|Astropaleobiology in the 21st Century
Sherry L. Cady (Department of Geology, Portland State University, P.O. Box 751, Portland OR 97207-0751, USA).
Malcolm R. Walter (School of Earth Sciences, Macquarie Univ., Macquarie N.S.W. 2109, AUSTRALIA)
David J. Des Marais (NASA Ames Research Center, Exobiology Branch, Mail Stop N239-4, Moffett Field, CA 94035-1000, USA)
Astropaleobiology, a new discipline just now emerging, rocketed into prominence in August 1996 with the announcement that a meteorite from Mars contains evidence indicative of early life on the red planet. In spite of the fact that every aspect of the evidence is disputed, the resultant burst of activity presented a challenge to paleobiologists worldwide to provide additional evidence testing the idea that the purported microfossils in the meteorite were of biological origin. A lack of knowledge with regard to ancient-Earth deposits analogous to Mars meteorite ALH84001 quickly became apparent. Given that five-sixths of life's history was ruled by microbial inhabitants, it is surprising that so little is known about the history of microbial life on Earth. Or is it? A decade ago, 50 of 5,000 paleontologists in the world identified themselves as microbial paleobiologists. In other words, only 1 percent of paleontologists is working on the history of what is probably more than 95 percent of the diversity of life.
The primary goal of astropaleobiology, to locate and interpret evidence of former life, requires a multidisciplinary approach that includes scientific efforts from a wide range of disciplines. We report here the results of a discussion held between a diverse group of scientists (paleobiologists, geochemists, and mineralogists), funding administrators, and scientific journalists focused on defining the contributions that paleontologists can make in the field of astropaleobiology.
As a preface to our discussion, Dr. David Des Marais summarized the concept of astrobiology as it has developed at NASA (e.g. http://astrobiology.arc.nasa.gov). Although the exploration for life and former life beyond Earth is clearly of global interest, by far the greatest activity is within NASA. We could not adequately answer whether space programs other than that of NASA are addressing astropaleobiology. While the concept of astrobiology reaches far beyond paleontology, it provides a framework within which the outgrowth of its sister discipline astropaleontology can be understood. Astrobiology is the scientific study of the origin, distribution, and destiny of life in the universe. How do habitable worlds form, and how do they evolve? How do living systems emerge? How can we recognize other biospheres? How have the Earth and its biosphere influenced each other over time? How do rapid changes in the environment affect emergent ecosystems and their evolution? What is the potential for biological evolution beyond the planet of origin? The search for answers to these questions leads to exploration across space and time and necessarily involves a combination of disciplines.
Life elsewhere is much more probable than previously believed. For example, the Drake Equation (http://www.seti.org) and modifications thereof provide a means to estimate the expected number of advanced civilizations in a typical galaxy the size of the Milky Way. Although this equation has no unique solution, it identifies those factors thought to play a role in the development of technological civilizations. If we assume that Darwinian evolution is a universal driving force that leads to the culmination of advanced life from a first cell, surely the possibility that intelligent life exists or evolved on other planets or their moons suggests that those planets are also likely to harbor a fossil record of their precursor microbial world: hence, the natural outgrowth and integration of paleobiology with astrobiology and the novel discipline we call astropaleobiology.
Three major themes emerged from our discussion.
- The nature of life on Earth, particularly as mapped by molecular biologists during the last three decades, requires an expanded view of the sort of life we might seek elsewhere. An important question is whether there is more than one tree of life. In any instance, the point here is that most life on Earth is microbial and apparently able to survive in any niche in which a source of energy and water exists. An improved understanding of the intrinsic and extrinsic physical and chemical parameters that limit the viability of life is needed. Two examples of the types of issues in need of experimental, theoretical, and field investigations include a determination of the lower size limit of microorganisms and the upper temperature limit of life. All three superkingdoms of life on Earth have very ancient origins and should be considered in any search strategy for evidence of life on Earth and elsewhere.
- An appreciation of biodiversity in the microbial world and the diversity of habitable environments here on Earth have led to exploration of previously unrecognized environments in which we might seek evidence of life. Therefore, a reevaluation is needed of the types of paleoenvironments that might host evidence of former life. Numerous paleobiological repositories remain untapped by paleontologists for evidence of biogenicity. Studies of extreme environments (from an anthropogenic viewpoint) have led to the recognition that microbes live in deeply buried subsurface sediments and in the fractures of deep-seated igneous rocks, in the frigid Arctic ocean, in Antarctic lakes, in boiling thermal springs, in acidic and alkaline lakes, and around mid-ocean ridges. By asking questions about where to look for life elsewhere, we recognize new places to look on Earth to reveal better our own fossil record. Although we all know that life is ubiquitous we rarely consider deposits such as paleosols, evaporites, hydrothermal mineral deposits, other ore deposits, desert varnish, endoliths, water ice, and clathrates.
- Given that the search for evidence of early life especially within our own Solar System is focused on microbes and that this search is particularly difficult, we must exploit all of the techniques available to us. We found ourselves discussing a gamut of issues with regards to the techniques necessary to characterize morphologically preserved fossils, chemical fossils (isotopic and organic), and even the composition and history of planetary atmospheres. This interlude led us to question conventional definitions of the discipline of paleontology. Paleontology is not limited to morphologically preserved fossils. Biological information is also stored in signatures and trends of isotopes, hydrocarbon molecules (biomarkers), sedimentary structures and biofabrics (stromatolites and thrombolites), biogenic ore deposits, and atmospheric compositions.
Paleobiologists must contribute to efforts underway to characterize the fundamental chemical, physical, and biological processes that constrain the limits of life. Significant advances require us to go beyond the show and tell approach that has served us well over the last century. There is a need for more comprehensive frameworks for interpreting biogenicity, especially in extreme environments. As previously mentioned, we do not know the upper temperature limit of life or the minimum size of a cell, yet these are essential pieces of knowledge in designing an exploration program as well as to elucidate our understanding of early Earth. Can we accurately determine the limits and metabolic strategies that might be involved in sustaining microbial ecosystems that extend kilometers down into the crust of the Earth? Is it even conceivable that 20-nanometer-wide objects such as those identified in Mars meteorite ALH84001 are fossil cells even though they are an order of magnitude smaller than almost all known bacteria? Participation in multidisciplinary studies that search for answers to such fundamental questions is also necessary to interpret problematic structures, the role of self-organizational processes in the development of sedimentary textures and microfossil-like objects, and issues related to mineral growth on organic templates.
The concept of astropaleobiology is inspirational, not only for us but also for the young people we want to attract into paleontology, to the public at large, and therefore for the politicians who control our funding. It can make a major contribution to programs of public outreach that will benefit all paleontologists.
- Develop a synergy with molecular biology.
- Systematize the collection of critical datasets such as the taxonomic distribution of biomarker molecules.
- Develop websites and links. Lobby appropriate agencies to support this. Build on sites such as those at NASA Ames Research Center and PaleoNet.
- Establish an electronic newsletter on PaleoNet (Jere Lipps volunteered).
- Conduct international workshops. There is a concern to make this an international effort. Dr. Sun Weiguo offered to start by proposing that a workshop be run in China next year. Names of organizers were proposed. Announce in society newsletters.
- Compile and disseminate a list of speakers willing to promote astropaleobiology.
- Facilitate development of traveling museum displays. A start has been made at the University of Colorado, at UC Berkeley, and at NASA Ames Research Center. It is important to give people links to further learning from such displays.
- Encourage contacts with journalists.
- What is paleontology?
- Link between paleontology and molecular biology.
- Inspiring youngsters towards paleontology by means of astropaleobiology.
- Explore habitable space for life on Earth.
- Strengthening fundamental research.
- Getting to the fundamentals of evolution. What is unique and what is not unique to earthly life?
- Develop the systematics of all categories of fossil information.