Mollusks evolved brains on four different occasions

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As far as invertebrates go, you don't get much smarter than the members of the mollusk phylum, which includes octopuses, cuttlefish, and squid. But mollusks apparently didn't think through their evolutionary path very well, developing brains four times over.

That's the finding of researchers at Auburn University. Kevin Kocot and his team examined the genetic sequences of the eight main branches of the mollusk phylum. They hoped to determine which branches are most closely related to which others, and in doing so provide a clearer history of the specifics of mollusk evolution. Until now, it was assumed that the two mollusk groups with the most highly organized central nervous systems, the cephalopods (octopus, cuttlefish, squid) and the gastropods (snails and slugs), are the most closely related.

Now it appears that that's actually almost the exact opposite of the truth. According to Kocot's analysis, the gastropods are most closely related to bivalves (clams, mussels, oysters, and scallops), which have far more rudimentary nervous systems and not much of a brain. Even more shockingly, cephalopods - the most intelligent of all the mollusk groups - comes from one of the earliest branches, meaning their evolutionary development predates that of snails, clams, and the rest.


There's no way that cephalopods and gastropods could have evolved together apart from all the other mollusks, which means that their similarly advanced nervous systems must have developed independently. That goes against a lot of longstanding assumptions about the evolution of sophisticated structures, as Kocot's colleague, University of Florida researcher Leonid Moroz, explains:

"Traditionally, most neuroscientists and biologists think complex structures usually evolve only once. 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."

A lot of evolutionary theory has been guided by something akin to Occam's Razor - it's simpler to assume that something as complex as the brain only evolved once in a given group, and that all brainy members of that group come from a single common ancestor. Mollusks appear to be pointing us towards a very different story of evolution, one governed by parallel developments and the repeated emergence of brains in wildly divergent groups. Evolution doesn't have any set goals, but it does appear that it has certain ideas and structures it just keeps coming back to.

Nature via New Scientist. Image by Albert Kok.