Paper View: Darwin, of All the Nerve
American neurons are due to get a workout this day. The taste buds and olfactory neurons will get their exercise first at Independence Day barbecues across the land, then the visual cortex and auditory neurons will max out as the fireworks start after dark. Escorted by the Editors of Science Magazine, Darwin is here in America and wants to get in on the action for his 200th birthday tour. Surely his handlers will have something evolutionary to say about all this electrical stimulation going on that makes the revolutionary holiday joyous. Picture a bandstand in the park with an audience eager to hear about the old man’s viewpoint on nervous systems.
Greg Miller, in a series celebrating the white beard of evolution for Science,1 tackled the question in Darwinian style with his essay, “On the Origin of the Nervous System.” Let’s see if it wins applause from the crowd. The editors give the introduction to the man of the hour: “What did the first neurons and nervous systems look like, and what advantages did they confer on the animals that possessed them? In the seventh essay in Science’s series in honor of the Year of Darwin, Greg Miller discusses some tantalizing clues that scientists have recently gained about the evolutionary origins of nervous systems.” Drum roll. Miller steps up to the mike. Will he be nervous? It always helps to start an essay with a grand parade of amazing facts:
The nervous systems of modern animals are amazingly diverse. A few hundred nerve cells are all a lowly nematode needs to find food and a mate. With about 100,000 neurons, a fruit fly can perform aerial acrobatics, dance to woo a mate, and throw kicks and punches to repel a rival. The sperm whale’s 8-kilogram brain, the largest on the planet, is the navigation system for cross-ocean travel and 1000-meter dives and enables these highly social creatures to communicate. The human brain—one-sixth that size—is the wellspring of art, literature, and scientific inquiry.
He’s won some applause for those lines, but has not yet answered the question. Might as well dive right in: “But how did they all get started? What did the first neurons and nervous systems look like, and what advantages did they confer on the animals that possessed them?” Miller detours briefly into history to absolve Mr. Darwin. The old man was ill-equipped to answer that question, he said. Neuroscience did not really begin till after he died. It has taken decades to develop the tools to even begin to understand the subject matter that needs explaining by Darwinian theory. With the father of evolution thus exonerated, how are modern researchers doing?
Using such modern tools, scientists have recently begun to gain some tantalizing clues about the evolutionary origins of nervous systems. They’ve found that some of the key molecular building blocks of neurons predate even the first multicellular organisms. By looking down the tree of life, they are concluding that assembling these components into a cell a modern neuroscientist would recognize as a neuron probably happened very early in animal evolution, more than 600 million years ago. Most scientists agree that circuits of interconnected neurons probably arose soon thereafter, first as diffuse webs and later as a centralized brain and nerves.
But the resolution of this picture is fuzzy. The order in which early branches split off the animal tree of life is controversial, and different arrangements imply different story lines for the origins and early evolution of nervous systems. The phylogeny is “a bit of a rat’s nest right now,” says Sally Leys of the University of Alberta in Edmonton, Canada. Scientists also disagree on which animals were the first to have a centralized nervous system and how many times neurons and nervous systems evolved independently. Peering back through the ages for a glimpse of the first nervous systems is no easy trick.
The audience, naturally, is only going to tolerate excuses for so long about how hard the question is. So far, they have only heard about “tantalizing clues” and “story lines” and low-resolution pictures compounding the problem – followed by the suggestion that this complicated system arose several times independently. No answer is yet in sight.
Surprisingly, Miller tries to soften up the audience on the radical idea of multiple independent origins by quoting an evolutionary colleague who said, “If you look at any other organ or structure, people easily assume it could evolve multiple times,” so, by implication, why the heartburn about nervous systems? The audience looks this way and that as if asking, What people are you talking about?
Miller next sets up the problem: “How to Build a Neuron.” He discusses the varieties of neurons, how they all transmit electricity in one direction, and other empirical facts. Dendrites, neurotransmitters, all the objects of study in the neuroscience lab get a brief mention. Then he tells what they’re good for:
Arranged in circuits, neurons open up new behavioral possibilities for an animal. Electrical conduction via axons is faster and more precise than the diffusion of chemical signals, enabling quick detection and a coordinated response to threats and opportunities. With a few upgrades, a nervous system can remember past experiences and anticipate the future.
The audience is sensing this is another distraction from the subject. Miller responds to their impatience as if to say I’m getting there, but gives another excuse: “Although the advantages of going neural are clear, how it first happened is anything but.”
On come the stories, or “plausible scenarios” as he calls them. Maybe jellyfish were the first pioneers to explore the possibilities of nerves. Back in 1970, “George Mackie of the University of Victoria in Canada envisioned something like the sheet of tissue that makes up the bell of a jellyfish as starting material.” Those cells respond to touch and contract. Perhaps these multifunctional cells “may have given rise to” additional cell types, and the ions began to flow. “With further specialization, the distance between the sensory and muscle cells grew and axons arose to bridge the gap,” he said, embellishing this story without any appeal to observational evidence. “Eventually, ‘interneurons’ appeared,” (how? from where?) “forming synapses with sensory neurons at one end and with muscle cells at the other end.” The audience is puzzled. He seems to have conjured up the evolutionary rabbit out of a hat of pure speculation.
So far this “plausible scenario” is tall on imagination and short on empirical support. Miller calls on another evolutionist who seems to have stronger faith in the power of convergent evolution: “Neurons may have appeared in multiple lineages in a relatively short time.” That doesn’t calm the rustling in the audience much.
Realizing he needs some factual support quick, Miller appeals to Paramecium and other single-celled organisms that can respond with a cascade of signals when they touch an obstacle. Voltage-gated channels in the membrane allow ions to flow as part of the response mechanism. It strikes some in the audience strange that Miller appeals to one complex system to explain the origin of another complex system. He works up his nerve to say, “Electrical excitability, it seems, evolved long before neurons made it their specialty.” A critic in the audience jots down a note that he has not described any of this in terms of mutations and natural selection. So far, it sounds Lamarckian.
Miller wipes his storyboard with a sponge. His next plot line is that sponges may have been a transitional link. After all “Many scientists think” that sponges “are the living creatures most similar to the common ancestor of all animals.” The audience shuffles restlessly again: who is he talking about? “And to many researchers, sponges look like animals on the verge of a nervous breakthrough.” That pun gets a brief courtesy giggle followed by furrowed brows. He continues, “Sponges don’t have a nervous system, or even neurons, but they do have a surprising number of the building blocks that would be needed to put a nervous system together.”
Miller shows they have these building blocks by referring to the genome of a marine sponge that can build some proteins used in synapses of neurons. These sponges, of course, lack synapses, but they appear to have some genes for neurotransmitter receptors. The audience perks up at this revelation. What does it mean? Miller is not sure: “the function of these synaptic scaffolding proteins in a sponge is a mystery….” Some in the audience are toying with alternative explanations. Simultaneously, Miller seems to realize his vulnerability. He just failed to explain why Darwinian selection would build equipment for an animal that appears to lack any use for it.
Surprising revelations might just keep the audience off guard. Miller describes some sponge larvae that “express a handful of genes that spur neural precursor cells to develop into full-fledged neurons in more complex animals.” These genes, he continues enthusiastically, stimulate the formation of extra neurons when inserted into fruit flies. Isn’t it therefore possible that these sponge larvae have “protoneurons”? The audience listens, but some are wondering what a protoneuron would be good for. “Bernard Degnan speculates that they may somehow help the free-floating larvae sense their environment and find a suitable place to settle down and metamorphose into their adult form.” The audience is listening intently now.
Miller continues with stories of how these sponges seem to have a kind of “neural foreshadowing”. The critic jots down another note: in Darwinian theory, evolution acts only for the present and cannot see possibilities down the line, so ‘neural foreshadowing’ makes no sense One sponge, Miller continues, seems to have a reaction potential and a slow-but-effective reflex response. How this improves on Paramecium is not clear, but he says Leys and Mackie think it’s interesting.
All in all, says Leys, sponges provide a tantalizing picture of what an animal on the brink of evolving a nervous system might look like. Their cells have many of the right components, but some assembly is still required. And although they have a wider behavioral repertoire than most people realize, Leys says, their “reflexes” are far slower than those of animals with a nervous system.
The thought of a sponge as a primitive link between single-celled organisms and animals with nervous systems is on the audience’s minds. But before they get too enthusiastic about this possible evolutionary transition, Miller pauses to caution them that “some researchers argue that sponges aren’t the most primitive living animals.” The audience goes from elation to deflation.
Now what? It might be, Miller continues, that comb jellies lie at the base of the evolutionary tree. This is very bad news for the sponge believers. “Like true jellies, ctenophores have bona fide neurons and a simple netlike nervous system,” Miller reveals. “Their position at the base of the animal family tree—if it stands up—would shake up many researchers’ views on nervous system evolution.” The audience gasps. This has other “unpalatable implications,” he moans: “if ctenophores came before sponges, the assorted nervous system components that have turned up in sponges may not be foreshadowing after all but rather the remnants of a nervous system that was lost after the sponge lineage split off from that of ctenophores.” The audience groans in disbelief. Sixteen paragraphs into the lecture and he is back at square one.
What will he do next? He takes a brief foray into discussing another contender for the earliest animals: cnidarians (which includes true jellyfish, sea anemones and corals). But that is not much help, because cnidarians have more complex neuronal components than sponges, just like ctenophores. Hemmed in by “unpalatable implications,” Miller abandons all pretence of empirical support, and projects an imaginary world on the screen:
Just as sponges, comb jellies, and sea anemones may hold clues to how the first nerves and nerve nets arose, other creatures may shed light on the evolution of more complex neural circuitry. “I think everybody agrees that nervous systems were at first diffuse and then evolved to be centralized,” with a concentration of neurons in the front end of the animal—that is, a brain—and a nerve cord connecting it to the rest of the body, says Arendt. “But there’s no consensus yet on exactly when this happened.” Arendt and others have argued that a centralized nervous system existed in the ancestor of all bilaterally symmetrical animals, or bilaterians.
To make the point, he alleges that genes that control development of the existing nervous system in fruit flies, worms and vertebrates all play similar roles. What does that mean? “That implies that these genes were already present in the last common ancestor of all these creatures—the ancestor of all bilaterians—and suggests to Arendt and others that this ancestor had a centralized nervous system.”
The audience appears poised to riot. Did Miller really just say that the evolution of the central nervous system happened because the ancestor already had one? Well, then, how did it evolve before that? It appears Miller has only pushed the problem further back in time to some mythical ancestor that already had a central nervous system.
This is certainly an embarrassing moment on stage. Miller backtracks: “But not everyone is so sure.” He presents a “range of possibilities” (from a range of disagreeing scientists). The responsibility for explaining the evolution of the nervous system passes back and forth between them like a hot potato. Miller employs Truman’s Rule: “If you can’t convince them, confuse them” —
Because most but not all modern bilaterians have a centralized nervous system, there will be awkward implications no matter what. If the bilaterian ancestor had a diffuse nervous system, centralized nervous systems must have originated multiple times in multiple bilaterian lineages—a far less parsimonious scenario than a single origin. On the other hand, if the ancestor had a centralized nervous system, several lineages, including that of Saccoglossus, must have later reverted to a diffuse nervous system—an apparent down-grade that’s hard to explain.
The puzzles don’t end there. Fastforwarding a bit in evolutionary time raises a new set of questions. What is the origin of the myelin insulation that speeds conduction down axons and ensures the fidelity of neural signals? Or of the glial cells that are proving to have important roles in brain function and appear to be more numerous in complex nervous systems?
The audience is reeling. It’s as if all the props on stage are falling apart and the stage hands are running in random directions not knowing what to do next. Miller reaches for a tried-and-true audience pleaser: prove that modern scientists are smarter than Aristotle.
The grand old Greek philosopher influenced people well into the 20th century, Miller says, by suggesting that animals could be “arranged in a linear series, with man and the angels at the top.” But of course, “we now know that’s just nonsense.” So even though we have no clue how a nervous system evolved, at least we are smarter than Aristotle.
As the audience sits down from its threatened riot, Miller lets loose with the whole evolutionary bag of tricks and miracle stories to end like a 4th of July Grand Finale:
Most researchers now agree that equally complex—but anatomically different—brains have evolved in birds, mammals, and other animal lineages, Northcutt says: “At least four or five times independently, … major radiations of vertebrates have evolved complex brain structure.” But whether brains that are put together differently operate on similar principles is still an open question. And then there is the enduring question of what, if anything, is special about the human brain. Perhaps the emerging clues about the long evolutionary path we’ve taken will one day help us decide where we are.
The audience leaves, shaking their heads. One jokes to another that if they weren’t enlightened, at least they were entertained.
1. Greg Miller, “On the Origin of the Nervous System,” Science, 3 July 2009: Vol. 325. no. 5936, pp. 24-26, DOI: 10.1126/science.325_24.
You have to laugh at the predicament of these Darwinists. We could dismiss this as a silly slapstick sideshow except for the fact that they have all power over the media, schools and scientific institutions with this malarkey and insist it is the only story fit to teach. What utter nonsense! It’s all fiction, imagination, speculation, futureware and miracles, with complex systems just emerging, giving rise to and appearing left and right without links, causes or evidence. This was exactly like the performance Marshall gave about the Cambrian explosion back in 045/23/2006. What the audience had come for, a scholarly scientific lecture on a matter of serious debate, turned into a circus of silliness camouflaged in jargon: Marshall’s explanation for the sudden emergence of all the major body plans in a geological instant was, in effect, “they evolved because they evolved!” Evolution gets served to the masses as its own circular justification.
Isn’t that exactly what Miller did here? He sidestepped this way, and that, more nimbly than Michael Jackson in a moonwalk, never getting around to answering the question except to say, in effect, “Nervous systems evolved, because… they evolved multiple times independently!” Clueless would be a compliment for this kind of answer. That’s really walking backward when appearing to walk forward. We could not possibly add to the shame the Darwinians should be feeling for giving Miller a clown act to have to play for Seventh Lecturer in the Darwinian Bicentennial than to let you read his words for yourself – that they published anyway. Of all the Nerf.