Organ level. Multicellular organism - an integrated system

All living organisms are conventionally divided into two groups - unicellular and multicellular. Man is multicellular. However, a person contains a couple of kilograms of microorganisms, so it is impossible to call a person simply multicellular, but rather a symbiosis of a multicellular organism and unicellular organisms!
I decided to start my story about man from the smallest thing - with a living cell.

I’m sitting here, looking at this picture and realizing that even in biology and medicine there are nothing but myths, simplified ideas, diagrams, pictures... which do not correspond to reality at all, but which form our worldview, our “understanding” of the world order, completely false, very far from reality.
What you see in the picture is just a very simplified diagram, well, a very simplified diagram!!! Is it possible to feel the scale of the city from a map of the Moscow metro? Get an idea of what kind of city this is, how it is structured? No, of course, the most important thing is lost - the feeling of a huge metropolis. A living cell, compared with its structural subdivisions, correlates in exactly the same way as, for example, the size of the Moscow Kremlin (cell core) with the rest of the city. Our ideas about a living cell are constructed in much the same way as if we look at Moscow from a satellite. With the advent of modern research methods, the detail of studying a cell can already be compared to good aerial photography!
Here are real photos of living cells...

The resolution is about the same...

Why do I compare a cell with a city, but because only a city can compare in complexity and versatility with a living cell.
The cell has a core like a CITY in a city - a think tank, control and documentation of everything that happens - DNA molecules in which production and self-reproduction technologies are written! Yes, a cell lives for a reason, it definitely does something, performs some general task!
I'll make a lyrical digression...
Single-celled microorganisms can very conditionally be considered as such; in fact, they are like a school of fish that obeys general laws and acts as a single whole. Microbes unite in communities with other microbes, putting their properties into new, common ones, and the actions of the cells are subordinated to some common task, most often survival.
In a person, all cells are united into a single organism - a person, therefore the cells are specialized, that is, they have different tasks and very often the same cell performs several different tasks! That’s why I compare a cell to a city in which there are different plants and factories. The cell does something for internal consumption to support itself, but basically the cell produces something for the benefit of the body as a whole.
Resources are constantly coming into the cell and production products and waste are being taken out, just like trains, cars and other vehicles, everything is checked at the entrance, controlled much more seriously than at our airports! The cell membrane is responsible for all this.
This is a schematic representation of a cell membrane with transport tubules and is actually just a guess and is simplified.

This is what a section of a cell looks like that is in contact with another cell... the thick wall is a cell membrane repeatedly folded like an accordion... the black dots are most likely finished products in “warehouses”

Orders are constantly received through the cell membrane that regulate the work of the cell; these are different orders, ranging from the simple one - “give more coal” to changing products and moving to a new quality!
And of course, the membrane is protection from the external environment, which outside the cell can be very aggressive - for example, if you remember the sensations in the mouth during vomiting... then these are the contents of the stomach with which the cells of the stomach wall come into contact and are not digested, kebab which you washed down with wine is digested, and the cells work in this environment!
But a cell is not a dumb worker, cells also send signals - reports on the work done, send requests for resources, report damage, coordinate common actions... how this happens is not completely known to science.
The cell itself does not hang in the air and everything inside it is filled with liquid, but in fact not just water, but a clearly structured solution in which the molecules are arranged in a certain order and it is the change in the position of the molecules in space that has a semantic load, we do not fully know how it is what happens, how many substances are transported inside cells, what currents roam there and how it all moves, but it’s all in motion!
Probably, if it were possible to look into a living cell, as cosmonauts look through their superpowers and see a newspaper in the hands of a person, then the picture would appear no less complex and interesting - everyone is in a hurry somewhere, cars, people enter, exit into houses, what that's what they do there.
In fact, it’s still not possible to look at living cells in such a resolution... the photos that I showed are a section! The cells are frozen in an array, and then an ultra-thin section is made and examined under. Well, it’s like filling a city with liquid nitrogen, then using a big saw to cut it off as necessary and try to understand how, for example, doctors live in this city or subway drivers, who may not even get into this cut! :::=)))
Well, in conclusion, I would like you to try to imagine how a person is made up of these cells! Can you imagine the distances on a cell scale, for example on the villi of the stomach and bone tissue cells in the right toe of the left foot??? This is probably further than from Earth to Proxima Centauri!
But all this is interconnected and governed by the same laws! Moreover, on a time scale it’s almost forever!!!
That's it. It is very difficult to write in simple words about an unimaginably complex system - MAN! The whole universe!

The body of multicellular animals consists of a large number of cells, varied in structure and function, which have lost their independence, since they constitute a single, integral organism.

Multicellular organisms can be divided into two large groups. Invertebrate animals are two-layer animals with radial symmetry, the body of which is formed by two tissues: the ectoderm, which covers the body from the outside, and the endoderm, which forms the internal organs - sponges and coelenterates. It also includes flat, round, annelids, arthropods, mollusks and echinoderms, bilaterally symmetrical and radial three-layered organisms, which in addition to ecto- and endoderm also have mesoderm, which in the process of individual development gives rise to muscle and connective tissues. The second group includes all animals that have an axial skeleton: notochord or vertebral column.

Multicellular animals

Coelenterates. Freshwater hydra.

Structure – Radial symmetry, ectoderm, endoderm, sole, tentacles.
Movement – Contraction of skin-muscle cells, attachment of the sole to the substrate.
Nutrition - Tentacles, mouth, intestines, cavity with digestive cells. Predator. Kills stinging cells with poison.
Breathing – Oxygen dissolved in water penetrates the entire surface of the body.
Reproduction - Hermaphrodites. Sexual: egg cells + sperm = egg. Asexual: budding.
Circulatory system - No.
Elimination - Food remains are removed through the mouth.
Nervous system – Nerve plexus of nerve cells.

Flatworms. White planaria.

Roundworms. Human roundworm.

Annelids. Earthworm.

Structure – Elongated worm-shaped mucous skin on the outside, a dissected body cavity inside, length 10–16 cm, 100–180 segments.
Movement – Contraction of the skin-muscle sac, mucus, elastic bristles.
Nutrition – Mouth pharynx esophagus crop stomach intestine anus. It feeds on particles of fresh or decaying plants.
Respiration – Diffusion of oxygen across the entire surface of the body.
Reproduction - Hermaphrodites. Exchange of sperm mucus with eggs cocoon of young worms.
Circulatory system – Closed circulatory system: capillaries, annular vessels, main vessels: dorsal and abdominal.
Excretion – Body cavity metanephridia (funnel with cilia) tubules excretory pair.
Nervous system – Nerves, ganglia, nerve chain, peripharyngeal ring. Sensitive cells in the skin.

Soft-bodied. Shellfish. Common pondweed.

Structure – Soft body enclosed in a helical shell = torso + leg.
Movement – Muscular leg.
Nutrition – Mouth, pharynx, tongue with teeth = grater, stomach, intestines, liver, anus.
Breathing - Breathing hole. Lung.
Reproduction - Hermaphrodites. Cross fertilization.
The circulatory system is not closed. Lung heart vessels body cavity.
Excretion – Kidney.
Nervous system – Peripharyngeal cluster of nerve nodes.

Arthropods. Crustaceans. Crayfish.

Structure – + belly.
Movement – Four pairs of walking legs, 5 pairs of ventral legs + caudal fin for swimming.
Nutrition - jaw mouth, pharynx, esophagus, stomach, section with chitinous teeth, filtering apparatus, intestines, food. gland - anus.
Breathing - gills.
Reproduction – Dioecious. Eggs on abdomen legs before hatching. During growth, chitin shedding is characteristic. There is a nauplius larval stage.
Circulatory system – Unclosed. Heart – blood vessels – body cavity.
Excretion - Glands with an excretory canal at the base of the antennae.
Nervous system – Periopharyngeal ring = suprapharyngeal and subpharyngeal node, ventral nerve cord. The organ of touch and smell is the base of the short antennae. The organs of vision are two compound eyes.

Arthropods. Arachnids. Cross spider.

Structure – Cephalothorax + abdomen.
Movement - Four pairs of legs, 3 pairs of arachnoid warts on the belly, arachnoid glands for weaving a fishing net.
Nutrition – Mouth = jaws with venom and claws. Poison is pre-digestion outside the body. Esophagus – stomach, intestines, anus.
Respiration - In the abdomen there are a pair of pulmonary sacs with folds. Two bundles of trachea respiratory openings.
Reproduction – Dioecious. Eggs in a cocoon - young spiders
Circulatory system – Unclosed. Heart – blood vessels – body cavity
Excretion – Malpischian vessels
Nervous system – Pairs of ganglia + ventral chain. The organs of vision are simple eyes.

Arthropods. Insects. Chafer.

Structure – Head + chest + abdomen (8 segments)
Movement – 3 pairs of legs with hard claws, a pair of wings, a pair of elytra
Nutrition – Mouth = upper lip + 4 jaws + lower lip esophagus, stomach with chitinous teeth, intestines, anus
Breathing – Spiracles on the abdominal segments of the trachea, all organs and tissues
Reproduction – Females: ovaries, oviducts, spermatic receptacles.
Males: 2 testes, vas deferens, canal, complete metamorphosis.
The circulatory system is not closed. Heart with valves, vessels, body cavity.
Excretion – Malpish vessels in the body cavity, fat body.
Nervous system – Circumpharyngeal ring + ventral chain. Brain. 2 compound eyes, olfactory organs - 2 antennae with plates at the end.

Echinoderms.

Structure – Star-shaped, spherical or human-shaped body shape. Underdeveloped skeleton. Two layers of integument - the outer one is single-layer, the inner one is fibrous connective tissue with elements of a calcareous skeleton.
Movement – Move slowly with the help of limbs, muscles are developed.
Nutrition - Mouth opening, short esophagus, intestine, anus.
Respiration - Skin gills, body coverings with the participation of the water-vascular system.
Reproduction – Two ring vessels. One surrounds the mouth, the other the anus. There are radial vessels.
Circulatory system – No special ones. Excretion occurs through the walls of the canals of the water-vascular system.
Discretion – The genital organs have different structures. Most echinoderms are dioecious, but some are hermaphrodites. Development occurs through a series of complex transformations. The larvae swim in the water column; during metamorphosis, the animals acquire radial symmetry.
Nervous system - The nervous system has a radial structure: radial nerve cords extend from the peripharyngeal nerve ring according to the number of people in the body.

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 the 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.

Origin

Multicellular animals may have appeared on Earth 2.1 billion years ago, shortly after the "oxygen revolution". Multicellular animals are a monophyletic group. 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.

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. In vegetative propagation, when an organism develops from a multicellular fragment of the mother organism, natural cloning usually 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

Artificial multicellular organisms

Currently, there is no information about the creation of truly multicellular artificial organisms, but experiments are being conducted to create artificial colonies of unicellular ones.

In 2009, Ravil Fakhrullin from Kazan (Volga Region) State University (Tatarstan, Russia) and Vesselin Paunov from the University of Hull (Yorkshire, UK) obtained new biological structures called “cellosomes” (eng. cellosome) and were artificially created colonies of single-celled 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, yeast cells remained active for two weeks at 4 °C.

In 2010, the same researchers, in collaboration with the University of North Carolina, announced the creation of a new artificial colonial organism called "yeastsome". yeastosome). Organisms were obtained by self-assembly on air bubbles that served as a template.

Notes

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 have dominated as a form of life 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.

Large 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 within the body allows damaged tissue to grow and repair faster.

During its division, a new cell copies the old one, which makes it possible 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.

The alternating generations are the sporophyte generation and the gametophyte generation. During the gametophyte phase, gametes are formed. The haploid gametes fuse to form a zygote, called a diploid cell because it has a complete set of chromosomes. From there, diploid individuals of the sporophyte generation grow.

Sporophytes go through a phase of meiosis (division) and form haploid spores.

Life on Earth appeared billions of years ago, and since then living organisms have become increasingly more complex and diverse. There is ample evidence that all life on our planet has a common origin. Although the mechanism of evolution is not yet fully understood by scientists, its very fact is beyond doubt. This post is about the path the development of life on Earth took from the simplest forms to humans, as our distant ancestors were many millions of years ago. So, from whom did man come?

The Earth arose 4.6 billion years ago from a cloud of gas and dust surrounding the Sun. In the initial period of the existence of our planet, the conditions on it were not very comfortable - there was still a lot of debris flying in the surrounding outer space, which constantly bombarded the Earth. It is believed that 4.5 billion years ago the Earth collided with another planet, resulting in the formation of the Moon. Initially, the Moon was very close to the Earth, but gradually moved away. Due to frequent collisions at this time, the Earth's surface was in a molten state, had a very dense atmosphere, and surface temperatures exceeded 200°C. After some time, the surface hardened, the earth's crust formed, and the first continents and oceans appeared. The oldest rocks studied are 4 billion years old.

1) The most ancient ancestor. Archaea.

Life on Earth appeared, according to modern ideas, 3.8-4.1 billion years ago (the earliest found traces of bacteria are 3.5 billion years old). How exactly life arose on Earth has not yet been reliably established. But probably already 3.5 billion years ago, there was a single-celled organism that had all the features inherent in all modern living organisms and was a common ancestor for all of them. From this organism, all its descendants inherited structural features (they all consist of cells surrounded by a membrane), a method of storing the genetic code (in DNA molecules twisted in a double helix), a method of storing energy (in ATP molecules), etc. From this common ancestor There were three main groups of single-celled organisms that still exist today. First, bacteria and archaea divided among themselves, and then eukaryotes evolved from archaea - organisms whose cells have a nucleus.

Archaea have hardly changed over billions of years of evolution; the most ancient ancestors of humans probably looked about the same

Although archaea gave rise to evolution, many of them have survived to this day almost unchanged. And this is not surprising - since ancient times, archaea have retained the ability to survive in the most extreme conditions - in the absence of oxygen and sunlight, in aggressive - acidic, salty and alkaline environments, at high (some species feel great even in boiling water) and low temperatures, at high pressures, they are also capable of feeding on a wide variety of organic and inorganic substances. Their distant, highly organized descendants cannot boast of this at all.

2) Eukaryotes. Flagellates.

For a long time, extreme conditions on the planet prevented the development of complex life forms, and bacteria and archaea reigned supreme. About 3 billion years ago, cyanobacteria appeared on Earth. They begin to use the process of photosynthesis to absorb carbon from the atmosphere, releasing oxygen in the process. The released oxygen is first consumed by the oxidation of rocks and iron in the ocean, and then begins to accumulate in the atmosphere. 2.4 billion years ago, an “oxygen catastrophe” occurs - a sharp increase in the oxygen content in the Earth’s atmosphere. This leads to big changes. For many organisms, oxygen turns out to be harmful, and they die out, being replaced by those that, on the contrary, use oxygen for respiration. The composition of the atmosphere and climate are changing, becoming much colder due to a drop in greenhouse gases, but an ozone layer appears, protecting the Earth from harmful ultraviolet radiation.

About 1.7 billion years ago, eukaryotes evolved from archaea - single-celled organisms whose cells had a more complex structure. Their cells, in particular, contained a nucleus. However, the emerging eukaryotes had more than one predecessor. For example, mitochondria, essential components of the cells of all complex living organisms, evolved from free-living bacteria captured by ancient eukaryotes.

There are many varieties of single-celled eukaryotes. It is believed that all animals, and therefore humans, descended from single-celled organisms that learned to move using a flagellum located at the back of the cell. The flagella also help filter water in search of food.

Choanoflagellates under a microscope, as scientists believe, it was from such creatures that all animals once descended

Some species of flagellates live united in colonies; it is believed that the first multicellular animals once arose from such colonies of protozoan flagellates.

3) Development of multicellular organisms. Bilateria.

Approximately 1.2 billion years ago, the first multicellular organisms appeared. But evolution is still progressing slowly, and in addition, the development of life is being hampered. Thus, 850 million years ago, global glaciation began. The planet is covered with ice and snow for more than 200 million years.

The exact details of the evolution of multicellular organisms are unfortunately unknown. But it is known that after some time the first multicellular animals divided into groups. Sponges and lamellar sponges that have survived to this day without any special changes do not have separate organs and tissues and filter nutrients from the water. The coelenterates are not much more complex, having only one cavity and a primitive nervous system. All other more developed animals, from worms to mammals, belong to the group of bilateria, and their distinguishing feature is the bilateral symmetry of the body. It is not known for certain when the first bilateria appeared; it probably happened shortly after the end of global glaciation. The formation of bilateral symmetry and the appearance of the first groups of bilateral animals probably occurred between 620 and 545 million years ago. Findings of fossil prints of the first bilateria date back to 558 million years ago.

Kimberella (imprint, appearance) - one of the first discovered species of Bilateria

Soon after their emergence, bilateria are divided into protostomes and deuterostomes. Almost all invertebrate animals descend from protostomes - worms, mollusks, arthropods, etc. The evolution of deuterostomes leads to the appearance of echinoderms (such as sea urchins and stars), hemichordates and chordates (which includes humans).

Recently, the remains of creatures called Saccorhytus coronarius. They lived approximately 540 million years ago. By all indications, this small (only about 1 mm in size) creature was the ancestor of all deuterostome animals, and therefore of humans.

Saccorhytus coronarius

4) The appearance of chordates. The first fish.

540 million years ago, the “Cambrian explosion” occurs - in a very short period of time, a huge number of different species of marine animals appear. The fauna of this period has been well studied thanks to the Burgess Shale in Canada, where the remains of a huge number of organisms from this period have been preserved.

Some of the Cambrian animals whose remains were found in the Burgess Shale

Many amazing animals, unfortunately long extinct, were found in the shale. But one of the most interesting finds was the discovery of the remains of a small animal called pikaia. This animal is the earliest found representative of the chordate phylum.

Pikaya (remains, drawing)

Pikaia had gills, a simple intestine and circulatory system, as well as small tentacles near the mouth. This small animal, about 4 cm in size, resembles modern lancelets.

It didn't take long for the fish to appear. The first animal found that can be classified as a fish is considered to be the Haikouichthys. He was even smaller than Pikaiya (only 2.5 cm), but he already had eyes and a brain.

This is what Haykowihthys looked like

Pikaia and Haikouihthys appeared between 540 and 530 million years ago.

Following them, many larger fish soon appeared in the seas.

First fossil fish

5) Evolution of fish. Armored and early bony fishes.

The evolution of fish lasted quite a long time, and at first they were not at all the dominant group of living creatures in the seas, as they are today. On the contrary, they had to escape from such large predators as crustaceans. Fish appeared in which the head and part of the body were protected by a shell (it is believed that the skull subsequently developed from such a shell).

The first fish were jawless; they probably fed on small organisms and organic debris, sucking in and filtering water. Only about 430 million years ago the first fish with jaws appeared - placoderms, or armored fish. Their head and part of their torso were covered with a bone shell covered with skin.

Ancient shell fish

Some of the armored fish became large and began to lead a predatory lifestyle, but a further step in evolution was made thanks to the appearance of bony fish. Presumably, the common ancestor of the cartilaginous and bony fish that inhabit modern seas originated from armored fish, and the armored fish themselves, the acanthodes that appeared around the same time, as well as almost all jawless fish subsequently became extinct.

Entelognathus primordialis - a probable intermediate form between armored and bony fishes, lived 419 million years ago

The very first discovered bony fish, and therefore the ancestor of all land vertebrates, including humans, is considered to be Guiyu Oneiros, who lived 415 million years ago. Compared to predatory armored fish, which reached a length of 10 m, this fish was small - only 33 cm.

Guiyu Oneiros

6) The fish come to land.

While fish continued to evolve in the sea, plants and animals of other classes had already reached land (traces of the presence of lichens and arthropods on it were discovered as early as 480 million years ago). But in the end, fish also began to develop land. From the first bony fishes two classes arose - ray-finned and lobe-finned. The majority of modern fish are ray-finned, and they are perfectly adapted for life in water. Lobe-finned fish, on the contrary, have adapted to life in shallow waters and small freshwater bodies, as a result of which their fins have lengthened and their swim bladder has gradually turned into primitive lungs. As a result, these fish learned to breathe air and crawl on land.

Eusthenopteron ( ) is one of the fossil lobe-finned fishes, which is considered the ancestor of land vertebrates. These fish lived 385 million years ago and reached a length of 1.8 m.

Eusthenopteron (reconstruction)

- another lobe-finned fish, which is considered a likely intermediate form of the evolution of fish into amphibians. She could already breathe with her lungs and crawl onto land.

Panderichthys (reconstruction)

Tiktaalik, whose remains were found dating back to 375 million years ago, was even closer to amphibians. He had ribs and lungs, he could turn his head separately from his body.

Tiktaalik (reconstruction)

One of the first animals that were no longer classified as fish, but as amphibians, were ichthyostegas. They lived about 365 million years ago. These small animals, about a meter long, although they already had paws instead of fins, still could hardly move on land and led a semi-aquatic lifestyle.

Ichthyostega (reconstruction)

At the time of the emergence of vertebrates on land, another mass extinction occurred - the Devonian. It began approximately 374 million years ago, and led to the extinction of almost all jawless fish, armored fish, many corals and other groups of living organisms. Nevertheless, the first amphibians survived, although it took them more than one million years to more or less adapt to life on land.

7) The first reptiles. Synapsids.

The Carboniferous period, which began approximately 360 million years ago and lasted 60 million years, was very favorable for amphibians. A significant part of the land was covered with swamps, the climate was warm and humid. Under such conditions, many amphibians continued to live in or near water. But approximately 340-330 million years ago, some of the amphibians decided to explore drier places. They developed stronger limbs, more developed lungs, and their skin, on the contrary, became dry so as not to lose moisture. But in order to live far from water for a really long time, another important change was needed, because amphibians, like fish, spawned, and their offspring had to develop in an aquatic environment. And about 330 million years ago, the first amniotes appeared, that is, animals capable of laying eggs. The shell of the first eggs was still soft and not hard, however, they could already be laid on land, which means that the offspring could already appear outside the reservoir, bypassing the tadpole stage.

Scientists are still confused about the classification of amphibians from the Carboniferous period, and whether some fossil species should be considered early reptiles or still amphibians that acquired only some reptilian features. One way or another, these either the first reptiles or reptilian amphibians looked something like this:

Westlotiana is a small animal about 20 cm long, combining the features of reptiles and amphibians. Lived approximately 338 million years ago.

And then the early reptiles split, giving rise to three large groups of animals. Paleontologists distinguish these groups by the structure of the skull - by the number of holes through which muscles can pass. In the picture from top to bottom there are skulls anapsid, synapsid And diapsid:

At the same time, anapsids and diapsids are often combined into a group sauropsids. It would seem that the difference is completely insignificant, however, the further evolution of these groups took completely different paths.

Sauropsids gave rise to more advanced reptiles, including dinosaurs, and then birds. Synapsids gave rise to a branch of animal-like lizards, and then to mammals.

300 million years ago the Permian period began. The climate became drier and colder, and early synapsids began to dominate on land - pelycosaurs. One of the pelycosaurs was Dimetrodon, which was up to 4 meters long. He had a large “sail” on his back, which helped regulate body temperature: to quickly cool down when overheated or, conversely, to quickly warm up by exposing his back to the sun.

The huge Dimetrodon is believed to be the ancestor of all mammals, and therefore of humans.

8) Cynodonts. The first mammals.

In the middle of the Permian period, therapsids evolved from pelycosaurs, more similar to animals than to lizards. Therapsids looked something like this:

A typical therapsid of the Permian period

During the Permian period, many species of therapsids, large and small, arose. But 250 million years ago a powerful cataclysm occurs. Due to a sharp increase in volcanic activity, the temperature rises, the climate becomes very dry and hot, large areas of land are filled with lava, and the atmosphere is filled with harmful volcanic gases. The Great Permian Extinction occurs, the largest mass extinction of species in the history of the Earth, up to 95% of marine and about 70% of land species become extinct. Of all the therapsids, only one group survives - cynodonts.

Cynodonts were predominantly small animals, from a few centimeters to 1-2 meters. Among them were both predators and herbivores.

Cynognathus is a species of predatory cynodont that lived about 240 million years ago. It was about 1.2 meters long, one of the possible ancestors of mammals.

However, after the climate improved, the cynodonts were not destined to take over the planet. Diapsids seized the initiative - dinosaurs evolved from small reptiles, which soon occupied most of the ecological niches. The cynodonts could not compete with them, they crushed them, they had to hide in holes and wait. It took a long time to get revenge.

However, the cynodonts survived as best they could and continued to evolve, becoming more and more similar to mammals:

Evolution of cynodonts

Finally, the first mammals evolved from cynodonts. They were small and presumably nocturnal. A dangerous existence among a large number of predators contributed to the strong development of all senses.

Megazostrodon is considered one of the first true mammals.

Megazostrodon lived approximately 200 million years ago. Its length was only about 10 cm. Megazostrodon fed on insects, worms and other small animals. Probably he or another similar animal was the ancestor of all modern mammals.

We will consider further evolution - from the first mammals to humans - in.