- 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,
Heckel proposed Three Kingdoms of life in 1866, plants,
animals, and protus, 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 domain's 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.