Lecture Abstracts
Lecture Abstracts and PowerPoint Presentations
Questions about life's origins and evolution are among the most profound and controversial topics in science. To tackle life's origin in the laboratory using the scientific method is a daunting challenge. The new field of emergence — the study of complex systems that arise through the interaction of many components — provides a powerful framework for that search. We are surrounded by emergent systems: molecules link to form cells, ants interact to form colonies, and brain cells network to form the conscious mind. In each instance, numerous interacting individual "agent" produce complex systems with new, often delightfully unexpected properties and behaviors. Life's origins can be modeled as a sequence of emergent steps, leading from geochemical simplicity to biochemical complexity.
Watch this lecture as videotaped at the Carnegie Institution, October 27, 2006:
(http://www.carnegieinstitution.org/CSE%20Webpage/)
2. Right and Left: Mineral Surfaces, Molecular Selection, and the Origin of Life's Homochirality
Life arose on Earth as a geochemical process from the interaction of rocks, water, and gases. Prior to the origin of life, the necessary organic molecules had formed abundantly, but indiscriminately, both in space and on Earth. A major mystery of life's origin is how an idiosyncratic subset of those diverse molecules was selected and concentrated from the prebiotic soup to form more complex structures leading to the development of life. Rocks and minerals are likely to have played several critical roles in this selection, especially as templates for the adsorption and organization of these molecules. Our recent experimental and theoretical studies on interactions between crystals and organic molecules reveal that crystals with chiral surface structures may have facilitated the separation of left- and right-handed biomolecules — the possible origin of life's distinctive homochirality. Application of DNA microarray technology to this problem now facilitates the simultaneous investigation of hundreds of mineral-molecule pairs.
3. Mineral Evolution of Terrestrial Planets
The mineralogy of terrestrial planets evolves as a consequence of varied physical, chemical and biological processes. Initial evolutionary stages include the transition from ~12 nano-scale mineral phases in pre-stellar dense molecular clouds, to ~60 primary chondrite minerals, to ~250 different minerals in altered chondrites, achondrites and differentiated asteroids. Earth’s subsequent prebiotic mineral evolution depended on a sequence of geochemical and petrologic processes, including volcanism and degassing, fractional crystallization, crystal settling, assimilation reactions, regional and contact metamorphism, plate tectonics and associated large-scale fluid-rock interactions. These processes resulted in perhaps 1500 different mineral species.
Biological processes began to affect Earth’s surface mineralogy by the Eoarchean, when large-scale surface mineral deposits, including carbonates and banded iron formations, were precipitated under the influences of changing atmospheric and ocean chemistry. The Paleoproterozoic “Great Oxidation Event” and Neoproterozoic increases in atmospheric O2 transformed Earth’s surface mineralogy and are responsible, directly or indirectly, for most of Earth’s 4300 known mineral species.
Mineral evolution arises from three primary mechanisms: (1) progressive separation and concentration of elements from their original relatively uniform distribution; (2) an increase in range of intensive variables such as pressure, temperature, and the activities of H2O, CO2 and O2; and (3) generation of far-from-equilibrium conditions by living systems. The sequential evolution of Earth’s mineralogy from chondritic simplicity to Phanerozoic complexity introduces the dimension of geologic time to mineralogy and thus provides a dynamic alternate approach to framing the mineral sciences.
4. Slow Life: Can High Pressure Enhance Long-Term Survival of Microorganisms?
Purported instances of long-term bacterial dormancy and survival have been ascribed to a variety of environmental factors, including desiccation, freezing, hypersalinity, or anoxia. Here we outline an experimental approach to investigate whether high pressures on the order of 0.1 to 1.0 GPa (1000 to 10,000 atmospheres) might also promote microbial stasis and longevity in some geochemical environments. Several lines of evidence point to the possible influence of high-pressure environments on long-term microbial survival. Experiments and field observations support the contention that microbes survive in a dormant state under high-pressure conditions. Sharma et al. (2002) documented significantly reduced microbial activity, but sustained viability, in strains of Escherichia coli (MG1655) and Shewanella oneidensis (MR1) at pressures to 1.6 GPa.
5. Emergence and the Ambiguity of What is Alive
The question "What is life?" is complicated by the fact that a continuum may exist between chemical systems that are unambiguously nonliving and those that are unambiguously alive. Metabolism and reproduction with variation (and, therefore, the potential for natural selection) seem to represent minimum requirements for a chemical system that is alive, but these criteria may be characteristic of plausible entities that are much more primitive than the simplest bacterium. These speculations are informed by recent experiments on the adsorption and organization of organic molecules on mineral surfaces. An organic film a few molecules thick on a mineral surface, for example, might grow laterally as a two-dimensional film, using oxidation-reduction couples for metabolism. Evolution of this coating could occur through selection and specificity of the most efficient organic species, with eventual vesiculation into individual protocells. The possibility of two-dimensional proto-life raises the interesting geochemical possibility that such a system might be more robust in terms of its pressure-temperature range of stability than life based on nucleic acids. If so, there might today exist volumes of Earth's crust in which such self-organized molecular systems thrive, because they are beyond the predation of more efficient cellular life. If so, how might such life forms be detected? The seeming ambiguity of the divide between nonlife and life is underscored by recent recognition of emergent properties. Many natural systems display complex, unexpected properties that arise from the self-organization of interacting "particles." Such emergent properties have been recognized, for example, in collections of atoms, molecules, cells, and people. This type of behavior is especially relevant to the chemical evolution of life, which may represent a sequence of successively more complex stages of emergence. One might establish a hierarchy of emergent properties — a sequence that leads, for example, through a number of steps from a cluster of organic molecules on a mineral surface to eukaryotic life. However, any defintion that distinguishes between nonliving and living becomes more arbitrary as the number of emergent steps to life increases. Nevertheless, the concept of a sequence of discrete emergences is useful and appealing in experimental and theoretical studies of the origin of life, because it reduces an immensely complex historical process to a succession of several, more manageable chemical episodes. Experimental programs are now elucidating the possible first steps in the geochemical emergence of life.
6. Philosophy of Science Association, Session on the Origin of Life
The hypothesis that life is a chemical process, initiated by the interaction of water, rock, and simple gaseous molecules, suggests an experimental strategy for origin of life research. Under this approach, prebiotic processes are modeled in laboratory environments in apparatus that sustain appropriate conditions of temperature, pressure, composition, and other environmental parameters. A half-century of such experiments, including a recently initiated program at the Carnegie Institution of Washington to investigate the possible role of deep hydrothermal systems in life's origins, raise questions about the context and meaning of origins research. Two of these questions will be addressed here. What is life? The experimental search for life's origins would seem to presuppose an unambiguous definition of living versus nonliving systems. Such a definition, however, has not yet been formulated. In fact, there may exist a continuum of states between non-life and life. Most plausible scenarios for life’s origin begin with the synthesis of organic molecules, which are then concentrated and become organized into larger structures, perhaps through the mediation of minerals as catalysts and templates. According to these models, such highly organized molecular systems, though not alive in the cellular sense, display such life-like properties as metabolism and growth (i.e., they obtain energy from their surroundings while fabricating new molecular structures). This "ur-life" may have been little more than a self-replicating coating of organic molecules on mineral surfaces, and may be undetectable by any current analytical techniques. The emergence of vastly more complex cellular life with nucleic acids is, presumably, a much later stage of development. Given that these "modern" cells probably ate all traces of the earlier stages of life's origins, what will constitute convincing evidence of life's origin? Is life inevitable? Assuming that life arose as a natural chemical process (as opposed to an act of divine intervention), there exists a continuum of possibilities from life being unique to our planet, to life being an inevitable consequence of planetary chemistry under a robust set of conditions. No meaningful observational evidence is presently available to address this question, and experiments on prebiotic chemistry need not be predicated on an answer. Nevertheless, many scientists have taken a strong public stand; many researchers conclude that life is a cosmic imperative. Perhaps this unsupported conclusion arises from a realization that unambiguous experimental results are possible only if life is an inevitable consequence of geochemical processes.