The brain, scientists have discovered, turned out to be so darned important that it has evolved independently four times (at least in molluscs):
"Traditionally, most neuroscientists and biologists think complex structures usually evolve only once," says Kocot's colleague Leonid Moroz of the University of Florida in Gainseville.
"We found that the evolution of the complex brain does not happen in a linear progression. Parallel evolution can achieve similar levels of complexity in different groups. I calculated it happened at least four times."
The four groups that independently evolved centralised nervous systems include the octopus, a freshwater snail genus called Helisoma, Tritonia – a genus of strikingly coloured sea slugs – and Dolabrifera, another genus of sea slugs, albeit less aesthetically interesting.
"If these results hold up, it suggests strongly that centralised nervous systems evolved more than once in Mollusca," says Paul Katz, a neurobiologist at Georgia State University in Atlanta. "This is more evidence that you can get complexity emerging multiple times."