Molecular complexity as a signature of life

An abundance of complex molecules has been proposed as a universal signature of life.

Molecular complexity is often measured by the “assembly index,” which calculates the minimum number of joining operations required to construct a molecule from basic components. Assembly indices from abiotic sources, such as meteoritic material or well-mixed chemical systems, are typically low.

But real-world environments are spatially complex. Porous rocks, rich with microcompartments, characterize many planetary bodies, and previous work shows such surfaces can influence polymerization, raising the question of whether spatial structure alone could push abiotic complexity high enough to mimic a biosignature.

Alexandre Champagne-Ruel and colleagues used a model to test how transport dynamics such as diffusion and network topology shape the distribution of complexity in the absence of life.

The authors found that ordered spatial structures, such as regular lattices, can shift abiotic assembly indices upward relative to a randomized control—but this effect is most pronounced under conditions that are unlikely to generate high-complexity molecules.

According to the authors, the findings, published in PNAS Nexus, help define the boundaries within which abiotic, spatially structured chemistry operates, supporting the reliability of life detection based on assembly indices and informing how future missions can be designed to minimize false positives.

The study may also provide design constraints for future prebiotic chemistry experiments attempting to produce lifelike systems in the lab.

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

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Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

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