Evolution__3rd_Edition

568 PART 5 / Macroevolution
Transposable elements may have
become quiescent in recent human
history
Table 19.1
Time of origin of repetitive DNA in (a) humans, and (b) the mouse. The entries in the
table show the percentage of the genome that belongs to the specified class of repetitive DNA
and originated at the specified time. Not all dates and repetitive DNA classes are shown so the
percentages do not add up to all the repetitive DNA in the genome. (a) In humans, note the
burst of Alu origination 25–75 million years ago, and the relative quiescence of repetitive
DNA origination in the past 25 million years. (b) In mice, neither feature is seen.
(a) Human.
Class of repetitive DNA
Time (Myr) SINE (Alu) LINE (LINE1) LTR
0–25 0.5 0.5 0
25–50 4.5 2 0.25
50–75 3.5 1.5 1.5
75–100 1 2.5 2
(b) Mouse.
Class of repetitive DNA
Time (Myr) SINE (Alu) LINE (LINE1) LTR
0–25 2 2.5 2
25–50 2 3 2
50–75 3 2.5 2
75–100 2 2.5 2
between 75 and 25 million years ago. About 10% of our DNA consists of Alu sequences,
and 80% of it originated in that 50 million-year period.
A second pattern is that all transposable elements seem to have become quiet in the
past 25 million years. Before 25 million years ago, new transposon-derived DNA was
added to our DNA at a relatively steady rate (though different kinds of transposon were
contributing more or less at different times). But little new DNA has been added in the
recent evolutionary past.
Both these patterns are provisional, because the International Sequencing Consortium
used a preliminary method of analysis. I said that we could infer the date of the
common ancestor for any two copies of Alu (for example) by counting the differences
and applying a molecular clock. The International Sequencing Consortium compared
each sequence to a consensus sequence a counting the differences between each Alu
and a consensus Alu. Ultimately, biologists will aim to reconstruct a gene tree for the
Alu sequences in our DNA, and estimate the number of sequence changes in the
branches of the tree. An analysis of that sort will give a more reliable history of our
non-coding DNA than the figures in Table 19.1.
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