All living organisms are classified into three major domains: Bacteria, Archaea, and Eukarya. This system helps biologists organize the enormous diversity of life using deep evolutionary relationships rather than just visible traits. The idea matters because organisms that look similar can be only distantly related, while organisms that look very different can share important molecular features.
A Tree of Life diagram shows these domains branching from a shared ancestral population called the Last Universal Common Ancestor, or LUCA.
The three-domain system was strongly supported by comparisons of ribosomal RNA, a molecule found in all cells that changes slowly over evolutionary time. Bacteria and Archaea are both prokaryotic, meaning they lack a nucleus, but their cell membranes, cell walls, and genetic machinery are different in important ways. Eukarya includes animals, plants, fungi, and protists, all of which have cells with nuclei and membrane-bound organelles.
Molecular evidence reshaped classification by showing that Archaea are not just unusual bacteria and are often more closely related to Eukarya in parts of their information-processing systems.
Key Facts
- The three domains of life are Bacteria, Archaea, and Eukarya.
- LUCA stands for Last Universal Common Ancestor, the ancestral population from which all modern life descended.
- Bacteria and Archaea are prokaryotes, meaning their cells do not contain a nucleus.
- Eukarya have nuclei and membrane-bound organelles such as mitochondria, chloroplasts, and the endoplasmic reticulum.
- Ribosomal RNA comparisons are used to infer deep evolutionary relationships because rRNA is found in all cells and evolves slowly.
- A simple relatedness idea is percent difference = number of differences / total positions x 100.
Vocabulary
- Domain
- A domain is the broadest commonly used category of life, grouping organisms by deep evolutionary relationships.
- Phylogenetic tree
- A phylogenetic tree is a branching diagram that shows hypotheses about evolutionary relationships among organisms.
- Prokaryote
- A prokaryote is an organism whose cells lack a nucleus and other membrane-bound organelles.
- Eukaryote
- A eukaryote is an organism whose cells contain a nucleus and usually other membrane-bound organelles.
- Ribosomal RNA
- Ribosomal RNA is a molecule in ribosomes that helps build proteins and is useful for comparing distant evolutionary lineages.
Common Mistakes to Avoid
- Calling Archaea a type of Bacteria is wrong because Archaea form a separate domain with distinct membranes, genes, and molecular machinery.
- Assuming all microbes are closely related is wrong because microscopic size does not show evolutionary relationship.
- Using only visible traits to build the Tree of Life is wrong because many deep relationships are best revealed by DNA and RNA sequence comparisons.
- Thinking LUCA was the first living cell is wrong because LUCA represents the last shared ancestor of modern life, not necessarily the earliest life that ever existed.
Practice Questions
- 1 A ribosomal RNA segment has 1200 positions. Two organisms differ at 96 positions. What is their percent difference?
- 2 In a sample of 500 organisms, 310 are Bacteria, 90 are Archaea, and the rest are Eukarya. How many are Eukarya, and what percent of the sample are Eukarya?
- 3 A newly discovered organism is single-celled, has no nucleus, has membrane lipids unlike those of bacteria, and lives in a hot spring. Which domain is the best initial classification, and what evidence supports your answer?