The majority of more than one million primate-specific Alu elements map to 
nonfunctional parts of introns or intergenic sequences. Once integrated, they 
have the potential to become exapted as functional modules, e.g., as 
protein-coding domains via alternative splicing. This particular process is also 
termed exonization and increases protein versatility. Here we investigate 153 
human chromosomal loci where Alu elements were conceivably exonized. In four 
selected examples, we generated, with the aid of representatives of all primate 
infraorders, phylogenetic reconstructions of the evolutionary steps presumably 
leading to exonization of Alu elements. We observed a variety of possible 
scenarios in which Alu elements led to novel mRNA splice forms and which, like 
most evolutionary processes, took different courses in different lineages. Our 
data show that, once acquired, some exonizations were lost again in some 
lineages. In general, Alu exonization occurred at various time points over the 
evolutionary history of primate lineages, and protein-coding potential was 
acquired either relatively soon after integration or millions of years 
thereafter. The course of these paths can probably be generalized to the 
exonization of other elements as well.