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
ago.
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.