The phylogenetic tree of life has been revised many times throughout the past several decades, especially with the discovery of new molecular bases for comparison of organisms. Most biologists now utilize the three domain
system, in which life is categorized into bacteria, archea, and eukarya. While bacteria and archea are both prokaryotes (with no true nucleus), rRNA evidence suggests that archea are more closely related to eukaryotes than to true bacteria. Furthermore, the traditional kingdom Protista is no longer used, since organisms that used to be included in this group do not constitute a monophyletic
group (i.e. a group consisting of a common ancestor and all the groups branching off from it). The tree used in this Demonstration can most closely be considered a cladogram
, which only shows branching patterns, and has no time axis. It is important to note that evolution of life on Earth does indeed constitute a tree rather than a ladder; evolution is in no sense a linear progression from the simple to the complex. Biologists tend to avoid the terms higher
when referring to evolutionary relationships (e.g. humans are higher than fish), instead using the more accurate terms more derived
or less derived
. Indeed, while bacteria, for instance, are less derived than, say, humans, they are in no sense lower, as they have been and remain the most evolutionarily successful form of life.
Phylogenetic data for this Demonstration was procured from the more robust "Tree of Life" web project
and the very popular undergraduate biology textbook, N. A. Campbell and J. B. Reece, Biology
, 7th ed., Benjamin Cummings, 2005.