Shipud writes "Mammalian genomes have been shrinking for about 65 million years, roughly since the dinosaur extinction. Why? And why were ancient mammalian genomes three times larger than they are today? A new article in Genome Biology and Evolution tries to explain this bizarre finding, and why the genomes of mammals (but not of other living groups) are still shrinking. 'Once [the dinosaurs] were gone, mammals started to radiate, fill those niches, and a whole new level of competition arose. The selective advantage of not having a genome encumbered by potentially damaging mobile DNA elements has probably become critical at this "be ye fruitful and multiply; bring forth abundantly in the earth, and multiply therein" stage. In effect, the genomes of mammals has been shrinking by removing mobile DNA elements, just after the KT boundary. And according to the model presented in this study, this process is still ongoing: mammalian genomes are not at an equilibrium size. Unlike flies, mammals are still cleaning up.'"