DON'T be offended, but you have the brain of a worm. Clusters of cells that are instrumental in building complex brains have been found in a simple worm that barely has a brain at all.
The discovery suggests that, around 600 million years ago, primitive worms had the machinery to develop complex brains. They may even have had complex brains themselves - which were later lost.
Vertebrates, such as humans and fish, have the biggest and most complex brains in the animal kingdom. Yet all their closest non-vertebrate relatives, such as the eel-like lancelets and sea squirts, have simple brains that lack the dozens of specialised nerve centres typical of complex brains. As a result, evolutionary biologists have long thought that complex brains only evolved after animals with backbones appeared.
Not so, says Christopher Lowe of Stanford University in California. His team studies a species of acorn worm, Saccoglossus kowalevskii, which has a rudimentary nervous system made up of two nerve cords and nerves spread out in its skin. The worms live in burrows in the seabed and pull in passing particles of food.
Lowe found that young S. kowalevskii have three clusters of cells identical to the ones vertebrates use to shape their brains. In developing vertebrate brains, these clusters - called signalling centres - make proteins that orchestrate the formation of specialised brain regions. The acorn worm, Lowe found, produces the same proteins, and they spread through its developing body in patterns similar to those they follow in the developing vertebrate brain (Nature, DOI: 10.1038/nature10838).
S. kowalevskii is more distantly related to vertebrates than lancelets and sea squirts. So the finding could mean that the last common ancestor of all these animals may already have had a relatively complex brain structure, some 590 million years ago, and some of its descendants later lost it.
Complex brains could go back even further, to about 630 million years ago, says Detlev Arendt of the European Molecular Biology Laboratory in Heidelberg, Germany. He studies an animal from a completely different group, an annelid worm called Platynereis dumerilii, which has barely changed in 600 million years. In 2010, he showed that this primitive worm has the molecular machinery to make the human cortex
- the hallmark of our big brains and the seat of our intelligence.
In unpublished research, Arendt found that his worm makes some of the same proteins that Lowe found in the acorn worm. He thinks that complex brains date back to the first worms, which used them to navigate the primordial seabed and find food. When some of their descendants took up stationary lifestyles, they no longer needed their brains - and so lost them.
There is an alternative explanation however. Lowe points out that, unlike vertebrates, which use signalling centres to structure their brains, acorn worms use them to build their entire body. So although the signalling centres are old, they might not have initially evolved to build big brains.
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