Brainy worms? Evolution of the cerebral cortex

Scientists have discovered a true counterpart of the cerebral cortex in an invertebrate (a marine worm), which gives an idea of what the most ancient higher brain centres looked like.

Our cerebral cortex, or pallium, is a big part of what makes us human: art, literature and science would not exist if this part of our brain had not emerged in a less intelligent ancestor in prehistoric times. But when did this occur and what were these ancestors?

Unexpectedly, scientists at the European Molecular Biology Laboratory in Heidelberg, Germany, have discovered a counterpart of the cerebral cortex in a marine worm. Their findings indicate not only what the most ancient higher brain centres looked like but also what our distant ancestors used them for.

The scientists have found the brain structures related to the vertebrate pallium in a very distant cousin – the marine ragworm Platynereis dumerilii, a relative of the earthworm – that last shared an ancestor with us approximately 600 million years ago.

‘Two stunning conclusions emerge from this finding,’ explains Detlev Arendt, who headed the study. ‘First, the pallium is much older than anyone would have assumed, probably as old as higher animals themselves. Second, we learn that it came “out of the blue” – as an adaptation to early marine life in Precambrian oceans.’

To uncover the evolutionary origins of our brain, Raju Tomer, who designed and conducted the work, took an unprecedentedly deep look at the regions of Platynereis dumerilii’s brain responsible for processing olfactory information – the mushroom-bodies.

He developed a new technique, called ‘cellular profiling by image registration’, which is the first to enable scientists to investigate a large number of genes in a compact brain and determine which are turned on simultaneously. This technique enabled Tomer to determine each cell’s molecular fingerprint, defining cell types according to the genes they express rather than their shape and location.

‘Comparing the molecular fingerprints of the developing ragworms’ mushroom-bodies to existing information on the vertebrate pallium,’ Tomer says, ‘ it became clear that they are too similar to be of independent origin and must share a common evolutionary precursor.’

This ancestral structure could have enabled our ancestors crawling over the sea floor to identify food sources, move towards them, and integrate previous experiences into some sort of learning.

‘Most people thought that invertebrate mushroom-bodies and vertebrate pallium had arisen independently during the course of evolution, but we have proven this was most probably not the case,’ says Tomer. Arendt concludes: ‘The evolutionary history of our cerebral cortex has to be rewritten.’

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