Trees versus Bushes
When Charles Darwin wrote On the Origin of Species in 1859, he drew a diagram of a tree to represent divergence through natural selection. His work focused on plants and animals—he had no clue what microbes were or how they would impact his conclusions (R. Kolter and S. Maloy, Microbes and Evolution: The World that Darwin Never Saw. ASM Press, Washington, D.C., 2012). However, it was the comparison of gene sequenc-es in microbes that provided the most conclusive evidence for a “universal tree of life.” In the late 1970s, Carl Woese argued that the 16S subunit of ribosomal RNA provided an ideal tool for evaluat-ing genetic relationships between organisms—16S rRNA from different organisms shared regions with sufficient sequence similarity to recognize the 16S gene from divergent organisms, while having regions that were sufficiently variable to distinguish closely related organisms. This discov-ery led to a paradigm shift in our understanding of evolution, dividing all living organisms into three Domains: Bacteria, Archaea, and Eukarya.