Multicellular organism. Subkingdom multicellular structure What are the first multicellular organisms called?
Most ideas about the origin of multicellular organisms are based on the long-standing belief that they originated from colonies of Protozoa and that, therefore, the body of a mononuclear protozoa corresponds morphologically to an individual cell of a multicellular animal. At the same time, it is believed that in the process of evolution a new individuality of a multicellular organism gradually developed, sharply subordinating it and suppressing the individuality of individual cells. In other words, a metazoon, in comparison with a protozoon, is recognized as an individual of a higher order. Colonial hypotheses, therefore, in full accordance with cell theory, consider the cell as an elementary structural unit that allows us to compare and analyze the organization of all Protozoa, Metazoa and Metaphyta (multicellular plants).
E. Haeckel's gastrea hypothesis. The first hypothesis about the colonial origin of multicellular organisms - the “gastrea” hypothesis - was proposed by E. Haeckel. The basis of this hypothesis, which he developed since the early 70s. XIX century, the idea of the homology of germ layers in all multicellular animals, first expressed by T. Huxley, was born. By the time the gastrea hypothesis appeared, the study of germ layers had made great strides thanks to the work of T. Huxley, K. F. Wolf, K. Baer and others. Haeckel relied on the achievements of embryology of his time and, in particular, on the research of A. O. Kovalevsky.
The most important “tool” in creating the theory of gastrea was the biogenetic law, substantiated almost simultaneously by F. Muller and Haeckel in the 60s. XIX century According to Haeckel, “ontogeny is a brief repetition of phylogeny, mechanically determined by the functions of heredity and adaptability” (Haeckel, 1874). He considered the so-called primary germ layers - ectoderm and gastrula endoderm as a manifestation in ontogenesis of the corresponding primitive organs of primitive ancestors. Haeckel also attributed absolute recapitulation significance to all other initial stages of ontogenesis. All characteristic stages of fragmentation "(Fig. 12) correspond, according to Haeckel, to similar stages of phylogenesis. Thus, the egg, or cistula, corresponds to the unicellular ancestor of Cytaea, the morula stage corresponds to the ancestral form of the "morea", etc. Particularly important and widespread In the animal world, Haeckel considered recapitulation (i.e., repetition of phylogeny in ontogenesis) to be a two-layer embryonic stage - the gastrula.He created the common hypothetical ancestor of all Metazoa in its image and likeness.
Fig. 12. Stages of embryonic development of coral
polyp (according to Haeckel)
The first phylogenetic stage, according to Haeckel, was a unicellular amoeba-like organism. From him came all animal-feeding organisms. A colony of identical amoeboid cells then gave rise to a “morea” - a dense spherical organism, the recapitulation of which in ontogenesis is represented by the morula. By accumulating liquid or gelatinous substance in the center of the seas, displacing the cells to the periphery, a free-floating “blastea” was gradually formed (in ontogenesis it corresponds to the blastula). The blastea cells were first covered with pseudopodia, which later acquired the ability to quickly move and bend and turned into ropes used for swimming.
The next very important stage was the gastraea, formed from the blastea by protrusion (invagination) of the body wall at the anterior pole. The outer cellular layer of the gastrea was equipped with flagella and retained the function of movement, the inner layer became intestinal. In the central, intestinal cavity, which communicated through the mouth with the external environment, digestion of swallowed prey took place. The two epithelial layers of gastrea - ectoderm and endoderm - represented the primary organs from which all of their organs and tissues arose in the descendants of gastrea.
Haeckel considered modern coelenterates and sponges as little changed descendants of gastrea, and the gastrula stage as a recapitulation of gastrea.
All multicellular organisms, according to Haeckel, unlike protozoa, have a monophyletic origin and developed from one ancestral form - gastrea, which, in turn, descended from unicellular ancestors. From Gastrea, all Metazoa inherited the primary intestine and two primary germ layers; their tissues are derivatives of these two leaves.
Haeckel characterized his hypothesis as an attempt to improve the phylogenetic foundations of natural classification and to clarify the paths of development of the main systematic groups of the animal world. Indeed, a number of significant conclusions for systematics, comparative anatomy, embryology and histology followed from the gastrea hypothesis. However, the most important consequence of the appearance of the gastrea hypothesis was the destruction of the doctrine of Cuvier's types, which still dominated zoology at that time.
From Haeckel's hypothesis it follows that true homologies of organs and their systems are possible in all descendants of Gastrea, i.e., in representatives of different types, whereas the theory of types denied this possibility. Since the gastrula is homologous in all Metazoa, the intestine is always homologous. Further, the skin is homologous in all Metazoa, since there is always a layer of epidermis that serves as a source for the development of other skin layers - cuticle, glandular formations, etc. - and corresponds to the gastrea ectoderm. The nervous system always develops from the ectoderm and is homologous in all groups of animals. Haeckel also saw grounds for homologizing the excretory organs, coelom and circulatory system in those animals that have them. For the sensory organs, skeleton and heart, he considered general homology unacceptable and believed that all these organs developed independently in different groups. He recognized as reliable the different origins of the oral opening in different groups of multicellular organisms. The blastopore of the gastrula, homologous to the mouth of the gastraea, is preserved in the adult state in coelenterates, in sponges (in the form of an aperture) and in lower worms. The mouth of echinoderms, arthropods and vertebrates, according to Haeckel, is a new formation.
Thus, Haeckel recognized the wide possibilities for the convergent development of various important features in the structure of animals.
He considered the single-layer flagellated epithelium to be the primary tissue, and all other tissues to be secondary derivatives of the epithelium. Haeckel considered the ectoderm and endoderm to be the primary germ layers. Mesoderm, in his opinion, arose much later in the process of evolution, since in ontogenesis it is always formed from ectoderm and endoderm and, in essence, does not even represent a single leaf, but has a dual nature, consisting of plates that developed independently from the skin muscular and intestinal-muscular plates.
Since the mesoderm always develops from paired primordia, then, according to Haeckel, in different groups of animals it has a common origin and can be considered homologous. The primary germ layers in the lower ones - sponges and coelenterates - in contrast to those in the higher types, are preserved as primary organs, just as was the case in the hypothetical gastrea.
Haeckel's hypothesis was dominant for a long time, and some prominent zoologists still adhere to it. Its positive role in zoology was extremely great, since it showed the unity and common origin of all multicellular organisms and thus contributed to the progress of Darwinism.
However, the gastrea hypothesis suffers from significant shortcomings, which were not hidden from some of Haeckel’s contemporaries and gave rise to sharp criticism.
Indeed, the gastrea hypothesis does not agree with many zoological data and must give way to a more advanced concept. However, the doctrine of the protozoan colonial ancestors of Metazoa, which lies at the basis of Haeckel’s generalizations, retains its entire significance to this day. The second “rational” grain of the gastrea hypothesis should be considered the doctrine of blastea, which was accepted by the authors of some other colonial hypotheses without any special changes.
The famous Russian embryologist V.V. Zalensky (1874) examined in detail the first stages of embryonic development of various animals from the point of view of compliance with their gastrea theory. He considered the first differentiation of the germ layers to be the most important moment in the ontogenesis of animals. The general course of V.V. Zalensky’s reasoning was as follows. In typical cases, in lower multicellular organisms, after cleavage and the morula stage, a two-layer intestinal form, the planula, is formed. If a hollow spherical blastula is formed, then endodermal cells appear in its cavity and a stage (diblastula) appears, quite comparable to the planula, since it, in essence, also has two germ layers and is devoid of epithelial intestine. In cases where a gastrula with a sac-like intestine and mouth is formed by invagination, we, in Zalensky’s opinion, have a secondary altered development, ensuring a very early appearance of the intestine and characterized by loss of the planula stage. Therefore, Zalensky thought that the common ancestor of Metazoa must have possessed the characteristics of a planula rather than a gastrea. Zalensky, in fact, was the predecessor of I.I. Mechnikov, who put forward the well-known phagocytella hypothesis.
Phagocytella hypothesis by I.I. Mechnikov. Like Zalensky, I.I. Mechnikov subjected the gastrea hypothesis to sharp criticism. In particular, he noticed that the identity of the primary gastrula in all Metazoa, assumed by Haeckel, does not actually exist. In different animals, this stage has different characteristics and develops differently, which cannot always be explained by secondary reasons. True bilayered, invaginating gastrula, as required by the gastrea theory, are in reality extremely rare. In its completed form, I. I. Mechnikov’s phagocytella hypothesis is presented in the final chapter of his monograph “Embryological Research on Jellyfish” (1886).
Being a supporter of colonial origins, I. I. Mechnikov, like Haeckel, saw the distant ancestors of multicellular organisms in flagellates with animal nutrition.
Mechnikov considered invagination, by which the gastrula is formed, to be a secondary method of formation of endoderm, which arose as a result of a long and complex evolution.
I. I. Mechnikov’s hypothesis is as follows. The primary metazoon was spherical and initially had a single-layer structure. In other words, blastea is recognized, and this coincides with Haeckel’s hypothesis.
Since in Metazoa the cleavage cavity usually appears very early and the embryo quickly turns into a blastula, the ancestor of multicellular Swordsmen was considered to be a blastula-shaped colony of flagellates. He believed that the total fragmentation of multicellular organisms should be inferred from the division of flagellates: the first meridional divisions of a fragmenting egg represent a legacy from flagellate ancestors, since longitudinal division is characteristic of flagellates. Mechnikov also tries to explain the initial spherical shape of the colony based on the longitudinal division of flagellates. If the cell division always occurs longitudinally, then a plate is obtained, but if the third division changes and becomes transverse, then the result is a spherical colony of cells. It is precisely this change in the direction of division that occurred in phylogeny. Thus, the ancestor of Metazoa was a colony in which the division directions alternated in three coordinate planes. Mechnikov thought that the formation of the two-layer stage occurred not by invagination, but by immigration - the introduction of individual cells into the cavity of the blastula, as a result of which the rudiment of the endoderm was formed. He saw the evolutionary origins of such immigration in the phenomenon of phagocytosis.
The nutrition of the primary metazoon, according to Mechnikov, was carried out by the same cells that served for movement, i.e. flagellar cells through intracellular digestion (phagocytosis). I. I. Mechnikov suggested that cells overloaded with food easily lost their flagellum and went into the body cavity, then they could again come to the surface and form a flagellum. This is how the first facultative differentiation into the outer layer of cells - “kinoblast” - and into the inner cell mass - “phagocytoblast” happened. This differentiation was then consolidated in evolution, and a compact organism was formed - parenchymella, the model of which he considered the sponge larva - parenchymella. Later, Mechnikov called this organism a phagocytelle. This was the common ancestor of multicellular animals.
The further fate of the phagocytella is as follows. Some of its descendants switched to a sedentary lifestyle and gave rise to sponges. Others began to crawl and acquired bilateral symmetry and oral openings. From They gave rise to the intestinal flatworms turbellaria, which do not yet have an intestine and digestion occurs in the lacunae of the parenchyma and in wandering cells - phagocytes. Still others, retaining a swimming lifestyle, acquired a mouth, experienced phagocytoblast epithelization and turned into primary coelenterates - the ancestors of sessile polyps.
Thus, the hypothesis of I.I. Mechnikov explained from an evolutionary point of view all the main stages of the ontogenesis of Metazoa and proposed new phylogenetically based ideas about the primary germ layers and their further evolution. On this basis, Mechnikov drew a completely plausible hypothetical picture of the evolutionary formation of Metazoa and the first stages of their phylogenetic development, a picture that well explains many embryological and comparative anatomical factors that are incomprehensible from the point of view of other hypotheses.
A. A. Zakhvatkin in 1949 put forward a hypothesis about the origin of multicellular organisms from colonial flagellates based on palintomy - a special form of asexual reproduction through successive cell divisions without stages of growth of the resulting daughter cells. This division is, in his opinion, a prototype of egg fragmentation in Metazoa.
Another path for the evolutionary formation of Metazoa was proposed by A.V. Ivanov in the late 60s, who believed that the hypothetical initial colonies of flagellates were not palintomic and generally differed little from the spherical colonies of modern collared flagellates.
Ivanov takes Mechnikov’s phagocytella theory as a basis. However, he considers the prototype of the phagocytella not to be a sponge larva, but a primitive flat multicellular Trichoplax, which is the only representative of the phylum Placozoa. The scheme of the emergence of multicellular organisms, according to Ivanov, is presented in Fig. 13.
Rice. 13. The main proposed stages of Metazoa phylogeny
according to A.V. Ivanov:
1 - colony of flagellates; B - inward migration of flagellate cells; IN - early phagocytella; G - late phagocytella; D - primary turbellaria - the appearance of a mouth and bilateral symmetry; E - primitive intestinal turbellaria - increased cell differentiation, displacement of the mouth to the ventral side; AND - primitive sponge - transition to a sedentary lifestyle, replacement of the locomotor function of kinocytes with hydrokinetic one; 3 - primary coelenterate gastrea type - formation of the mouth, epithelization of the phagocytoblast
Since in the embryogenesis of lower multicellular bilayers the embryo is formed more often by immigration, most zoologists believe that this is precisely the way the transformation of the spherical colony of flagellates into the first multicellular organism took place. Moreover, in the ancestral forms of multicellular organisms, the formation of two cell layers was accompanied by cell specialization and the colony of flagellates turned into a single multicellular organism. The outer layer retained motor and sensory functions, while the inner layer retained digestive and reproductive functions.
That is, they differ in structure and functions.
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Subtitles
Differences from coloniality
It should be distinguished multicellularity And coloniality. Colonial organisms lack true differentiated cells and, consequently, the division of the body into tissues. The boundary between multicellularity and coloniality is unclear. For example, Volvox is often classified as a colonial organism, although in its “colonies” there is a clear division of cells into generative and somatic. A. A. Zakhvatkin considered the secretion of a mortal “soma” to be an important sign of the multicellularity of Volvox. In addition to cell differentiation, multicellular organisms are also characterized by a higher level of integration than colonial forms. However, some scientists consider multicellularity to be a more advanced form of coloniality [ ] .
Origin
The most ancient multicellular organisms currently known are worm-like organisms up to 12 cm long, discovered in 2010 in sediments of the formation Francevillian B in Gabon. Their age is estimated at 2.1 billion years. Grypania spiralis, a suspected eukaryotic algae up to 10 mm long, found in sediments of the Negaunee Ferrous Formation at the Empire Mine is about 1.9 billion years old. (English) Russian near the city of Marquette (English) Russian, Michigan.
In general, multicellularity arose several dozen times in different evolutionary lines of the organic world. For reasons that are not entirely clear, multicellularity is more characteristic of eukaryotes, although the rudiments of multicellularity are also found among prokaryotes. Thus, in some filamentous cyanobacteria, three types of clearly differentiated cells are found in the filaments, and when moving, the filaments demonstrate a high level of integrity. Multicellular fruiting bodies are characteristic of myxobacteria.
According to modern data, the main prerequisites for the emergence of multicellularity are:
- intercellular space filler proteins, types of collagen and proteoglycan;
- “molecular glue” or “molecular rivets” for connecting cells;
- signaling substances to ensure interaction between cells,
arose long before the advent of multicellularity, but performed other functions in unicellular organisms. "Molecular rivets" were supposedly used by single-celled predators to capture and hold prey, and signaling substances were used to attract potential victims and scare away predators.
The reason for the appearance of multicellular organisms is considered to be the evolutionary expediency of enlarging the size of individuals, which allows them to more successfully resist predators, as well as absorb and digest larger prey. However, conditions for the mass emergence of multicellular organisms appeared only in the Ediacaran period, when the level of oxygen in the atmosphere reached a level that made it possible to cover the increasing energy costs of maintaining multicellularity.
Ontogenesis
The development of many multicellular organisms begins with a single cell (for example, zygotes in animals or spores in the case of gametophytes of higher plants). In this case, most cells of a multicellular organism have the same genome. During vegetative propagation, when an organism develops from a multicellular fragment of the maternal organism, as a rule, natural cloning also occurs.
In some primitive multicellular organisms (for example, cellular slime molds and myxobacteria), the emergence of multicellular stages of the life cycle occurs in a fundamentally different way - cells, often having very different genotypes, are combined into a single organism.
Evolution
Six hundred million years ago, in the late Precambrian (Vendian), multicellular organisms began to flourish. The diversity of the Vendian fauna is surprising: different types and classes of animals appear as if suddenly, but the number of genera and species is small. In the Vendian, a biosphere mechanism of interaction between unicellular and multicellular organisms arose - the former became a food product for the latter. Plankton, abundant in cold waters, using light energy, became food for floating and bottom microorganisms, as well as for multicellular animals. Gradual warming and an increase in oxygen content led to the fact that eukaryotes, including multicellular animals, began to populate the carbonate belt of the planet, displacing cyanobacteria. The beginning of the Paleozoic era brought two mysteries: the disappearance of the Vendian fauna and the “Cambrian explosion” - the appearance of skeletal forms.
The evolution of life in the Phanerozoic (the last 545 million years of earth's history) is the process of increasing complexity in the organization of multicellular forms in the plant and animal world.
The line between unicellular and multicellular
There is no clear line between unicellular and multicellular organisms. Many unicellular organisms have the means to create multicellular colonies, while individual cells of some multicellular organisms have the ability to exist independently.
Sponges
Choanoflagellates
A detailed study of choanoflagellates was undertaken by Nicole King from the University of California at Berkeley.
Bacteria
In many bacteria, for example, steptococci, proteins are found that are similar to collagen and proteoglycan, but do not form ropes and sheets, as in animals. Sugars that are part of the proteoglycan complex that forms cartilage have been found in the walls of bacteria.
Evolutionary experiments
Yeast
Experiments on the evolution of multicellularity conducted in 2012 by University of Minnesota researchers led by William Ratcliffe and Michael Travisano used baker's yeast as a model object. These single-celled fungi reproduce by budding; When the mother cell reaches a certain size, a smaller daughter cell separates from it and becomes an independent organism. Daughter cells may also stick together to form clusters. The researchers carried out an artificial selection of cells included in the largest clusters. The selection criterion was the rate at which clusters settled to the bottom of the tank. The clusters that passed the selection filter were again cultivated, and the largest ones were again selected.
Over time, the yeast clusters began to behave like single organisms: after the juvenile stage, when cell growth occurred, there followed a reproduction stage, during which the cluster was divided into large and small parts. In this case, the cells located at the border died, allowing the parent and daughter clusters to disperse.
The experiment took 60 days. The result was individual clusters of yeast cells that lived and died as a single organism.
The researchers themselves do not consider the experiment pure, since yeast in the past had multicellular ancestors, from which they could have inherited some mechanisms of multicellularity.
Seaweed Chlamydomonas reinhardtii
In 2013, a group of researchers at the University of Minnesota led by William Ratcliffe, previously known for evolutionary experiments with yeast, conducted similar experiments with single-celled algae Chlamydomonas reinhardtii. 10 cultures of these organisms were cultivated for 50 generations, centrifuging them from time to time and selecting the largest clusters. After 50 generations, multicellular aggregations with synchronized life cycles of individual cells developed in one of the cultures. Remaining together for several hours, the clusters then dispersed into individual cells, which, remaining inside the common mucous membrane, began to divide and form new clusters.
Unlike yeast, Chlamydomonas never had multicellular ancestors and could not inherit the mechanisms of multicellularity from them, however, as a result of artificial selection over several dozen generations, primitive multicellularity appears in them. However, unlike yeast clusters, which remained a single organism during the budding process, chlamydomonas clusters are divided into separate cells during reproduction. This indicates that the mechanisms of multicellularity could arise independently in different groups of unicellular organisms and vary from case to case cellosome) and represented artificially created colonies of unicellular organisms. A layer of yeast cells was applied to aragonite and calcite crystals using polymer electrolytes as a binder, then the crystals were dissolved with acid and hollow closed cellosomes were obtained that retained the shape of the template used. In the resulting cellosomes, the yeast cells retained their activity and template shape
The origin of multicellular organisms has not yet been fully elucidated. Even in the last century, scientists debated the origin of multicellular organisms, putting forward various, sometimes even fantastic, hypotheses. To this day, only a few of them have retained their significance, primarily those that recognize that the ancestors of multicellular organisms were simple ones. The most famous hypotheses for the origin of multicellular organisms are:
- Gastreus hypothesis (E. Haeckel).
- Placula hypothesis (A. Büchli).
- Bilatogastrea hypothesis (T. Jägersten).
- Phagocytella hypothesis (I. I. Mechnikov).
gastrea hypothesis
Thus, in the 70s of the last century, the famous German biologist E. Haeckel developed a system of views on the origin of multicellular organisms from colonial flagellates - the gastrea hypothesis.
According to this hypothesis, the ancestors of multicellular organisms were colonies of flagellates, similar to modern ones. Haeckel relied on embryological data and provided the main stage of embryonic development of an organism with phylogenetic significance. Just as in ontogenesis a multicellular organism is formed from one fertilized egg, as a result of fragmentation it turns into multicellular stages - morulae, then blastula and gastrula, so in historical development - first unicellular amoeba-like organisms - cytaea - arose, then from such organisms colonies of several individuals - the sea, which subsequently turned into spherical single-layer colonies - blastea, which had flagella on the surface and swam in the water column.
Finally, protrusion of the wall of the blastea inward (intussusception) led to the emergence of a two-layered organism - the gastrea. The outer layer of its cells had flagella and performed locomotor functions. the inner one lined the primary intestine and performed the function of digestion. Thus, according to Haeckel’s hypothesis, the primary mouth (blastopore) and the closed primary gut arose simultaneously. Since at the time this hypothesis was created, the only method of gastrulation was considered to be intussusception, characteristic of more highly organized animals (lancelet, ascidians), Haeckel argued that in the phylogeny of multicellular gastritis, the formation occurred in exactly this way. The development of coelenterates began with a two-layer floating organism - gastrea, which settled on the substrate at the aboral pole, which, according to Haeckel, is the most primitive multicellular, from which all other multicellular organisms arose.
At one time, the gastraea hypothesis was quite substantiated. Haeckel put forward it even before I. I. Mechnikov’s discovery of intracellular digestion. Then it was believed that food was digested only in the intestinal cavity, therefore the primary endoderm was represented as the epithelium of the primary intestine.
Note 1
The gastraea hypothesis played a major role in the development of evolutionary zoology. It was the first to substantiate the unity of origin of all multicellular animals.
The hypothesis was supported by a number of zoologists, and with certain additions it is accepted by many modern scientists, particularly in Western Europe; it is also presented in many foreign zoology textbooks.
Placula hypothesis
One of the modifications of the gastraea hypothesis was the placule hypothesis, proposed by the English scientist O. Büchli (1884), who believed that multicellular organisms originate from a two-layered flat colony of protozoa (placula). The placule layer facing the substrate performed the function of nutrition, absorbing food particles from the bottom. Curving one side upward, the two-layer placule turned into a gastrea-like organism.
Bilaterogastrea hypothesis
Quite popular among modern scientists is another modification of the gastrea hypothesis put forward by the Swedish scientist T. Jägersten in 1955-1972, known as the bilaterogastrea hypothesis. According to this hypothesis, the distant ancestor of multicellular animals was a spherical colony of plant flagellates, similar to Volvox, which floated in the surface layers of water and could feed autotrophically and heterotrophically - due to the phagocytosis of small organic particles. The colony, like modern Volvox, had anterior-posterior polarity. According to Jägersten, such a blastea switched to the opthos type of life, settling to the bottom on its side, which became flat.
Thus, a benthic bilaterally symmetrical (one through whose body one can draw one plane of symmetry, dividing it into two mirror-like halves) blastulo-like animal - bilateroblastea - arose. Since the illumination at the bottom is insufficient for photosynthesis, the bilateroblast fed predominantly heterotrophically, with ventral epithelial cells phagocytosing nutrient particles from the bottom. During the transition to feeding on large prey, these animals retracted the ventral layer, forming a temporary cavity into which the prey fell and where it was digested. Gradually, this temporary cavity became a permanent intestinal cavity.
From bilaterogastrea come cuttings, which, according to Jägersten, have an intestinal cavity. Later, during the evolution of bilaterogastrea, three pairs of lateral invaginations appeared in the intestinal walls. From such a complicated bilaterogastrea all other types of animals descend:
- coelenterates (primary coral polyps) with three pairs of septa in the gastric cavity,
- Coelomic animals with three pairs of coelom.
Parenchymal and primary animals, according to this hypothesis, have secondarily lost their coelom.
Mechnikov's hypothesis
Now the most substantiated and alternative to the gastray hypothesis can be considered the hypothesis of the domestic scientist I. I. Mechnikov, developed in 1877-1886. Studying the embryonic development of lower multicellular organisms - sponges and coelenterates, Mechnikov established that during the formation of the two-layer stage they do not experience invagination, but mainly immigration - the crawling of individual cells of the blastula wall into its cavity. Mechnikov considered this primitive process of gastrula formation to be primary, and invagination as a consequence of the reduction and simplification of development that took place in the process of evolution.
Note 2
The ancestors of multicellular organisms, according to Mechnikov’s hypothesis, were spherical colonies of heterotrophic flagellates that swam in water and fed on phagocytic tiny particles.
The prototype of such a colony could be pelagic spherical colonies of collared flagellates (Sphaeroeca volvox). Individual cells, having captured the nutrient lobule, lost their flagellum, turning into amoeboids, and sank deep into the colony filled with structureless jelly. They could then return to the surface.
This phenomenon is observed in modern sponges, the flagellar cells of which, choanocytes, can, when filled with food, turn into amoeboid cells and migrate to the parenchyma, where digestion occurs, and then return to their place. Over time, the cells differentiated into those that primarily provided for the colonies, and those that fed and fed others. The colony no longer had the appearance of a hollow ball - there was an accumulation of phagocytes inside.
Of modern animals, the closest to organisms of this type is the collar flagella (Choanofiagellida) Proterospongia haeckeli, which form a colony, the outer layer of which contains collar flagellates, and the inner layer contains amoeboid cells. Gradually, the temporary differentiation of cells acquired a permanent character and the colony of unicellular organisms turned into a multicellular organism, which must have two layers of cells:
- external (basal) - kinoblast
- internal (amoeboid) - phagocytoblast.
The nutrition of such an organism occurred due to the capture of organic particles from the water column by the flagellar cells of the kinoblast and their transfer to the amoeboid cells of the phagocytoblast. Mechnikov called this hypothetical multicellular organism a phagocyte, wanting to emphasize the role of phagocytosis in its occurrence.
The earth reached 1% of its current level. This was sufficient for the life activity of some microorganisms, but for multicellular plants and especially animals, a noticeably higher level is required concentration oxygen (that is, its amount in each cubic meter of air).
In any case, there were no predators in the ecosystems of that time. The world of the most ancient multicellular organisms remains extremely mysterious, and paleontologists studying them are actually in the position of astronauts faced with the fauna of an alien planet.
Apparently, the first multicellular animals did not leave direct descendants. And the skeletal organisms that were familiar to us that replaced them arose on a completely different basis and spread widely throughout our planet.
On this page there is material on the following topics:
The first multicellular plants were
The first large multicellular organisms
Aramarthoses of the Proterozoic era
Proterozoic era report briefly
The ancestors of the first multicellular organisms were
Questions for this article:
What is the origin of multicellular plants and animals?
Why do multicellular organisms need a higher concentration of oxygen than single-celled organisms?
Why do animals require more oxygen for their metabolism than plants?
Has the total mass of all living organisms on Earth decreased or increased as a result of the “oxygen revolution”?
Did the emergence of multicellular organisms lead to the extinction of single-celled organisms? Why?
How are some ancient multicellular organisms similar to modern lichens?
Is it possible to meet the first multicellular animals on our planet now?
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Probably 700-900 million years ago the first multicellular animals and plants appeared on Earth. In plants, the emergence of a multicellular level of organization probably occurred on the basis of differentiation of ribbon-shaped colonies formed by lateral fusion of attached filamentous forms or due to cell division of the latter in two mutually perpendicular directions (in the same plane). In colonies attached at one end to the substrate, different areas were in different conditions with respect to the incident light, substrate and aquatic environment. In this regard, natural selection favored the emergence of some differentiation of parts of the colony. The first step was the emergence of colony polarity; at one end there were cells that served for attachment to the substrate (they are characterized by a weakening of photosynthesis, loss of the ability to divide), at the other end there were apical cells that intensively divided and formed a kind of “growth point” of the colony. Natural selection favored the acquisition by cells of the colony of the ability to divide in different directions; this led to branching, which increased the surface area of the colony. The division of cells along three mutually perpendicular axes or the interweaving of individual threads led to the emergence of a multilayered “volumetric” body. In the process of its further differentiation, multicellular organs were formed that performed different functions (fixation on the substrate, photosynthesis, reproduction). At the same time, a certain interdependence developed between different cells of the plant, which, in fact, marks the achievement of a multicellular level of organization.
In animals, an active lifestyle required more advanced and complex differentiation of the organism than in plants. The complexity of the organization of multicellular animals (Metazoa) and the diversity of its specific forms have stimulated the development of various hypotheses about the origin of Metazoa.
The first of them originates in the works of E. Haeckel, who, in developing his theory of gastrea, was based on the formulated by him biogenetic law, according to which the ontogeny of a given type of organism is a compressed and abbreviated repetition (recapitulation) of the course of the phylogeny of its ancestors (for more details, see Part GU). In accordance with this, E. Haeckel believed that the phylogeny of the most ancient Metazoa is to a certain extent repeated in the ontogeny of modern lower multicellular animals (Fig. 28). According to Haeckel, the ancestors of Metazoa were colonial protozoa, which had spherical colonies with a single-layer wall, similar to the blastula - one of the early
Rice. 28.
A- blastula; 6 - gastrulation; v-g- gastrula (appearance and longitudinal
section) stages of embryonic development of modern multicellular animals. Haeckel called this hypothetical ancestral form "blastea". During directed swimming, the spherical colony - the blastea - was oriented with one pole forward, as is observed in modern colonial protozoans, for example, Volvox. According to Haeckel, at the anterior pole of the colony, an invagination of its wall appeared inward, similar to what occurs during invagination gastrulation in the ontogenesis of some modern Metazoa. As a result, a multicellular organism was formed - “gastrea”, the body wall of which consists of two layers, ecto- and endoderm. The endoderm surrounds an internal cavity - the primary intestine, open to the outside by a single opening - the primary mouth. The organization of the gastrea corresponds to the fundamental plan of the structure of the coelenterata (phylum Coelenterata), which Haeckel considered as the most primitive multicellular animals.
I. I. Mechnikov drew attention to the fact that in primitive coelenterates, gastrulation occurs not through invagination (invagination of one pole of a single-layer embryo - blastula), which is typical for more highly organized groups, but through the migration of some cells from the single-layer body wall inward (Fig. 29 ). There they form a loose accumulation, which later organizes itself in the form of the walls of the gastric cavity, which breaks out through the oral opening. This method of gastrulation is much simpler than intussusception, since it does not require complex directed and coordinated displacement of an entire layer of cells, and is probably more primitive than intussusception. In this regard, Mechnikov modified Haeckel's hypothesis as follows. In a spheroid colony of protozoa - flagellates, the cells of its single-layer wall, capturing (phagocytosing)
Rice. 29. Gastrulation of the embryo of the hydroid polyp Stomateca (from I.A. Ioff) food, migrated for its digestion inside, into the cavity of the colonies (similar to the migration of cells of the future endoderm during the gastrulation process of coelenterates). These cells formed a loose internal accumulation - the phagocytoblast, the function of which was to provide the entire body with food, including its digestion and distribution, while the surface layer of cells - the kinoblast - carried out the functions of protection and movement of the body. To capture new food particles, the phagocytoblast cells, according to Mechnikov, did not need to return to the surface layer: located directly under the kinoblast, the phagocytoblast cells captured food particles with pseudopodia that extended outward in the spaces between the phagocytoblast cells. This hypothetical stage of Metazoan evolution was named by Metchnikoff phagocytella(or parenchymella); its structure corresponds to that of the parenchyma, the larvae of some coelenterates and sponges. Subsequently, as an adaptation to increased feeding activity, the descendants of the phagocytella underwent epithelialization of the phagocytoblast with the formation of the primary intestine and the appearance of an oral opening in the place where the predominant migration of cells inward occurred. According to some scientists, this place probably corresponded to the rear pole of the body in the direction of movement, where turbulence in the water flow occurs during swimming, and therefore conditions are most favorable for capturing food particles. Mechnikov's hypothesis, like Haeckel's hypothesis, considers coelenterates and sponges as the most primitive multicellular animals.
Important information for understanding the early stages of the evolution of Metazoa was obtained from the study of the extremely primitive multicellular animal Trichoplax adhaerens, discovered in the Red Sea by F. Schulze back in 1883, but studied in detail only in the 1970s. our century by K. Grell and A.V. Ivanov. Trichoplax (Fig. 30) has a flattened body, lacking polarity. The surface of the body facing upward is covered with flat, and the bottom - with columnar ciliated epithelium. Inside, between the epithelial layers corresponding to the kinoblast, there is a cavity with liquid contents, in which spindle-shaped and stellate cells are located. These latter can be considered as a phagocytoblast. Trichoplax reproduces asexually - by division and budding. A.V. Ivanov pointed out that Trichoplax is, as it were, a living model of phagocytella, and proposed to distinguish this form into a special type of animal Phagocytellozoa. Apparently, Trichoplax reinforces the position of the phagocytella hypothesis of I.I. Mechnikov. However, according to modern ideas, direct
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And- changes in the body shape of one individual (according to F. Schulze); b- incision perpendicular to the edges of the body (according to A.V. Ivanov): 1 - amoeboid cells; 2 - dorsal epithelium; 3 - spindle cells; 4 - fatty inclusions; 5 - digestive vacuoles; 6 - abdominal epithelium
The descendants of phagocytellozoans among metazoans were not coelenterates, but primitive worm-like animals, similar in level of organization to flat ciliated worms - turbellaria.
The first fossil traces of the life activity of worm-like multicellular animals are known from Late Riphean deposits. In Vendian times (650-570 million years ago), a variety of animals already existed, probably belonging to different types. A few prints of soft-bodied Vendian animals are known from different regions of all continents of the globe, except for the still little-explored Antarctica. A number of interesting finds were made in Late Proterozoic deposits in Russia - on the Kola Peninsula, in the Arkhangelsk region, on the Maya River and on the Olenek uplift in Yakutia, etc.
Rice. 31.
1-10 - coelenterates (/ - Ediacara; 2 - Beltanella; 3 - Mcdusinitcs; 4 - Mawsonites; 5-6- Cyclomedusa; 7 - Conomedusites; 8 - Rangea; 9- Arborea; 10 - Pteridinium); 11-14 - flat and annelid worms (11 - Spriggina; 12-14 - Dickinsonia); 15-16 - arthropods (15 - Parvancorina; 16 - Praecambridium); 17 - echinoderm Tribrachidium; 18 - spherical gelatinous organisms
The most famous is the rich Late Proterozoic fauna discovered in Central Australia in the Ediacara region north of Adelaide. M. Glessner, who studied this fauna, believes that it includes several dozen species of very diverse multicellular animals belonging to different types (Fig. 31). Most forms probably belong to the coelenterates. These are jellyfish-like organisms, probably “hovering” in the water column (Ediacara flindersi, Beltanella gilesi, Medusinites asteroides, etc.), and polypoid forms attached to the seabed, solitary or colonial, reminiscent of modern alcyonarian corals, or sea feathers (Rangea longa, Arborea arborea, Pteridinium simplex, etc.). It is remarkable that all of them, like other animals of the Ediacaran fauna, lack a hard skeleton.
In addition to coelenterates, the Ediacaran fauna contains remains of worm-like animals classified as flatworms and annelids (Spriggina floundcri and various species of Dickinsonia). Some species of organisms are interpreted as possible ancestors of arthropods (Praecambridium sigillum, reminiscent in the nature of body segmentation of trilobites and chelicerates) and echinoderms (Tribrachidium heraldicum with a disc-shaped body, on the flat surface of which three ridges protrude, and Arkaria
adami with a five-rayed star-shaped cavity on the oral side of the body and with a semblance of ambulacral grooves). Finally, there are a number of fossil organisms of unknown taxonomic affiliation.
Many Vendian organisms were also discovered in Vendian deposits of different regions of Russia: jellyfish-like Ediacarans and Medusinites - on the Rybachy Peninsula, Pteridinium - in the north of Yakutia, Spriggina-like Vendia - in the Yarensk region of the Arkhangelsk region, etc. Locations of the Vendian fauna, by richness not inferior to the Ediacaran, were found on the Syuzma River on the Onega Peninsula and on the Zimny Coast of the White Sea. Fossil remains of over 30 species of non-skeletal multicellular animals were discovered here, the sizes of which varied from 3 mm to 30 cm. Among them are probable representatives of coelenterates, flat and annelids, arthropods, echinoderms, as well as a number of forms belonging to some unknown groups. In general, the relationship of Vendian organisms with modern groups, most of which have been reliably known since the Cambrian, remains problematic - the differences are very large, and some researchers believe that the currently known Vendian organisms are not directly related to the later Cambrian, but represent blind evolutionary branches.
M.A. Fedonkin, who studied the White Sea fauna of Vendian animals, believes that some of these organisms have characteristics of several different types of animals and may represent the original forms occupying an intermediate position. Fedonkin also drew attention to the similarity of a number of Vendian organisms with the larval stages of some modern animals, although Vendian organisms are much larger in size than the corresponding larvae. With all the diversity of body plans in organisms of the Vendian fauna (“vendobionts”), they are united by some common organizational features: the absence of a skeleton, limbs, and probably also respiratory and digestive organs. Many vendobionts led a stationary, attached lifestyle. Some researchers believe that Vendian organisms fed osmotically through the surface of the body or with the help of photo- or chemosynthetic symbionts living in their bodies - unicellular algae and bacteria.
Although soft-bodied, skeletonless forms predominate among Vendian animals, it is likely that at that time there were already a few species that had a shell. Such, for example, is Cloudina, which had a simple tubular shell consisting of organic matter and calcite. Claudine was found in carbonate rocks that are interbedded with sediments containing the remains of the Ediacaran fauna of soft-bodied animals.
All these data indicate a wide distribution of soft-bodied faunas in Vendian times. The accumulation of materials on Vendian fossil organisms allowed some researchers to raise the question of expanding the scope of the Phanerozoic, with the inclusion of the “Ediacaran” - a period covering the period of time from 670 to 550 million years ago (in the geochronology diagram shown on p. 149, this period corresponds to the Vendian as part of the Proterozoic).
Since the Vendian fauna is so diverse and includes quite highly organized animals, it is obvious that the evolution of Metazoa had already been going on for a very long time before its emergence. Probably, multicellular animals appeared much earlier - somewhere between 700-900 million years ago.
Thus, in the late Proterozoic (600-650 million years ago), groups of multicellular animals such as sponges, coelenterates, flat and annelids, and even, possibly, the ancestors of arthropods already existed. Judging by the achieved level of organization, it can be assumed that by this time the evolutionary trunks of filamentous worms (type Nemathelminthes), the ancestors of mollusks and the ancestors of deuterostomes - oligomeric worms, had also become separated.
The Precambrian phylogeny of Metazoa can be hypothetically represented as follows (Fig. 32). From colonial flagellates (according to some authors, from heterotrophic forms belonging to the order Protomonadida), through differentiation and integration of the colony, with migration into the colony of phagocytoblast cells at the posterior pole of the body, the first multicellular animals arose, the organization of which corresponded to the phagocytella (according to I. I. Mechnikov). The little changed descendants of these ancient multicellular organisms are modern Phagocytellozoa (Trichoplax adhaerens). Primitive multicellular animals were free-swimming (due to the work of the ciliated epithelium - kinoblast) animals that fed on various microorganisms - protozoa and unicellular algae.
With the further development of adaptations to active feeding, a gradual epitlization of the phagocytoblast occurred, i.e.
Rice. 32.
transformation of a loose accumulation of cells into an organized cellular layer - intestinal epithelium. Epithelization of the phagocytoblast probably began with the development of a permanent oral opening at the posterior pole of the body. As K.V. Beklemishev noted, at this stage of phylogenesis the organism began to feed as a whole, and not as a collection of individual independently phagocytic cells. Probably, by this time, the nervous system integrating the body in the form of an epithelial nerve plexus had also formed. Active swimming required the ability to navigate in space and coordinate the work of all organs. To carry out these functions, a neuro-receptor complex arose on the aboral (opposite the oral opening) pole of the body, which included a nerve ganglion, tactile setae and a statocyst (equilibrium organ). A similar aboral organ is present in modern ctenophores (type Ctenophora), as well as in free-swimming larvae of many groups of animals: flat and annelids, mollusks, arthropods, hemichordates, echinoderms, etc. This hypothetical stage of the phylogeny of ancient Metazoa can be called “stomophagocytella” (emphasizing the epithelization of only the oral part of the phagocytoblast).
Perhaps, at this stage of phylogeny, the first major divergence of the phylogenetic trunk of ancient multicellular organisms occurred, associated with the fact that some groups of these animals moved on to the development of the seabed, while others continued to improve adaptations to active life in the water column.
Modern lower flatworms - intestinal turbellarians (Acoela) have generally retained the level of organization that was probably characteristic of the most ancient multicellular organisms, which first began to develop a mobile lifestyle at the bottom of reservoirs. From the Vendian representatives of these turbellarians, phylogenetic trunks could have arisen leading to other groups of flatworms, to filamentous worms and to the ancestors of annelids (protoannelids). The ancestors of mollusks, on the one hand, and the ancestors of arthropods, on the other, separated from the protoannelids. In all these groups, further differentiation of the phagocytoblast occurred. In lower worms, only its central part was epithelialized, which led in flatworms to the formation of a branched intestine with a single opening - the “mouth”, leading to the external environment, and in filamentous worms - to the formation of a through intestine with oral and anal openings. In higher groups (annelids, mollusks and arthropods), the entire phagocytoblast became epithelial: not only its central part (endodermal intestine), but also its peripheral part (mesoderm and its derivatives). The latter led to the development of a secondary body cavity - the coelom, the walls of which are formed by mesodermal coelomic epithelium. More primitive representatives of annelids, mollusks and arthropods have a characteristic larval stage - the trochophore. In this regard, these groups are sometimes combined under the name Trochozoa.
In those descendants of Stomophagocytella who continued to improve adaptations to life in the water column, epithelization of the central and partly peripheral phagocytoblast also occurred: the gastric cavity (primary intestine) and its peripheral branches (gastrovascular canals) arose. Among modern animals, the closest to this level of organization are ctenophores, which probably retained a primitive way of life in the water column. From their Late Proterozoic ancestors, which can be called “proctenophores,” with the transition to attached life on the seabed, cnidarians (phylum Coelenterata, or Cnidaria) arose.
Other phylogenetic lines that branched off from proctenophores also explored the seabed, but with the development of adaptations for active movement along the substrate, like turbellarians and their descendants, but at a different initial level of organization. In these forms, as a result of the completion of epithelization of the peripheral phagocytoblast, a secondary body cavity was also formed - the coelom, but it arose in a completely different way than in Trochozoa. In the ontogenesis of animals descended from proctenophores, the secondary body cavity is separated from the primary intestine, like its lateral pocket-like protrusions (initially there were probably three pairs of such protrusions), which are then laced from the intestinal walls (Fig. 33). This method of development of the coelom is called enterocoelous - in contrast to the schizocoelous method characteristic of Trochozoa, in which the coelom arises as a result of the appearance of cavities inside accumulations of mesodermal cells, without any connection with the primary intestine. O. and R. Hertwig and I.I. Mechnikov substantiated the hypothesis according to which the enterocoelous coelom arose in evolution from the gastrovascular canals of proctenophore ancestors (enterocoelous theory of the origin of the coelom). The enterocoelous coelom is characteristic of the types of pogonophora (Pogonophora), chaetognatha, brachiopods (Brachiopoda), bryozoans (Bryozoa) and a number of others, including the group of so-called deuterostomes (Deuterostomia), which unites the types of chordates (Chordata), echinoderms (Echinodermata) and hemichordata (Hemichordata). Deuterostome animals have much in common, in particular the special position of the definitive (inherent in adult organisms) mouth, which appears at the pole of the body opposite the primary embryonic mouth - the blastopore. In place of the latter, the anus develops. Deuterostomes undoubtedly have a common origin; their ancestors indicate a hypothetical group of oligomeric worms, whose body was divided into three sections, had a secondary mouth and an enterocelous coelom. Among modern deuterostomes, the closest to the level of organization of oligomeric worms are the free-living hemichordates, of which the acorn worm (Balanoglossus) is a representative.
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I- ectoderm; 2 - endoderm; 3 - mesoderm; 4 - primary intestine; 5 - coelomic pockets; 6 - neural plate; 7- overall; 8 - secondary intestine; 9 - neural tube; 10 - chord
Sponges (phylum Porifera, or Spongia) occupy a special position among multicellular animals. This group is characterized by a very primitive general level of organization: sponges essentially do not have an epithelialized phagocytoblast, an ordered internal structure, a real intestine, a nervous system, receptors, etc. Sponges differ from all other Metazoa in their extremely unique ontogenesis, during which inversion occurs germ layers (ecto- and endoderm, so to speak, change places). The latest molecular research data have shown that sponges have a common origin with all multicellular organisms. They probably represent a very early lateral branch that separated at the level of the phagopitella. The oldest fossil remains of sponges are known from Vendian (Ediacaran) deposits in Australia.
- See: Ivanov A.V. Origin of multicellular organisms. - L., 1968.
- Recently, the first data have appeared on the fossil remains of metazoans found in Canada and China in rocks of this geological age and separated from rocks with remains of Ediacaran organisms by layers of glacial sediments (tillites).
