Illustration showing the tree of life

Tree of life

Living things fall into three major divisions: Bacteria, Archaea and Eukarya

The idea that all living things arose from a common ancestor has given rise to the analogy of the ‘tree of life’. The trunk represents this ancestor, major branches are the high-level taxonomic groups, and the twigs are the individual species themselves.

It used to be thought there were two main groups: the prokaryotes without a nucleus and the eukaryotes with a nucleus. This view has now been significantly revised. A new group has emerged: the Archaea. Although they are single-celled and lack a nucleus, genetic comparisons suggest they are more closely related to eukaryotes than to bacteria.

Because the vast majority of microbial life forms have not yet been grown in the lab, we know little about them. A new approach, ‘metagenomics’, is to collect samples from particular environments – such as the sea or parts of the gut – and to sequence every genome within them.

These kinds of studies are revealing incredible microbial diversity and new species never described before. They have also identified staggering numbers of viruses in the world’s oceans, with profound influence on global ecosystems.

Lead image:

Illustration © Glen McBeth


Further reading

About this resource

This resource was first published in ‘Evolution’ in January 2007 and reviewed and updated in December 2014.

Microbiology, Ecology and environment, Cell biology, Genetics and genomics
Education levels:
16–19, Continuing professional development