Introduction to the Tree of Life

Richard A. Paselk, Associate Curator

In redesigning the exhibits pages for the Life Through Time exhibits, I have grouped fossils in a biologically relevant way. For each group an abbreviated diagram indicates their approximate position on a main branch of the tree of life. For example for echinoderms the diagram:

(ToL: Echinodermata<Deuterostomia<Bilateria<Metazoa<Eukaryota)

shows that echinoderms are members of the deuterostomes, which are members of the bilaterians, etc., each group in the diagram includes the group to the left. The groupings themselves are based on degree of relatedness as currently understood (2006), using morphology, ribosomal RNA sequences, DNA sequences, protein amino acid sequences, etc. This method of classification, called cladistics, groups organisms by the fundamental characteristics organisms share. Such characteristics in turn are used to determine which organisms share a common ancestor. All organisms sharing a common ancestor are included within a clade. Clades can be very large, including many different species (e.g. the vertebrates), or very small (e.g. modern humans, with a single recognized species).

Unfortunately, cladistic and traditional classifications do not always agree. One of the most obvious examples is the classification of birds and reptiles. Traditionally birds and reptiles are each given their own class. However, modern research shows that birds are more closely related to lizards, snakes and crocodilians than any of these organisms are to turtles. So either birds are reptiles, or turtles are not! In our classification we include birds in the clade "reptilia" as a subgroup, aves, within the reptilia, while noting it is traditionally a class of its own.

My diagrams are simplified interpretations of the information provided on the international Tree of Life Project (tolweb.org), a web based effort to show the relationships of all life on Earth, past and present. For more detail, references etc. the user should go to the Tree of Life (tolweb.org).

Within our museum site the tree of life can be explored through time via the links on the Life Through Time Exhibit pages. You may also access the life through time cases via links on the Life Through Time Mural or the Geological Timeline (click on the inappropriate icon below). Other displays focused on ancient or modern life may be accessed through the main Exhibit page.

There are a myriad of sites on the web focussing on fossils, evolution etc. Two recommended sites are the Paleontology Portal and Understanding Evolution. Both of these sites provide additional links to many other sites and resources.