The CBC has an article about “Origins of nervous system found in sponges.”
Researchers at the University of California, Santa Barbara, have discovered that while sponges remain the only multicellular animals without a nervous system, they do possess most of the genetic components of synapses, one of the essential building blocks of a nervous system.
The results are being published in PLoS ONE, a journal of the
U.S. Public Library of Science.
Scientists compared the genes of a species of sponge to human genes for expressing a synapse (a connecting point between nerve cells). This suggests that the sponges developed many of the building blocks for constructing a true nervous system.
Ken Kosik, senior author and the co-director of the university’s Neuroscience Research Institute, said:
We look at the evolutionary period between sponges and cnidarians as the period when the nervous system came into existence, about 600 million years ago. It is clear that evolution was able to take this entire structure [in sponges], and, with small modifications, direct its use toward a new function.
NOTE: Fossil sponges go back to the Vendian era, about 680 million years ago. About 300 genera of fossil sponges are known. (I don’t know how many species.) See geological time line.
Going further back, the paramecium, a relatively large single-celled animal, uses the same ions as all other animals to generate coordinating signals to its thousands of cilia.
Addendum: P.Z. Myers had posted a detailed description of the discovery in terms of proteins and gene expression. He also has a nice diagram of a synapse.