Specifically, he focused on the small subunit ribosomal RNA (SSU rRNA), which is a crucial component of the ribosome, the cellular machinery responsible for protein synthesis. rRNA is highly conserved across all life forms, meaning it changes very slowly over evolutionary time. This makes it a good molecular clock for tracing evolutionary relationships between organisms.

Woese's groundbreaking work led to the discovery of the third domain of life, the Archaea, which were previously grouped with bacteria. His research revolutionized our understanding of the evolutionary history of life on Earth.