L1 retrotransposons replicate (amplify) by copying (reverse transcribing) their RNA transcript into genomic DNA. The evolutionary history of L1 in mammals has been unique. In mice and humans ~80 million years of L1 evolution and replication produced a single evolutionary lineage of L1 elements while generating ~20% of the genomic mass in each species. By contrast, zebrafish contain >30 distinct L1 lineages that have generated approximately one-tenth as much DNA. We contend that, by becoming far more permissive of interspersed repeated DNA than other organisms, mammals are conducive to competition between L1 families for replicative dominance, and that this competition, perhaps for the host factors required for L1 replication, results in a single L1 lineage.