Carl Woese, a biophysicist and evolutionary microbiologist whose discovery 35 years ago of a “third domain” of life in the vast realm of micro-organisms altered scientific understanding of evolution, died on Sunday at his home in Urbana, Ill. He was 84.

His death was announced by the University of Illinois, where Dr. Woese (pronounced woes) joined the faculty in 1964 and spent his entire academic career.

In 1977, Dr. Woese and colleagues at the university startled the scientific world by announcing the discovery of what would be called archaea, a category of single-cell microbes genetically distinct from the two groups previously believed to comprise living organisms: prokaryotes, which include bacteria, and eukaryotes, which include plants and animals.

While other evolutionary biologists had long studied physical traits of species to determine their relationships, Dr. Woese spent years laboriously comparing the genetic sequences of protein-building structures in cells, known as ribosomes and ribosomal DNA. In the process, he established that archaea, which had previously been thought to be within the prokaryote group, had in fact evolved separately from a universal ancestor shared by all three groups.