The Azimuth Project
Tree of life



The tree of life is a guide to all life on Earth. Apart from humans it represents what there is to be saved. Like any guide, it is an approximation to the truth. Approximately 1.7 million species have been identified, but the total number may be 5 or 50 times that. Some simplification is necessary!

This page is a very rough guide to the tree of life, based largely on the book The Variety of Life by Colin Tudge.

Here is a picture of the tree of life.

The Three Domains

The three domains which encompass all life are Bacteria (aka eubacteria), Archaea (aka archaebacteria), and Eukaryotes. Viruses are not regarded as being alive. Bacteria and Archaea are small single celled organisms with little internal structure. In particular, they have no nucleus.

Eukaryotic cells have nuclei, and a lot more organization. They are typically 10 times as long or 1000 times the volume of Archaea and Bacteria cells. Despite the similarity of Archaea and Bacteria, Archaea resemble Eukaryotic cells more than bacteria in some important ways.

Since Archaea and Bacteria reproduce asexually, it is difficult to define what is meant by a ‘species’ of bacteria. They are also affected by a large amount of horizontal gene transfer which makes the model of a tree for their organization questionable. However they do tend to form clusters - populations of closely related and very similar individual organisms with few if any intermediates between different populations.


Bacteria tend to attract attention when they cause disease in humans, farm animals or crops. They also live in the gut, the soil, and water, performing important functions. They have a much larger diversity of biochemistry than Eukaryotes.


Archaea are a recent discovery, first found near hot vents, but later found in many other places.


The greater complexity of the Eukaryotic cell includes histones, and a cytoskeleton, which enable much more precise modes of cell division, in particular meiosis which is necessary for sexual reproduction. Most Eukaryotes practice sex.

Most Eukaryotic cells contain mitochondria which are the ‘power plants’ of the cell, and contain their own DNA; they resemble certain bacteria. Plant cells contain plastids which carry out photosynthesis; these resemble other bacteria. In many ways, a typical Eukaryotic cell looks like an archae which swallowed (or was invaded by) one or more bacteria a long time ago, which is very likely what happened.

The most familiar Eukaryotes are plants, animals and fungi. These groups are known as kingdoms. At least seventeen further kingdoms are identified by Tudge. The structure of the tree near the root of the Eukaryotes is controversial, and has beeen revised several times in the last decade.

Most Eukaryotes are animals, most animals are insects, and most insects are beetles.

The image below shows a tree for insects, with the approximate number of known species at the ends of branches. It is very imbalanced: the first split is into 2,000 versus over 800,000 and most of the other splits are similarly extreme.

Phylogenetic tree for insects


  • Colin Tudge, The Variety of Life: A Survey and a Celebration of All the Creatures That Have Ever Lived, Oxford University Press, Oxford and New York, 2000. Paperback, March 2002.

  • Andrew J. Roger and Alastair G.B. Simpson, Evolution: Revisiting the Root of the Eukaryote Tree, 2010.

  • Keeling et al., The tree of eukaryotes, 2005.

category: biodiversity