The Worm In Your Brain

By Carl Zimmer | September 3, 2010 4:56 pm

mushroom and brainOne of the most fascinating things about the history of life is the way distantly related species can look alike. In some cases, the similarities are superficial, and in other cases they are signs of a common ancestry. And sometimes–as in the case of our brain and the brains of worms–it’s a little of both.

The biggest feature of our brains is a massive stack of densely woven neurons called the cerebral cortex. Once our brains take in sensory information, it’s the cortex that integrates it, makes sense of it, learns from it, and decides how to respond. If you compare our cortex to those of our close relatives, the apes, they’re nearly identical in structure, although our cortex is very big for our body size. If you look further afield, you’ll find the same basic architecture of the cortex in all vertebrates, although different parts are different size in different species. Because these similarities are so consistent in so many different ways, and because you can trace the changes to the cortex along different lines of descent, they’re strong evidence that the common ancestor of all vertebrates had a cortex.

insect diagramVertebrates are not the only animals with a nervous system, however. Insects, crustaceans, worms, and other invertebrates have nervous systems that are also organized around a central cord. These invertebrates typically have a large cluster of neurons at the front of the cord that functions like our brain does: it’s where sensory information goes in, and various commands go out. And in some invertebrates, such as insects and spiders, these brains have tightly packed clumps of neurons that are essential for letting these animals learn associations between odors and food and other important lessons. These clumps are known as mushroom bodies.

The similarities between mushroom bodies and our cortex are not overwhelming, but they are tantalizing. The cortex and mushroom bodies play similar roles, and their arrangements are somewhat similar. Mushroom bodies are even parcelled up into distinct regions, just as we have regions for handling sight, smell, and other tasks.

On the other hand, mushroom bodies are missing a lot of the landmarks of the vertebrate cortex. The brain regions they connect to don’t have counterparts in our brains. And while all vertebrates have a cortex, a lot of invertebrates have no known mushroom bodies.

Traditionally, scientists have concluded that mushroom bodies and the cortex are an example of convergence. Birds and bats both have wings, for example, but their common ancestor did not. Instead, both lineages evolved different wings much later. After many of the major branches of animals split apart between 600 and 550 million years ago, the vertebrate lineage evolved a brain with a cortex, and some invertebrates evolved mushroom bodies.

Over the past thirty years, scientists have added a new line of evidence to the search for the origin of brains and other traits. They can now identify the genes that build the traits. As a mouse embryo develops, certain genes switch on in the brain to start building the cortex. The same genes build up our own brains as well, which is not surprising given all the other evidence that the common ancestor of mice and humans had a cortex.

But scientists have had some wonderful surprises when they’ve compared genes of more distantly related species. Jellyfish, grasshoppers, and humans all have eyes, for example, but they’re radically different from each other–at least anatomically. Yet they also share some of the same genes for building light receptors and other parts. So they’re actually a mix of convergence and a shared ancestry. I wrote about the evolution of eyes in The Tangled Bank, in a section excerpted by the New York Academy of Science here.

ragwormThe cortex now turns out to follow the same story as the eye. Detlev Arendt of the European Molecular Biology Laboratory and his colleagues decided to compare the genes that build the cortex in mammals to the genes that build mushroom bodies in invertebrates. They studied a pretty little creature called the ragworm. They chose it because it has huge, easy-to-study mushroom bodies, and because it has evolved more slowly since the vertebrate-invertebrate split than flies and other well-studied species. The scientists carried out an exquisitely detailed survey, mapping where a number of genes were becoming active in the developing ragworm brain, down to the individual cell.

The figure here shows a remarkable similarity. On the left is a developing mouse cortex. Below it is a chart showing where a group of genes are expressed. The colored strip on the brain matches the vertical axis of the chart. And to the right is a diagram of the developing ragworm brain. If you duplicate the strip in the mouse cortex and join two ends together into a fork, you get a region in which many of the same genes are being expressed in a nearly identical pattern. And that forked regions–da dum!–eventually becomes the mushroom bodies.

cortex diagram

So our cortex turns out to be a lot older than previously thought. The common ancestor of us and ragworms–a wormy creature that lived 600 million years ago–not only had a brain, but had an ur-cortex. And it probably used that ur-cortex to learn about its world–most likely learning about the odors it sniffed. That animal’s descendants diverged into different forms, and the ur-cortex changed along the way. Yet they still used many of the same genes their ancestor did long ago. So next time you splat a fly against a wall, remember: there was a cortex in there.

mushroombody tree

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Comments (17)

  1. Aurora

    Thanks, Carl, you’re the 1st person I’ve seen really go into to the convergent vs. divergent evolution aspect of this – and I love the ending!

  2. Hmm, one may also speculate that they first learned how to move around, before it made any sense for them to learn about their world. We are just now in the process of collecting evidence that genes involved in this form of learning are also conserved: http://bjoern.brembs.net/comment-n632.html
    Still in its early stages, so there won’t be any peer-reviewed publication coming out before next year, at the earliest. Just thought it may be interesting in this context.

  3. “Yet they also share some of the same genes for building light receptors and other parts. So they’re actually a mix of convergence and a shared ancestry”
    If I remember correctly the same also applies to the general body plan and its division in segments both in Arthropoda and in Chordata due to hox genes… The convergence vs synapomorphy matter is a tough one…

  4. David Zuccaro

    “Once our brains take in sensory information, it’s the cortex that integrates it, makes sense of it, learns from it, and decides how to respond.”

    Great article. Just one minor nit pick. I take issue with your use of the word “decides”. To me this word has a human, all too human connotation. I would have preferred to the use of a less anthropomorphic word such as “*calculates* how to respond”.

  5. johnk

    Not sure I buy it. I’ve been struggling to figure out the origins of the vertebrate nervous system since graduate school. I’m a neuroscientist, but evolution of the nervous system is not my principal area.

    The origin of the vertebrate CNS has always been a mystery, from spinal cord on up. There seem to be many fundamental differences between the vertebrate CNS and invertebrate nervous systems.

    Three things that occur to me:
    1. pyramidal cells have particular characteristics and are characteristic of vertebrate cortex. It would be interesting — and more convincing — if the mushroom bodies had neurons with characteristics of pyramidal cells.
    2. Olfactory receptors seem ancient, and similar across vertebrates and invertebrates. Olfaction enters the front-most part of the vertebrate brain, and it looks as if the vertebrate forebrain evolved to coordinate olfaction with other modalities and behavior. If the vertebrate and invertebrate forebrains are homologous, one would expect great similarities in the organization of olfactory processing regions: ie., olfactory bulb, olfactory cortex and olfactory projections to hypothalamus. My understanding is the vertebrate and invertebrate eyes are not homologous, so one would not expect homologies in visual processing.

    Interesting stuff.

  6. amphiox

    1. pyramidal cells have particular characteristics and are characteristic of vertebrate cortex. It would be interesting — and more convincing — if the mushroom bodies had neurons with characteristics of pyramidal cells.

    I’m not sure if this is necessary to establish the connection. The ur-cortex may have had more generalized cell types that went on to evolve the characteristics of pyramidal neurons in the vertebrate lineage after the split, and other sorts of characteristics in the invertebrate lineage, even while both retained the ancestral developmental master genes. It would be analogous to how versions of Pax-6 govern the development of eyes with different structures and cell types in vertebrates and invertebrates.

    My understanding is the vertebrate and invertebrate eyes are not homologous, so one would not expect homologies in visual processing.

    See above. The same developmental genes are responsible for eye development in both vertebrate and invertebrate, even though the resultant eyes end up appearing completely different.

  7. amphiox

    I’m curious to know where cephalopod brains fit in with this. Do they have a cortex, or mushroom bodies, or something else?

  8. Great post.
    Pyramidal cells in the mushroom bodies would be nice, but they might have evolved later. It would make sense if the ur-cortex started out as just another lump of the same types of cells, and then, once that clump was established, the cells became optimized. Makes sense to me anyway.

  9. So next time you splat a fly against a wall, remember: there was a cortex in there.

    Synchronicity: I just saw the old flick “The Fly” that has a fly with a whole human head.

    I was already fretting about stepping on unseen insects in the grass that might have the capacity for suffering. Next we’ll be finding that bacteria are sentient.

    Oh, the humanity!

    Poor Richard’s Almanack 2010

  10. Skep Tical

    I’ve not much doubt that you folks are far superior to me in many ways, but it’s certainly not in intellect. I’m sure that God may have used a lot of common elements in our creation, that’s his prerogative. But evolution takes more faith than I have in my Lord Jesus. Your religion is a complex way to deny Gods will in your lives. How can you look at the wondrous things He has created, and deny Him?

    It takes this kind of denial to justify the evil and deviance in mankind, and let’s you off the hook for it. How does evolution account music and beauty appreciation? Or even your uncanny ability to seek triviality and pass it off as knowledge? Too bad about Hawking. Our Lord would have him whole again and give him all the answers he’s seeking if he would just admit and submit to what he already knows in his heart better than any of us. There is an intelligent design in everything, and He is God.

  11. Bernie Laguna

    Excellent article. To know evolution look at your parents, grandparents, great grandparents……. As an aside: humans should not confuse “GOD” with religion. “GOD” is the “Creation Itself” from the very beginning.

  12. Phil Ardery

    September 9, 2010, Amphiox asked, “I’m curious to know where cephalopod brains fit in with this. Do they have a cortex, or mushroom bodies, or something else?”

    According to a presentation “Invertebrate Neural Development” (Manny Garcha, Kabir Bains, and Amber McEnerney — Sacramento State), the answer to Amphiox’s question is “Something Else.” Garcha/Bains/McEnerney write, “Cephalopods … have a highly decentralized nervous system. For example, there are more neurons in the tentacles of the octopus that there are total neurons in the brain.

    “It [raises] the question whether or not we need to rethink our concept of what a brain is, given that cephalopods may be thought of as having multiple ‘mini brains’ in conjunction with a more centralized processing unit”

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The Loom

A blog about life, past and future. Written by DISCOVER contributing editor and columnist Carl Zimmer.

About Carl Zimmer

Carl Zimmer writes about science regularly for The New York Times and magazines such as DISCOVER, which also hosts his blog, The LoomHe is the author of 12 books, the most recent of which is Science Ink: Tattoos of the Science Obsessed.

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