The basic “scaffolding” for the vertebrate brain has been found in an unexpected distant relative: a marine worm, a new study reports.
The worm’s brain is much simpler than that of even the simplest vertebrates – but it contains three signaling centers almost identical to those found in the brains of vertebrate embryos.
I leaped up and did a happy dance when I read this news, because it’s a clue to one of the greatest mysteries in neuroscience today. See, for most of science history, evolutionary biologists have found the vertebrate brain to be pretty enigmatic. The closest relatives of vertebrates – animals like lancelets and sea squirts – don’t seem to have a brain that’s structured much like ours at all (they’re mainly just bundles of neurons), but even the simplest fish have the same brain layout that all vertebrates more or less inherited.
Somewhere in the distant past, some animal had to develop the basic structure on which all that complexity is built. But when, and who? The mystery went unsolved for more than a century.
The humble acorn worm (Saccoglossus kowalevskii) doesn’t appear to have much in common with vertebrates – it’s closely related to starfish and it has no eyes or ears – or even much of a centralized brain to speak of.
But it does have three genetic traits that are known to underlie vertebrate brain anatomy:
Genetic programs homologous to three vertebrate signalling centres—the anterior neural ridge, zona limitans intrathalamica and isthmic organizer—are present in the hemichordate Saccoglossus kowalevskii.
These three areas are pretty fundamental to the vertebrate brain layout. In vertebrate embryos, the anterior neural ridge develops into the forebrain, the zona limitans intrathalamica divides the cerebrum from the thalamus (helping complex feedback loops develop between these areas), and the isthmic organizer supports development of the midbrain and cerebellum. In short, without these three structures in your embryo, you wouldn’t have much of a brain to speak of either.
As I mentioned above, these signaling centers aren’t found in the brains of vertebrates’ closest living chordate relatives – and this implies something else interesting: that those comparatively more complex animals actually lost some features of their vertebrate-esque brains as they continued to evolve:
We propose that these genetic programs were components of an unexpectedly complex, ancient genetic regulatory scaffold for deuterostome body patterning that degenerated in amphioxus and ascidians, but was retained to pattern divergent structures in hemichordates and vertebrates.
The researchers say they’ll still study those animals for more clues about our brains, though – apart from the genetic similarities between our brains and the acorn worm’s, its nervous system doesn’t have much else in common with ours. Since the acorn worm doesn’t have any sense organs (besides some simple taste receptors) its “brain” is really just a hollow tube of nerves that tells its body to keep squirming toward food. But this structure is also intriguingly similar to one found in vertebrate embryos: the neural crest that eventually develops into the spinal cord.
A lot of questions still need to be answered before we understand the exact relationship between the acorn worm and the earliest vertebrates. We can be pretty sure that the most recent common ancestor (MRCA) of all vertebrates had a similar organ to the acorn worm’s brain, but that knowledge alone doesn’t throw much light on why that structure originally evolved, or what its ancestor organ was like, or why vertebrate and invertebrate brains started evolving along such different paths.
Those are all questions for future researchers to tackle – that is, if this worm is willing to give up its secrets. I dunno…it looks like a tough little guy.