Humble beginnings of the brain?: New research suggests that the genetic blueprints for our big brains might be present in this simple acorn worm. But not all invertebrate researchers agree. Image: Ariel Pani
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.