The Astrobiology Primer 3.0: The dynamic field of modern astrobiology
By Catherine Maggiori
Synopsis: The Astrobiology Primer 3.0 is a brief introduction for scientists new to the discipline and all those curious about astrobiology. Over 12 sections, the Primer outlines the chief hypotheses and areas of research in this growing and interdisciplinary field, complete with broad swaths of references and other research materials. Here, SAGANet provides a summary and review so you can decide if diving into the Primer is right for you.
The cover art from the new Astrobiology Primer 3.0 special issue of the journal Astrobiology
I recently attended AbSciCon24 (the Astrobiology Science Conference), which is probably my personal favourite academic conference to attend.
I’m a dyed-in-the-wool biologist, with the vast majority of my professional work done in the context of environmental microbiology and biomolecule detection. That means that while I love hearing about all of the different facets and recent discoveries of astrobiology (AbSciCon is a constant battle of “should I attend Talk X that’s relevant to my current work on life detection and microbiology, or Talk Y that’s about hydrogen cyanide chemistry?”), I'm not always able to deeply comprehend the finer points of things outside of my area of expertise.
That issue is not the fault of the presenter or the subject matter; it just means I have a specialized research background. Depending on how you look at it, it’s either a crisis of knowledge or an opportunity to educate myself (or a cris-atunity!) and I try to view it as an opportunity.
The Astrobiology Primer was born of a similar attitude from author Dr. Lucas Mix, who describes feeling overwhelmed by the scope and specialized jargon of astrobiology as a graduate student.
Dr. Mix wanted to compile an accessible, relatively brief compendium of current knowledge and innovations in the interdisciplinary and growing field of astrobiology. This led to the original release of the first Astrobiology Primer in 2006. A second edition, with expanded chapters and more ideas from across the realm of astrobiology, followed in 2016. And now, the Astrobiology Primer 3.0 has been released this year and is available open access on the Astrobiology journal website.
The Primer 3.0 is broken down into subject-specific chapters that incorporate knowledge from 60 authors and 12 editors.
(Please note: direct quotes from the Primer will be indicated in italics and quotation marks, “like so.” This review is a simplified summary of the Primer; points have been condensed for clarity and ease of communication. It is not an exhaustive summary of the Primer, which itself is not an exhaustive list of the literature. I encourage you to check out the Primer and its surfeit of supplementary data for yourself.)
Astrobiology is the study of the origin, future, and distribution of life in our universe, from lenses of geology, chemistry, biology, astrophysics, and more, as well as across space, time, and scale. The search for and study of life in our universe is a fundamentally human one; people have been looking up at the sky and wondering about where we come from and what else is out there far beyond modern space missions. So much of astrobiology is an active area of research and requires critical investigation and cooperative community involvement.
The Astrobiology Primer 3.0 describes 3 key commitments and driving principles for astrobiologists:
“Complex questions require collaboration.” Astrobiology poses some inherently complicated questions that challenge current breadths of knowledge and worldviews. (Where did we come from? Where are we going? What else is out there? What is life?) It necessitates cooperation from not just within STEM fields, but also social scientists, philosophers, ethicists, and historians.
“Support for early career scholars pays off.” The astrobiology community is broad and involves cross-disciplinary and cross-generational interactions inherently. As stewards of knowledge, astrobiologists should seek to support new colleagues and junior members of the community; we all stand on the shoulders of giants.
“Jargon can be avoided.” Clear and effective communication benefits not only specialized members of the astrobiology community, but also early career researchers, students, and members of the general public. Avoiding exclusionary and overly technical jargon ensures that our research reaches a broad audience and makes us better communicators.
Chapter 1 of the Primer outlines its “unifying concepts”, which are discussed in depth in the remaining 10 chapters.
In Chapter 2, the Primer begins by introducing “life” as a scientific concept and framework. NASA defines astrobiology as “the study of the origin, evolution, and distribution of life in the universe” (https://astrobiology.nasa.gov/about/), but what exactly is “life”? This may depend on the historical context in which the question is being asked and who you’re asking.
The Tree of Life: a schematic representation of the connectivity of all currently known living things on Earth. From the Primer 3.0.
There is no unified definition of life. Is life the ability to reproduce or respond to stimuli? Does it have DNA or proteins or cells? Does it have a “soul”? Can life be tested with the scientific method? What about viruses? The definition of life cannot be a static one and should not rely solely on our current single data point (i.e. us).
NASA’s working definition of life is that “life is a self-sustaining chemical system capable of Darwinian evolution.”
Features that enable Darwinism (i.e. genetic biomolecules) can thus be used as the beginning of a framework that recognizes the diverse phenomena that comprise “life” and its requirements. This working definition of life is particularly useful for astrobiologists, when we want to be both as specific and inclusive as possible. For instance, when you’re designing a life detection experiment, what can you detect that both unambiguously signifies the presence of life and is broad enough in an environment to remain stable spatially and temporally? (A lot of my previous work focused on using DNA as a biosignature based on this definition of life and the polyelectrolyte theory of the gene. DNA is wonderfully definitive as a sign of life, but not super stable over long timescales. Having an operational definition of life keeps the context of your target biosignature and its environment in mind.)
We may never have a fully comprehensive, generalized, and singular definition of life; instead, astrobiologists work to understand life in the universe in the context of its dynamism, complexity, and potential for emergent, unpredictable qualities, as well as within the limits of our current knowledge. We can use constraints of chemistry, physics, and geology to better predict life, its products, and the repeatability thereof.
The modes of origin of the chemical elements. From Chatper 3 of the Primer 3.0.
Chapter 3 and Chapter 4 of the Primer take a step backwards (literally) to the origins of planetary systems and life on early Earth, respectively.
Understanding the environmental context in which life emerged on Earth can further constrain our working definitions of life and contribute to how we search for life elsewhere. Life was likely already present on Earth ~3.5 bya based on stromatolite evidence from the Dresser Formation and the Strelley Pool Formation (both in Australia) and may have even been present some 4.1 bya (there’s some controversial isotopic and morphological evidence “suggestive of biological activity” in Hadean zircon samples from Jack Hills, Australia).
The origin of life from non-life on early Earth (aka abiogenesis) could have occurred in submarine hydrothermal vents, hot springs, oil slick-type ocean surfaces, or deep subsurface environments beneath oceanic crusts. Some environmental conditions on early Earth were also conducive to life as we know it: salts to form complexes with organic molecules, high ambient temperatures to increase reaction yields, acidic pH, and water-rock interactions to form minerals. Water-rock interactions like serpentinization could also have contributed to the formation of biologically relevant monomers (i.e. the precursors of biomolecules like proteins and nucleic acids). Large proto-metabolic networks could then have risen from a series of linked chemical reactions and autocatalysis to create feedback loops that generate more and more products and catalyst intermediaries.
The shift from prebiotic chemistry to cellular biology continues in Chapter 5, which describes terrestrial life’s major biological innovations over time: the emergence of LUCA (the Last Universal Common Ancestor), the blueprint for all life today; oxygenic photosynthesis, which resulted in an explosive increase in the diversity and expansion of life on Earth; the emergence of eukaryotes, the foundation for complex multicellularity, and; multiple independent origins of multicellularity.
So, what was LUCA? At its core, LUCA was an entity or population of beings that harnessed the energy of prebiotic chemical reactions for its own growth and maintenance. It probably had some kind of lipid membrane, a genetic code, and an informational aspect that linked DNA and RNA to metabolism. It was also probably chemolithoautotrophic, not dissimilar genetically to modern microbes that live in and around hydrothermal vents.
There are energetic limits to chemoautotrophy, but these limits can be expanded when using light as an energy source (i.e. phototrophy). Phototrophy and photosynthesis allowed for new ecological niches and metabolic byproducts (i.e. oxygen), increasing carbon cycling, contributing to the formation of Earth’s ozone layer, increasing the bioavailability of other elements like nitrogen, phosphorus, and sulfur, and resulting in the Great Oxidation Event.
Eukaryogenesis occurred from the consolidation of a bacterial endosymbiont and archaeal host, resulting in a completely new cell type and the third domain of life. Asgardarchaeota and proto-eukaryotic lineages probably diverged ~2.5 bya and the last Eukaryotic common ancestor (LECA) probably emerged ~1.8 - 1.6 bya. The transition from single to multicellular organisms can occur via aggregative multicellularity or clonal multicellularity, but only clonal multicellularity could have resulted in large, complex organisms like animals and plants.
Both types of multicellularity have arisen independently multiple times in bacteria and eukaryotes. Simple multicellularity probably occurred relatively early in the development of bacteria and eukaryotes, but complex multicellularity arose much later. This delay could be the result of changing global environmental conditions but this is among the many open questions remaining in astrobiology.
Most terrestrial life is bacterial or archaeal, and the abundance of extreme environments beyond Earth indicates that any putative extraterrestrial life is equally as extreme and probably microbial. Chapter 6 discusses the breadth and limits of life on Earth, which are important constraints for astrobiology.
Life on Earth is diverse in terms of size, structure, metabolism, and environments inhabited, but there are common threads, including shared major biomolecules, cellular structure, and preference for certain elements. These commonalities, as well as studying life in terrestrial analog environments, can contribute to the development of astrobiology missions and life detection experiments.
Factors impacting habitability of environments. From Primer 3.0.
Chapter 7 introduces defining and searching for habitable environments beyond Earth (although it is often easier to rule out environments that are not habitable for life as we know it): regions with available CHNOPS elements, energy gradients, and relatively mild temperatures, pressures, and pH. Many bodies in our Solar System have potentially habitable environments, including Mars, Europa, Enceladus, and other icy worlds. Constraining exoplanet habitability is far more challenging, but there may be potentially habitable Earth-like exoplanets in the TRAPPIST-1 system and around Proxima Centauri.
Searching for life beyond Earth (Chapter 8) necessitates searching for specific biosignatures. Biosignatures are a diverse class of phenomena that can include chemical features, morphology, spectra, distribution of elements/molecules, physical processes, and mineralogy that indicate the presence or past presence of life in an environment. Biosignatures vary in abundance, temporal stability, reliability, and ubiquity across life; thus, verifying that a potential biosignature is indeed a sign of life requires multiple lines of evidence and confidence that it was not produced abiotically. “As said in Sagan et al. (1993): “life is the hypothesis of last resort””.
“Terran biosignatures” (i.e. those associated with Earth life) may be common to all life-forms or limited to terrestrial life; we don’t know. Life exists along a continuum, resulting from a metamorphosis of non-living chemistry into biochemistry. Chapter 9 considers life as we don’t know it in astrobiology and how to search for signs of life without assuming any kind of biochemistry. Agnostic biosignatures can involve searching for hypothetical alternative biomolecules, enantiomeric excesses, environmental disequilibrium, and specific isotopic ratios.
When searching for life beyond Earth, how do we ensure that we are not contaminating a foreign body with Earth life (i.e. forward contamination) or bringing any potential extraterrestrial lifeforms back with us (i.e. backward contamination)? Chapter 10 discusses the history, future, and science of planetary protection, and the logistical and ethical questions that arise when preparing to explore another world.
Decontamination procedures for modern spacecraft include dry heat microbial reduction, wet heat microbial reduction, alcohol treatments, radiation treatments, and ethylene dioxide treatments. International space law is primarily outlined in the Outer Space Treaty (OST) and the Committee on Space Research (COSPAR) has formulated international planetary protection guidelines, which are used by national agencies to establish their own planetary protection policies. In the United States, for instance, this work is directed by NASA’s Office of Planetary Protection (OPP).
But how should we view the importance of disturbing a potential ecosystem on another planet, either with remote scientific operations or a prospective human settlement? Is the potential for a cataclysmic event affecting the survival of humankind or humanity’s innate curiosity enough of a reason to risk forward contamination of an extraterrestrial body? Additionally, how do we ensure that all peoples, regardless of nation or stake in private entities, are included in addressing these questions? “Beyond scientific, environmental, and economic motivations of planetary protection, some scholars critically engage with the historical and ongoing treatment of indigenous populations, as well as the prioritization of industrial benefit over moral considerations, in the context of interplanetary travel and exploration.” Astrobiologists should seek to develop missions/concepts outside of settler colonialism and “non-exploitive systems for potential humans inhabiting other worlds that are not dominated by the interests of a select few.”
Chapter 11: Astrobiology Education, Engagement, and Resources provides lists of institutions with astrobiology programs and courses, relevant granting agencies, and funding opportunities. Example course materials are supplied for a variety of education levels: high school, undergraduate, graduate and beyond, as well as nano/short courses and informal education (i.e. science centers, museums, planetariums, citizen science projects, and social media). The Primer also discusses public engagement in astrobiology and its societal impacts. Astrobiology can be explored not just through STEM studies, but also from cultural significance and spirituality.
Some of the research institutions around the globe known for astrobiology relevant research. From Primer 3.0.
So, is this a successful “overview of the investigations and driving hypotheses that make up this interdisciplinary field”? Without a doubt, yes.
The Primer includes relevant discussions of current literature, the historical development of the field, and the ever-evolving nature of astrobiology. As Mix notes in the foreword, science changes over time. Information and current knowledge changes and grows. We are standing on the shoulders of giants, building on what’s come before - from Metrodorus, to Sagan, to messages sent out beyond our Solar System.
Astrobiology is constantly changing in scope, in knowledge, and in technology.
At AbSciCon24, I attended a plenary session on “Communicating Discoveries in the Search for Evidence of Extraterrestrial Life” featuring author Jaime Green. Jaime said something that really stuck with me as an astrobiology postdoc struggling with tough experiments; essentially, searching for life and detecting biosignatures is not a binary process yielding a yes-or-no answer. Science is ambiguous; it exists on a scale of probability and understanding. We'll probably never have a definitive answer on what “life” is and we'll always be searching for it.
What an amazing sentiment; our scientific journey, like astrobiology, is dynamic, constantly growing, and compelling us forward.
Author info: Dr. Catherine Maggiori is an astrobiologist and microbiologist. You can find her at the bench, lurking on Twitter, or at the climbing gym.