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The grayness of the origin of life — 2021 — Life

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In the search for life beyond Earth, distinguishing the living from the non-living is paramount. However, this distinction is often elusive, as the origin of life is likely a stepwise evolutionary process, not a singular event. Regardless of the favored origin of life model, an inherent “grayness” blurs the theorized threshold defining life. Here, we explore the ambiguities between the biotic and the abiotic at the origin of life. The role of grayness extends into later transitions as well. By recognizing the limitations posed by grayness, life detection researchers will be better able to develop methods sensitive to prebiotic chemical systems and life with alternative biochemistries.

Citation: Smith HH, AS Hyde, DN Simkus, E Libby, SE Maurer, HV Graham, CP Kempes, B Sherwood Lollar, L Chou, AD Ellington, GM Fricke, PR Girguis, NM Grefenstette, CI Pozarycki, CH House, and SS Johnson, 2021. The grayness of the origin of life. Life 11(6):498.

Image description: Figure 1 from the paper cited above, whose caption reads: "Illustration of the three macromolecules used by life on Earth, as both constituent parts (a,c,e) and larger-scale structures (b,d,f), which fulfill the roles described in the chemoton model: metabolism, compartmentalization, and information storage. (a) A polypeptide consisting of eight amino acids. (b) Spinach ferredoxin protein structure, PDB 1A70, showing alpha-helices, beta-pleated sheets and loops structures [19]. (c) A phospholipid containing fatty acid tails made of repeating 2C units. (d) A segment of a phospholipid bilayer, which forms cell membranes. (e) A short chain of DNA, illustrating the Watson–Crick nitrogenous bases. (f) An A-form double helix showing the structure of DNA storage."

Identifying molecules as biosignatures with assembly theory and mass spectrometry - 2021 - Nature Communications


The search for alien life is hard because we do not know what signatures are unique to life. 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). To do this we calculate the complexity of several million molecules and validate that their complexity can be experimentally determined by mass spectrometry. This approach allows us to identify molecular biosignatures from a set of diverse samples from around the world, outer space, and the laboratory, demonstrating it is possible to build a life detection experiment based on MA that could be deployed to extraterrestrial locations, and used as a complexity scale to quantify constraints needed to direct prebiotically plausible processes in the laboratory. Such an approach is vital for finding life elsewhere in the universe or creating de-novo life in the lab.

Citation: Marshall, SM, C Mathis, E Carrick, G Keenan, GJT Cooper, H Graham, M Craven, PS Gromski, DG Moore, SI Walker, and L Cronin. Identifying molecules as biosignatures with assembly theory and mass spectrometry. Nature Commununications 12, 3033 (2021).

Image Description: Figure 1 from the paper cited above, showing the assembly pathways of an object made of red/blue blocks, a word, and a molecule as stepwise additions of new units or repeated blocks. The figure also shows a model of pathway assembly as a random walk along a weighted tree and a chart showing the probability of the most likely path through the tree as a function of the path length.

Generalized stoichiometry and biogeochemistry for astrobiological applications - 2021 - Bulletin of Mathematical Biology


A central need in the field of astrobiology is generalized perspectives on life that make it possible to differentiate abiotic and biotic chemical systems McKay (2008). A key component of many past and future astrobiological measurements is the elemental ratio of various samples. Classic work on Earth’s oceans has shown that life displays a striking regularity in the ratio of elements as originally characterized by Redfield (Redfield 1958; Geider and La Roche 2002; Eighty years of Redfield 2014). The body of work since the original observations has connected this ratio with basic ecological dynamics and cell physiology, while also documenting the range of elemental ratios found in a variety of environments. Several key questions remain in considering how to best apply this knowledge to astrobiological contexts: How can the observed variation of the elemental ratios be more formally systematized using basic biological physiology and ecological or environmental dynamics? How can these elemental ratios be generalized beyond the life that we have observed on our own planet? Here, we expand recently developed generalized physiological models (Kempes et al. 2012, 2016, 2017, 2019) to create a simple framework for predicting the variation of elemental ratios found in various environments. We then discuss further generalizing the physiology for astrobiological applications. Much of our theoretical treatment is designed for in situ measurements applicable to future planetary missions. We imagine scenarios where three measurements can be made—particle/cell sizes, particle/cell stoichiometry, and fluid or environmental stoichiometry—and develop our theory in connection with these often deployed measurements.

Citation: Kempes, CP, MJ Follows, H Smith, H Graham, CH House, and SA Levin, 2021. Generalized stoichiometry and biogeochemistry for astrobiological applications. Bulletin of Mathematical Biology 83, article 73.

Image description: Figure 5 from the paper cited above, showing the ratio of the cellular to environmental nitrogen as function of the size spectrum exponent, the minimum quota (cellular requirement) scaling exponent, and the growth rate scaling exponent.

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