Junk DNA—or, as scientists call it nowadays, noncoding DNA—remains a mystery: No one knows how much of it is essential for life. As one scientist mused, “Is the genome a trash novel, from which you can remove a hundred pages and it doesn’t matter, or is it more like a Hemingway, where if you remove a page, the story line is lost?” However enigmatic, though, noncoding DNA has proved mighty useful for scientists in one way—it’s great for tracking evolution, through so-called DNA clocks.
DNA clocks take advantage of the fact that DNA mutates at a constant rate: Every so many years, a new mutation should pop up along a stretch of DNA. So in examining the natural history of two related species—which once had the same DNA sequence—a scientist can count the number of different mutations that have accumulated along a stretch, and estimate from that how many years have passed since the species started drifting apart. Except it’s not quite that simple. Mutations can arise anywhere in the genome, in gene DNA and noncoding DNA alike. But mutations to genes have bigger consequences: They can disable proteins and kill a creature. As a result, mutations within genes often get weeded out and don’t get passed on to future generations. Noncoding DNA faces fewer constraints—it can mutate more freely without causing problems when it’s passed along. Counting mutations in noncoding DNA therefore provides more accurate estimates in many cases because the timer there isn’t getting reset.