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."