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Project Progress

Habitable Planets and Evolution of Bio-Complexity (dm)

The University of Washington team will carry out a research program focused on planetary habitability and the evolution of biological complexity. This is integrated multidisciplinary effort with concentration on four broad problems:

1. How often do planets with truly Earth-like properties form?
2. How important is plate tectonics in the formation and maintenance of metazoan life?
3. How important are mass extinctions for the evolution and extinction of complex life: are mass extinctions fertilizer or poison (or both) in the garden of complex organisms?
4. What are the evolutionary pathways by which complex organisms originate from microbes?


A habitable planet is defined as one capable of supporting life as we know it on Earth; that is, life that requires liquid water, is carbon-based, and is capable of self-replicating and evolving. The search for extra-solar habitable planets involves a broad interdisciplinary approach that spans an understanding of how planets are formed with the conditions that allow such life to originate, sustain, and evolve. At the same time, it is becoming clear that there will be a broader spectrum of planets habitable for microbial life than for metazoan life, which apparently can exist only under a narrow subset of the range of the conditions conducive for microbial life.

The search for extra-solar habitable planets will first involve the identification of stars with chemical compositions that favor the formation of gas-giant planets. Although gas-giant planets are not habitable (although their moons may be), their presence greatly effects solar-system evolution and may be necessary for the formation of habitable planets. They also provide conditions for creating asteroids and meteor clouds that would deliver water and organic compounds to a fledgling habitable planet. However, the formation of evolving life requires more than liquid water and organic compounds. A habitable planet must make available a full range of energy sources and chemical compounds and elements that allow for oxidation and reduction reactions necessary for life, and it must have the flexibility to co-evolve with organisms to form and maintain multiple habitats. Earth provides all of the clues as to how geophysical, geochemical and astrophysical processes help create and sustain habitable planets. A differentiated planet with liquid water, active tectonism and volcanism could sustain microbial life. On earth, the accumulation of oxygen was key to the origin and evolution of metazoans. A habitable planet that could support metazoans as we know them would have to develop a carbon dioxide atmosphere that is modulated to create temperate conditions and allow the evolution of oxygenic photosynthesis and the formation of an ozone layer to limit ultra violet effects on organisms. It can also be argued that periodic catastrophic events including bolide impacts are also necessary to create and maintain high variability of habitable conditions which result in increased biodiversity and biocomplexity.

Our research will draw on two major sources of evidence to study the rise of biocomplexity. The first is from the fossil record on Earth. Very little is known about the origin of metazoans other than they appear in the fossil record between 700 and 600 million years ago, in conjunction with the accumulation of relatively high concentrations of oxygen in the atmosphere. One of our hypotheses is that the earliest group of heterotrophic metazoans used anaerobic microbial communities (consortia) in their gut to decompose the complex organic compounds left behind by dead microbes. Many invertebrates and some vertebrates still employ microbes as their source of digestive enzymes. The metabolic community of microorganisms that comprised this gut flora still exists today, probably in aquatic microbial mats. Did lateral gene transfer occur between these gut microbes and metazoans, and what genes would likely be transferred?

The second major source of information will come from the study of living organims. Genomics has already proven to be extremely useful for addressing questions of early life and the evolution of eukaryotes. To date, more than 50 bacterial and archaeal genomes have been sequenced or are in progress. One of the conundrums from the genome sequences is that up to 50% of the open reading frames in genomes from microorganisms encode for unknown proteins. Moreover, some of the enzymes involved in metabolic pathways known to be present in the organism are absent in the genome suggesting that either the pathways are incomplete or that enzymes with unknown sequences or low sequence homology are involved.