L1 (LINE-1) retrotransposon evolution and amplification in recent human history

L1 (LINE-I) elements constitute a large family of mammalian retrotransposons that have been replicating and evolving in mammals for more than 100 Myr and now compose 20% or more of the DNA of some mammals. Here, we investigated the evolutionary dynamics of the active human Ta L1 family and found that it arose similar to 4 MYA and subsequently differentiated into two major subfamilies, Ta-0 and Ta-l, each of which contain additional subsets. Ta-l, which has not heretofore been described, is younger than Ta-0 and now accounts for at least 50% of the Ta family. Although Ta-0 contains some active elements, the Ta-l subfamily has replaced it as the replicatively dominant subfamily in humans; 69% of the loci that contain Ta-l inserts are polymorphic for the presence or absence of the insert in human populations, as compared with 29% of the loci that contain Ta-0 inserts. This value is 90% for loci that contain Ta-ld inserts, which are the youngest subset of Ta-l and now account for about two thirds of the Ta-l subfamily. The successive emergence and amplification of distinct Ta L1 subfamilies shows that L1 evolution has been as active in recent human history as it has been found to be for rodent L1 families. In addition, Ta-l elements have been accumulating in humans at about the same rate per generation as recently evolved active rodent L1 subfamilies.