Why The Chromosomal Fusion Argument Doesn’t Wash
|February 21, 2011||Posted by Jonathan M under Intelligent Design|
Recently, I purchased and read Daniel Fairbanks’ relatively recent (February 2010) book, Relics of Eden — The Powerful Evidence of Evolution in Human DNA. There was a time when I would have been compelled by many of the arguments for common descent articulated in that book. I have always been skeptical, in large measure, of the proposition of the unlimited causal efficacy which is often so casually ascribed to the neo-Darwinian synthesis. But there was a time when I would have strongly favoured a paradigm consistent with common descent. More recently, however, a deeper delve into the scientific literature has given me cause for caution with respect to these types of argumentation, as compelling to the uninitiated as they may at first glance superficially appear. As with many modern popular science writers, Fairbanks provides a gripping read, and is a very effective communicator to a lay audience. Having read the book, however, it quickly became clear that, on multiple levels, Fairbanks was seemingly out of touch with much of the scientific literature which succeeds in providing potent counter-examples to many of the arguments for common descent which he raises (an example of which is detailed below).
“It is very hard to dissuade them, to tell them that this process is indeed real and ubiquitous but cannot explain the origin of species, pace Darwin. An opinion now shared by many distinguished biologists (please read our book to see by whom and why). The sheer brilliance of the idea, and the elation one feels for having grasped it, voids the minds of any attention to counterexamples.”
I intend to write a full book-review of Fairbanks’ volume when time permits.
Those of us who regularly read and delve into the wealth of literature which has been produced by our critics will be accustomed to the oft-made argument for human/chimp common ancestry concerned with the fusion evidence with respect to chromosome 2 in humans. Brown University biologist, Kenneth Miller, is reputed for making this argument in his lectures (see, for instance, here) and also raised this point at the Dover trial in 2005 (see Casey Luskin’s commentary here). Daniel Fairbanks, in the book alluded to above, spends his entire first chapter discussing this evidence.
Briefly, the chromosomal fusion argument for human-chimp common descent begins with the observation that humans possess 23 pairs of chromosomes, whereas apes possess 24 pairs, thus allowing one to predict that – evolution being true – a chromosomal fusion must have taken place at some point in our lineage. And, indeed, this is what we observe. Chromosome 2 possesses two centromeres. It also possesses a section where there are two telomeres in the middle of the chromosome, which are oriented in such a way so as to suggest that the ends of the two chromosomes were fused together. Every telomere in human and great-ape chromosomes has the six base-pair sequence, TTAGGG repeated over and over approximately fifty to one hundred times in tandem. Such telomeric repetitive units, when they are found not in the telomeres at the end of the chromosome, but rather in the middle of the chromosome (perhaps near the centromere), are referred to in the literature as “interstitial telomeric sequences” (or ITS). At the supposed fusion site in chromosome 2, the sequence in the upper strand abruptly changes from TTAGGG repeats to CCCTAA repeats (the complementary sequence of the inversion). This is taken to be indicative that the DNA in a telomere of one chromosome and the DNA in a telomere of the other chromosome broke and subsequently the two chromosomes fused at the broken ends. This site is referred to in the literature as 2q13 (‘2′ referring to the chromosome number, ‘q’ referring to the long arm, and ’13’ referring to the position on the arm).
Now, there are, in fact, plausible alternative explanations for this observation (envision, for instance, a scenario where our genus Homo, originally possessing 48 chromosomes, underwent a chromosomal fusion event within its own independent lineage). This point is often made on the ID blogosphere in response to this type of argument, but this is not the point which I wish to raise in this blog.
I recently encountered a peer-reviewed paper which appeared in the journal, Cytogenetic and Genome Research in 2009 by Farre, Ponsa and Bosch [Farré, M., M. Ponsà, et al. (2009). “Interstitial telomeric sequences (ITSs) are not located at the exact evolutionary breakpoints in primates.” Cytogenet Genome Res. 124(2): 128-131.]
The abstract reports,
“Although their function has not yet been clearly elucidated, interstitial telomeric sequences (ITSs) have been cytogenetically associated with chromosomal reorganizations, fragile sites, and recombination hotspots. In this paper, we show that ITSs are not located at the exact evolutionary breakpoints of the inversions between human and chimpanzee and between human and rhesus macaque chromosomes. We proved that ITSs are not signs of repair in the breakpoints of the chromosome reorganizations analyzed. We found ITSs in the region (0.7–2.7 Mb) flanking one of the two breakpoints in all the inversions assessed. The presence of ITSs in those locations is not by chance. They are short (up to 7.83 repeats) and almost perfect (82.5–97.1% matches). The ITSs are conserved in the species compared, showing that they were present before the reorganizations occurred.” [My emphasis]
So, what is the significance of the cited paper? Though there are many documented instances of these interstitial telomeric sequences in the genomes of humans and chimps, the 2q13 interstitial telomeric sequence is the only one which is able to be associated with an evolutionary breakage point or fusion. The other ones do not square up with chromosomal breakpoints in primates at all!
As the authors of the paper note,
“The availability of complete genome sequences (Hubbard et al., 2007) offers the opportunity to characterize the regions flanking the breakpoints of chromosomal reorganizations at the molecular level. However, to our knowledge, only the head-to-head ITS located in the human 2q13 region, which is a relic of an ancient telomeretelomere fusion, is precisely associated with an evolutionary breakpoint (Ijdo et al., 1991). Here, we used bioinformatic tools to analyze, in the current genome releases, the presence of short ITSs in the chromosomal inversions that do not involve terminal regions and that occurred between human and chimpanzee and between human and rhesus macaque during evolution.”
The pro-ID evolutionary biologist, Richard Sternberg, has also briefly weighed in on this paper here.
Thus, the take-home message is this: To make much of the 2q13 interstitial telomeric sequence and portray it as typical of what is observed in chimp and human genomes may be considered careful cherry-picking of data. A phenomenon which we, sadly, observe all too frequently in evolutionary biology. Are people like Miller and Fairbanks not aware of these things? Or do they opt just not to mention them? Aren’t these potent counter-examples (and there are similar instances for most of the arguments developed by Fairbanks in his book) of relevance to any sincere and intellectually honest discussion of such matters? It is the failure to objectively consider all the evidence which leads the Darwinian camp into many snares.