Several distinct families of endogenous retrovirus-like elements (ERVs) exist in 
the genomes of primates. Despite the important evolutionary consequences that 
carrying these intragenomic parasites may have for their hosts, our knowledge 
about their evolution is still scarce. A matter of particular interest is 
whether evolution of ERVs occurs via a master lineage or through several 
lineages coexisting over long periods of time. In this work, the paleogenomic 
approach has been applied to the study of the evolution of ERV9, one of the 
human endogenous retrovirus families mobilized during primate evolution. By 
searching the GenBank database with the first 676 bp of the ERV9 long terminal 
repeat, we identified 156 different element insertions into the human genome. 
These elements were grouped into 14 subfamilies based on several characteristic 
nucleotide differences. The age of each subfamily was roughly estimated based on 
the average sequence divergence of its members from the subfamily consensus 
sequence. Determination of the sequential order of diagnostic substitutions led 
to the identification of four distinct lineages, which retained their capacity 
of transposition over extended periods of evolution. Strong evidence for mosaic 
evolution of some of these lineages is presented. Taken altogether, the 
available data indicate that the possibility of ERV9 still being active in the 
human lineage can not be discarded.