(plant and animal communities)

The most striking event in the evolution of living forms was the emergence of plants and living creatures from water and the subsequent formation of a wide variety of land plants and animals. From these, highly organized forms of life subsequently arise.

The transition to a terrestrial habitat required corresponding changes, because body weight on land is greater than in water, and air, unlike water, does not contain nutrients. In addition, dry air transmits light and sound differently than water.

The latest evolution of eukaryotes was associated with the division into plant and animal cells. An important stage in the development of life and its complexity was the emergence of approximately 900 million years ago sexual reproduction. Sexual reproduction consists of the mechanism of fusion of the DNA of two individuals and the subsequent redistribution of genetic material, in which the offspring are similar to the parents, but not identical to them. The advantage of sexual reproduction is that it significantly increases species diversity and dramatically accelerates evolution, allowing faster and more efficient adaptation to environmental changes.

The embryo could remain inside the seed for quite a long time until the plant disperses the seeds and they find themselves in favorable growing conditions. And then the sprout inflates the seed coat, germinates and feeds on reserves until its roots and leaves themselves begin to support and nourish the plant. Thus, in all seed plants, the dependence of the process of sexual reproduction on the presence of an aquatic environment disappears.

The transition to seed reproduction is associated with a number of evolutionary advantages: The diploid embryo in the seeds is protected from unfavorable conditions by the presence of integuments and is provided with food, and the seeds have adaptations for distribution by animals, etc.

What happens next is pollination specialization(with the help of insects) and the distribution of seeds and fruits by animals, strengthening the protection of the embryo from unfavorable conditions, providing food, forming integuments, etc. In the Early Cretaceous period, some plants improved their seed protection system by forming an additional shell.

The emergence of angiosperms was associated with the improvement of the fertilization process: with the transition to the fact that pollen was carried not by the wind, but by animals (insects). This required significant transformations of the plant organism. Such an organism must contain means of signaling animals about itself, attracting animals to itself, in order to then transfer pollen to another plant of the same species, and, in the end, the animal itself must receive something for itself (nectar or pollen).

This whole complex of issues was resolved on the path to the emergence of a huge variety of beautiful and diverse angiosperms (flowering) plants: the flowers of each plant should differ in appearance (shape, color) from the flowers of other plants.

Flowering plants are characterized by high evolutionary plasticity and diversity generated by pollination by insects. Gradually spreading, flowering plants conquered all continents and won the struggle for land. The flower played a major role in this, ensuring the attraction of pollinating insects. In addition, flowering plants have a developed conducting system: the fruit and embryo of flowering plants have significant food reserves, which ensures the development of the embryo and seed. In the Cenozoic, botanical and geological areas close to modern ones were formed. Angiosperms achieve dominance. Forests are the most widespread on Earth. The territory of Europe was covered with lush forests: coniferous trees predominated in the north, and chestnut-beech forests with giant sequoias in the south.

Geographical areas (landscapes) changed depending on climate changes. With warming, heat-loving plants spread to the north, and with cooling - to the south.

A significant step in the further complication of the organization of living beings was the appearance, approximately 700-800 million years ago, of multicellular organisms with a differentiated body, developed tissues, and organs that performed specific functions. The first multicellular animals are represented by several types: sponges, coelenterates, brachiopods, and arthropods.

Multicellular organisms descend from colonial forms of unicellular flagellates. The evolution of multicellular organisms went in the direction of improving methods of movement and better coordination of cell activity, improving methods of respiration, etc.

In the Proterozoic and early Paleozoic, plants inhabited mainly seas. Among those attached to the bottom there are green and brown algae, and in the water column there are golden, red and other algae. Almost all the main types of animals already existed in the Cambrian seas, which subsequently only specialized and improved. The appearance of the marine fauna was determined by numerous crustaceans, sponges, corals, echinoderms, various mollusks, brachiopods, and trilobites. Numerous corals lived in the warm and shallow seas, and cephalopods, creatures similar to modern squids, several meters long, reached significant development. At the end of the Ordovician, large carnivores appeared in the sea, reaching 10-11 m in length. In the Ordovician, approximately 500 million years ago, the first animals with skeletons, vertebrates, appeared. This was a significant milestone in the history of life on Earth.

The first vertebrates arose in shallow fresh water bodies, and only then these freshwater forms conquered the seas and oceans. The first vertebrates are small (about 10 cm long) creatures, jawless fish-like creatures, covered with scales, which helped protect themselves from large predators (octopuses, squids).

Further evolution of vertebrates went towards the formation of jawed fish-like fish, which quickly replaced the majority of jawless fish. In the Devonian, lungfishes also appeared, which were adapted to breathing in water, but had lungs. As you know, sharks are cartilaginous. Bony fish are the most numerous group of fish, currently predominant in the seas, oceans, rivers, and lakes. Some freshwater fish (lungfishes) apparently gave birth to first the primary stegocephalians, and then to land vertebrates. Thus, the first amphibians appear in the Devonian. In the Devonian, another extremely progressive group of animals appeared - insects.

Table 6.1.

Development of living organisms on the planet

Eras, periods (time from the formation of the Earth)	The main stages of development of flora and fauna
Katarhey 5.0 – 3.5 billion years
Archean era 3.5 – 2.6 billion years (duration 800 million years)	The appearance of the first simple living creatures, algae and bacteria. The first calcareous algal structures, stromatolites
Proterozoic era 2.6 – 0.57 billion years (duration 2030 million years)	Massive development of blue-green algae. The appearance (about 1 billion years) of the first animals - coelenterates, worms, etc.
Paleozoic era 570-230 million years (duration 340 million years)
Cambrian period 570 – 500 million years (duration 70 million years)	At the beginning of the period, there was a massive appearance of skeletons (internal and external shells) in various groups of animals. Mass development of calcareous algae
Ordovician period 500 – 440 million years (duration 60 million years)	A significant part of the Russian Platform is dry. Siberia has a shallow open sea. Distribution of trilobites and graptolites. The first jawless vertebrates. Armored and cartilaginous fishes, graptolites and brachiopods are widespread
Silurian period 440 – 410 million years (duration 30 million years)	Land plants—psilophytes—appear.
Devonian 410 – 350 million years (duration 60 million years)	Psilophyte flora is widespread and ferns appear. Widespread development of lobe-finned and lungfishes. The first amphibians - stegocephalians
Carboniferous period, or Carboniferous 350 – 280 million years (duration 65 million years)	Dominance of giant club mosses. Development of amphibians, insects, emergence of reptiles

Continuation of Table 6.1.

Permian period 285 – 230 million years (duration 55 million years)	Giant ferns grow, the first gymnosperms appear. Reptiles and large amphibians develop. Tabulates, trilobites and many brachiopods become extinct
Mesozoic era 230 - 67 million years (duration 163 million years)
Triassic 230 – 195 million years (duration 35 million years)
Jurassic period 195 – 137 million years (duration 58 million years)
Cretaceous period 137 – 67 million years (duration 70 million years)	The appearance and sharp increase at the end of the period of angiosperms. The rise and extinction of large lizards. The appearance of toothless birds. Rare primitive mammals. Ammonites and belemnites are dying out
Cenozoic era 67 – 0 million years (duration 67 million years)
Paleogene period 67 – 27 million years (duration 40 million years)	Distribution of angiosperms. Development of various groups of mammals, artiodactyls, predators, and cetaceans appear. Toothless birds are widespread
Neogene period 27 - 3 million years (duration 25 million years).	Rich and varied vegetation. Horses, giraffes, saber-toothed tigers appear
Quaternary period 3 – 0 million years (duration 3 million years) Pleisotcene (3 million years – 20 thousand years) Holocene (20 thousand years – 0)	From the beginning of the period, the animal and plant worlds are close to modern ones. Mammoths and rhinoceroses were found in Europe and Siberia. A man appeared

The formation of insects indicated that in the course of evolution two different ways of solving the problems of strengthening the body and improving forms of reflection had developed. In vertebrates, the role of the frame is performed by the internal skeleton, in higher forms of invertebrate insects - by the external one. As for the forms of reflection, insects have an extremely complex nervous system with huge and relatively independent nerve centers scattered throughout the body. In vertebrates, the development of the brain and the predominance of conditioned reflexes over unconditioned ones are noted. The difference between these two different ways of structuring the most important evolutionary tasks was fully manifested before the transition to life on land. Reptiles that came onto land turned out to be a promising form. They mastered the land. Some reptiles become carnivorous, others become vegetative.

In the Cretaceous period, giant herbivorous dinosaurs appeared (Fig. 6.2). Marine reptiles (ichthyosaurs) developed especially intensively in the Jurassic. The conquest of the air environment is also gradually progressing. Insects began to fly back in the Carboniferous and for about 100 million years they were sovereign in the air. And only in the Triassic did the first flying dinosaurs appear. Reptiles successfully master the air environment. Large insects appear. Some flying lizards had a wingspan of up to 20m. At the end of the Mesozoic, placental mammals appeared.

Rice. 6.2. Diplodocus reached 30 m in length and was one of the largest animals that ever lived on Earth.

At the end of the Mesozoic, due to cooling conditions, the spaces occupied by rich vegetation were reduced. This entails the extinction of first the herbivorous dinosaurs, and then the predatory dinosaurs that hunted them. In cold weather, warm-blooded animals – birds and mammals – receive exceptional benefits. In the Paleocene, the first predatory mammals appeared. At the same time, some species of mammals “go” to the sea (cetaceans, pinnipeds). The order of primates is separated from some species of insectivores. In the Pliocene, all modern families of mammals were already found.

In the Cenozoic, the most important trends were formed that led to the emergence person. This concerns the emergence of a herd lifestyle, which was a stepping stone to the emergence of social communication. Moreover, if in insects biosociality led to the loss of individuality; then in mammals, on the contrary, to emphasizing the individual traits of the individual. In the Neogene, numerous species of monkeys appeared in the vast open spaces of the savannas of Africa. Some species of primates adopt upright walking. The development of consciousness led to the fact that they began to plan their actions.

Thus, in the biological world, the prerequisites for the emergence of Human and the world of culture.

Geology and geochemistry have made it possible to determine the time of existence of transitional forms between humans and those animals from which humans descended. Archeology, by studying the material monuments of ancient human material culture, reveals the history of the development of human society. The most important thing that distinguishes a person from an animal is a highly developed consciousness, with the help of which a person began to plan his actions, consciously produce all the necessary means of existence and articulate speech. However, despite many common characteristics between humans and apes, none of the living apes was the ancestor of humans.

MOSCOW, December 12 – RIA Novosti. The oldest multicellular organisms discovered in the mid-20th century in the Ediacaran Hills of Australia may not be primitive marine invertebrates, but land lichens, an American paleontologist says in a paper published in the journal Nature.

The first multicellular organisms arose on Earth in the Proterozoic, a period of geological history spanning the period from 2500 to 550 million years ago. To date, scientists have discovered very few fossils dating back to this period. The most famous of these are the prints of multicellular organisms found in the rocks of the Ediacaran Hills in Australia in 1947.

Gregory Retallack from the University of Oregon in Eugene (USA) doubted that these organisms were marine invertebrates, and offered his explanation of their nature by studying the chemical composition of the rocks in which the prints of the most ancient living creatures lay.

Retallak's attention was drawn to the fact that the rocks surrounding the remains of the Ediacaran creatures were not similar in their structure and mineral composition to the sedimentary deposits that formed on the bottom of the sea. The scientist decided to test his suspicions by studying the chemical composition of samples from the Ediacaran hills and their microstructure using an electron microscope.

The chemical composition of the soil, as well as the shape and size of the mineral grains, indicate that this part of Australia was not in a tropical climate, but in a temperate or even subarctic climate. The water off the coast of the future Ediacaran Hills would have frozen during winter, casting doubt on the possibility of primitive multicellular organisms existing within it.

On the other hand, the mineral composition of the rocks surrounding the imprints is very similar to paleosols - fossilized fragments of ancient soils. In particular, samples from the Ediacaran Hills and other fragments of paleosols have the same isotopic composition, and on the surface of the samples there are microscopic grooves similar to film colonies of bacteria or primitive roots of lichens or fungi.

According to Retallack, soil and similar "roots" should not have existed at the bottom of shallow bays or other parts of the primordial ocean. This allowed him to suggest that the found prints were in fact not marine multicellular organisms, but the fossilized remains of lichens that lived on the surface of the land. Some of the “multicellular animals,” according to the researcher, are actually traces of ice crystals frozen inside the ancient soil.

This conclusion has already met criticism from the scientific community. In particular, paleontologist Shuhai Xiao from Virginia Tech (USA) noted in comments to an article in the journal Nature that microscopic depressions on the surface of Ediacaran rocks could only have been left by moving organisms, and not stationary lichens. According to him, similar remains of multicellular organisms were found in other deposits of the end of the Proterozoic, whose “marine” origin is beyond doubt.

The emergence of multicellularity is a natural process in the evolution of living forms, since in this case the organism acquires a number of advantages in the struggle for existence. At the dawn of the existence of eukaryotes, multicellularity arose more than once. Today's multicellular life forms on Earth have several different single-celled ancestors. For example, sponges are thought to have a different single-celled ancestor than other organisms.

The ancestors of multicellular organisms were colonial forms of protozoa. In colonies, cells are usually not so differentiated (if their specialization is observed at all) and can exist independently when separated.

The flowering of multicellular forms began about 600 million years ago. However, they could have appeared much earlier, about 2 billion years ago. This is indicated by archaeological finds of worm-like organisms and multicellular algae.

True multicellularity (with the separation of tissues) is characteristic only of eukaryotes (prokaryotes have colonies). This may be due to the complexity of the genome of eukaryotic cells, which provides flexibility (“customizability”) of cells, and hence their ability to change their metabolism and structure. Hereditary variability, mitosis, and meiosis could play an important role.

Multicellularity allows the most complete use of the reserve of hereditary variability, which accelerates evolutionary changes. Sexual reproduction plays a big role in this, in which the sexual process and reproduction are combined.

Biological evolution involves the improvement of the vital functions of organisms, which is largely achieved through their differentiation. As a result of the isolation of various life processes, special structures arise. These can be either cell structures or parts of a multicellular organism. The division and specialization of functions and structures can be considered as one of the properties of living things.

Unicellular eukaryotes (ciliates) have digestive vacuoles that specialize in the digestion, utilization and excretion of substances, which resembles a kind of digestive system. There are contractile vacuoles that regulate water balance (excretory system). The cilia and flagella of unicellular organisms can be considered as organs of movement that allow them to search for food and avoid danger.

However, the separation of structures and functions is much more efficient in a multicellular organism. The interconnection of cells enhances the vitality of the system through the repetition of cellular processes, separation of functions, and the formation of special structures (tissues, organs, organ systems).

Multicellular organisms are usually larger than unicellular ones. This allows them to eat larger foods, on the other hand, they themselves are eaten less often.

Maintaining multicellularity requires more energy. Therefore, it could only arise when the oxygen level in the atmosphere reached a certain value.

An important role in the emergence of multicellularity was played by the appearance of a number of properties and features in unicellular eukaryotes. Thus, predatory protozoa could secrete certain substances to “stick” the victim to themselves. Such compounds (collagen, etc.) could subsequently begin to act as a filler for the intercellular space, as well as for holding cells together.

Signal substances secreted by protozoa (to attract prey or scare away predators) in the process of evolution began to be used for the interaction of cells within the same organism.

That is, they differ in structure and functions.

Encyclopedic YouTube

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✪ Unicellular and multicellular organisms (biologist Evgeniy Sheval tells)

✪ Sponges. Biology video lesson 7th grade

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✪ The most important moment in the history of life on Earth

✪ Volvox. Online preparation for the Unified State Exam in Biology.

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

All living organisms are divided into subkingdoms of multicellular and unicellular creatures. The latter are one cell and belong to the simplest, while plants and animals are those structures in which a more complex organization has developed over the centuries. The number of cells varies depending on the variety to which the individual belongs. Most are so small that they can only be seen under a microscope. Cells appeared on Earth approximately 3.5 billion years ago.

Nowadays, all processes occurring with living organisms are studied by biology. This science deals with the subkingdom of multicellular and unicellular organisms.

Unicellular organisms

Unicellularity is determined by the presence in the body of a single cell that performs all vital functions. The well-known amoeba and slipper ciliates are primitive and, at the same time, the most ancient forms of life that are representatives of this species. They were the first living creatures to live on Earth. This also includes groups such as Sporozoans, Sarcodaceae and bacteria. They are all small and mostly invisible to the naked eye. They are usually divided into two general categories: prokaryotic and eukaryotic.

Prokaryotes are represented by protozoa or some species of fungi. Some of them live in colonies, where all individuals are the same. The entire process of life is carried out in each individual cell in order for it to survive.

Prokaryotic organisms do not have membrane-bound nuclei and cellular organelles. These are usually bacteria and cyanobacteria, such as E. coli, salmonella, nostoca, etc.

All representatives of these groups vary in size. The smallest bacterium is only 300 nanometers long. Unicellular organisms usually have special flagella or cilia that are involved in their movement. They have a simple body with pronounced basic features. Nutrition, as a rule, occurs during the process of absorption (phagocytosis) of food and is stored in special cell organelles.

Single-celled organisms dominated as life forms on Earth for billions of years. However, evolution from the simplest to the more complex individuals changed the entire landscape, as it led to the emergence of biologically evolved connections. In addition, the emergence of new species has created new environments with diverse ecological interactions.

Multicellular organisms

The main characteristic of the metazoan subkingdom is the presence of a large number of cells in one individual. They are fastened together, thereby creating a completely new organization, which consists of many derivative parts. The majority of them can be seen without any special equipment. Plants, fish, birds and animals emerge from a single cell. All creatures included in the subkingdom of multicellular organisms regenerate new individuals from embryos that are formed from two opposite gametes.

Any part of an individual or a whole organism, which is determined by a large number of components, is a complex, highly developed structure. In the subkingdom of multicellular organisms, the classification clearly separates the functions in which each of the individual particles performs its task. They engage in vital processes, thereby supporting the existence of the entire organism.

The subkingdom Multicellular in Latin sounds like Metazoa. To form a complex organism, cells must be identified and joined to others. Only a dozen protozoa can be seen individually with the naked eye. The remaining nearly two million visible individuals are multicellular.

Pluricellular animals are created by the union of individuals through the formation of colonies, filaments, or aggregation. Pluricellular organisms developed independently, like Volvox and some flagellated green algae.

A sign of the subkingdom metazoans, that is, its early primitive species, was the absence of bones, shells and other hard parts of the body. Therefore, no traces of them have survived to this day. The exception is sponges, which still live in the seas and oceans. Perhaps their remains are found in some ancient rocks, such as Grypania spiralis, whose fossils were found in the oldest layers of black shale dating back to the early Proterozoic era.

In the table below, the multicellular subkingdom is presented in all its diversity.

Complex relationships arose as a result of the evolution of protozoa and the emergence of the ability of cells to divide into groups and organize tissues and organs. There are many theories explaining the mechanisms by which single-celled organisms may have evolved.

Theories of origin

Today, there are three main theories of the origin of the multicellular subkingdom. A brief summary of the syncytial theory, without going into details, can be described in a few words. Its essence is that a primitive organism, which had several nuclei in its cells, could eventually separate each of them with an internal membrane. For example, several nuclei contain mold fungus, as well as slipper ciliates, which confirm this theory. However, having several nuclei is not enough for science. To confirm the theory of their multiplicity, it is necessary to demonstrate the transformation of the simplest eukaryote into a well-developed animal.

Colony theory says that symbiosis, consisting of different organisms of the same species, led to their change and the emergence of more advanced creatures. Haeckel was the first scientist to introduce this theory in 1874. The complexity of the organization arises because cells stay together rather than separate as they divide. Examples of this theory can be seen in such protozoan multicellular organisms as green algae called Eudorina or Volvaxa. They form colonies of up to 50,000 cells, depending on the species.

Colony theory proposes the fusion of different organisms of the same species. The advantage of this theory is that during times of food shortage, amoebas have been observed to group into a colony, which moves as one unit to a new location. Some of these amoebas are slightly different from each other.

However, the problem with this theory is that it is unknown how the DNA of different individuals can be included in a single genome.

For example, mitochondria and chloroplasts can be endosymbionts (organisms within a body). This happens extremely rarely, and even then the genomes of endosymbionts retain differences among themselves. They separately synchronize their DNA during mitosis of host species.

The two or three symbiotic individuals that make up a lichen, although dependent on each other for survival, must reproduce separately and then recombine, again creating a single organism.

Other theories that also consider the emergence of the metazoan subkingdom:

• GK-PID theory. About 800 million years ago, a small genetic change in a single molecule called GK-PID may have allowed individuals to move from a single cell to a more complex structure.
• The role of viruses. It has recently been recognized that genes borrowed from viruses play a crucial role in the division of tissues, organs, and even in sexual reproduction, during the fusion of egg and sperm. The first protein, syncytin-1, was found to be transmitted from a virus to humans. It is found in the intercellular membranes that separate the placenta and brain. A second protein was identified in 2007 and named EFF1. It helps form the skin of nematode roundworms and is part of the entire FF family of proteins. Dr. Felix Rey at the Pasteur Institute in Paris built a 3D model of the EFF1 structure and showed that it is what binds the particles together. This experience confirms the fact that all known fusions of tiny particles into molecules are of viral origin. This also suggests that viruses were vital for the communication of internal structures, and without them the emergence of colonies in the subkingdom of multicellular sponges would have been impossible.

All these theories, as well as many others that famous scientists continue to propose, are very interesting. However, none of them can clearly and unambiguously answer the question: how could such a huge variety of species arise from a single cell that originated on Earth? Or: why did single individuals decide to unite and begin to exist together?

Maybe in a few years, new discoveries will be able to give us answers to each of these questions.

Organs and tissues

Complex organisms have biological functions such as defense, circulation, digestion, respiration, and sexual reproduction. They are performed by specific organs such as the skin, heart, stomach, lungs and reproductive system. They are made up of many different types of cells that work together to perform specific tasks.

For example, heart muscle has a large number of mitochondria. They produce adenosine triphosphate, which keeps blood moving continuously through the circulatory system. Skin cells, on the contrary, have fewer mitochondria. Instead, they have dense proteins and produce keratin, which protects the soft internal tissues from damage and external factors.

Reproduction

While all simple organisms, without exception, reproduce asexually, many of the subkingdom metazoans prefer sexual reproduction. Humans, for example, are highly complex structures created by the fusion of two single cells called an egg and a sperm. The fusion of one egg with a gamete (gametes are special sex cells containing one set of chromosomes) of a sperm leads to the formation of a zygote.

The zygote contains the genetic material of both the sperm and the egg. Its division leads to the development of a completely new, separate organism. During development and division, cells, according to the program laid down in the genes, begin to differentiate into groups. This will further allow them to perform completely different functions, despite the fact that they are genetically identical to each other.

Thus, all the organs and tissues of the body that form nerves, bones, muscles, tendons, blood - they all arose from one zygote, which appeared due to the fusion of two single gametes.

Multicellular advantage

There are several main advantages of the sub-kingdom of multicellular organisms, due to which they dominate our planet.

As the complex internal structure allows for increased size, it also helps develop higher order structures and tissues with multiple functions.

Larger organisms have better protection from predators. They also have greater mobility, which allows them to migrate to more favorable places to live.

There is another undeniable advantage of the multicellular subkingdom. A common characteristic of all its species is a fairly long life expectancy. The cell body is exposed to the environment from all sides, and any damage to it can lead to the death of the individual. A multicellular organism will continue to exist even if one cell dies or is damaged. DNA duplication is also an advantage. The division of particles inside the body allows damaged tissues to grow and repair faster.

During its division, a new cell copies the old one, which allows it to preserve favorable features in subsequent generations, as well as improve them over time. In other words, duplication allows for the retention and adaptation of traits that will improve the survival or fitness of an organism, especially in the animal kingdom, a subkingdom of metazoans.

Disadvantages of multicellular

Complex organisms also have disadvantages. For example, they are susceptible to various diseases arising from their complex biological composition and functions. Protozoa, on the contrary, lack developed organ systems. This means that their risks of dangerous diseases are minimized.

It is important to note that, unlike multicellular organisms, primitive individuals have the ability to reproduce asexually. This helps them not waste resources and energy on finding a partner and sexual activity.

Protozoa also have the ability to take in energy by diffusion or osmosis. This frees them from the need to move around to find food. Almost anything can be a potential food source for a single-celled creature.

Vertebrates and invertebrates

The classification divides all multicellular creatures without exception into the subkingdom into two species: vertebrates (chordates) and invertebrates.

Invertebrates do not have a hard frame, while chordates have a well-developed internal skeleton of cartilage, bones and a highly developed brain, which is protected by the skull. Vertebrates have well-developed sensory organs, a respiratory system with gills or lungs, and a developed nervous system, which further distinguishes them from their more primitive counterparts.

Both types of animals live in different habitats, but chordates, thanks to their developed nervous system, can adapt to land, sea and air. However, invertebrates also occur in a wide range, from forests and deserts to caves and the mud of the seafloor.

To date, almost two million species of the subkingdom of multicellular invertebrates have been identified. These two million make up about 98% of all living beings, that is, 98 out of 100 species of organisms living in the world are invertebrates. Humans belong to the chordate family.

Vertebrates are divided into fish, amphibians, reptiles, birds and mammals. Animals without a backbone include phyla such as arthropods, echinoderms, worms, coelenterates and molluscs.

One of the biggest differences between these species is their size. Invertebrates, such as insects or coelenterates, are small and slow because they cannot develop large bodies and strong muscles. There are a few exceptions, such as the squid, which can reach 15 meters in length. Vertebrates have a universal support system, and therefore can develop faster and become larger than invertebrates.

Chordates also have a highly developed nervous system. With the help of specialized connections between nerve fibers, they can respond very quickly to changes in the environment, which gives them a distinct advantage.

Compared to vertebrates, most spineless animals use a simple nervous system and behave almost entirely instinctively. Such a system works well most of the time, although these creatures are often unable to learn from their mistakes. The exceptions are octopuses and their close relatives, which are considered among the most intelligent animals in the invertebrate world.

All chordates, as we know, have a backbone. However, a feature of the subkingdom of multicellular invertebrate animals is their similarity to their relatives. It lies in the fact that at a certain stage of life, vertebrates also have a flexible supporting rod, a notochord, which subsequently becomes the spine. The first life developed as single cells in water. Invertebrates were the initial link in the evolution of other organisms. Their gradual changes led to the emergence of complex creatures with well-developed skeletons.

Coelenterates

Today there are about eleven thousand species of coelenterates. These are some of the oldest complex animals to appear on earth. The smallest of the coelenterates cannot be seen without a microscope, and the largest known jellyfish is 2.5 meters in diameter.

So, let's take a closer look at the subkingdom of multicellular organisms, such as the coelenterates. The description of the main characteristics of habitats can be determined by the presence of an aquatic or marine environment. They live alone or in colonies that can move freely or live in one place.

The body shape of coelenterates is called a “bag”. The mouth connects to a blind sac called the gastrovascular cavity. This sac functions in the process of digestion, gas exchange and acts as a hydrostatic skeleton. The single opening serves as both the mouth and anus. Tentacles are long, hollow structures used to move and capture food. All coelenterates have tentacles covered with suckers. They are equipped with special cells - nemocysts, which can inject toxins into their prey. The suction cups also allow them to capture large prey, which the animals place in their mouths by retracting their tentacles. Nematocysts are responsible for the burns that some jellyfish cause to humans.

Animals of the subkingdom are multicellular, such as coelenterates, and have both intracellular and extracellular digestion. Respiration occurs by simple diffusion. They have a network of nerves that spread throughout the body.

Many forms exhibit polymorphism, which is a variety of genes in which different types of creatures are present in the colony for different functions. These individuals are called zooids. Reproduction can be called random (external budding) or sexual (formation of gametes).

Jellyfish, for example, produce eggs and sperm and then release them into the water. When the egg is fertilized, it develops into a free-swimming, ciliated larva called a planla.

Typical examples of the subkingdom Multicellular coelenterates are hydra, obelia, Portuguese man-of-war, sailfish, aurelia jellyfish, cabbage jellyfish, sea anemones, corals, sea pens, gorgonians, etc.

Plants

In the subkingdom Multicellular plants are eukaryotic organisms that are able to feed themselves through the process of photosynthesis. Algae were originally considered plants, but they are now classified as protists, a special group that is excluded from all known species. The modern definition of plants refers to organisms that live primarily on land (and sometimes in water).

Another distinctive feature of plants is the green pigment - chlorophyll. It is used to absorb solar energy during the process of photosynthesis.

Every plant has haploid and diploid phases that characterize its life cycle. It is called alternation of generations because all phases in it are multicellular.