---
slug: paper-marshall-s-m-et-al-2021-identifying-molecules-as-biosignatures-with-assembly-the
title: Marshall et al. 2021: Assembly Theory and Molecular Biosignatures
canonical: https://miscsubjects.com/a/paper-marshall-s-m-et-al-2021-identifying-molecules-as-biosignatures-with-assembly-the
---

## What the work saw and its core results

Marshall, Mathis, Carrick et al. measured molecular complexity by counting the shortest sequence of joining steps needed to build a molecule from bonds. They called this the molecular assembly index (MA). They showed that molecules with high MA appear in detectable numbers only when produced by living systems.

The paper calculates MA for millions of molecules. It validates that tandem mass spectrometry recovers the theoretical MA values. Samples from biology, the lab, Earth environments, and one meteorite separate cleanly by MA threshold. High-MA molecules in abundance serve as a universal biosignature because random chemistry cannot produce them at observable copy numbers.

## Exact primary work and load-bearing passages

The source is Marshall, S.M., Mathis, C., Carrick, E. et al. Identifying molecules as biosignatures with assembly theory and mass spectrometry. Nat Commun 12, 3033 (2021). https://doi.org/10.1038/s41467-021-23258-x

Key passage from the abstract: “We show why complex molecules found in high abundance are universal biosignatures and demonstrate the first intrinsic experimentally tractable measure of molecular complexity, called the molecular assembly index (MA).”

From the results section on defining molecular assembly: “The MA of an object is the length of the shortest of those pathways, i.e. the smallest number of joining operations required to construct the object, where objects created in the process can subsequently be reused.”

From the probabilistic bounding section: “Our central thesis is that molecules with high MA are very unlikely to form abiotically, and the probability of abiotic formation goes down as MA increases.”

Taxol example: “the natural product Taxol, is an example of a molecule that could be a biosignature—this is because it is so complicated, that the probability of its formation abiotically in any detectable abundance (>10,000 identical copies) would be very small.”

## Convergence patterns touched

The work maps assembly pathways as directed multigraphs that branch from shared bonds. It tracks copy number as evidence of selection. It treats MA as a quantitative record of constraints imposed by information-encoding processes. These patterns align with branching structures, memory through reuse of sub-assemblies, and selection that concentrates high-MA objects.

## Distance from the full OIP/GRAIN synthesis

The paper supplies a mechanistic bridge from physical assembly rules to the emergence of life-like selection. It stops at molecular biosignatures. It does not address the full Ladder from difference through flow, structure, memory, and mind, nor the Mirror Layer in which the observer sits inside the system. The measure remains agnostic to specific biochemistry and therefore sits one step short of a complete account of life as a constrained assembly process.

## Honest limits and disconfirming edges

The model assumes molecular life and counts only the shortest assembly path under valence rules. It does not incorporate full thermodynamic or kinetic barriers, so some high-MA molecules may prove easier to form abiotically than the random-walk model predicts. Experimental validation covers a finite set of samples; extraterrestrial deployment remains untested. The probability calculations rest on simulated trees rather than exhaustive enumeration of chemical space.

## Atomic claims

- Claim: High-MA molecules in abundance cannot form abiotically at detectable levels. Tier: mechanistic. Source: paper abstract and results.
- Claim: MA is recovered experimentally from tandem mass spectra. Tier: human (experimental validation on standards and samples). Source: results section.
- Claim: Assembly pathways are sequences of joining operations starting from bonds. Tier: mechanistic. Source: results on defining molecular assembly.
- Claim: The approach distinguishes biological from non-biological samples across tested sets. Tier: human. Source: results on sample application.
- Claim: The work provides no account of observer-system reflexivity. Tier: speculative (absence in source). Source: paper scope.

## Sources

Source s1: Marshall et al., Nat Commun 12, 3033 (2021). URL: https://www.nature.com/articles/s41467-021-23258-x. Quote: “We show why complex molecules found in high abundance are universal biosignatures...” Summary: Primary paper establishing MA as biosignature metric.

All other references in the paper are cited only where they appear in the 2021 text; no external sources are invented.
