Biologists have taken another whack at the human ego, showing that our brain’s cerebral cortex—the seat of higher thought—is eerily similar to a clump of neurons inside the head of the lowly marine ragworm. The ragworm’s brain, which evolved some 600 million years ago, is so similar to the cortex that humans and worms must share a common ancestor.
Scientists knew that fruit flies, cockroaches, and other simple organisms have sensory processors that resemble a cortex, but these were “always interpreted as a striking example of convergent evolution of unrelated structures,” says molecular biologist Raju Tomer, who led the study at the European Molecular Biology Laboratory in Germany.
To test that idea in the ragworm, Tomer used a technique he had developed to examine the complex brains of small creatures with unprecedented clarity: He created a high-resolution map of the worm’s brain cells according to the genes they express, not just their shape and location. When Tomer compared the worm’s cells with those in a vertebrate cerebral cortex, he found they were too similar to be of independent origin.
That result, published in an article in the September issue of Cell, challenges the standard notion that the ability to think evolved from complex vertebrate behaviors like predation, Tomer says. Thought now appears to spring from something far more basic, he argues, like the ability “to distinguish between food and nonfood”—a feat the ragworm accomplishes with aplomb.