Nature the tinkerer
(Top Posts - Science - 020808)

- - -
February 2, 2008
  http://books.guardian.co.uk/review/story/0,,2250705,00.html
- - -

Excerpts:

Your Inner Fish: A Journey into the 3.5 Billion-
Year History of the Human Body
by Neil Shubin

The Making of the Fittest: DNA and the Ultimate
Forensic Record of Evolution
by Sean B Carroll

Remember the old story about modern science:
knowing more and more about less and less?

It's not true any more.

We are living in the age of the great biological
synthesis. Both Neil Shubin and Sean B Carroll
thrillingly show us how, in the last 10 years,
work on fossils, on DNA sequencing and on
embryological development have combined to
piece together the story of how we got here.

But for Carroll celebration is not enough. He is
troubled by the remorseless attacks on evolution
in the US by rightwing evangelicals and says:
"The body of new evidence I will describe in
this book clinches the case for biological evo-
lution as the basis for life's diversity, beyond
any reasonable doubt."

Carroll, at the University of Wisconsin, is one
of the pioneers of evolutionary developmental
biology ("Evo Devo"), the science of how genes
tell organisms to form the shapes they do, both
in growing from the egg and in evolving over
time.

Shubin is a palaeontologist at the University of
Chicago who has eagerly embraced the corro-
borative evidence that Evo Devo provides.

Genetics has transformed fossil hunting, because
the record of every creature's evolution kept by
the genes can be read alongside the fossil forms
to paint a more complete picture. Here, Shubin
traces the descent of mammals - especially us -
from fishes.

Both authors provide stunning case histories of
evolution in action, which can now be traced
down to the actual shifts of single letters in the
DNA code.

Shubin's quest has been for fossils representing
the transition from fish to land animals about
375m years ago.

In 2006 he reported the important finding of
Tiktaalik, a transitional fish/land mammal from
Ellesmere Island, Canada. The fin-bone structure
of Tiktaalik is so developed and animal-like that
this "fish" was capable of doing press-ups on
the mudflats.

As Shubin points out, when all animals were in
the sea, predation was a murderous business.
The first creature to be able to get out of the
water opened a window into countless new
niches.

The fossil evidence of limbs-from-fins is corro-
borated by the genes. A control gene whimsically
called sonic hedgehog is involved in creating the
digits of the hands and feet in mammals, reptiles
and birds.

Errors in sonic hedgehog can create extra digits
or digits that don't differentiate properly between
thumb and pinky. But sonic hedgehog is already
present in the fins of sharks and skates - very
primitive, ie early, fishes.

Many genes can be inserted into creatures remote
from them in evolutionary terms by hundreds of
millions of years. The genes still work.

When researchers inserted a mouse sonic hedge-
hog gene into a skate the fins were transformed:
not into mouse feet (an inserted control gene is
always expressed in the character of the recip-
ient), but by the differentiation of the normally
identical fin-rods of the skate into something
more like mammal digits.

Shubin makes more of body plans and control
genes than Carroll, although this is Carroll's
speciality (he has already written a classic book
on the subject: Endless Forms Most Beautiful).

Carroll has his own astounding fish story: the
icefish of Antarctica. Unique among the higher
animals, these fishes have no red blood cells.
Antarctica went through a profound cooling
(from 68F to about 30F), some 33-34m years
ago. It became too cold in the sea for red blood
cells - at this temperature they would gum up
the works.

To circumvent this, icefish have evolved a suite
of adaptations including a protein "antifreeze".
This has been identified as the product of a mu-
tated copy of a digestive gene (duplicated genes
that can then evolve new functions are one of
the key engines of evolution; nature, as Carroll
points out, is a tinkerer, a bricoleur).

...

Understanding evolution at the level of the de-
tailed DNA code has brought many surprises.

The most vivid and bounteous evidence we
have for natural selection concerns two kinds
of genes: those that never change (immortals)
and are going strong at more than 2 billion
years old; and those that are no longer used
(fossil genes) but live on, moth-eaten, accu-
mulating more and more deleterious mutations.

The immortal genes are the 500 or so that are
vital for the life processes of every cell. These
are virtually identical in every living creature,
from primitive archaeobacteria that can live
in the boiling geysers of Yellowstone Park to
Einstein's brain cells.

They have been preserved intact by selection
because most mutations to these would be
fatal: if a cell stops working it cannot repro-
duce.

The fossil genes are at the opposite pole: they
hang around, gathering mutations, making them
even more useless. Because they are no longer
used, selection cannot keep them in trim.

Moles still have rudimentary eyes but because
they are not needed underground they have
furred over. All of the eye genes are still there
but they are shot to pieces.

We humans have lost the functionality of half
of our odour genes, confirming what we always
knew about animal senses. We still have all the
genes dogs use to sniff out their world - we can
tell because most of the DNA sequences are
still there - but again the holes, insertions and
other damage have disabled them.

The elegance of this double whammy - immortal
and fossil genes - for natural selection is almost
beyond poetry.

Which brings me to so-called "intelligent design",
the idea that some biological structures are too
complex to have evolved under natural selection.

Fossil genes are the nemesis of intelligent design.
What sort of grand designer would litter his crea-
tions with decayed copies of genes that we know
are still functional in other creatures?

There is a simpler explanation. Fossil genes have
decayed because they are no longer under selec-
tion pressure. We humans use our eyes more than
our noses.

As Carroll says: "the rule of DNA code is use it
or lose it."

Evolutionary science has a coherent rationale,
with an interlocking interpretation of vast amounts
of information.

Intelligent design has no rationale. It merely throws
up its hands in despair: "this is too complex and
wonderful to have evolved!"

Although there is inevitably some overlap between
these two books, there is no contest. The dazzling
work described in them is so new to the general
reader that both books are entirely justified.

...

Nothing of more lasting importance than the core
narratives of these books is likely to be published
this year.

--- end excerpts ---