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Executive Summary

Habitable Planets and Evolution of Biological Complexity

Earth is the prototype by which the feasibility of life on other planets in the Solar System or on planets in other star systems is measured. It is reasonable to assume that if life exists in other planetary systems, it will exist in conditions that will, to a great extent, mimic those that created and sustained life on Earth. By understanding the early-Earth environment and the evolutionary processes that allowed life to appear in the form of simple and then more complex organisms, scientists can better establish the criteria by which the search for other habitable planets can be guided.

Our research at the University of Washington has centered on three important astrobiological questions: What are the characteristics of planets that can evolve complex organisms? Where might such planets occur? How does biological complexity evolve on a planet, and how might it end?

Earth is our only example so far of a life-supporting planet. We therefore have initiated programs of research identifying key times and processes in Earth history (its formation, early bombardment, plate tectonics, evolution of surface and atmosphere) and likewise in the history of its life over billions of years: early evolution, continual adaptation to changing conditions, extinctions, and resurgences. We then try to generalize as much as possible to other possible sites for life and how life in those environments would compare to Earth’s. From this rationale we have examined the following questions, with new results.

How often, where, and under which conditions do habitable planets form and persist?

We define a habitable planet as a solid body capable of supporting life as we know it on Earth — carbon-based, requiring liquid water, and capable of self-replication and evolution. The study of extrasolar habitable planets involves a broad interdisciplinary approach that extends from understanding how planets are formed to understanding the conditions that allow such life to originate, survive, and evolve.

There exists a huge gap (by a factor of 100 in mass) between the extrasolar giant planets discovered with current observational techniques and Earth-like planets. Although gas-giant planets are not habitable (but their moons may be), they greatly affect planetary system formation and evolution, and thus their presence may prove important for the existence of habitable worlds. These connections include the overall dynamical stability of the planetary system, the influence of gas giants on planet formation in the terrestrial zone, and the gravitational influence that giant planets have on small bodies such as comets and asteroids (and therefore on where and how they impact other bodies). All these issues profoundly affect the type of environment that any potential life must face. Examples of the factors involved include (1) the size of the planet (large enough to retain volatiles, as well as drive long-term active tectonism and volcanism that could support microbial life), (2) the early delivery of water and organic compounds to the planet by means of comets, (3) the presence or absence of a stable orbit and obliquity in order to maintain a relatively stable climate, and (4) periodic catastrophic events such as bolide impacts that are perhaps necessary to create and maintain high variability of habitable conditions, producing increased biodiversity and biocomplexity. During the past year simulations have been performed of massive protoplanetary gaseous disks using unprecedented resolution. It has been shown that such disks become gravitationally unstable and form condensations along spiral arms that contract to protoplanetary densities in a few hundred years. These protoplanets have masses and orbital eccentricities remarkably similar to those of observed extrasolar planets. The unprecedented resolution of these simulations has shown the viability of a new paradigm for planet formation: the gravitational instability model.

What caused the delivery and retention of organics and volatiles through Earth’s history?

The volatile and organic composition of impacting bodies is a key factor in the evolution of habitable planets. What are the relative roles of large comet and asteroid fragments and interplanetary dust particles (IDPs) in bringing these materials to a planet? Although the role of IDPs may be important, it is poorly understood. Detailed measurements of IDPs in our own Solar System, combined with interpretation of infrared-emitting dust clouds observed to surround young solar-like stars, will be the basis for developing a detailed theoretical model of the delivery of organics to Earth-like planets. Not only must a habitable planet be supplied with volatile materials, it must also retain them. Retention of volatile materials is a function of a planet’s size, internal evolution, and orbital parameters. Because of its small size, its distance from the parent star, and its relative accessibility, Mars provides an excellent case for investigating the limits of habitability.

Our recent work has focused on the role of 10-µm and 200-µm particles as carriers of organic material to early Earth. We are studying the 10-µm cometary and asteroidal particles collected from the stratosphere and 50-300-µm extraterrestrial particles collected from the 140- by 25- by 25-m volume of ice melted to provide water for the South Pole station. We are employing a variety of techniques to characterize the organic contents and compare the small particles with those of 200-µm size that dominate Earth’s mass accretion of extraterrestrial material. Particular emphasis is on the role of atmospheric entry heating in modifying the organic materials, and a special oven has been built to simulate the effects of hypervelocity entry. From the collected particles and laboratory heating experiments we see that the organics in small particles meet three fates: (1) survival with only moderate alteration, (2) sublimation into the atmosphere, and (3) conversion to refractory char (similar activated carbon). All three of these forms must have been abundant on the pre-biotic Earth and could have played critical roles in the evolution of life.

What are favored stellar and galactic environments for the origin and existence of life?

Our exploration of habitability has also considered processes that span the size and time scales of the Milky Way galaxy. Our goal is to determine the optimal places and times in the Milky Way for the formation of planets that are suitable for life. So far, optical spectra have been obtained of planet-hosting candidate stars at the McDonald Observatory’s 2.7-meter telescope in Dec. 2001, and in March and April 2002. Making use of very-high-resolution spectra of eight stars with planets that were obtained by our colleagues in Texas, they set upper limits on the lithium-6/7 isotope ratios, finding no evidence for lithium-6 in any of the stars. In particular, the detection of Lithium-6 in the atmosphere of HD 82943 could not be confirmed.

What is the role of plate tectonics in maintaining planetary habitability, and how common might it be on other planets?

The importance of plate tectonics in planetary habitability is controversial. For microorganisms it is unclear, but for complex metazoans there is evidence suggesting that plate tectonics may be a key to their appearance and preservation. For more than two decades it has been understood that plate tectonics provides a climate feedback mechanism allowing relatively stable global temperatures over long periods, and that it may have been the single most important mechanism allowing Earth to maintain liquid water on its surface for the past 4 billion years. Plate tectonics is also the dominant force causing changes in sea level, in creating continents, and even, perhaps, in maintaining in an indirect way magnetic fields of the terrestrial planets.

The major thrust of this work has been modeling the interaction of key tectonic processes. The results obtained point to a very small critical yield stress required for initiation of plate tectonics. Second, a number of arguments indicate that the difference between the strength of a wet lithosphere and that of a dry lithosphere can be big enough to control the very existence of plate tectonics, and that the presence of liquid water can be essential for sustaining plate tectonics. We constructed a simple box model to understand the feedback between mantle convection, rheology, and water and carbon cycles. By considering such competitive processes as partial melting, induced dehydration of the mantle, and recycling of water through percolation into the lithosphere from the surface, we find that under certain conditions the system tends to “avoid” temperatures below the freezing point or above the boiling point of water. Outside this temperature range percolation of water into the lithosphere is inefficient and plate tectonics ceases. Our results to date thus show that plate tectonics requires a very weak lithosphere. This might be the result of water. Also, the presence of a sufficient amount of liquid water on a planetary surface can be essential for sustaining plate tectonics. Feedback between plate tectonics and climate can result in a stable equilibrium surface temperature within the stability range of liquid water.

How do mass extinctions and impacts affect the evolution and survival of complex organisms, that is, the long-term habitability of planets?

Mass extinctions are short-term events that kill off a significant proportion of a planet’s biota; on Earth they have been of greatest consequence to more complex organisms such as metazoans. Surface life is vulnerable to major planetary catastrophes, for example, the impact of a large comet or asteroid, radiation, and particles from a nearby supernova, or catastrophic climate changes such as intense intervals of greenhouse heating or Snowball-Earth-type episodes. It may even be that life on the surface of Earth was repeatedly sterilized during the period of heavy bombardment about 4 billion years ago (4 Ga), only to be re-seeded by a possible "reserve" of deep-Earth microbes. But if the animals and higher plants are ever wiped out by a catastrophe, they cannot be immediately restocked from some underground reserve; rather, biocomplexity must ”revolve,” a process lasting at least hundreds of millions of years.

Judging from the history of life on Earth, mass extinction events (MEEs) have the potential to end animal life on any planet where it has arisen. On Earth there have been about 15 such episodes during the last 500 million years: five eliminated more than half of all species then inhabiting our planet. There were also an unknown number during the Archaean and Proterozoic. The frequency and severity of MEEs thus influence the biocomplexity of a planet, and are appropriate areas of astrobiological study.

MEEs have significantly affected the evolutionary history of Earth’s biota in two competing ways. After each of the major events of the Phanerozoic, biotic diversity was substantially reduced for several million years. Yet these same extinctions and diversity depressions were each followed by periods of rapid diversification, composed of different assemblages of organisms, that resulted in more global biodiversity existed prior to the MEE. MEEs thus seem linked to diversity enhancement, as well as to formation of biotic novelty.

During the past year two major mass-extinction boundaries were studied: the Permian-Triassic boundary in Australia and South Africa, and the Triassic-Jurassic boundary in the Queen Charlotte Islands and the Newark Basin. Isotopic and paleontological results from these study areas indicate that both of these extinction may have been protracted rather than sudden, and that single-impact models for mass extinction do not pertain to these events. While impact may have been one source of environmental stress, it was not the only cause of the mass extinctions.

What can we learn from the geological and fossil record about the evolution of eukaryotes and metazoans?

Though we can extrapolate back from existing organisms or build theoretical biogeochemical models, the only robust empirical data that show how early life on Earth evolved and interacted with its environment come from the study of early Precambrian rocks. Despite popular misconceptions, globally there is in fact a moderate abundance of well-preserved Archaean (>2.5 Ga) and Palaeoproterozoic (2.5-1.6 Ga) rocks, which can serve as our clearest windows on the events that occurred shortly after life’s origin. Some of these rocks formed during the period when Mars, the best current candidate for having once developed extraterrestrial life, was evidently warm and wet, potentially allowing life to radiate there too. Thus, the primordial geological, geochemical, and paleontological records of Earth might serve as proxies for what will be encountered on the initial astrobiological missions to Mars. Moreover, the advent of eukaryotes, a major step in the development of biological complexity, now appears to have occurred during this time interval It is important to understand the state of the biosphere before, during and after this evolutionary transition, especially with regard to studying the relationship between oxygenation of the surficial environment around 2 Ga and the introduction of eukaryotes.

Research was conducted on Archean U-Pb geochronology, basalt trace-element geochemistry, hydrocarbon biomarker geochemistry, sulfur isotopic fractionation, microfossil recognition, and Paleoproterozoic hydrocarbon preservation in fluid inclusions. The principal outcomes were as follows:

1. Determination of a precise age for the Sulphur Springs hydrothermal base-metal deposit at 3.235 Ga, thus dating the oldest known oil and bitumen, constraining the oldest known fossilized subsurface microbiota, and establishing the existence of thermophile organisms very early in Earth’s history.
2. Documentation of Paleoproterozoic oil/methane/carbon dioxide/water fluid inclusions from Elliott Lake, thus demonstrating the survival of complex hydrocarbon molecules for billions of years under closed-system and high-pressure conditions and establishing a base-line for preservation of such biosignatures on other planets. These findings resolved a controversy about the presence of widespread continental crust in the early Archean based on basalt geochemical evidence for crustal contamination, thus showing that Earth’s surface differentiated early in its history and hence provided a source of refractory elements such as phosphorus.
3. Completion of a major study of hydrocarbon biomarker molecules in late Archean sediments from the Hamersley Basin, showing that cyanobacterial and eukaryotic lipids are present in rocks half a billion years before body fossils of these groups appear in the geologic record, and that molecular fossils can survive for much longer under high thermal regimes than previously expected.

Thus, the age of the Sulphur Springs hydrothermal deposit has been precisely determined, showing it to be the oldest known site with a fossilized subsurface microbiota that metabolized hydrocarbons, and a detailed study of hydrocarbon fluid inclusions from Elliott Lake shows that complex organic molecules can survive metamorphic conditions in closed systems at high pressures for ~2.4 billion years.

What can we learn from the physiology and molecular characteristics of extant life about the evolutionary pathways by which microbes and their communities evolve, and by which complex organisms originate?

All extant life on Earth arose from a common genetic ancestor. Although there may have been competing genetic codes, only one code survived. The very first stage in creating biocomplexity was the building of the microbial genome to a size that would allow independent growth and reproduction. This may well have been accomplished by extensive lateral transfer of genes between "cells" that were dependent because of limited genetic information on other "cells" for growth and survival. The 0.5-billion-year period prior to 3.5 Ga probably experienced extensive evolutionary experimentation and very limited physiological diversity. Recent evidence also points to lateral gene transfer between Bacteria and Archaea as the main mechanism involved in the formation of eukaryotes. Subsequently, symbioses between specific kinds of bacteria and eukaryotes contributed to the rise of oxygen-respiring and oxygen-producing multicellular biota. Today, both lateral gene transfer and interdependence of different species living in communities are ancient processes that continue in the microbial world.

One way to study this important period in ancient Earth history is through gene sequencing of extant organisms. At the end of this first year of our funding the genome sequence of Methanococcus maripaludis has recently reached the final, tenfold coverage stage of shotgun sequencing, and is entering the finishing phase. This species is important because it may be a good model for the type of early Earth organisms flourishing in the Archean. We have manually annotated (inferred function) for 10% of the 1,800 genes. Recently other workers have generated a partial sequence for two species that are closely related to M. maripaludis, and we have begun to discuss a collaboration to study the role of lateral transfer and gene loss in the evolution of the methanococcal lineage. Work also continues on the development of methods for random mutagenesis of the genome.

What was the pathway to multicellularity?

The invention of multicellularity was a major biological innovation contributing to new states of biocomplexity. This was preceded, however, by the invention of eukaryotic cellular organization. The genome sequences from bacteria have yielded many surprises, including the presence of genes thought to be present only in eukaryotes. We have found genes for tubulin in four species of Prosthecobacter, a genus of the bacterial division Verrucomicrobia. This is the first report of tubulin genes in a prokaryotic organism. Two tubulin genes are found including i>btuba and btubb, which appear to be homologs for alpha- and beta- tubulin genes of eukaryotes, respectively. The genes occur in an operon along with a gene for light chain kinesin. RT-PCR has been used to demonstrate that the genes are being expressed. Using deduced amino acid sequences, we have constructed the three-dimensional proteins. The BtubA and BtubB do not appear to form a dimer as is found in eukaryotic tubulin. This finding is consistent with the apparent absence of microtubules in the cells of Prosthecobacter dejongeii when observed in thin sections.

The discovery of tubulin genesin bacteria raises questions as to their origin. They could have been transferred horizontally from a eukaryotic organism to these bacteria or vice versa. The goal of our current research is to better understand the origin of the bacterial tubulin genes that are very distantly related to those from eukaryotes, indicating that the transfer, if it occurred, must have happened a long time ago.

What was the nature of early Earth communities?

A second way to examine the ancient Earth is through study of microbial communities that likely resemble those of the Precambrian. These communities include those found in (1) anaerobic and photosynthetic microbial mats and biofilms, (2) the sub-seafloor associated with deep-sea hydrothermal vents, and (3) water ice. It is possible that hidden in the presently unknown diversity of these ecosystems there exist organisms with metabolic pathways that are relics of common metabolisms of the past.

Members of the University of Washington’s NAI group participated in two Ecogenomics EMERG group field trips in 2001 (June and October) to the Guerrero Negro salt company evaporation ponds in Mexico. This highly collaborative project involves geochemical and molecular biological analysis of the microbial mat populations that exist in these ponds. These mat populations are visually homogeneous over kilometers of extent and display steep geochemical gradients (e.g., light, oxygen) with depth. We are primarily focusing on the molecular analyses. Core samples of microbial mats were frozen and returned for analysis. DNA was subsequently extracted from these samples for use in two projects investigating microbial diversity in these samples. We have measured the distribution and diversity of bacteria in these microbial mat samples; used standard PCR techniques to amplify the conserved bacterial 16S rRNA gene; and assessed diversity over two scales: horizontally, using cores sampled at varying spatial scales over a kilometer distance, and vertically, using individual cores sliced at submillimetric scales. Results indicate that bacterial communities are remarkably stable across even the largest spatial scales (~1 km); however, variation at even fine scale (cm) was also detected. Significant variation in community structure with depth was also observed from different depth intervals in the mat, suggesting that different microbial populations exist in different depth zones.

How do microorganisms survive in extreme environments?

Another approach to understanding the habitability of planets is to consider the range of extreme environmental conditions on Earth that support life. For example, the detection of water ice or submarine hydrothermal-vent systems on another planetary body would satisfy some of the key criteria for habitability. Other criteria needed to support life would include a source of carbon, nitrogen, and phosphorus. The sub-seafloor environment associated with present-day hydrothermal systems harbors hyperthermophilic microbial communities (growth temperatures higher than 80° C) that could be analogous to the microbial communities present on early Earth.

In the first year of this award, we have begun to build conceptual models and rationales for first-stage biofilm formation by bacteria under the extreme conditions in ice, considering in particular the role of bacterial motility and attachment as the eutectic point of water is approached. Since salinity influences the eutectic point as well as bacterial activity, the composition and concentration of salts in the parent water become a central issue. Building upon our previous work with sea ice, we adapted our in situ microscopic imaging methods to freshwater ice, using natural Arctic lake ice as the test medium. We obtained the first images of bacteria packed within the fine veins of fluid between the microscale ice crystals of lake ice. The space available for the bacteria, however, was less than that in sea ice at a comparable temperature of 5° C. We also conducted tests for bacterial motility at subzero temperatures and documented unambiguous movement at 10° C in the presence of glycerol; salt constraints are next to be investigated.