For 20 years Leonid Moroz tried to deal with one unimaginable idea: scientists are busy searching for life on other planets, but “strangers” can live here on Earth, having a paradoxical difference from our biology and the brain. For thousands of years these aliens were all in sight. They can tell a lot about the nature of evolution and what to expect when we finally discover life on other planets.
Frost, a neuroscientist, discovered the first hint of his future discovery in the summer of 1995, shortly after moving to the United States from his native Russia. That summer he spent at the Marine Laboratory of Freidia Harbor in Washington. It was located in the middle of a forest-covered archipelago in the Puget Sound Bay system – at the crossroads of opposing streams and currents carrying hundreds of species of marine animals passing along the rocky shores: flocks of jellyfish, amphipods, wavy sea lilies, nudibranchs, flatworms and tadpoles, sea stars and etc. These organisms embody not only the marine nature of the remote areas of Puget Sound, but also the farthest branch of the evolutionary tree. Frost spent hours on the pier in the laboratory, collecting animals to study their nervous system. He spent years studying the nervous systems of the animal kingdom in the hope of understanding the evolution of the intellect and the brain. However, he came to Freetown Harbor for one particular animal.
He learned to distinguish his bulbous, transparent body in the sun-drenched water: iridescent shine and subtle shades of rainbow light scattered by the rhythmic flapping of the “crests” of the cilia holding the body afloat. This species of organisms, called the ctenophore, was long considered another species of jellyfish. But that summer in Freetown Harbor, Moroz made a terrific discovery: the unremarkable appearance of this animal hid an unprecedented case of erroneous identification. Already after the first experiments, the scientist was able to understand that the ctenophores did not belong to the jellyfish. In fact, they are radically different from any other animals on Earth.
Frost came to this conclusion by conducting experiments on neural cells of ctenophores in an attempt to detect neurotransmitters – serotonin, dopamine and nitric oxide – chemical conductors, considered the universal language of the nervous systems of all animals. No matter how he looked for them, he could not find these molecules. The conclusions were overwhelming.
Already before that, it was known that the ctenophores had a relatively advanced nervous system; However, the first experiments of Moroz showed that their nerves were built from a special set of molecular building blocks – different from the set of any other animal organism – using, according to Moroz, “another chemical language”: these animals are “marine aliens”.
If Frost is right, the comb jellies are an evolutionary experiment of incredible proportions that has been going on for more than a billion years. This separate evolutionary path – Evolution 2.0 – has formed neurons, muscles and other specialized tissues regardless of the rest of the animal world, using other building materials.
The characteristics of ctenophores are the answer to the question: what could evolution have come to, if not to the appearance of vertebrates, mammals and people dominating the Earth’s ecosystem? The structure of these organisms also sheds light on the great dispute that raged for decades: if we talk about the present state of life on Earth, how much has happened by chance and how much has been predetermined from the very beginning?
If evolution were restarted, would intelligence come again? And if so, could another large branch of the animal kingdom be formed? Ctenophores offer a promising response, being a living example of a carrier of the brain, different from the brain of other animals. The brain became the crown of the creation of convergent evolution – a process in which unrelated species develop similar features to adapt to one of the habitat conditions. People have an unprecedentedly developed intellect, but the physiology of the ctenophores shows that we can not be alone. The tendency of complex nervous systems to evolve, perhaps, is universal – not only on Earth, but also on other planets.
Compared with the main groups of organisms, the ctenophores have been poorly studied. Their bodies at first glance resemble bodies of jellyfish – gelatinous, oblong or spherical, with a round mouth opening at the other end. Ctenophore abundantly live in the oceans, but the scientists did not pay attention to them for a long time. At the beginning of the 20th century, schematic sketches most often depicted an animal with its head down, with a mouth opening facing the seabed, like a jellyfish, while in reality during movement their mouths point upwards.
While jellyfish move in the water thanks to muscle contractions, the comb combs use thousands of cilia for swimming. Ctenophora is a voracious predator known for its ambush tactics. Unlike the stinging tentacles of jellyfish, the comb jelly hunts two sticky, glue-releasing tentacles – instruments that have no analogues in the animal kingdom. Hunting, he spreads them like a web and methodically catches one victim after another.
When scientists in the second half of the 19th century began to investigate the nervous system of the ctenophores, they did not find anything unusual in what they saw in the microscope. A dense tangle of neurons was located near the lower part of the animal’s body, a diffuse network of nerves spread throughout the body, and a handful of thick nerve bundles retreated to each of the tentacles and to each of the eight ligament cilia. During an electron microscope study conducted in the 1960s, one discovered what served as the synapses of these neurons – vesicle-like branches that released neurotransmitters that stimulated the neighboring cell.
Scientists have introduced into the neurons of a living ctenophora calcium, provoking the appearance of electrical impulses, similar to those that occur in the nerves of rats, worms, flies, snails and other animals. Stimulating certain nerves, the researchers were able to make the cilia move in different directions – forward or backward.
In general, it seemed that the nerves of ctenophores behave like the nerves of any other animal. Therefore, biologists assumed that there was nothing special about them. This point of view contributed to the formation of a general picture of the animal kingdom – and it turned out to be wrong.
By the early 90’s, scientists placed the comb jelly below the evolutionary tree on a branch near the creeping coelenterates, into a group including jellyfish, sea anemones and corals. Jellyfish and ctenophore have muscles, in both species there is a diffuse nervous system that is not completely condensed in the brain. And, of course, both are well known for their soft, swaying and often transparent bodies.
Over the comb jelly and jellyfish on the evolutionary tree are two other branches of animals, which, apparently, were more primitive: plate and sea sponges, in which the nervous system was absent in principle. It seemed that the sponges were beyond the animal kingdom: until in 1866 the English biologist James Clarke proved that the sponges are really animals.
This helped to fix the sponge as our closest living connection with the ancient, pre-animal world of unicellular protists, related to modern amoeba and infusoria. Researchers argued this by the fact that the sponges evolved when the ancient protists gathered in fast-growing colonies, where each cell used its flagellum – a filamentary structure that is cognate to cilia – for sustenance instead of movement.
Such rhetoric supported a convenient point of view on the gradual process of evolution of the nervous system, which became more complex along with each branch of the evolutionary tree. All animals were sons and daughters of one second of evolutionary creation: the birth of the nerve cell. And only once, at a subsequent stage of evolution, these neurons crossed the second most important threshold – their unification into a centralized system. This view is supported by another evidence: striking similarities in the organization of nerve cells in insects and in humans, where neural circuits underlie episodic memory, spatial navigation and general behavior. In fact, scientists were of the opinion that the first brain appeared early enough before the ancestors of insects and vertebrates dispersed on the evolutionary path. If this were true, then the elapsed 550 or 650 million years from now would be a single line where a variety of animal genera develop along the same pattern.
This picture of the evolution of the brain looked plausible, but when observing the ctenophora in Freidia Harbor in 1995, Moroz began to suspect that she was fundamentally untrue. To demonstrate his guesses, he collected several species of ctenophores, cut their nerve tissues into thin sections and treated them with special chemical compounds, trying to identify the presence of dopamine, serotonin, or nitric oxide, the three neurotransmitters that were widely distributed within the animal kingdom. Again and again he looked into the microscope and did not see any traces of yellow, red or green spots on the preparation.
As soon as you repeat the experiment, says Frost, you begin to understand that these are really completely different animals. He suggested that the nervous system of the ctenophora not only differed from the system of the supposed related group, the jellyfish – it was also extremely different from any other nervous system on Earth.
It seemed that the comb combs followed a completely different evolutionary path, but Moroz was not yet sure. If he had published the results then, after researching only a couple of important molecules, no one would have paid attention to him. “Outstanding statements require outstanding evidence,” Moroz says. And so he entered on a long and difficult path, which took much longer than he originally intended.
He applied for funding to study comb jelly using other techniques – for example, by examining their genes – but gave up after several vain attempts. At that time he was still young, a few years before he left the Soviet Union, and only began to publish his work in English-language magazines, where they were to arouse wider interest. Because Moroz postponed the study of ctenophores in a long box and returned to his main work – the study of neural signals in snails, octopuses and other mollusks. Only after 12 years, by chance, the scientist returned to the project of interest to him.
In 2007, Moroz attended a scientific conference in Freetown Harbor. One evening he came across the same pier, where he spent so much time in 1995. There, the scientist accidentally noticed the iridescent sparks of the ctenophores floating downstream in the light of the lantern. By that time, science had made a big step forward and allowed the whole genome to be deciphered in a few days, not years, as before. And Moroz himself by that time had become a recognized scientist, he had his own laboratory at the University of Florida. At last he could afford to amuse his curiosity.
With the help of a network scientist caught with a dozen ctenophores of the species Pleurobrachia bachei, or sea gooseberry. He froze them and sent them to his laboratory in Florida. Three weeks later he had a partial ctenophore transcript – about 5-6 thousand gene sequences, which were directly related to the nervous activity of the animal. The results were amazing.
First, it was found that the ctenophores of the Pleurobrachia species lacked the genes and enzymes necessary to create a large number of neurotransmitters, widely distributed among other animals. Neurotransmitters included not only those that Moroz noticed in 1995 – serotonin, dopamine and nitric oxide – but also acetylcholine, octopamine, noradrenaline, etc. In addition, the ctenophores had no genes for receptors that allow neurons to perceive these neurotransmitters .
This confirmed what Moroz had expected to find over the years: when in 1995 he could not find the widely distributed neurotransmitters in the world of ctenophores, it was not the errors in methodolgy, but the fact that the animal did not use them in any way. According to the scientist, this became a “grandiose shock”.
“We all use neurotransmitters. And the jellyfish, worms, mollusks, humans, and sea urchins have a very strong set of signaling molecules, “the scientist says. But somehow the nervous system of the ctenophores evolved so that the functions of the neurotransmitters took upon themselves other sets of molecules that had not yet been studied.
The transcript and DNA sequencing showed that the ctenophores also lacked many other genes inherent in the animal kingdom necessary for the creation and functioning of the nervous system. Pleurobrachia did not have many animal-specific proteins, which are known as ion channels and serve to pass along the nerves of electrical signals. The ctenophores lacked genes that are responsible for the process of transformation of germ cells into mature nerve cells, as well as genes that are responsible for the phased organization of these neurons into mature functioning chains. “It was not just about the presence or absence of several genes. There was a truly grandiose design here, “the scientist asserts.
This meant that the nervous system of the ctenophores evolved from scratch, using a combination of molecules and genes different from any animal known on Earth. It was a classic example of convergence: the nervous system in the genus of ctenophores formed due to the available raw materials. In a sense, it was an alien nervous system – it evolved separately from the rest of the animal world.
But the surprises did not end there. It turned out that the ctenophores are unique animals not only from the point of view of the nervous system. The genes involved in the development and functioning of the muscles were also completely different. In addition, the ctenophores did not observe several varieties of common genes responsible for the formation of the body, which had previously been considered universal for all animals. These include the so-called microRNAs that help in creating special types of cells in the organs, and Hox genes that divide the body into separate parts, whether it’s segmentation of the body of a worm or a lobster, or differentiation of the vertebrae and the bones of a human finger. These types of genes are even in the simplest sponges and lamellar, but they are not found in ctenophores.
All of the above led to an incredible conclusion: despite the more complex structure than the sponges and lamellar ones – which are deprived of nerve cells and muscles, as well as practically any other specialized cells – the ctenophores are in fact the oldest branch of the evolutionary tree. Somehow during the period from 550 to 750 million years the nervous system and muscles, similar in complexity to those possessed by jellyfish, anemones, starfish and various types of worms and mollusks, but formed on the basis of a different set of genes, developed in the ctenophores.
Frost tried to publish the results of his research in 2009, but his article was not accepted. Then he continued his experiments.
Even when Moroz confirmed the findings at the end of the 2000s, other research groups were just beginning to collect the pieces of data already known to him: this suggested the worrying thought that after so many years someone else could come to a similar conclusion before Moroz himself will be able to publish his research.
Firstly, a study published in 2008 in Nature called into question the basic structure of the evolutionary tree, undermining the long-held assumption that sponges were his first, most primitive branch. Scientists compared DNA sequences from 150 genes to restore the evolutionary relationships of 77 different species of animals, including two species of ctenophores. In the article, it was publicized for the first time that intricate ctenophores – but not simple sponges – could actually be the earliest branch. According to the biologist from the Institute of Marine Research at the Monterey Bay Oceanarium of Stephen Haddock, one of the authors of the work, the mere proposal for this created a hurricane in the scientific community.
In December 2013 another team of researchers published the world’s first ctenophore genome – the species Mnemiopsis leidyi, which is different from the one studied in detail by Moroz. An article published in Science also concluded that it was the ctenophores, and not the sponges, that were the evolutionary branch closest to the source of the origin of all animals.
Over the next few months, the entrenched belief that the sponges were the earliest animals continued to crack at the seams. In January 2014, Sally Leis from the University of Alberta in Edmonton, one of the world’s leading biologists investigating sponges, questioned the 150-year-old statement that sponges were only a colonial version of unicellular organisms that are considered the ancestors of all animals. Detailed studies have shown that the sponge and protista, called hoanoflagellate, used a different set of genes and proteins to create similar structures. Therefore, the sponges could not develop from anything resembling hoanoflagellates. Their similarity under the microscope was yet another deceptive example of convergent evolution: two unrelated organisms developing in the course of evolution similar structures for performing similar functions, but using different genes as a basis.
These studies dispelled indirect evidence that the sponges were the earliest branch of the animal tree of life. What seemed a strong argument was simply a case of erroneous identity. Despite the fact that the ctenophores were much more complicated than sponges, with nervous systems, muscles and other organs, they now seemed the earliest branch.
But none of these studies have studied nerve cells. Thus, the rest of the world still did not know the essence of the discovery of Frost – an independently developed nervous system.
In subsequent years, Moroz filled the gaps in the evidence base. His team sequenced the last few percent of the Pleurobrachia ctenophore genome, making its way through complex DNA sections that were difficult to cope with even with modern technology. Frost hired three dozen students to conduct in-depth studies of which genes were expressed in individual neuromuscular jaundices, how these cells were connected in chains, and how the animal developed, starting from the germinal cell itself.
In June 2014, the scientist finally published the results of decoding the genome Pleurobrachia in the journal Nature. In his work, which took seven years, it was firmly established: the nerve cells and nervous system of the ctenophores evolved separately from other animals. In his vision, the comb jaws represented the closest structure to the extraterrestrial consciousness on Earth.
Ctenophores are a striking example of what is probably a common model: eyes, wings and fins have repeatedly appeared in the evolution of animals – the same has happened to nerve cells. Currently, Moroz has from 9 to 12 independent evolutionary sources of the nervous system: at least one – among the creeping coelenterates (this group includes jellyfish and anemones), three – among the echinoderms (these include sea stars, sea lilies and sea urchins) , one among arthropods (insects, spiders and crustaceans), one – in mollusks (snails, squid and octopuses are also included), one in vertebrates – and now at least one source is found among the ctenophores.
“There are several ways of originating neurons and at least two ways of originating the brain,” says Moroz. In each of these evolutionary branches, a different set of genes, proteins and molecules emerged by accident, through duplication and mutations, and then participated in the construction of the nervous system.
The most interesting thing is that different ways of evolution led to the emergence of nervous systems, which are very similar among all representatives of the evolutionary tree. Take, for example, the work of Nicholas Strausfeld, a neuroscientist from the University of Arizona in Tucson. Together with a group of colleagues, he discovered that in insects the neural chains responsible for perception of odors, episodic memory, spatial navigation, behavior and vision are almost identical to those that perform the same functions in mammals – despite the fact that for the emergence of each of them different sets of genes are used.
These similarities reflect the two key principles of evolution, which are probably important for any planet where life has arisen. The first is convergence, convergence in one point: the distant branches of the evolutionary tree came to the general construction of the nervous system, because before each of them there are the same basic tasks. The second is a common history: the idea that all these differently constructed nervous systems share at least some element of common origin. On our planet, all living beings are made up of molecular building blocks that have arisen in the physico-chemical environment of the early Earth.
In fact, much of the basic mechanism of all nervous systems has most likely evolved from the life-or-death adaptation that originated in the first cells on Earth four billion years ago. These cells probably lived in aquatic environments such as hot springs or salt pools that contained a mixture of dissolved minerals that threatened life, such as calcium. (It is known that important biological molecules, DNA, RNA and ATP, under the influence of calcium, merge into an unreceptive broth – like foam in the bathroom.) Therefore, biologists believe that in the early stages of life, organisms had to develop ways to prevent the increase in the level of calcium inside cells . Such a defense system can include proteins that pump calcium atoms out of the cell, and a “signaling system” that is activated when the level of calcium is raised. Evolution later used this exceptional susceptibility to calcium in order to conduct signals inside and between cells, to control the beating of cilia and flagella that use microbes to move, control muscle contraction, or to conduct signals on nerve cells in organisms such as ours. By the time the nervous systems began to appear, about half a billion years ago, the foundation of many building blocks needed for this has already been laid.
These principles are of great importance for understanding the evolution and life forms that may appear on Earth or other planets. They shed light on the importance of chance and inevitability in the formation of the evolution vector for billions of years ahead.
The late paleontologist Harvard Stephen Jay Gould noted in his book “The Amazing Life” (1989) the high importance of randomness: the evolutionary history of animals was formed through devastation to the same extent as through innovations. He noted that during the Cambrian period, 570 million years ago, there were significantly more types of living beings than there are today. These diverse branches on the early evolutionary tree disappeared due to mass extinctions. They, in turn, spurred evolution, opening ecological niches that surviving groups of animals could master, opening up opportunities for new evolutionary solutions.
At the same time, Simon Conway Morris, a paleontologist at the University of Cambridge, emphasized the importance of evolutionary convergence: evolution tends to come back to the same decisions over and over again, even in remote branches of the evolutionary tree, even when proteins or genes used to create a similar structure themselves are not connected to themselves.
Bring these two thoughts to a logical conclusion – and come to a startling conclusion. If the Earth’s history is rewound to the very beginning and reproduced anew, evolution could have gone differently, and by this point in time could have come to completely different groups of animals. Mammals or birds, perhaps even all vertebrates, might be absent. But evolution could still come to the majority, if not all of the innovations that allowed the complex brain to appear, but on other branches of the evolutionary tree.
Since scientists are speculating about the form in which life can exist on other planets, a provocative idea arises: alien organisms that are little like those we are familiar with can already exist here on Earth. The idea is that life could arise two or even more times on our planet, rather than one, as previously thought. Our life form began to dominate, and other forms retreated to the side. This “shadow biosphere” will be difficult to detect, as it may not contain DNA, proteins or other molecules, by which we usually define it.
Ctenophores are not so unusual. They are based on the same chemical base as we do, but nevertheless represent a shadow biological form. Ctenophores are our long-lost ancestors, whose existence we did not even know.
Since the ctenophores reinvented the brain and muscles, using a different set of proteins and genes than any previously studied, they give a unique opportunity to study some global questions: how different can the structure of nervous systems be? Do we really understand how a living organism feels its surroundings and how does it behave?
Ctenophore also could provide useful information for predicting the development of nervous systems on other planets, in more exotic life forms, not based on DNA or proteins. Evolutionary biologists believe that even a life that is based on an unusual biochemical base will continue to be built on the same principles of organization. Nick Lane, a biochemist at University College London, wrote that extraterrestrial life probably separates himself from the outside world with a kind of cell membrane, and uses energy to use electrochemical differences in pH or ion concentrations from different sides of this membrane, like cells on Earth. Chemicals extracted from ancient meteorites can easily form membranes – even if these membranes do not consist of the same molecules. And as soon as the structure of the cell membrane of living creatures from another planet will settle, the process of development of the nervous system will probably pass the same way as on Earth.
The frost is still trying to find out about the ctenophores all that is possible. These animals were forgotten by scientists, because they were too fragile and difficult to sustain life in the laboratory. Frost solved this problem by equipping the research vessel with modern equipment for sequencing the genome, growing embryos and stimulating neurons in living animals during field work. He hopes that studying the nervous system of the ctenophores will help to learn more about the principles of the structure of the brain as a whole and to check whether these principles are truly universal.
The researcher took a very long time to come to this point. To understand that the comb jellies were really so alien, Moroz had to renounce what he had learned from earlier studies. Since his original hypothesis did not differ from that written in textbooks, the transition to a new way of thinking took 20 years.