In the vast expanse of the cosmos, the search for extraterrestrial life has long been divided into two distinct camps: the quest for biosignatures and the search for technosignatures. These two approaches, while seemingly disparate, leave a significant gap in our understanding of the universe's potential for intelligence. Astrobiologist Julia DeMarines, in her groundbreaking paper, "Signs and Signatures of Intelligence," introduces a new concept to bridge this divide: noosignatures. This innovative idea opens up a fascinating middle ground in astrobiology, offering a fresh perspective on the search for extraterrestrial intelligence (SETI).
The traditional approach to astrobiology has been to look for biosignatures, such as chemical traces of biological activity, and technosignatures, which are the observable products of advanced technology. However, the transition from microbial life to a civilization capable of transmitting radio waves is a long and complex process. As DeMarines points out, an alien civilization observing Earth 10,000 years ago would not have seen radio waves, but they would have witnessed a world far beyond simple microbial life. This missing middle ground is where noosignatures come into play.
Noosignatures, as defined by DeMarines, are structured traces left by a mind on a medium. These traces can be physical, like stone tools or architectural marvels, or signal-based, such as complex animal communication. The key is that they must be detectable as the product of intelligence, even if their meaning is beyond our comprehension. For instance, the Indus Valley script, despite remaining undeciphered, is a physical residue of cognitive activity, easily distinguishable from biological processes.
To measure noosignatures, DeMarines proposes Assembly Theory, which calculates the "assembly index" of an object. This index represents the number of joining operations required to construct the object from basic elemental components. An object with a high assembly index suggests that it could not have arisen by random chance and, therefore, required a mind to create it. The Lomekwian assemblage, dating back 3.3 million years, would pass this test, as would the impact of agriculture on Earth's nitrogen cycle around 8,000 years ago.
The beauty of noosignatures lies in their ability to capture the worlds that developed intelligence but failed to solve the coordination problem necessary for a cooperative planetary civilization. These civilizations might have lasted for geological time scales but never transmitted radio signals. In such cases, noosignatures could be the only evidence of intelligence on a given world.
However, DeMarines acknowledges the challenges and limitations of this concept. Noosignatures can decay over time, and natural self-organization can be difficult to distinguish from noosignatures. Assembly Theory is still in its infancy when applied to macroscale archaeological structures, and relatively few scientists have explored this area of research. At the Astrobiology Science Conference, there were 23 sessions on biosignatures, one on technosignatures, and none dedicated to intelligence research.
Despite these challenges, the potential of noosignatures is exciting. By recognizing the continuum of astrobiological signatures, scientists may start to discover planets where life falls into this middle category. This opens up a new frontier in SETI, one that could reveal the existence of intelligent life in the universe in ways we have yet to imagine. As DeMarines suggests, the time is ripe for astrobiologists to embrace this new perspective and explore the possibilities it offers.
In my opinion, the introduction of noosignatures is a significant step forward in our understanding of the universe's potential for intelligence. It encourages us to think beyond the traditional biosignatures and technosignatures and consider the rich tapestry of possibilities that lie in the middle ground. As we continue to explore the cosmos, the search for noosignatures may just be the key to unlocking the secrets of extraterrestrial intelligence.
