Time-course of integration of mobile elements in the human genome. The percent substitution of various elements relative to their consensus sequence (x-axis) is plotted against the portion of the modern human genome encompassed by elements with those levels of divergence. To a first approximation, percent substitution relative to the consensus correlates with the age of the individual element insertions in the genome. Thus, elements such as L2 and MIR appear to be old and to not have had any recent activity. Most of the recent activity has been from L1 and Alu, although there has been a sharp decrease in their amplification in recent time. To provide an approximate timeframe, the peak of Alu amplification was 40–50 million years ago. Reprinted with minor modification with permission from Nature 409:860–921 ©2001, Macmillan Magazines.