, but not in the way a mind would arrange them. The Indus Valley script is the perfect example—we can't read it, but we know someone wrote it.
But couldn't a planet have intelligence and still leave almost no trace? Wouldn't most of it decay?
Yes. That's the hard part. Noosignatures need maintenance or they vanish. But agriculture changed the nitrogen cycle. That's detectable. A civilization doesn't have to build monuments to leave a mark—it just has to reshape its world in ways that don't happen naturally.
And if we found something like that, what would it tell us?
It would tell us that intelligence is not rare, but that radio-broadcasting civilizations might be. It would mean the universe is full of worlds where minds emerged and did remarkable things—and then either failed or chose not to broadcast. That's a completely different story than the one we've been telling ourselves.