My latest Mind and Matter column in the Wall
Street Journal, with added links:
Don’t look for the soul in the language of DNA
Back in the genomic bronze age-the 1990s-scientists used to
think that there would prove to be lots of unique human genes found
in no other animal. They assumed that different species would have
many different genes. One of the big shocks of sequencing genomes
was not just the humiliating news that human beings have the same
number of genes as a mouse, but that we have the same genes, give
or take a handful.
This humiliation deepened recently when David Knowles and Aoife McLysaght at Trinity
College, Dublin, tracked down, at last, some uniquely human genes:
just three of them. They estimate that there are, altogether,
probably no more than 18 of this wholly unique kind-out of 22,568
genes in total. Over the span of our history, human beings seem to
have acquired a brand-new gene only every third of a million
years.
The three that Drs. Knowles and McLysaght identified lie in
stretches of DNA that are gobbledegook in chimpanzees, gorillas,
gibbons and macaques, so the chances are they have sprung to life
as protein-coding sequences in human beings uniquely. (A gene is
the digital recipe for making a protein molecule.)
The functions of these three genes are not yet known (since they
don’t exist in mice, experiments are tricky), though one seems to
be slightly more active in people with a form of leukemia. They are
small and simple genes, however-unlikely candidates to hold the
recipe for the human soul.
This might seem to leave a small hook upon which philosophers
could hang the uniqueness of the human race. But we have long known
that our uniqueness lies in the order and combination of our genes,
not in the ingredients themselves. DNA is not only like a language;
it is a language, a linear sequence of recombinable
digital characters of infinite variety.
There are close parallels between DNA and a language like
English. Just as evolution uses the same 22,000 genes in a
different order to make a rhinoceros or a rabbit, so Shakespeare
used many of the same 18,000 words in each of his plays. The 10
most common words in “Othello” and “King Lear” are the same (I,
and, the, to, you, of, my, that, a, in), yet the plays are very
different. The English language, like the human genome, contains
very few brand new words that were invented recently from
scratch.
Most new English words arise by different means: by borrowing
from foreign languages (Schadenfreude, pajama); by recombining
existing words (blogosphere, download); or by the addition of
second meanings to existing words (green, mouse).
All of these habits are common in the genome, too. Lateral gene
transfer brings genes from one species into another, especially
among bacteria (less often among mammals like us). This is just
like borrowing a foreign word. Genes recombine by fusing in whole
or in part, by a process known as exon shuffling (exons are the
separate stretches of code that are used to make one protein in
split genes). This is just like recombining existing words. And
genes often duplicate themselves and then diverge into different
functions, just as old words acquire new meanings.
About 800 million years ago, a gene for a simple pigment protein enabling
worm-like creatures to see was duplicated, and the daughter genes
diverged to give the different proteins used in the rods and cones
of the eye. About 500 million years ago, in a lamprey-like fish,
the gene used in cones duplicated and diverged again to give us
blue versus yellow color vision. About 30 million years ago, in a
tree-climbing primate, the yellow gene duplicated and diverged
again to give us green-red color vision.
Genes also die out, just as words do. When they are still
recognizable but no longer in use, they are called pseudogenes. The
word “theatrophone” is a forgotten linguistic pseudogene, and the
word “minidisc” is becoming one. The words “trebuchet” and
“cenotaph” are examples of extinct words that sprang back to
life-something that pseudogenes sometimes do as well.