Our earliest invertebrate ancestors did not have brains. Yet, over hundreds of millions of years, we and other vertebrates have developed amazingly complicated mental machinery. "It must have evolutionary roots somewhere, but where?" wrote Henry Gee, an editor at Nature, in an essay published in the journal's March 15 issue. (Scientific American is part of Nature Publishing Group.)
Years of study of common invertebrate lab subjects, such as amphioxus (Branchiostoma lanceolatum) or nematodes, have yielded scant evidence as to the origins of the big, centralized brains we all develop as embryos. Until, that is, researchers turned their gaze to the humble acorn worm (Saccoglossus kowaleskii).
These unlovely, simple little worms live most of their brainless lives buried in deep-sea beds. Researchers have probed the genetic patterns of their developing larvae and think they might have discovered a set of signals similar to the ones we use to build our central nervous system. The findings are reported online in the same issue of Nature.
But not everyone in the invertebrate community is convinced that the early antecedent to the vertebrate brain has been discovered. And these little worms seem to be stirring up controversy in the quest to find the beginnings of our own brains.
Complexity from simplicity
All of our features—from our brains to our bones—emerged from elaboration on the simplest of genetic patterns found in primitive gunk. But scientists have been keen to find out just how far back they can trace key developments, such as the signals that spurred our central nervous system to develop.
"The vertebrate brain is really exquisitely complex and elaborate," says Ariel Pani, a graduate researcher at Stanford University and co-author of the new paper. The brain is prompted into being during development by a long chain of genetically determined signals. "There are particular developmental processes in vertebrates that seem to be absent in other species"—or at least those that have been most commonly studied, such as the amphioxus, Pani notes. Thus, many scientists had presumed that these genetic tools had only emerged with the vertebrate line itself.
That is where members of the hemichordate group, such as S. kowaleskii, can broaden the view into our joint invertebrate past. The last common ancestor this worm had with vertebrates probably lived more than 500 million years ago. So is it possible that this ancient ancestor already contained the genetic groundwork for big brains—and that this ability has since been lost in more common invertebrate subjects?
The path from a few cells to a full brain has taken hundreds of millions of years in evolutionary time. But during embryonic development, the elaborate process takes just days or months. During an animal's embryonic phase, clusters of proteins—called signaling centers—help spur the creation of different parts of the body. Three major signaling areas in vertebrates—the anterior neural ridge, the zona limitans intrathalamica and the isthmic organizer—are responsible for starting off the major divisions within the central nervous system, such as separating the mid and hind parts of the brain.
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."