DNA fragments scattered about the human genome that don't make anything except more of themselves. The human genome is home to about a half a million of these entities, which take up fully 14 percent of the human genome. But ALUs living near genes are preferentially retained, according to the new genome analysis, while ALUs that live in other areas of the genome are gradually weeded out. Recent but controversial experiments had suggested that ALUs might help their human hosts, perhaps by turning on protective genes in stressful situations. The new finding that ALUs are especially welcomed around genes is strong evidence that they are indeed helpful neighbors.
Many mysteries remain to be solved.
Inexplicably, for example, few new families of genetic interlopers seem to have taken root in the human lineage in the past 50 million years. Also unexplained, some types of genetic parasites are becoming gradually extinct within the genome while others are apparently thriving. Entire civilizations, it seems, have risen and fallen within the human genome and are continuing to do so. It's a humbling perspective, said Robert Waterston, director of Washington University's genome center in St. Louis. But not just any vehicle.