Slimy and often sluggish they may be, but some molluscs deserve credit for their brains – which, it now appears, they managed to evolve independently, four times.
The mollusc family includes the most intelligent invertebrates on the planet: octopuses, squid and cuttlefish. Now, the latest and most sophisticated genetic analysis of their evolutionary history overturns our previous understanding of how they got so brainy.
The new findings expand a growing body of evidence that in very different groups of animals – molluscs and mammals, for instance – central nervous systems evolved not once, but several times, in parallel.
Kevin Kocot of Auburn University, Alabama, and his colleagues are responsible for the new evolutionary history of the mollusc family, which includes 100,000 living species in eight lineages. They analysed genetic sequences common to all molluscs and looked for differences that have accumulated over time: the more a shared sequence differs between two species, the less related they are.
The findings, which rely on advanced statistical analyses, fundamentally rearrange branches on the mollusc family tree. In the traditional tree, snails and slugs (gastropods) are most closely related to octopuses, squid, cuttlefish and nautiluses (cephalopods), which appears to make sense in terms of their nervous systems: both groups have highly centralised nervous systems compared with other molluscs and invertebrates. Snails and slugs have clusters of ganglia – bundles of nerve cells – which, in many species, are fused into a single organ; cephalopods have highly developed central nervous systems that enable them to navigate a maze, use tools, mimic other species, learn from each other and solve complex problems.
SLIMY COUSINS
But in Kocot's new family tree, snails and slugs sit next to clams, oysters, mussels and scallops (bivalves), which have much simpler nervous systems. The new genetic tree also places cephalopods on one of the earliest branches, meaning they evolved before snails, slugs, clams or oysters.
All this means that gastropods and cephalopods are not as closely related as once thought, so they must have evolved their centralised nervous systems independently, at different times.
That's a remarkable evolutionary feat. "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."
