Are we missing the universe's 'noosignatures'? Astrobiology has long been divided between the search for 'biosignatures' and 'intelligence'. These two approaches look for very different things, but they also leave a significant gap in between. It took 3.5 billion years for us to progress from the first microbe to a civilization capable of sending radio waves into space. Detecting life at any stage between these two milestones is a relatively unexplored area of astrobiology, which is the focus of a new paper, 'Signs and Signatures of Intelligence', available in pre-print on arXiv, by astrobiologist Julia DeMarines.
Before delving into this missing middle ground, it's essential to understand the two typical astrobiological categories. Biosignature searches focus on chemical traces like oxygen and methane that suggest biological activity. In contrast, 'technosignatures' represent the observable products of advanced technology, such as radio waves or massive planetary-scale engineering projects.
However, civilizations don't simply emerge from microbes and start emitting radio waves. It's an evolutionary process that takes billions of years. If an alien civilization had observed Earth 10,000 years ago or viewed it from 10,000 light-years away, they wouldn't have seen any radio waves. But they also wouldn't have witnessed a world dominated by simple microbes. So, how do we quantify this 'middle ground' and integrate it into our broader study of astrobiology?
DeMarines introduces the concept of 'noosemiotics', an empirical research framework for the search for 'noosignatures'. A noosignature, according to the paper, is a structured trace that a mind leaves on a medium. This definition is philosophical, but it has practical applications. Noosignatures can be physical, such as stone tools or architecture, or signal-based, like complex animal communication. Crucially, they must remain detectable as the product of intelligence, even if we can't decipher their meaning.
Examples of noosignatures on Earth include the Indus Valley script, which we may never understand, but we know was created by intelligent beings. It's a physical residue of cognitive activity, easily distinguished from biological processes.
Measuring noosignatures is a challenge. DeMarines proposes 'Assembly Theory', which calculates the 'assembly index' of an object. This index represents the number of joining operations required to construct it from basic elemental components. If an object's assembly index exceeds a certain threshold, it cannot have arisen by random chance; it requires a mind to create it.
Earth's Lomekwian assemblage, dating back 3.3 million years, would pass this test. However, noosignatures aren't limited to tools. DeMarines highlights how agriculture significantly impacted Earth's nitrogen cycle around 8,000 years ago, leaving a detectable trace of intelligence thousands of years before we invented radio dishes.
The beauty of this concept is that it captures the worlds that developed intelligence but failed to solve the coordination problem to sustain a moderately cooperative planetary civilization. These civilizations might have lasted for geological time scales but never sent a single radio signal. In such cases, a noosignature could be the only evidence of intelligence on a given world.
DeMarines acknowledges that this idea is still in its infancy and has many kinks to work out. Noosignatures can decay over time if not maintained, and information requires a physical substrate to persist long enough for our telescopes to detect it. Additionally, natural self-organization can be challenging to distinguish from noosignatures. Assembly Theory is still in its early stages when applied to macroscale archaeological structures like complex crystals that can be naturally grown.
Moreover, relatively few scientists have explored this problem. At this year's Astrobiology Science Conference, there were 23 sessions on biosignatures, 1 on technosignatures, and 0 dedicated to intelligence research, with only one abstract. Perhaps, with the publication of 'Signs and Signatures of Intelligence', astrobiologists will begin to view astrobiological signatures as a continuum rather than a graph with two distinct peaks. If they do, there's a chance they'll start discovering planets where life falls into this middle category. This possibility should excite everyone in the field.
Learn More:
- J. DeMarines - 'Signs and Signatures of Intelligence' (https://arxiv.org/abs/2606.28437)
- UT - The Search for Advanced Civilizations is Going Real-Time (https://www.universetoday.com/articles/the-search-for-advanced-civilizations-is-going-real-time)
- UT - SETI Scientists Scan TRAPPIST-1 for Technosignatures (https://www.universetoday.com/articles/seti-scientists-scan-trappist-1-for-technosignatures)
- UT - Technosignatures are NASA's New Target for Detecting Other Civilizations in Space. Wait. What's a Technosignature? (https://www.universetoday.com/articles/technosignatures-are-nasas-new-target-for-detecting-other-civilizations-in-space-wait-whats-a-technosignature)
