"As well as containing lots of
semi-decrepit retroviruses that can be stripped for parts, the human genome
also holds a great many copies of a "retrotransposon" called LINE-1.
This a piece of DNA with a surprisingly virus-like way of life; it is thought
by some biologists to have, like ERVs, a viral origin. In its full form, LINE-1
is a 6,000-letter sequence of DNA which describes a "reverse
transcriptase" of the sort that retroviruses use to make DNA from their
RNA genomes. When LINE-1 is transcribed into an mRNA and that mRNA subsequently
translated to make proteins, the reverse transcriptase thus created immediately
sets to work on the mRNA used to create it, using it as the template for a new
piece of DNA which is then inserted back into the genome. That new piece of DNA
is in principle identical to the piece that acted as the mRNA's original
template. The LINE-1 element has made a copy of itself.
In the 100m years or so that this
has been going on in humans and the species from which they are descended the
LINE-1 element has managed to pepper the genome with a staggering 500,000
copies of itself. All told, 17% of the human genome is taken up by these
copies--twice as much as by the ERVs.
Most of the copies are severely
truncated and incapable of copying themselves further. But some still have the
knack, and this capability may be being put to good use. Fred Gage and his
colleagues at the Salk Institute for Biological Studies, in San Diego, argue
that LINE-1 elements have an important role in the development of the brain. In
2005 Dr Gage discovered that in mouse embryos--specifically, in the brains of
those embryos--about 3,000 LINE-1 elements are still able to operate as
retrotransposons, putting new copies of themselves into the genome of a cell
and thus of all its descendants.
Brains develop through proliferation
followed by pruning. First, nerve cells multiply pell-mell; then the
cell-suicide process that makes complex life possible prunes them back in a way
that looks a lot like natural selection. Dr Gage suspects that the movement of
LINE-1 transposons provides the variety in the cell population needed for this
selection process. Choosing between cells with LINE-1 in different places, he
thinks, could be a key part of the process from which the eventual neural
architecture emerges. What is true in mice is, as he showed in 2009, true in
humans, too. He is currently developing a technique for looking at the process
in detail by comparing, post mortem, the genomes of different brain cells from
single individuals to see if their LINE-1 patterns vary in the ways that his
theory would predict."[1]
Thus, Dr. Gage’s brief response is as follows: the diversity of brain cells is created by the different sites of transposon LINE-1 insertion in the genomes of those cells. Natural selection, which preserves only properly functioning cells, uses this diversity to create complex brain structures. That is why you are so smart, my dear.
1. "Leaving their mark." The
Economist, 22 Aug. 2020, p. 20(US).
