One of the most important
findings in the evolution of signalling pathways is that
they evolved before the occurrence of the bilaterians.
Although no comprehensive searches for signalling
molecules have been carried out in any metazoan phyla,
it is fair to say that the basic components of most pathways
have been found in all studied bilaterian taxa.
However, different numbers of PARALOGOUS molecules
were identified in organisms of divergent animal phyla.
So, a second major conclusion in the evolution of signalling
systems is the importance of gene duplication
and subsequent protein sequence divergence (BOX 2; for
a review, see REF. 34).