The Tree of Life - Bacteria, Archaea, and Eukaryotes

Charles Darwin famously compared the evolutionary relationships between different species to the branches of a tree, all originating from a common ancestor.

Historically, an organism’s position on the tree was simply determined based upon its observable characteristics or phenotype, such as the presence of leaves, a spine, or eyes.

Extinct species could be placed on the tree in a similar fashion – using phenotypic traits preserved in the fossil record.

Shortly after Darwin published On the Origin of Species, Haeckel proposed three ‘kingdoms’ of life in 1866, plants, animals, and protists – which were neither plant nor animal - based on easily observable phenotypes.

Eventually, the distinction between plants and animals was found to be less important than previously proposed.

Thus, in 1937, the tree of life evolved to contain two large ‘empires’ – the eukaryotes with membrane-bound nuclei and the prokaryotes without.

This paradigm, however, resulted in some difficulty when it came to placing prokaryotes on the tree: one rod-shaped microbe is phenotypically hard to distinguish from another.

Advancements in genetic analysis, particularly 16S rRNA sequencing, soon toppled this tree when organisms traditionally classed together as prokaryotes were found to have genetically divergent evolutionary origins.

So, the tree of life evolved once more in 1990 and the prokaryotes were divided into two domains - giving us the three domains of the modern tree of life: bacteria, archaea, and eukaryotes.

The first of these domains, bacteria, consists of unicellular, prokaryotic organisms that lack a membrane-bound nucleus.

The next domain, archaea, is also made up of unicellular, prokaryotic organisms distinguished from bacteria by their unique evolutionary history.

Finally, the eukaryotes consist of uni- and multicellular organisms distinguished by their membrane-bound nucleus. This domain includes fungi, plants, and animals.

The evolutionary tree of life, much like an actual tree, is a ‘living’ metaphor and the classification of many organisms, even the distinction between archaea and bacteria, is often contested and occasionally changed.