Source: David M. Hillis, Derrick Zwickl, and Robin Gutell, University of Texas
In my previous post I mentioned “Pale Blue Dot,” a photograph taken by Voyager 1 from the outer reaches of the solar system showing Earth as a barely visible dot of light. But even on Earth, we humans are just one in a family of millions.
Brett Keller discusses in his blog Brett Keller & the World a remarkable “tree of life” published a few years ago in the prestigious journal Science. This “tree,” arranged in a circular format, was developed by David Hillis and colleagues at the University of Texas. They selected around three thousand species, trying to include representatives from all major groups. You may read more about it and see where our species, Homo sapiens, fits in at Mr. Keller’s blog.
Possible phylogenetic tree
These representations, more formally called phylogenetic trees, are intended to illustrate the evolutionary relationships between species. Lines are drawn from two species (call them A and B) that are closely related, meeting at a vertex called a “node” representing the most recent species that is ancestral to the both of them. Another node may occur further up, representing the most recent common ancestor of the A-B ancestor and species C, and lines will be drawn to both of those. And perhaps even further up is the ancestor to species A, B, C, and D, with one line going to the A-B-C ancestor and one going to species D.
Sometimes, however, it is difficult to determine how closely two species are related. In the past, similarity was determined largely on the basis of morphological characteristics such as body structure and appearance. This is a relatively crude technique, since minor variations in genes can produce large physical differences (phenotypes). Nowadays, these determinations are supplemented by molecular studies of genes and proteins. Since genes continually undergo mutations (random changes), comparison of genes or the resulting proteins between two species that diverged recently would be expected to show fewer differences than that between two species that diverged longer ago.
Alternate phylogenetic tree
Sometimes this means that we refine our tree. For instance, more detailed evidence may suggest that C and D are actually quite closely related. In this case, lines of them would meet at a node representing their common ancestor, and then lines from the A-B ancestor and the C-D ancestor meet at the ancestor of all four. None of these methods is exact, and sometimes different analyses yield slightly different results. Still, overall, there is remarkable agreement between the different methods. The tree created by Mr. Hillis and colleagues compared ribosomal RNA (rRNA), and instead of arranging current species in a row, they arranged them in a circle.
This discussion is only an overview, and omits many details. In particular, I have ignored the problem of rooting the tree. If you are interested in this sort of thing, I recommend reading Richard Dawkins’ The Ancestor’s Tale. In it, Mr. Dawkins essentially guides us along the tree of life, starting at Homo sapiens and working backwards. He calls each node a “rendezvous,” where our ancestors meet the ancestors of other species, all the way back to the origin of life. It’s a fascinating look back at the history of Earth.