Carl Woese, the University of Illinois microbiology professor credited with the discovery of a third domain of life, died Sunday at his home in Urbana. He was 84.

In 1977, Mr. Woese and his colleagues overturned a universally held assumption about the basic structure of the tree of life. Microbes, now known as archaea, are as distinct from bacteria as plants and animals are, they wrote in a published paper. Prior to this finding, scientists had lumped archaea together with bacteria and asserted that the tree of life had two main branches bacteria (called prokarya), and everything else (eukarya). Their discovery added archaea as a third main branch of the evolutionary family tree.

We are all saddened at the passing of Dr. Woese and our collective thoughts are with his family, Chancellor Phyllis Wise said in a press release. It is truly impossible to adequately describe or to categorize his contributions to the University of Illinois, to biology and to the world during his long and distinguished career here. The campus community has lost one of our giants this week, she said.

Mr. Woese was born on July 15, 1928, in Syracuse, N.Y. He earned bachelors degrees in math and physics from Amherst College and a Ph.D. in biophysics at Yale University. He studied medicine at the University of Rochester, was a postdoctoral researcher in biophysics at Yale and worked as a biophysicist at the General Electric Research Laboratory in Schenectady, N.Y., before he joined the microbiology faculty at the UI in 1964. He was also a professor at the UI's Institute for Genomic Biology.

Carl was truly a man of vision, creativity and passion, with a deep love of this university, said Gene Robinson, director of the UI's Institute for Genomic Biology in a statement. Carl not only rewrote the textbook in evolutionary biology, but his discovery also has given us the tools today to study the human microbiome, the incredibly diverse and complex assemblages of microorganisms in our bodies that contribute so much to both health and disease.

Woese received a number of awards for his research: a MacArthur Foundation grant in 1984, election to the National Academy of Sciences in 1988, the Leeuwenhoek Medal (microbiologys premier honor from the Royal Netherlands Academy of Arts and Sciences) in 1992, a National Medal of Science in 2000 and many more.

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Visionary UI biologist Carl Woese, 84, dies