Did neurons evolve twice on Earth? :: Catherine Brahic, New Scientist, 11 April 2015
Summary and review of the above article
A recent conference discussed whether neurons could have evolved twice on Earth. Neurons in the nervous systems of comb jelly fish are argued to be sufficiently unlike other neurons in nature, to suggest that neurons may have evolved multiple times. If neurons evolved more than once on Earth, there is a greater likelihood that they could evolve elsewhere in the universe.
The more conventional view has been that simple neural networks emerged about 600 million years ago, and subsequently evolved into modern nervous systems/brains. The recently published full genome of the comb jelly fish suggested that they split off from the animals earlier than other jelly fish, and earlier than sponges that lack neurons. If sponges come later than comb jellies, either sponges lost neurons, or neurons evolved twice.
Some biologists reject the claimed evidence of the comb jelly genome, and view the brainless sponges as the most primitive form of animal. The difference in the comb jelly neurons could be the result of a period of accelerated evolution. Some take the view that it is brains rather than neurons that evolved several times from simple underlying neural networks. Animal groups are argued to derive from different creatures with neural networks, and the brains of these different groups may have evolved separately.