PHOTOGRAPH BY GEORGE GRALL, NATIONAL GEOGRAPHIC CREATIVE
Published May 21, 2014
A close look at the nervous system of the gorgeously iridescent animal known as the comb jelly has led a team of scientists to propose a new evolutionary history: one for the comb jelly, and one for everybody else.
"It's a paradox," said Leonid Moroz, a neurobiologist at the University of Florida in Gainesville and lead author of a paper in today's Nature about the biology of the comb jelly nervous system. "These are animals with a complex nervous system, but they basically use a completely different chemical language" from every other animal. "You have to explain it one way or another."
The way Moroz explains it is with an evolutionary scenario—one that's at odds with traditional accounts of animal evolution.
Moroz and his colleagues have been studying comb jellies, whose scientific name is ctenophores (pronounced TEN-o-fors), for many years, beginning with the sequencing of the genome of one species, the Pacific sea gooseberry, in 2007. The sea gooseberry has 19,523 genes, about the same number as are found in the human genome.
The scientists enlarged their library to the genes of ten other species of comb jelly (out of the 150 or so species known to exist) and compared them to the analogous genes in other animals. And when they looked at the genes involved in the nervous system, they found that many considered essential for the development and function of neurons were simply missing in the comb jelly.
Some of those missing genes are involved in building neurons in embryos. The cells in any animal start out in the embryo as stem cells, looking pretty much identical to one another and capable of turning into any particular type of cell. Only later in embryonic development do some stem cells switch on specific genes that transform them into neurons. This process is much the same in humans as it is in flies, slugs, and just about every other animal with a nervous system.
But comb jellies, Moroz and his colleagues found, lack those neuron-building genes altogether. Which means that comb jelly embryos must build their neurons from a different set of instructions—instructions no one yet understands.
Nor do comb jellies use the standard complement of neurotransmitters found in other animals, the scientists found. The genes for most of the neurotransmitters in other animals are either missing or silent in the comb jelly—except for one, the gene for the neurotransmitter glutamate. No wonder Moroz likes to call these creatures "aliens of the sea."
Instead of the typical neurotransmitter genes, the scientists found, comb jellies produce a huge diversity of receptors on the surface of their neurons. Moroz can't say yet what the receptors are doing there, but he says they're probably grabbing neurotransmitters, maybe as many as 50 to 100 neurotransmitters in all (comparable to the number of neurotransmitters in the human brain).
Rewriting Evolutionary History
The unique nature of the comb jelly nervous system led the Florida scientists to hypothesize a new evolutionary history for these marine animals, which they laid out in the Nature paper. The earliest animals, according to this new theory, had no nervous system at all. The cells of these early animals could sense their environment directly, and could send signals directly to neighboring cells.
Millions of years later, those signals and receptors became the raw material for the nervous system. But its evolution, according to Moroz, took place in two separate lineages. One led to today's ctenophores. The other led to all other animals with nervous systems—from jellyfish to us.
If there was indeed a parallel evolution with two separate lineages, the split would have happened long ago. Fossils that look a lot like modern-day ctenophores date back some 550 million years, making them among the oldest traces of complex animal life.
But precisely how and when the comb jelly split off from other animal lineages remains controversial. To draw the animal evolutionary tree, Moroz and his colleagues analyzed the similarity of DNA in different species. According to the authors, ctenophores belong to a lineage all their own that split off from the others at the tree's base.
In finding that relationship, the new paper confirms the findings of a team led by Andy Baxevanis, head of the Computational Genomics Unit at the National Human Genome Research Institute, who arrived at a similar conclusion in December after sequencing the genome of another ctenophore species, the American comb jelly (Mnemiopsis leidyi). "You couldn't ask for a better outcome," he said about Moroz's research. "It really shakes up how we think animal complexity evolved."
Gert Woerheide, an evolutionary geobiologist at Ludwig-Maximilians-Universität in Munich, who was not involved in the research, agreed that Moroz and his colleagues have made a thorough case for their revised view of brain evolution. "I think, in this respect, this is a great paper," he said.
But in terms of the actual shape of the animal family tree, Woerheide is less convinced. He isn't sure that comb jellies branched off at the base of the tree, he said; sponges, for example, might have branched off first. In Woerheide's view, the exact reconstruction of the tree reaching so far back in evolutionary history remains an open question.
No matter how the nervous systems of comb jellies evolved, though, everyone agrees that they are weird—and thus worth getting to know better. As Casey Dunn, an evolutionary biologist at Brown University in Providence, Rhode Island, who was not involved in the research, pointed out, comb jellies are turning out to be "even more different from other animals than had previously been appreciated."
This new family tree makes me wonder what the last common ancestor of cnidarians and bilaterians looked like. Something like a primitive flatworm or trichoplax perhaps. After all, the cnidarian larvae (the planula) has some similarities with a tiny flatworm. There are even some planula larvae that perform phototaxis without neurons just like sponge larvae do:
Complex behavior, chemical senses, ability to detect light and vibrations, and coordinated movements evolved long before multicellularity and a nervous system. Take a look at an unicellular creature like Paramecium for instance. Except from their modest size, the lifestyle and behavior of this protozoa is not that different from most comb jellies. The first nervous system probably evolved to make the interaction between the sensory organs and the cilia (the only way to get around before the muscles evolved) more efficient. And the first muscles probably had a different role than locomotion in the first animals. Later both the muscles and nervous systems would get others and more sophisticated roles. Even today there are many animals who depends more on cilia than muscles for locomotion as adults, both comb jellies and bilaterians like flatworms and free living hemichordates (Torquaratoridae).
If the comb jellies branched off before or after the sponges, it either way looks like they evolved their nervous system independently of others.
Some scientists have argued that the idea about a nervous system evolving more than once is too unlikely. Another scientist made a good point by comparing these skeptics with creationists who claim the origin and evolution of complex organisms is way to amazing to have happened on its own. What the skeptics says is "We can agree that a nervous system and more complex body tissue can evolve ones, but no way can it evolve twice". By saying so, they are not much better than creationists who refuse to believe it couldn't even have evolved once.
As the newer research points out, the nervous system of both comb jellies and our own ancestors probably have the same genetic origin, and used genes that were already present and most likely to be given a new function. They just used a different recipe to build their neurons with more or less the same genetic material.
Comb jellies are just the descendants of an early branch of the multicellular animals (and most likley much larger than their ancestors). But once this branch gave rise to a nervous system different than our own, did they already look like comb jellies, or are they just the last and specialized survivors of a larger Group?
They look like overgrown bacteria, it doesn't seem surprising they don't have a central nervous system.
In stead of declaring a different family tree, taking note to parts of dna strains and their function, one might ask if all the results of research until now have been interpreted the right way, i.e.: Is our current knowledge of those genes correct, or should we go back to the beginning and start all over?
just my 2 cents...
Very interesting information. Very good example of both a living fossil as well as possibly the most amazing example of parallel evolution.
See also: Citation: Schierwater B, Eitel M, Jakob W, Osigus HJ, Hadrys H, et al. (2009) Concatenated Analysis Sheds Light on Early Metazoan Evolution and Fuels a Modern “Urmetazoon” Hypothesis. PLoS Biol 7(1): e1000020
Mistaken Identity: 'Sea Anemone' Is Actually New Type of Animal
By Elizabeth Howell, Live Science Contributor | May 16, 2014
"Stranded on a Late Cambrian shoreline: Medusae from central Wisconsin"
Geology; February 2002; v. 30; no. 2; p. 147-150
500-Million-Year-Old Animal Looked Like a Tulip
By Stephanie Pappas, LiveScience Senior Writer | LiveScience.com – Thu, 17 Jan, 2013.
Oldest Horseshoe Crab Fossil Found, 445 Million Years Old
ScienceDaily (Feb. 8, 2008) — "Few modern animals are as deserving of the title “living fossil” as the lowly horseshoe crab. ..."
LiveScience:Skull Confirms Older Origin for 'Living Fossil' Fish
Update time: 04/11/2012
"A group of ancient fish, called coelacanths, have changed so little over time they are known as "living fossils." Now, the remains of a skull found in the Yunnan Province of China, confirms these creatures have been around, largely unchanged, for more than 400 million years. "
Is it possible that the comb jellies followed animal linage farther but then lost the genes that weren't as necessary as we previously thought? Maybe since the comb jellies function well without the the similar nervous system, those genes faced mutations without negative repercussions and they became fossil genes.
I read "The Making of the Fittest: DNA and the Ultimate Forensic Record of Evolution" for school. Good book!
Could these be "space" jellies? Maybe early bacteria traveling on an asteroid from another planet? That could explain why the DNA doesn't fit or there was just an evolutionary split. But I like the idea of something evolving separately from outer space. It could tell us that life is similar on other planets, but different. (Just pondering the possibilities)
Does anyone else think these beautiful creatures look an awful lot like the 'aliens' in the film "The Abyss"?
I flat love the organic transmission entities that compare to electronic control apparatus! Amazing study---we have so much more to learn from our planets inhabitants.
For anyone interested in the beauty and study of these and other gelatinous creatures, please check the Aquatilis Expedition, a 3-year-long journey through the oceans led by Alexander Semenov, who has produced beautiful macrophotography.
Reading this, I am wondering not "How did comb jellies evolve?" but rather, "When will the discordant evidence like this be sufficient to realize that the paradigm of evolution no longer fits the data?"
Increasingly, it seems, evolution is being given and more super powers and soon will be able to do anything, and that without actual data. When did it become forbidden for a favored theory to be falsified, or even considered for falsification, within the science of origins? That's the scientific method, is it not?
If this is insufficient, is there, even theoretically, an amount of evidence that would be sufficient to falsify evolution? From my viewpoint, it seems that would be permitted about the same time that the sun is a dark chunk of coal in the sky.
Forbidding the falsification of a favored theory is more a sign of weakness than of strength.
"...everyone agrees that they are weird- and thus worth getting to know better." Carl Zimmer you are such a hero of mine!
Very interesting to read the article and to know that even today we are still discovering so many new things about the life around us . Amazing oceans
I have not read the paper, but it strikes me as odd to propose that the nervous system in comb jellies is part of distinct and separate evolutionary history than the rest of animals when they still have some (abliet silent) genes for neurotransmitters found in animals. Why would they have them at all if they didn't inherit them from a common ancestor with animals?
Being less familiar than the majority of you on the subject, I Googled "Glutamate" and got this:
Glutamate is generally acknowledged to be the most important transmitter for normal brain function. Nearly all excitatory neurons in the central nervous system are glutamatergic, and it is estimated that over half of all brain synapses release this agent. Glutamate plays an especially important role in clinical neurology because elevated concentrations of extracellular glutamate, released as a result of neural injury, are toxic to neurons.
So something tells me Leo seems to be on to something. But I wouldn't really know.
Glutamate is an amino acid. There is no "gene" for it. I wonder if he meant they found a gene that resembled the Glu receptor??
@Tasha G. Systems that function, even poorly, tend to remain in place as long as they keep the animal alive above the terminal gene pool number. EX - the human eye. It works, but just OK. The retina is upside down, signalling is slow, they are in a very vulnerable position, the lenses and focusing system ages prematurely. I believe there were a few other defects. EX2 - human heart. Right ventricle typically blows out unannounced, earning it the nickname 'widowmaker'. This is another defect we live with because it doesn't kill us before we reproduce. Having an alien nervous system should not come as a surprise, since DNA theoretically *could* be composed of other combinations of about 23 amino acids instead of GATC. DNA could also be triangular instead of ladder shaped, or it could be circular (see 'plasmid'). Etc. I have not studied archons, but these little buggers are ancient bacteria that are still around today, living separate lives.
What actually does surprise me is the planet supports animal life at all. Our (reading all species) digestive systems are set up, and internal chemistry based around L- enantiomers. Plants can and do produce both L- and R- polysaccharides, but L- is favored. If you were to eat nothing but R- based food, you would starve to death.
You had a good idea though. Losing a gene expression is not without precedent. After a certain number of generations, cave fish become blind. The eye is still there, but non-functional. And in the case of this jellyfish, it is quite possible that it already had a normal neural system but lost it in favor of the present one due to evolutionary pressure. Well - not lost, but changed.
@Tasha G. Loss is certainly more likely than gain; however, that would still leave all of the receptors and neurotransmitters they do have -- these are apparently distinct from those in other living things and require a separate ancestry.
@Heather Noe Do you mean, maybe they evolved from space bacteria? Because these are relatively complex, multi-celled eukaryotes, not at all like bacteria.
@Heather Noe It seems there's good enough room in this article to confuse readers towards thinking ctenophores are somehow strangely distant from the rest of metazoans. Ctenophores had already been considered to be at the base of diverging metazoa.
And, even though I'm ignorant of molecular biology, I'd say this conundrum is more about understanding functionality than it is about origin.
@Bonni B. What about the reverse? That particular lifeforms influenced the creation of the creatures in the film?
@Dean Bruckner Do you have a better theory to explain the observations and data we do have?
@Sarah Longo I can only assume that the molecules that the article labels "neurotransmitters" serve a different function once. It is not at all strange for the same molecule to be involved in separate processes (neurotransmitter, hormone, intracellular signal.. all a matter of context). Possibly these molecules served a purpose in the common ancestors of comb jellies + everyone else, and then were adapted to be neurotransmitters in our evolutionary line, but not in the comb jellies'.
All speculation though.
Or perhaps he meant a gene for one of the proteins involved in synthesizing glutamate.
(Most, if not all, the neurotransmitters are not proteins or RNA or peptides (think acetylcholine, dopamine, noradrenaline, etc), and so would not have a gene for them, but rather genes for the proteins that synthesize them, and genes for their receptors.)
Irrelevant. If a hypothesis is proven to be false to a specified level of significance, it must be rejected whether or not there is another theory in view. Anything else is unscientific.
That attitude also reminds me of my time as a Coast Guard officer. When a ship is sinking, some people will stay on it until it goes under, even if they know it may pull them down with it, because jumping into the water is just too terrifying. If a Coast Guard helicopter pilot can't perform a hoist, for example, due to interferences in the rigging, the Coast Guard crew may direct the crew of the stricken vessel to don survival suits or life jackets (personal flotation devices) and jump overboard where they can be rescued. Some people freeze and just won't. The implementation of the Coast Guard rescue swimmer capability went a long way to prevent this.
No scientist wants to be without a theory, but holding onto one that can't work is unscientific.
@Eric Allen @Dean Bruckner The current scientific paradigm doesn't allow any plausible theory except some sort of evolution. Thus, the best we can do is posit that some processes or conditions not amenable to scientific investigation were involved, not unlike the finding the the universe had a beginning beyond which it not scientific to inquire, likewise there is not much we can do to investigate the realm beyond the event horizon of a black hole.
@Bonni B. Eh... I can see how I might have "sounded" both presumptuous and picky by presuming the intent of the comment. On the other hand though, I was just commenting as well, no particular point either. Without "negative intent".
Minor correction: There are 64 permutations of 4 things taken 3 at a time, with repeats. Not combinations. This web site only allows 5 minutes to edit your post, and I wasn't quick enough. Guess I don't feel too bad mixing up combinations and permutations, since we use these things called "combination" locks, which are actually permutation locks!
What part of the new comb jellies data strikes you as inconsistent with Darwinian evolution? All we are doing is using the latest techniques to fill in formerly unknown detail in the evolutionary tree. You seem to be confused about modern biology and the evidence for evolution, so here's a quick tutorial.
Every animal cell on Earth, including those of comb jellies and humans, is a copy of a parent cell, and has a near exact copy of the parent's DNA. Cells in a growing embryo become specialized, depending on their location in the embryo, but all (except red blood cells) have a near exact copy of the parent cell's DNA, which holds the recipes for the cell's proteins. Two animals of the same species share much the same DNA recipes (genes), but slight spelling differences (mutations) are often used to track a person's heredity, or migratory the patterns of populations.
What works for members of a species works just as well for different species in the same genus, or different genera in the same order or family. In fact, DNA is the perfect clock! By comparing differences among all animals, biologists have constructed a tree of heredity that tells how long ago any two animals shared a common ancestor.
This all works because of the UNIVERSALITY of the genetic code. Darwin of course never heard of DNA, but his theory essentially predicted its existence. In the genetic code, 3 DNA letters specify 1 amino acid in a protein. For example, both GAA and GAG code for glutamic acid. There are 64 combinations of 4 things (DNA bases) taken 3 at a time, yet only 20 amino acids in animal proteins. That's because multiple 3-letter codons encode the same amino acid, like glutamic acid. What is truly remarkable is that the EXACT same code is used throughout the animal kingdom! That is strong evidence for evolution and common ancestry.
Even stronger evidence is in the details of how animal genomes differ. Some mutations, like GAA -> GAG have no affect on a protein's recipe, yet these "silent" mutations accumulate with time. Combined with "expressed" mutations like GAA -> GAC, DNA provides truly massive evidence for Darwinian evolution in general, and the specific paths that evolution took.
Not only that, but the tree of life as determined by comparative genomics is the same tree that we see looking at the fossil record, and radioactive decay, and geological history, and geomagnetic reversal, and ice cores, and comparative embryology, anatomy and physiology. The shear volume of evidence FOR Darwinian evolution dwarfs that for any other scientific theory. Each time some new mystery appears in biology (such as 23 pairs of chromosomes in humans, vs 24 in other primates) what seemed like a failure of evolution turns out to be new evidence supporting it!
To see the perfect example that "Forbidding the falsification of a favored theory is more a sign of weakness than of strength." look no further than Creationism, or "Intelligent Design". Now there's a web of falsehoods that survives countless refutations, solely on the back of wishful thinking!
@Dean Bruckner @David Bump @Eric Allen
Evolution is a widely accepted Scientific theory because it is consistent with the current information and evidence we have gathered thus far.
The genetic evidence alone is irrefutable, unless you can provide evidence that contradicts this statement, I can't take your suggestion that evolution is not true seriously.
Thanks for your reply. However, just because a paradigm is current doesn't mean it's right or that it is even-handedly taking into account all the available evidence.
As to whether some sort of evolution is mandatory, I politely say that is nonsense in the truest definition of the word. That is a metaphysical assumption, not a physical or scientific one. There are accepted methods of inference to the best historical explanation based on uniformitarian assumptions, and evolutionists seem to be uniformly but selectively ignoring them to uphold their own metaphysical assumptions.
To borrow a well-known popular quip,"Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth." Evolution not only is not true, the odds say it cannot be true. The fact that evolutionists see nothing else beyond evolution is not an indicator that truly nothing else is there; it is, I assert, instead an indication that their own a priori metaphysical commitments have ruled out any other possibilities, and that not in accordance with reality.
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