Alu elements are the most successful SINEs (Short INterspersed Elements) in 
primate genomes and have reached more than 1,000,000 copies in the human genome. 
The amplification of most Alu elements is thought to occur through a limited 
number of hyperactive "master" genes that produce a high number of copies during 
long evolutionary periods of time. However, the existence of long-lived, 
low-activity Alu lineages in the human genome suggests a more complex 
propagation mechanism. Using both computational and wet-bench approaches, we 
reconstructed the evolutionary history of the AluYb lineage, one of the most 
active Alu lineages in the human genome. We show that the major AluYb lineage 
expansion in humans is a species-specific event, as nonhuman primates possess 
only a handful of AluYb elements. However, the oldest existing AluYb element 
resided in an orthologous position in all hominoid primate genomes examined, 
demonstrating that the AluYb lineage originated 18-25 million years ago. Thus, 
the history of the AluYb lineage is characterized by approximately 20 million 
years of retrotranspositional quiescence preceding a major expansion in the 
human genome within the past few million years. We suggest that the evolutionary 
success of the Alu family may be driven at least in part by "stealth-driver" 
elements that maintain low retrotranspositional activity over extended periods 
of time and occasionally produce short-lived hyperactive copies responsible for 
the formation and remarkable expansion of Alu elements within the genome.