Pani and his colleagues report that they have found three similar signaling centers in embryonic acorn worms. What are these genetic cues doing in a brainless worm? They appear to be directing the animal's ectoderm, which contains sensory cells, around the circumference of the animal (unlike the centralized nerve bundle found in vertebrates). According to Pani and his colleagues, these cues are not present in amphioxus and its fellow invertebrate chordates, suggesting that they lost them over time.
That would mean that the vertebrate brain "didn't invent entirely new mechanisms—it took existing ones to develop a completely new structure," Pani explains. If the genome is the proverbial set of blueprints for an organism, the signaling centers involved in embryonic development are like the early pieces of the scaffolds. "Vertebrates have that same sort of framework and are turning it into a very fancy Frank Lloyd Wright house, and hemichordates have turned it into a little cottage."
But not everyone is convinced that these reminiscent signaling centers really are the original beacons signaling early nervous system complexity. The researchers have found gene interactions that are involved in body patterning dividing their heads from their tails—and not much more, says Linda Holland, a research biologist at Scripps Institution of Oceanography who was not involved in the new research. "It's not uncommon for an animal to have part of a gene network" without possessing the entire workup, she notes.
She works with amphioxus and is not convinced that all of these signaling centers are absent from her subjects after all. The suggestion that amphioxus and other invertebrates have lost these signaling centers is "way out of bounds," she says. "I think the amphioxus community is going to be up in arms."
She also is skeptical that the signals Pani and his colleagues found are quite as clear and simple as the paper describes. The genes might be helping to distinguish the worm's front from its back, but possibly no more than that, she notes. "Hemichordates probably have some of [this] machinery in place, but it's much messier" than the findings suggest, she says.
Holland notes that a lot more research—on acorn worms, amphioxus and other extant invertebrates—is needed. And Pani does not write off amphioxus as a first-choice organism for evolutionary research. But researchers shouldn't limit themselves to one model subject, he argues. "If you see things that were different between it and vertebrates, you can't conclude they're unique to vertebrates without looking further."
Looking for ancient ancestors can also help to elucidate how early animals developed their basic body plan and nervous system. "It's still very controversial as to what the nervous system of that common ancestor would have looked like," Pani says. Fossils of 500-million-year-old small, soft-bodied invertebrates can be difficult to interpret. "Not having a time machine, we're stuck," Holland says.
The origin of our complex brains remains controversial, and, Pani says, "I imagine it will stay that way."