National Academy of Sciences
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Election Year: 1991
Primary Section: 15, Geology
Secondary Section: 27, Evolutionary Biology
Membership Type: Member
I am broadly interested in the evolution of life, the evolution of Earth surface environments, and the relationships between the two. I am particularly interested in Archean and Proterozoic paleontology, carbonate sedimentology, and biogeochemistry. Past and current projects include investigations of selected problems in Phanerozoic Earth history. Current areas of research include both the early evolution of life and the neo-proterozoic-Cambrian diversification of animals.
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 a decade, 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, diverse fossil assemblages from ca. 850 million-year-old rocks in the Grand Canyon, multicellular chromophyte algae from 1000 million-year-old shales in eastern Siberia, and beautifully preserved microfossils from ca. 1700 million-year-old successions in northern Australia. The "Cambrian Explosion" has attracted the interest of scientists since the time of 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; and 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.