What about the other 98% of DNA? Until the past decade, it had been thought that it was ‘junk’ that had just hitched a ride evolutionarily. A discovery with far-reaching implications is that more than half of all human DNA is transcribed into RNA but this RNA is not mRNA that is translated into amino acid sequences (Kapranov, Cawley, Drenkow, Bekiranov, Strausberg et al., 2002; Cheng, Kapranov, Drenkow, Dike, Brubaker et al., 2005). It is called non-coding RNA (ncRNA) in the sense that the DNA is transcribed into RNA but the RNA is not translated into amino acid sequences. It seems unlikely that such a complicated mechanism would have evolved unless it served some function. Indeed, the proportion of ncRNA is correlated with the complexity of species, whereas the number of traditional genes is not. For example, the number of traditional genes is similar in humans (~20–25,000) and nematodes (~19,000), but the proportion of ncRNA differs greatly: 57% and 33%, respectively (Frith, Pheasant, & Mattick, 2005). In addition, there is some evidence that ncRNA may have evolved more rapidly in primates than traditional protein-coding genes (Pang, Frith, & Mattick, 2006; Pollard, Salama, Lambert, Lambot, Coppens et al., 2006).
