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All animal vision derives from one common ancestor

My latest Mind and Matter column is on the origin
of vision in animals and a vindication for Darwin:

Until recently it was possible, even plausible, to think that
the faculty of vision had originated several times during the
course of animal evolution. New research suggests not: vision arose
only once and earlier than expected, before 700 million years

Davide Pisani and colleagues from the National University of
Ireland have traced the ancestry of the three kinds of
“opsin” protein that animals use, in combination with a pigment, to
detect light. By comparing the genome sequences of sponges,
jellyfish and other animals, they tracked the origin of opsins back
to the common ancestor of all animals except sponges, but including
a flat, shapeless thing called a placozoan. Some time after 755
million years ago, the common ancestor of ourselves and the
placozoa duplicated a gene and changed one of the copies into a
recognizable opsin.

Placozoans still have just that one kind of opsin, and it lacks
the key amino acid change at position 296 that makes light
detection possible, so Dr. Pisani concludes that the last opsin
common ancestor, dubbed LOCA, had no vision. But on the other
branch, the common ancestor of ourselves, insects and jellyfish
made the change to light detection, then experienced two more
duplications some time between 711 million and 700 million years
ago to give the three kinds of light-sensing opsins we still
possess today.

That vision was a single evolutionary innovation is a discovery
that would have surprised an earlier generation of evolutionary
biologists, who contrasted the compound eye of the insect with the
camera-like eye of human beings and imagined several parallel
inventions. But some years ago it emerged that the very same gene, called Pax6,
commands the development of the insect eye and the human eye,
hinting at a common origin. Still more surprising, a version of a
Pax gene was then found directing the development of simple eyes in
jellyfish. So the single origin of vision has become gradually more

All this would come as a relief to Charles Darwin, who worried
about eyes, because their perfect complexity seemed to defy gradual
evolutionary assembly: What use is half an eye? In 1860 he wrote to the American botanist Asa Gray: “The
eye to this day gives me a cold shudder, but when I think of the
fine known gradation my reason tells me I ought to conquer the odd
shudder.” In fact, the anatomy of eyes shows every gradation
between simple light-sensitive spots and full cameras. The detailed
genetic evidence of descent with modification from a single common
ancestor further vindicates Darwin and has largely silenced the
Intelligent Design movement’s use of the eye as a favored

After the duplications that led to working opsin molecules,
there seems to have been a long pause before complex eyes

The first lensed eyes that fossilized belonged to the trilobites
which dominated the Cambrian oceans after 525 million years ago.
Andrew Parker of Oxford University argued in a book a few years ago that newly
perfected eyes explain the sudden appearance of many kinds of
hard-bodied animals, the so-called Cambrian explosion. With
predators hunting by sight for the first time, prey needed
protection and mobility, so an arms race led to a plethora of new
hard-body designs.

Just as eyes suddenly enabled our ancestors to see the world
around them, so the capacity to read genomes enables us to see deep
into the past. Long before LOCA there lived a creature called LUCA,
the last universal common ancestor. It was only about 50 years ago
that the unity of life became apparent for the first time. The
molecular biologist Francis Crick, surveying the experiments that
were deciphering the genetic code in bacteria, animals and yeast
cells, and seeing that they were all converging on the same
universal cipher, concluded that there is only one kind of life on
the planet: that plants, animals and microbes must once have shared
a common ancestor.

By Matt Ridley | Tagged:  rational-optimist  wall-street-journal