Why All the Fuss Over Some Junk? - Evolution News & Views

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Why All the Fuss Over Some Junk?

When the ENCODE Project (ENCyclopedia Of DNA Elements) announced recently that more human DNA is functional than biologists previously knew, most people congratulated the several hundred scientists who had worked for five years to produce their results. A few Darwinists, however, lit up the blogosphere with angry denunciations -- including Larry Moran, Nick Matzke, P.Z. Myers, and T. Ryan Gregory.

Now, probably none of the ENCODE scientists regard their work as posing any challenge to Darwinian evolution, so why are Moran, Matzke, Myers and Gregory so upset? In their opinion, the ENCODE people overstated the percentage of functional DNA. According to project coordinator Ewan Birney,

It's clear that 80% of the genome has a specific biochemical activity -- whatever that might be... [We used] different classes of assay; in decreasing order of coverage these are: RNA, "broad" histone modifications, "narrow" histone modifications, DNaseI hypersensitive sites, Transcription Factor ChIP-seq peaks, DNaseI Footprints, Transcription Factor bound motifs, and finally Exons.
The first class, RNA transcription, is the largest; the other classes (as Birney implied) are smaller. Moran, Matzke, Myers and Gregory (and a few other critics) argue that RNA transcription is too broad an assay, because some RNAs might not serve any function. Possibly, but why set the blogosphere ablaze over it?

Some historical context might help. After James Watson and Francis Crick discovered the molecular structure of DNA in 1953, Crick announced that they had found "the secret of life," a popular formulation of which became "DNA makes RNA makes protein makes us." But biologists discovered that about 98% of our DNA does not code for protein, and in 1972 Susumu Ohno and David Comings independently used the term "junk" to refer to non-protein-coding DNA (though neither man excluded the possibility that some of it might turn out to be functional).

Why didn't biologists simply call non-protein-coding sequences "DNA of unknown function" rather than "junk DNA?" For some, it was because "junk DNA" seemed more suited to the defense of Darwinism and survival of the fittest. In 1976, Richard Dawkins wrote in The Selfish Gene that "the true 'purpose' of DNA is to survive, no more and no less. The simplest way to explain the surplus [i.e., non-protein-coding] DNA is to suppose that it is a parasite, or at best a harmless but useless passenger, hitching a ride in the survival machines created by the other DNA."

In 1980, W. Ford Doolittle and Carmen Sapienza wrote in Nature (284:601) that many organisms contain "DNAs whose only 'function' is survival within genomes," and that "the search for other explanations may prove, if not intellectually sterile, ultimately futile." In the same issue of Nature (284:604), Leslie Orgel and Francis Crick wrote that "much DNA in higher organisms is little better than junk," and its accumulation in the course of evolution "can be compared to the spread of a not-too-harmful parasite within its host." Since it is unlikely that such DNA has a function, Orgel and Crick concluded, "it would be folly in such cases to hunt obsessively for one."

Two biologists then wrote to Nature (285:617,618) expressing their disagreement. Thomas Cavalier-Smith considered it "premature" to dismiss non-protein-coding DNA as junk, and Gabriel Dover wrote that "we should not abandon all hope of arriving at an understanding of the manner in which some sequences might affect the biology of organisms in completely novel and somewhat unconventional ways." Cavalier-Smith and Dover were not criticizing evolutionary theory; they were merely questioning the claim that non-protein-coding DNA is non-functional.

After the rise of intelligent design (ID) in the 1990s, "junk DNA" became a favorite weapon against ID in the hands of some Darwinists, including Richard Dawkins and the four bloggers mentioned above. According to ID, it is possible to infer from evidence in nature that some features of the world, including some features of living things, are explained better by an intelligent cause than by unguided natural processes. The Darwinists' argument was that an intelligent designer would not have filled our genomes with so much junk, but that it could have accumulated as an accidental by-product of unguided evolution. In 2004, Dawkins wrote in A Devil's Chaplain that much of our genome "consists of multiple copies of junk, 'tandem repeats,' and other nonsense which may be useful for forensic detectives but which doesn't seem to be used in the body itself." Dawkins suggested that creationists (among whom he included ID advocates) "might spend some earnest time speculating on why the Creator should bother to litter genomes with untranslated pseudogenes and junk tandem repeat DNA."

Dawkins continued to rely on junk DNA in his 2009 book The Greatest Show on Earth: The Evidence for Evolution. "It is a remarkable fact," he wrote, "that the greater part (95 per cent in the case of humans) of the genome might as well not be there, for all the difference it makes." In particular, pseudogenes "are genes that once did something useful but have now been sidelined and are never transcribed or translated." Dawkins concluded: "What pseudogenes are useful for is embarrassing creationists. It stretches even their creative ingenuity to make up a convincing reason why an intelligent designer should have created a pseudogene... unless he was deliberately setting out to fool us."

But if most of our DNA is functional, as the ENCODE results suggest, then the "junk DNA" argument against ID collapses.

So the four bloggers listed above are doing everything they can to discredit the ENCODE project's estimate of functional DNA. Yet whatever the estimate may currently be, it is certain to increase with further research. In 2007, the ENCODE pilot project reported on the basis of about 200 datasets that our DNA is "pervasively transcribed," suggesting functionality. The 2012 results, based on 1,640 datasets, documented that "the vast majority (80.4%) of the human genome" is biochemically functional in at least one cell type. But ENCODE has so far sampled only a fraction of the cell types in the human body.

Clearly, we have a lot more to learn about our genome -- but not if we start by assuming that most of it is junk.


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