About the ToL Navigation Picture
We've been getting a lot of inquiries about the tree of life picture on our home page. It is important to note that the major function of this picture is to help visitors to the ToL web site to quickly navigate to pages of some of the major groups of organisms. In order to serve this purpose, we had to use a greatly simplified representation of the tree of life.
Here's some guidance for the interpretation of the image, explaining which scientific facts are represented in the image and what conclusions should not be drawn from it.
- Life on Earth shares a common, genetic history with complex origins
- Our picture shows that all organisms on earth share a common, genetic history. Lineages of organisms are represented by branches diversifying into descendent lineages as the tree grows from the bottom (representing the origin of life) to the tips (representing species living today). The very early history of life is kept obscure on purpose. Recent evidence indicates that rather than descending from a single "primordial cell", the first organisms may have evolved in an environment characterized by loosely constructed cellular organization and frequent exchange of genetic materials between individuals (see Tree of Life Turns Out to Have Complex Roots). Even after distinct genetic lineages were established, exchange of genetic materials between distantly related organisms (also called horizontal or lateral gene transfer) continued, which is indiciated in our picture by lines connecting distant branches of the tree of life. While horizontal gene transfer has clearly been an important feature of genome evolution in Eubacteria and Archaea, the significance of this process in eukaryotic evolution is much less clear. It is, however, well accepted that eukaryotes gained important organelles such as mitochondria and chloroplasts by engulfing and retaining certain symbiotic bacteria.
- Don't draw conclusions about the relative diversity of different groups of organisms.
- While our picture hints at the diversification of early life into the millions of different creatures we see today, it does not provide a good representation of the relative diversity (i.e., number of species) of different groups. If this had been our goal, the image would have been dominated by the arthropod branch. Taking into account recent projections for the number of yet-to-be-described species in groups like bacteria, protists, fungi, and nematodes, these branches would have filled the remainder of the diagram, with vertebrates and plants hardly visible. Of course, such an image would not be very useful as a navigation tool for the ToL web project. Therefore, we created a diagram that is more of a general representation of people's interest in different groups as well as their coverage on the ToL project. If you are uncomfortable with such utilitarian liberties, simply imagine the image to be three-dimensional and viewed from an angle focusing on some of the less speciose groups such as plants and vertebrates, while the majority of eubacterial diversity, for example, is hidden in the background.
- Don't interpret relative branch lengths as indicators of levels of evolutionary "advancement".
- It is also important to note that the placement of the tips of the eubacterial and archaean branches lower down on the image does not mean that these organisms are in any way more "primitive" or "ancient." We have drawn these branches shorter and relegated them to the corner of the image, in order to have more space for the display of the big, showy creatures in other parts of the tree. All of the organisms at the tips of the tree are, of course, living today and should therefore be shown at the same time level. In addition, any two creatures living today, for example a Eubacterium and a frog, have had exactly the same amount of time to diversify since their common ancestor. It's just that their evolutionary history has taken them into vastly different directions.
- No organism alive today represents the ancestor of any other living creature.
- For practical reasons, we have placed an extant (living) annelid worm smack in the middle of our tree. Of course, this worm is not meant to represent the ancestor of organisms shown higher up in the tree; rather, it stands as a representative of animals in general. The most recent common ancestor of arthropods and vertebrates was some kind of worm, but it certainly wasn't an annelid; in fact, nobody knows what this creature may have looked like.
There is currently great interest in developing visual representations of the tree of life as a whole. However, summarizing the entire tree in a picture is quite challenging because there are a vast number of species (about 1.8 million have been described, but many more are known to exist), and the relationships among many groups remain yet to be resolved. Have a look at Rebecca Shapley's work on Visualizing the Tree of Life to get an idea of current efforts to produce representations of the tree of life that are accessible to a general audience. Also, if you are looking for a wall poster of the tree of life, David M. Hillis, Derrick Zwickl, and Robin Gutell (University of Texas) have generated a pdf file featuring an image of a tree (based on rRNA sequence data) that includes about 3,000 species. You can get a copy of the file from the Hillis & Bull Lab Download Page. Also, a high-resolution picture of a tree based on the analysis of complete genomes (Ciccarelli & al. 2006) is available from the European Molecular Biology Laboratory Heidelberg's A Phylogeny of Complete Genomes: Data Repository.
The Tree of Life navigation picture is licensed under the Creative Commons Attribution-No Derivative Works License - Version 3.0. You can download a larger JPEG version or a high resolution TIFF version of the file. Thanks to Nick Kurzenko , Greg Rouse , and the U. S. Fish and Wildlife Service for making available images of organisms for this graphic.
Note that you may not modify the ToL navigation image. You can only use it as is. However, if you would like to create a tree of life graphic, using your own organism images, you can use the basic tree image.
Ciccarelli, F. D., T. Doerks, C. von Mering, C. J. Creevey, B. Snel, and P. Bork. 2006. Toward automatic reconstruction of a highly resolved tree of life. Science 311(5765):1283-1287.