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THE hallmark of the human brain – the thing that sets us apart from all other animals and is presumed to be the source of our intellectual superiority – can be traced back 600 million years to the ancestor of a primitive worm.

The discovery comes from a study of how genes are switched on and off inside brain cells. It suggests that the ancestor of the ragworm already possessed the seed of what would become the cortex. There are also tantalising hints of how, armed with proto-brains, worms could have given rise to the staggering diversity of animals alive today, from millipedes to horseshoe crabs and the great apes.

The cortex is the thick layer of folded brain tissue that gives mammalian brains their distinctive appearance. It is involved in integrating large amounts of information and is responsible for processes such as memory, awareness, thought and language. It is thought that the denser and larger the cortex is relative to body size, the more cognitively advanced is its owner. By this measure humans are crowned kings of the brainiacs.

Now it seems this remarkable structure had its humble beginnings in the primitive brain of a marine worm. of the European Molecular Biology Laboratory in Heidelberg, Germany, studies a species of ragworm called , which has hardly changed since it first evolved. Just a few centimetres long, it is unusually cerebral for such a primitive animal. “Its brain is more complex than an earthworm’s,” Arendt says. In particular, it has two unusually large structures – called mushroom bodies – which pull together all the information from the worm’s many smell receptors. These mushroom bodies may also be involved in its (admittedly ) ability to learn.

Although they are similar in function to the cortex, evolutionary biologists have long thought that mushroom bodies evolved separately to this more complex structure.

To test this theory, Arendt and his team looked at 42 genes that play a crucial role in the development of the brain. They used fluorescent markers to tag mushroom body cells where each of the 42 genes was switched on, repeating the tagging throughout every stage of the worm’s development.

The team were stunned by the pattern of gene expression they found. Not only was it similar to the one found in mushroom bodies in insect brains, it also resembled the pattern in the complex vertebrate cortex “in shocking detail”, says Arendt. What’s more, this held true for each stage of development up to adulthood ().

The results are “lovely and very convincing”, says of University College London. “They show the brain cells forming in the same place, being patterned by the same genes, and performing similar functions.”

The odds are extremely low that such a complex genetic pattern evolved twice – once to give rise to simple worm brains, and once to give rise to vertebrate brains. “The simplest explanation is that it only evolved once,” says Telford. That suggests Arendt and his team have identified the earliest ancestor of the human cortex.

The implications don’t stop there. Arendt and others believe that the emergence of a proto-cortex in early worms was a driver of the Cambrian explosion, the sudden diversification of complex multicellular life, some 530 million years ago.

Ragworms split from the other complex animals around 600 million years ago. Arendt’s study suggests that their common ancestor, nicknamed “ur-bilaterian”, already had a proto-cortex. This may have given the ur-bilaterian a key advantage.

According to Arendt it would have provided a link between the animal’s senses and its muscles. The crucial thing, he says, was “being a predator that smelled something and went for it”.

of the University of Arizona in Tucson, points out another potential advantage. In vertebrates, the pallium – a precursor to the cortex – develops into a structure called the hippocampus, which is involved in spatial awareness. “It would be a big advantage to have something that told the animal where it was, where it had been, and where it wanted to go,” he says.

He points to horseshoe crabs which evolved in the early Cambrian. Every year they come back to spawn on the same bit of beach in Massachusetts, after an underwater journey of many miles. Their sight is poor, so they may rely on smell to navigate – with the help of their mushroom bodies, which are the largest of any invertebrate.

In light of Arendt’s findings, the ur-bilaterian may even have had some form of primitive memory, says of the Wellcome Trust Sanger Institute in Hinxton, UK. “It was probably capable of most of the things modern insects can do,” he says.

All this is speculation for now, but what seems clear is that the seed for what would eventually give us the likes of da Vinci and Darwin was planted inside a tiny worm, 600 million years ago.

“The ancestral worm may have had a memory, similar in capability to that of a modern insect”