Tweaking the Tree of Life - Again! August 19 2016, 0 Comments

If you look closely at my Tree of Life chart, you may notice changes. Knowledge about the early branches of the eukaryotes has grown, and it was time for another adjustment in the protists. This time I changed the label on the unikonts to also include a newer term for them, the Amorphea. Some biologists wanted this change because the original hypothesis about what makes the unikonts unique failed. The unikonts do not always have one flagellum, and they have two basal bodies (the part from which flagella grow), like other eukaryotes. What they do have is a unique fusion of three genes. This condition is so rare that it is unlikely to arise twice. The lineage of amoebas, animals, and fungi is still called the unikonts by many biologists, so I left that name on the chart.

The other main branch of eukaryotes, known informally as the bikonts, has a fusion of two different genes, another rare feature. The branch that includes chromalveolates (brown algae, diatoms, ciliates, etc.) and rhizarians (foraminiferans, radiolarians, etc.) has a much less wieldy name. It is now known as SAR (or Sar), an abbreviation for stramenopiles, alveolates, and rhizarians, and I added this to the chart. The evidence now points to some associations that I wanted to include on my Tree of Life. It appears that the Archaeplastida and SAR are more closely related to each other than they are to the Excavata (euglenas, Giardia, etc.). I’ve moved the branch positions on the chart to show this.

I like a newer term for the main branches of the eukaryotes. They are called the eukaryotic supergroups, which is a good descriptor for them.

You may be wondering what to do with your Tree of Life chart if you printed it from the older files. At lower elementary, I would do little more than adding the SAR and Amorphea labels. At that level, it is about showing a broad sweep of life, not the more exacting details. At upper elementary, you may wish to briefly explain about the changes since your chart was printed. Secondary students can learn more about these changes and modify their chart if they are interested.

And then there is that little fact we like to ignore. There are at least as many organisms not shown on our charts (even the more sophisticated scientific ones) as we show there. DNA studies show as many or more bacteria that have never been cultured or named as known bacteria. There are many named, but unplaced protists. Life isn’t simple! Is this the last version of the Tree of Life? Not likely, but it works for now.

You may also be wondering why I bothered to change the chart. Why not start new users of it with the most up-to-date information? As the flood of information continues, it will be best to go forward, not back. The most important thing is that children understand the Tree of Life and the evolutionary history it reflects. As a recent article in Nature Microbiology (2016, article number 16048) states “The tree of life is one of the most important organizing principles in biology.”