Domains

For most of the 20th century, scientists believed life was split into just two categories: Prokaryotes (simple organisms like bacteria) and Eukaryotes (complex organisms like us). However, in 1977, a discovery by microbiologist Carl Woese changed everything.

By looking at the "molecular clock" inside cells, Woese found a third group of life. While they looked like bacteria under a microscope, they were genetically as different from bacteria as a human is from a piece of mould. He named them Archaea.

The New Penthouse: The Domain

In the original Linnaean system, the Kingdom was the highest level of classification. Today, the Kingdom has been demoted to the second level, and the Domain is now the "penthouse suite" of biological organisation.

1. Domain Bacteria: The Ancient Recyclers

Bacteria are likely the oldest lineage on the Tree of Life. These single-celled organisms lack a nucleus.

• Roles: They do everything from helping you digest food to causing a sore throat.

• Structure: They are microscopic and sensitive to traditional antibiotics.

2. Domain Archaea: The Extreme Survivors

Archaea look exactly like bacteria—tiny, single-celled blobs—but their internal machinery for building proteins and copying DNA is actually more similar to ours.

• Habitats: They are famous for living in "impossible" places: boiling hydrothermal vents, salt lakes, and cow stomachs.

• Friendly Neighbours: No known species of Archaea causes disease in humans; they are simply peaceful, extreme-living survivors.

3. Domain Eukarya: The Complex Builders

This is the domain humans belong to. It includes every organism that has a nucleus in its cells to protect its DNA.

• The Four Kingdoms: Animals, Plants, Fungi, and Protists (like amoebas).

• The Origin Story: Modern DNA suggests the first Eukaryotic cell was born when an Archaea merged with a Bacteria.

Comparison at a Glance

Feature Cell Nucleus Membrane Lipids Antibiotic Sensitive Size

Bacteria No Unbranched Yes Microscopic

Archaea No Branched No Microscopic

Eukarya Yes Unbranched No Micro to Massive

A Microbe-Major World

The modern Tree of Life is no longer "human-centric". If you look at a truly accurate tree, humans, lions, and oak trees are all just tiny sub-twigs on the Eukarya branch. The vast majority of the living history of our planet belongs to the microbes in the other two domains.