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Andrew Knoll
Organismic and Evolutionary Biology
Harvard University

Research Interests

Members of the Paleobotany lab at Harvard are broadly interested in the evolution of life, the evolution of Earth surface environments, and the relationships between the two. We are particularly interested in Archean and Proterozoic paleontology, carbonate sedimentology, and biogeochemistry; however, both past and current projects include investigations of selected problems in Phanerozoic Earth history. Current areas of research include the following:

1. The Early Evolution of Life. The Universal Tree of Life is a depiction of the genealogical relationships of all extant organisms, based primarily on comparisons of molecular sequence data. It is properly read as an inferred evolutionary history, inviting comparison with the direct evolutionary record encrypted in sedimentary rocks. Insofar as plants and animals reside on the uppermost principal branches of the tree, paleontological and biogeochemical evidence of life's much deeper history must be sought in Archean and Proterozoic rocks. For more than two decades, we have conducted research on the systematic paleontology, paleoecology, and evolutionary interpretation of Proterozoic fossils. At present, we are engaged in detailed studies of exceptionally preserved unicellular and multicellular algae from phosphorites in China that just antedate the Ediacaran radiation of animals, and beautifully preserved microfossils from 1450-1700 million year old successions in northern Australia.

2. The Neoproterozoic-Cambrian Diversification of Animals. The so-called "Cambrian Explosion" has attracted the interest of scientists since the time of Charles Darwin, but only in the past few years have enough data on the molecular biology of development and Neoproterozoic-Cambrian environmental change accumulated to begin critical evaluation of this remarkable interval of evolutionary change. At Harvard, we have contributed to the development of chemostratigraphic methods that provide a means of correlating PC-C boundary successions independently of the animals we wish to evaluate. We have also participated in stratigraphic and radiometric studies that have sharply constrained the timing of these events; produced geochemical data that support the hypothesis that atmospheric oxygen levels increased just before macroscopic animals evolved; demonstrated that phytoplankton show evolutionary dynamics much like those of PC-C animals, indicating that ecology played an important role in a Cambrian explosion that extends across kingdoms. With colleagues at MIT, we are working on early skeletal animals in latest Proterozoic reefs.

3. The Genesis and Diagenesis of Proterozoic Carbonate Rocks. Carbonates can be viewed as multitrack tapes that record aspects of tectonic, biological and environmental evolution. Deconvolving these signals is challenging, especially in carbonates formed before the diversification of skeleton-forming algae and animals. We are focusing on Proterozoic carbonates as records of evolving biology and ocean chemistry. Stromatolites have traditionally been viewed as proxies for microbial communities, and therefore, changes in stromatolite macrostructure, microstructure, and distribution have been used to draw evolutionary inferences. Current studies suggest that the evolutionary signal in the stromatolite record has been overestimated, whereas their importance as a record of environmental change has been underappreciated. Specific projects on Proterozoic carbonates are currently underway in Siberia and northern Australia.

4. Physiological Selectivity in Mass Extinction. At times of mass extinction, some species survive, while many others disappear. For years, paleontologists have sought commonalities among victims or survivors in terms of taxonomy, paleoenvironments, or biogeography. Stimulated by a novel hypothesis to explain Permo-Triassic extinction, we have begun to evaluate Phanerozoic fossils as a record of physiologies. Preliminary results are encouraging -- there is strong physiological bias at the P-Tr boundary, which places biological constraints on extinction scenarios. We are also engaged in petrological and geochemical analyses capable of distinguishing among, and therefore testing, various hypotheses for end-Permian extinction and subsequnet Triassic recovery.

5. The Paleozoic Diversification of Land Plants. Vascular plant evolution is a long term interest in the Paleobotany Lab. Past studies included early attempts to quantify patterns of plant diversity through time and character-based evolutionary studies of Devonian evolution. Current interest centers on the developmental basis of Paleozoic vascular plant diversification.

6. The Mesozoic Plankton Revolution. Primary production in most areas of the modern oceans is dominated by three algal clades: the diatoms, coccolithophorids, and dinoflagellates. Molecular phylogenies suggest that these groups belong to protistan lineages that diverged from one another during the Proterozoic Eon, but none of them rose to taxonomic and ecological prominence until the Mesozoic. As part of an NSF Biocomplexity project centered at Rutgers University, we are participating in an interdisciplinary exploration of this evolutionary pattern.

7. Astrobiology and Mars Exploration. Globally, interest in questions of life beyond the Earth is growing. In accord with this interest, a new round of planetary exploration is beginning. We participate in this effort through membership in the NASA Astrobiology Institute and by membership on the rover science team for NASA's 2003 lander mission to Mars.