Goal 3: Understand how life emerges from cosmic and planetary precursors
Perform observational, experimental and theoretical investigations to understand the general physical and chemical principles underlying the origins of life.
How life begins remains a fundamental unsolved mystery. The origin of life on Earth is likely to represent only one pathway among many along which life can emerge. Thus the universal principles must be understood that underlie not only the origins of life on Earth, but also the possible origins of life elsewhere. These principles will be sought by determining what raw materials of life can be produced by chemical evolution in space and on planets. It should be understood how organic compounds are assembled into more complex molecular systems and the processes by which complex systems evolve those basic properties that are critical to life's origins. Such properties include capturing energy and nutrients from the environment, and manufacturing copies of key biomolecules. Clues from the biomolecular and fossil records, as well as from diverse microorganisms, should be explored in order to define better the fundamental properties of the living state.
Background
We must move beyond the circumstances of our own particular origins in order to develop a broader discipline, a "Universal Biology." Although this discipline will benefit from an understanding of the origins and limits of terrestrial life, it also requires that we define the environmental conditions and the chemical structures and reactions that could support life on other habitable planets. These may be very different than what we have learned to expect from the biology of Earth. For example, liquid water is essential for all life on Earth, however, at least under laboratory conditions, certain chemical systems can undergo a form of replication in non-aqueous solvents. Furthermore, laboratory experiments that involve analogs of the nucleic acids, proteins, sugars, and lipids indicate that the particular molecular structures found in Earth-based life would not be essential in life forms having a genesis independent of life on Earth. The perspectives gained from such research will improve both the search for habitable environments in the Solar System and the recognition of biosignatures within those environments. The invention of translation, the creation of new metabolic pathways, the adaptation of organisms to extreme environments, and the emergence of multi-cellular life forms and other higher order functions, are all constrained by the intrinsic chemistry of the molecules that supported the particular example of life that achieved these innovations. Given this abundance of chemical opportunity, it seems likely that an expanded research effort will lead to novel molecular systems having the combination of properties that we associate with life processes. Such research will help us to understand better the link between molecular evolution and chemistry that is central to astrobiology.
Sources of organic compounds required for the origin of life. To understand how life can begin on a habitable planet such as the Earth, it is essential to know what organic compounds were likely to have been available, and how they interacted with the planetary environment. Chemical syntheses that occur within the solid crust, hydrosphere and atmosphere are potentially important sources of organic compounds, therefore they continue to be an important focus of research on this question. Prebiotic chemistry might begin in interstellar clouds. Laboratory simulations have recently demonstrated that key molecules can be synthesized in interstellar ices that are incorporated into nascent solar systems, and astronomical observations and analyses of extraterrestrial materials have shown that many compounds relevant to life processes are also present in meteorites, interplanetary dust particles and comets. It is likely that substantial amounts of such organic material were delivered to the Earth during late accretion, thereby providing organic compounds that could be directly incorporated into early forms of life or serve as a feedstock for further chemical evolution. An important research objective within this goal is to establish sources of prebiotic organic compounds and to understand their history in terms of universal processes that would take place on any newly formed planet. This will require an integrated program of pan-spectral astronomical observations, sample return missions, laboratory studies of extraterrestrial materials, and realistic laboratory simulations of inaccessible cosmic environments.
Origins and evolution of functional biomolecules. Life can be understood as a chemical system that links a common property of organic molecules—the ability to undergo spontaneous chemical transformation—with the uncommon property of synthesizing a copy of that system. This process, unique to life, allows changes in a living molecular system to be copied, thereby permitting Darwinian-like selection and evolution to occur. At the core of the life process are polymers composed of monomeric species such as amino acids, carbohydrates, and nucleotides. The pathways by which monomers were first incorporated into primitive polymers on the early Earth remain unknown, and physical properties of the products are largely unexplored. A primary goal of research on the origin of life must be to understand better the sources and properties of primitive polymers on the early Earth, and the evolutionary pathway by which polymerization reactions of peptides and oligonucleotides became genetically linked.
Origins of energy transduction. Axiomatically, life cannot exist in an environment at thermodynamic equilibrium. If the environment were at equilibrium, then, by the second law of thermodynamics, no net chemical transformation would be possible. Thus we assume that life began in an environment that was far from thermodynamic equilibrium, so that free energy was available to drive the chemical transformations required for life processes. A fundamental question concerns the mechanisms by which this energy was captured by the earliest forms of life. The forms of available energy include light, chemical bond energy, and the energy of electron transfer reactions involving compounds with different redox potentials. It seems likely that photosynthesis appeared very early in evolutionary history, thus it is important to identify primitive pigment systems. Hydrothermal vents and other geothermal environments offer a second potential source of energy in the form of dissolved gases such as hydrogen and hydrogen sulfide, and mechanisms by which reduced gases in solution can deliver energy to living systems should be investigated. In contemporary cells, the energy present in chemical bonds is captured by metabolism, and the first forms of life must have incorporated linked chemical reactions as simple metabolic pathways. A primary research objective will be to identify mechanisms by which any of these energy sources were coupled to polymerization chemistry.
Origins of cellularity and protobiological systems. For life to begin in a natural setting such as a planetary surface there must be mechanisms that concentrate and maintain interacting molecular species in a microenvironment. From this perspective, life began as a bounded system of interacting molecules, none of which has the full property of life outside of that system. A bounded system of replicating, catalytic molecules is by definition a cell, and at some point life became cellular, either from its inception or soon thereafter. Besides separating the contents of a cell from the environment, membranes have the capacity to develop substantial ion gradients that represent a central energy source for virtually all life today. Boundary membranes also divide complex molecular mixtures into large numbers of individual structures that can undergo selective processes required to initiate biological evolution. A primary objective of research is to assemble laboratory versions of model cells. These will incorporate systems of interacting molecules within membrane-bounded environments. They will have the capability to capture energy and nutrients from the environment, grow through polymerization, and reproduce some of their polymeric components. Approaching this challenging problem will lead to a more refined definition of the living state, and will clarify the hurdles faced by self-assembled systems of organic molecules as they evolved toward the first life on the Earth.
Objective 3.1
Sources of prebiotic materials and catalysts
Characterize the cosmic and endogenous sources of matter (organic and inorganic) for potentially habitable environments in the Solar System and in other planetary and protoplanetary systems.
Example investigations
- Trace the cosmic formation of prebiotic materials
from the formation of interstellar molecules and solids through the
processing of these materials to produce more complex compounds.
- Conduct laboratory experiments and simulations
to provide a framework for analyzing meteorites and samples returned
from asteroids and comets, and for interpreting spectra of interstellar
clouds. Analyze meteorites and returned samples to understand the nature
of extraterrestrial organic compounds.
- Identify the organic compounds and complexes produced under primordial planetary conditions through laboratory simulation experiments.
Objective 3.2
Origins and evolution of functional biomolecules
Identify multiple plausible pathways for the condensation of prebiotic monomers into polymers. Identify the potential for creating catalytic and genetic functions, and mechanisms for their assembly into more complex molecular systems having specific properties of the living state. Examine the evolution of artificial chemical systems that model processes of natural selection to understand better the molecular processes associated with prebiological evolution in the universe.
Example investigations
- Search for mechanisms of enantiomeric enhancement
that introduced chirality into biological systems.
- Investigate polymers other than nucleic acids
that have the potential to have been precursor molecules capable of
containing genetic information.
- Investigate the RNA-catalyzed active site in ribosomes to better understand how RNA could have first evolved to mediate translation in early forms of life.
Objective 3.3
Origins of energy transduction
Identify prebiotic mechanisms by which available energy can be captured by molecular systems and used to drive primitive metabolism and polymerization reactions.
Example investigations
- Search for pigments that were plausible components
of the prebiotic environment and have the capacity to capture and
transduce light energy into chemical energy.
- Investigate redox reactions in which hydrogen
serves as a source of free energy that could plausibly be available
for early forms of life.
- Investigate mechanisms by which early boundary
membranes could couple the energy available in ion gradients to the
synthesis of high-energy compounds such as pyrophosphate.
Objective 3.4
Origins of cellularity and protobiological
systems
Investigate both the origins of membranous boundaries on the early Earth and the associated properties of energy transduction, transport of nutrients, growth, and division. Investigate the origins and early coordination of key cellular processes such as metabolism, energy transduction, translation and transcription. Without regard to how life actually emerged on Earth, create and study artificial chemical systems that undergo mutation and natural selection in the laboratory.
Example investigations
- Determine how ionic and polar nutrients could
permeate membrane boundaries to supply monomers and energy for intracellular
metabolism and biosynthesis.
- Investigate polymerase reactions that can
take place in membrane-bounded microenvironments, using external sources
of monomers and chemical energy.
- Establish membrane bounded protein synthesis
systems that incorporate ribosomes and mRNA in lipid vesicles.