If you’ve been reading the evolution websites, you’ll know about the very nice paper in this week’s Nature by Douglas Theobald. (You may remember Theobald as the author of one of the greatest creationism-refuting websites of all: 29+ Evidences for Macroevolution: The Scientific Case for Common Descent. If you haven’t seen it, you should.) In the new paper, Theobald makes a few conservative assumptions to show that the probability that all living species descend from a universal common ancestor is infinitely higher than any other hypothesis, including those of multiple origins of the kingdoms (Bacteria, Eukarya, and Archaea) or of rampant horizontal gene transfer betweeen species that would, by mixing genomes, make life look as though it had a single origin when it didn’t.
Fortunately, I delayed posting on this long enough so that others did the job for me: these include Nick Matzke at Panda’s Thumb, P. Z. Myers at Pharyngula, and Ker Than at National Geographic. It’s pretty airtight evidence for evolution, since the hypothesis that trumped all others is that of a single origin of life, with the proteins of existing species showing a pattern of similarity and divergence reflecting the branching bush of evolution.
I’m not sure how creationists will respond to this, but I suppose they could maintain that the data show only that God created life in this way because he needed to give similar proteins to similar species. That, of course, would require one to believe that those similarities just happen to mimic the similarities expected under evolution. Proteins group not by lifestyle, but by ancestry. Bats have proteins that resemble those of rats more than those of birds, and whales have mammal-like rather than fish-like proteins. A marsupial mole has virtually the same niche, and looks almost the same as, a placental mole, but its proteins are more similar to those of a kangaroo.
As P. Z. (and Theobald) point out, the advantage of this new study is that it gives us a number—a probability—with which to gauge the likelihood of modern life descending from multiple origins.
But let us remember (and Theobald mentions this) that we already had incontrovertible evidence for a single origin of all species: the near-universality of the genetic code.
If you’ve studied biology at all, you’ll know that the genetic code—the triplet sequence of DNA (and RNA) that codes for the amino acids of proteins—is virtually identical across all species. There are 64 triplet codons coding for around 20 amino acids (as well as protein-terminating “stop positions”), and the correspondence between the code and the amino acid is nearly identical across animals, plants, and bacteria. There are a few exceptions to this, but they are minor: no species deviates from the code by more than a few amino acids, though some mitochondrial DNA deviates by as many as 8 codons. You can find a list of these deviations (last updated in 2008) at the National Center for Biotechnology Information at the NIH.
Although we can’t attach numbers to the likelihood that the “universal code” reflects a single rather than a multiple origin of life (that would require a model of how the code might have evolved), only a moron or a creationist would deny that this similarity reflects a single origin. There are simply too many genes involved in producing the code and turning it into proteins to think that the code’s universality merely reflects evolutionary convergence in lineages that originated independently. The universality results from ancestry, not coincidence.
In the end, I expect that creationists will find a way around Theobald’s arguments, just as they have around the universal genetic code (Nick Matzke mentions that creationist Paul Nelson has long battled the universal-code argument), but they do so at the expense of their own credibility.
Fig. 1. The “universal” genetic code (read the triplet code starting with the letter on the left, then the top, and then the right). Courtesy of Harvard’s Department of Molecular and Cellular Biology.
Theobald, D. L. 2010. A formal test of the theory of universal common ancestry. Nature 465:219-223.