The first animals to land. First land animals

At the beginning of the next, Silurian period (or Silurian ) the seas and continents retained approximately the same outlines as in the Cambrian. marine fauna Silurian resembles the Cambrian, but appear and new groups of invertebrates - corals, graptolites, worms, bryozoans, sea ​​urchins.

Fauna and flora of the late Paleozoic (click to enlarge)

Corals belong to the type of so-called intestinal animals - exclusively aquatic organisms. In addition to corals, the well-known jellyfish and hydra also belong to the intestinal cavities. Corals still exist today; many of them are reef-formers in the tropical Pacific and Indian Oceans. Corals are arranged very simply. Like other coelenterates, their body has only one internal cavity representing the intestines (which is why they are called coelenterates). Externally, the body of a coral, or rather, a coral polyp, is a pouch that opens outwards (above) with a mouth opening around which there is a rim of tentacles that help capture prey. Coral polyps feed on small floating organisms - plankton. Waste products are also ejected through the mouth opening. The body of a coral polyp is enclosed in a skeleton - a calcareous chamber secreted by the walls of the polyp. As the chamber is built up, the polyp itself rises higher and higher, the lower wall of which (the bottom of the sac) deposits horizontal partitions called bottoms.

Coral polyps can live alone (solitary corals) or in groups (colonial corals). Solitary corals reach sizes of 15-20 cm. Like colonial corals, they still grow to the bottom. All corals are inhabitants of the sea. They live in warm clear water, rich in oxygen and well lit, that is, no deeper than 45 m.

Peculiar animals - graptolites . They are known from the Silurian deposits - the so-called graptolitic shales, common in our country near Leningrad, in the Baltic states and in Central Asia, and in Western Europe in England, Germany and Sweden. Graptolites have the appearance of fan-shaped threads or twigs, on the sides of which are numerous tiny cells of polyps. Above, where the ends of the threads converged, during the life of the graptolites there was an air-bearing bell, the imprints of which have been preserved. Probably, graptolites were either passive swimming animals, or some of them crawled along the bottom. Graptolites are classified as hemichordates.

Bryozoans, as the name suggests, are more like plants (mosses) than animals. Bryozoans form colonies that look like crusts and raids on pitfalls or twigs that look like corals. Like coral polyps, each bryozoan sits in a separate cell, but bryozoans are more highly organized animals than corals. Their gastrointestinal tract has not only an inlet, but also an outlet; in addition, they already have a real nervous system(and in corals - only individual nerve cells).

The mouth opening of the bryozoan, like that of corals, is surrounded by a corolla of tentacles, the movement of which drives food into the mouth - unicellular algae and unicellular animals. Interestingly, some individuals of bryozoans have the appearance of flagella, continuously vibrating, or bird heads that constantly flap their “beak”. This is a “guard” that drives away enemies of bryozoans, and at the same time silt cleaners. Bryozoans have never been a particularly numerous group, but some of their detachments have survived to the present day.

Sea urchins resemble with their needles real urchins - land mammals, but they have no relationship with them. The body of the sea urchin is enclosed in a spherical calcareous shell, consisting of many plates. These plates form fields, some of which bear needles, while others carry tiny holes. Hundreds of microscopic legs protrude through such holes in the form of soft tubes filled with water. Water is pumped into them through special channels inside the body of animals. With the help of its legs, the hedgehog slowly moves or sticks tightly to some underwater object. In the movement of the sea urchin, spines are also involved, which also serve for protection. Some sea urchins have reached the size of a child's head. Modern marine eyash are found in our northern and eastern seas. They feed on algae and tiny animals.
In the region of the current Scandinavian Peninsula, in Scotland and Ireland, on the site of Svalbard and along the eastern coast of Greenland - where the sea existed for many millions of years - high mountain ranges. Their remnants are the Scandinavian Mountains, the Grampian Mountains of Scotland, layers crumpled into folds along the eastern outskirts of Greenland and the island of Svalbard. In the second half of the Silurian, powerful mountain-building movements took place - the so-called Caledonian folding.

The mountainous land rose in the region of present-day Kazakhstan and the northern ranges of the Tien Shan, and the Sayano-Baikal mountain arc was formed.

Caledonian mountain building led to the rise of the continents and the gradual shallowing of the seas, the appearance of numerous small bays and lagoons. Some of them were desalinated by the rivers flowing into them, in others the salinity of the water increased and even the deposition of salts occurred.

Most marine animals do not tolerate changes in the salinity of sea water in either direction. Therefore, only a few of the inhabitants of the Silurian Sea have adapted to life in the lagoons.

The "crowding in living space" of the marine population served as an impetus for the development of land as a new additional area life. It was from the dying areas of the sea - lagoons - that the first plants began to land on land, and then animals that fed on these plants, and later predatory animals came to land.

In the Silurian land plants - psilophytes - have already been distributed; apparently, they originated from algae, most likely from green ones.

Their body, like algae, has not yet been dissected into the main organs - root, stem and leaves. Instead of roots, they had peculiar underground unicellular outgrowths - rhizoids. The most primitive of the psilophytes did not even have a stem that would carry true leaves. Psilophytes reproduced with the help of spores placed in sporangia - at the ends of branches. Some psilophytes were marsh plants, while others were real land inhabitants, sometimes reaching considerable sizes - 3 m in height. Psilophytes did not last long. They are known in the next period - the Devonian. Many paleobotanists attribute to them two more genera of modern tropical plants- psilots. In the Silurian, another group of plants is also common (also, apparently, descended from algae) - mushrooms, which, perhaps, were first aquatic forms, and then came to land. In the same period, more highly organized plants also existed - fern-like, in particular, primitive club mosses. Scorpions appear in the Silurian. These ancient scorpions, perhaps, were not yet terrestrial animals, but first inhabited various water bodies - rivers, lakes and swamps.

And another remarkable event occurred in the Silurian: the first vertebrates appeared - the so-called armored fish, the remains of which are found together with giant crustacean scorpions. Both those and others were inhabitants of lagoons - laced bays of the sea. Probably, armored fish, and after them their enemies - giant crustacean scorpions, climbed up the river deltas, gradually mastering fresh water.

Until now, there are two points of view on the question of where the first vertebrates appeared - in the seas or rivers. AT sea ​​water contains a lot of dissolved calcium, and calcium is part of the bones of animals; in addition, all lower vertebrates live in the sea. This is strong evidence for the marine origin of vertebrates. But supporters of the theory of freshwater origin believe that the skeleton must have appeared in rivers where there is a current: the skeleton is a stable support for the body, necessary to counteract the movement of water.

One thing is certain: the ancestors of vertebrates lived in a zone where fresh waters bordered on sea waters, and their remains are found there. The oldest vertebrates known to us already possessed bone tissue- a shell, while their internal skeleton was, apparently, cartilaginous (it is not preserved in a fossil state). Replacement of cartilage by bone, its ossification occurs much later - in higher groups of fish. The ancient armored fish were not, in essence, true fish yet, they only had a fish-like shape. This body shape - in the form of a torpedo - is generally characteristic of actively swimming aquatic animals, since it provides the least resistance when moving in water.

The ancient armored fishes belong to the group of so-called jawless fishes, which are contrasted with the jawless ones, which include other classes of vertebrates.

Armored jawless are known only from the Silurian and Devonian, but some jawless have survived to the present day; these are lampreys and hagfishes. All jawless, as their name indicates, lacked jaws, as well as paired limbs (fins), and usually had only one nostril. Ancient jawless animals, the remains of which are often found here in the Baltic, on the Yenisei and in the Kolyma basin, as well as in Northern Europe and North America, were quite large animals - half a meter or more in length. Their body in front or almost entirely (except for the tail) was enclosed in a shell, consisting of bone plates and scales. This armor protected them from dangerous pursuers - racoscorpions, reaching a length of 3 m.

The armored jawless ones fed on plankton. Probably some of the jawless were benthic forms. Picking their snout in the silt, they stirred it up and caught small organic remains.

Thus, the Silurian was not only the heyday of various groups of invertebrates, but also the time of the appearance of the first vertebrates. In the Silurian, the resettlement of terrestrial plants and the emergence of the first animals on land began.

The generally accepted story of the origin of life on Earth is outdated. Two scientists, Peter Ward and Joseph Kirschvink, offer a book that summarizes all the findings of recent research. The authors show that many of our previous ideas about the history of the origin of life are incorrect. First, the development of life was not a slow, gradual process: cataclysms contributed to the formation of life more than all other forces combined. Secondly, the basis of life is carbon, but what other elements determined its evolution? Third, since Darwin we have been thinking in terms of the evolution of species. In fact, the evolution of ecosystems has taken place - from underwater volcanoes to rainforest who have shaped the world as we know it. Drawing on their years of experience in paleontology, biology, chemistry, astrobiology, Ward and Kirschvink tell a story of life on Earth that is so fantastic that it is difficult to imagine, and at the same time, so familiar that it is impossible to pass by.

Book:

The first land animals

The first land animals

The main problem for any first land animal was an acute shortage of water. All living cells must have water, and the aquatic lifestyle easily provided this need. Living on land, however, requires a dense top coat to keep water inside the body. The difficulty is that solutions to reduce water loss to air are at odds with the needs of skin respiration. Here's a challenge for you: on the one hand, having an outer cover that retains water is an advantage, but at the same time there is a risk of dying from suffocation. An alternative would be a breathing system in which oxygen permeates through the outer covering, but there is an increased risk of moisture loss through the same system. This dilemma had to be resolved by all the pioneers of land. Obviously, the process was so difficult that only a very small number of groups of animals, plants and protozoa managed to do it. Some of the many and most common modern marine life, apparently, they could not conquer the land: there are no terrestrial sponges, cnidarians, brachiopods, bryozoans and echinoderms, and many others too.

The oldest land animal fossils are probably small arthropods resembling modern spiders, scorpions, mites, isopods, and primitive insects. It is not clear which of the listed groups of arthropods was the first, however, the primacy did not last long, since representatives of all these taxonomic groups are found in the fossil record.

The classification of these first terrestrial animals inevitably had to be based on fossils, which did not guarantee accuracy, since they were small terrestrial arthropods that have very weakly hardening exoskeletons and therefore are rarely preserved in sediments. By the end of the Silurian period or by the early Devonian, about 400 million years ago, however, the spread of plants on land allowed the avant-garde of the animal kingdom to emerge from the water. It is quite clear that, independently of each other, arthropods from various taxa acquired in the process of evolution respiratory systems capable of supporting life in the air.

The respiratory system of modern spiders and scorpions explains how they evolved from thriving sea creatures to equally thriving land dwellers. For such a step - from water to land - no other body system requires such important changes like breathing. It also seems quite clear that the lungs of the first terrestrial arthropods were a transitional link in evolution, almost as efficient as those of later species. But in an atmosphere with a lot of oxygen, it was possible to breathe with the whole body - the air penetrated the entire surface of these small land creatures (they were definitely very small), and oxygen freely entered their primitive lungs.

Of all the types of animals that moved to land first - this includes many groups of arthropods, molluscs, annelids, chordates (and with them very small creatures like nematodes), - arthropods were still the very first, since their bodies already had a dense outer covering that ensured the retention of water in the body. However, they still faced the problem of breathing. It has already been mentioned that the external skeleton of arthropods required the evolution of large gills on almost all segments of the body in order to ensure survival in the Cambrian (this is when most highly developed fossil arthropods appeared) with a low oxygen content in environment. But the gills do not function in the air. The first land arthropods - spiders and scorpions - developed the new kind respiratory system called "lung book" ( internal structure so light it resembles book pages).

This “book”, the “pages” of which are sheets of tissue filled with hemolymph (a fluid that plays the role of blood in arthropods), is inserted into the lung sac (atrium), communicating with the outside atmosphere through breathing holes in the shell. This is a passive lung, since there is no inflow of air inhaled through such a structure, therefore its work depends on a certain minimum of oxygen.

Some very small spiders are known to be able to be blown to great heights by the wind, which is why they are called aeroplankton. This fact proves that the book lungs of spiders are able to extract oxygen in an environment with a low oxygen content. However, the representatives of aeroplankton are so small that their respiratory needs can be satisfied by the passive penetration of gas into the body. More large spiders because of their light-books are very vulnerable.

More efficient than respiratory system insects, consisting of tracheae, possibly book gills. The respiratory system of insects is passive in the sense that there is no or very weak air pumping mechanism, although recent studies show that some pumping is still present, but with very little pressure. The lung-book system of arachnids has a much larger surface area than that of insects and can therefore function in low oxygen environments.

The time of the first stage of penetration of spiders and scorpions on land is very difficult to determine, since the ancient spiders and scorpions were very small and left almost no fossils. Modern scorpions are more hardened than spiders and are therefore more common in sediments.

The earliest evidence of land animals is from the late Silurian (fossils in Wales) - about 420 million years ago - this is almost the end of the Silurian period. At that time, oxygen levels reached the highest levels in the entire history of the Earth. Fossils from this period are few and show little diversity. However, they were recognized and classified as centipedes.

A much richer collection of fossils is represented by the famous Rhine trait in Scotland, dated 410 million years ago. This deposit contains fossils of very early plants, as well as small arthropods, most of which probably belong to modern mites and springtails - both groups feed on plant remains and therefore, most likely, were well adapted to life in the new land conditions where they reigned in mostly small and primitive plants. Ticks are related to spiders. Springtails, however, are insects and probably the first of this most numerous class of animals to date. It would be quite logical to assume that insects immediately developed such a huge variety of life forms on land. However, this is not the case, everything happened just the opposite.

Paleoentomologists have found that insects remained a small group of terrestrial fauna until the end of the early Carboniferous period, when oxygen levels reached modern levels - about 330 million years ago. Insect fossils become more abundant by the Late Carboniferous, about 310 million years ago. Insects began to fly much later than the moment of their birth - undoubted signs of flying insects can be found in sediments dating back 330 million years ago. Shortly after their first flight, insects made an incredible evolutionary leap, giving birth to many new species, mostly flying. This is a classic case of evolutionary radiation, when the rapid (on a geological scale) and massive increase in the taxonomic diversity of certain groups of organisms allows them to occupy new ecological niches. However, such radiation occurred during a period when very high level oxygen, and, undoubtedly, it was precisely this state of the atmosphere that ensured the success of these processes.

Insects, however, were not the first animals on land; the primacy, apparently, belongs to scorpions. In the middle of the Silurian period, about 430 million years ago, the first protoscorpions crawled out of freshwater swamps and lakes. They had gills adapted to life in the water, and they probably fed on the remains of dead animals, such as fish washed ashore by a wave. The gills remained moist, and their very big square surface provided some breathing. They definitely didn't have any lungs, only gills.

The order of appearance of animals on land can be represented as follows: scorpions - about 430 million years ago, but they were most likely strongly attached to water due to reproduction and, possibly, even breathing; centipedes - 420 million years ago; insects - 410 million years ago. However, the insects familiar to us appeared no earlier than 330 million years ago. How is this order related to changes in the level of oxygen in the atmosphere?

The latest methods for determining the level of oxygen in the atmosphere allow us to determine that the maximum level of oxygen in the atmosphere falls on a period of about 410 million years ago. Then a sharp decline followed, after which the rise began again - from very low rates (12%) at the end of the Devonian to the highest in the entire history of the planet in the Permian period (more than 30%). Today, we recall, the oxygen content in the atmosphere is 21%. The Rhine line, in which numerous accumulations of insects and arachnids were first discovered, belongs to the period of the oxygen maximum in the Devonian. Then, according to the reports of paleontologists studying the diversity of insects, the latter are rare in fossils. This situation persists up to a jump in the oxygen level of up to 20% in the interval between the Early and Late Carboniferous, 330–310 Ma ago, during the period of the spread of winged insects.

The distribution of vertebrates on land became possible due, most likely, to an increase in the oxygen content in the atmosphere in the Ordovician and Silurian periods. If not for this circumstance, perhaps the history of the development of animals on land, and the forms of land animals would be completely different. Or maybe there would be no land animals at all. We also know that immediately after leaving the water, surviving in conditions with a low oxygen content in the atmosphere, the animals were very few.

There are three possible explanations for the distribution of fossils observed in the rocks of those periods.

First, this apparent pause in the development of land animals did not actually exist; just a very poor fossil record from 400-370 million years ago.

Second: there really was a pause - there was little oxygen, and very few arthropods lived on land, especially insects. But the few that managed to survive were able to give rise to a huge variety of forms when, 30 million years later, oxygen levels rose.

Third, the first migrants from aquatic habitats to land were swept away by falling oxygen levels. True, in places some people survived. And already the second wave of land conquerors was a real swarm of migrants who took advantage of the increase in oxygen levels. The development of land by animals (arthropods and, as we will see, vertebrates) took place in two distinct stages: 430-410 million years ago, and then - 370 million years ago and later.

Arthropods were not the only ones to adapt to life on land. Gastropods also made an evolutionary rush to land, but not earlier than the Late Carboniferous, that is, they were part of the second stage of land development by animals, when the level of oxygen became high enough. Another group of animals - horseshoe crabs - arrived on land at about the same time as molluscs. However, these were all small colonists compared to the group that interests us the most - ours, that is, vertebrates.

But amphibians did not just jump out of the sea. They were the culmination of a very long evolutionary journey, and before they appear on land and in our narrative, imagine the Devonian period, which has long been called the age of fish. An example is our favorite place, the Devonian Canning Basin. (canning bass) in Western Australia, where the authors of this book have done extensive fieldwork. Canning Basin is one of the most beautiful (very hot!) places in the world, where the fossils of the barrier reef are best preserved - like a modern Great barrier reef suddenly turned into stone, and the water suddenly left. Although most of the writings on the Canning Basin are devoted to this giant reef of the Devonian period, nevertheless, the rocks formed in the deeper sea places of that period contain particularly impressive fossils that certainly deserve to be included in the pages of any new history of life on earth.

This article focuses on another type of animal that lives almost exclusively on land, in terrestrial habitats, and which we call terrestrial or terrestrial animals. As you will see below, "terrestrial" does not mean that the animal does not come into contact with water at all; it simply indicates that the aquatic environment cannot ensure the survival of the organism.

Terrestrial or land animals are those that live predominantly or entirely on land (for example, cats, ants, land snails) compared to aquatic animals that live predominantly or completely in water (for example, lobsters), and amphibians or semiaquatic, relying on combination of water and ground places habitats (eg frogs, newts or beavers, otters). Terrestrial examples include crickets, grasshoppers, snails, and slugs.

Arthropods (such as flies) are the most common land animals in terms of number of species.

Taxonomy

The exit of animals from the ocean to land is one of the most important events in the history of life on our planet. Land lines have developed from several types of animals, among which , and , represent the most successful groups of land animals.

Terrestrial animals do not form a single clade (they do not have a common ancestor); rather, they are separated only by the fact that they all live on land. The transition from aquatic to terrestrial life in the process occurred independently and successfully many times in different ways. Most terrestrial lines originated in or during the temperate period, while during the period some animals became completely terrestrial.

The labeling "terrestrial" or "aquatic" is often obscure and subject to controversy. Many animals considered terrestrial have a life cycle that depends in part on being in the water. , seals and walruses sleep on land and feed in, but they are all considered terrestrial. Many insects, such as mosquitoes, and all land crabs, as well as other types of animals, have an aquatic stage. life cycle: their eggs must develop and hatch in aquatic environment; after hatching, they are characterized by an early aquatic life phase (nymph or larva).

There are types of crabs that are both fully aquatic and semi-aquatic or terrestrial. beckoning crabs ( Uca) are called "semi-aquatic" because they make holes in the mud substrate, to which they retreat during high tides. When the tide is out, these crabs come to the beach to search for food. The same is true of mollusks: hundreds of genera and species of gastropods live in intermediate environments, for example, Truncatella. Some gastropods with gills live on land, while others with lungs prefer water.

As entirely terrestrial or aquatic animals, numerous borderline species also exist. There are no generally accepted criteria for determining where to place these species, so the designation of some animals is disputed.

Obvious examples

There are animals that, obviously, can be called terrestrial. It's not often that you see a swimming chicken or a flying pig. Most, including humans, horses, dogs, and cats (among many others), are terrestrial. All of them may temporarily move in the aquatic environment for various reasons such as feeding, migration or recreation, but their primary and most important places habitats are on dry land.

Small organisms like earthworms, crickets, ants and beetles are also land animals. In each terrestrial, from to, there lives an uncountable number of animals that are considered terrestrial. And with the exception of fish and frogs, almost all pets kept by humans are terrestrial. While they may enjoy swimming in the water, land animals such as dogs do not actually live in it.

Non-obvious examples

We have learned that terrestrial animals must live on solid ground to be considered terrestrial, but what about creatures like penguins, crabs, or snails, all of which spend part of their Everyday life in aquatic environments? Since their main habitat is on land, and their dependence on water is usually justified by feeding, they are all generally considered terrestrial creatures.

What about birds? Just as amphibians blur the line between aquatic and terrestrial animals, so it is with birds. They can carry out most their life on land, but at the same time, because they can fly and live in the crown of trees, they are considered a special group of land animals, known as "arboreal animals". Since they do not rest or nest while flying, they must have some sort of solid habitat, and it is directly or indirectly tied to the earth's surface.

First land animals

Fossil evidence shows that sea ​​creatures, probably related to arthropods, first began to come ashore about 530 million years ago. However, there is no reason to believe that animals first became full-fledged inhabitants of the land during the same period of time. A more plausible hypothesis is that the motivation of these early arthropods to land was to breed (as modern horseshoe crabs do) or lay their eggs out of the reach of predators.

Over time, evidence suggests that at about 375 million years ago, bony fish(for example, Tiktaalik ( Tiktaalik roseae)), most adapted to life in shallow coastal and swampy waters, were much more viable than amphibians and their arthropod predecessors. With relatively strong muscular limbs and lungs combined with gills, Tiktaaliks and animals like them laid the foundation for life on land by the end of the Devonian period. Thus, they are probably the last common ancestor of all modern .

Summarizing

Since the biological community has not developed an officially recognized classification system for terrestrial, aquatic, or semi-aquatic organisms (as for families, genera, species, etc.), there is room for debate as to whether a particular animal belongs to terrestrial species or not. As a rule, most biologists recognize animals as: terrestrial if they mainly live on land; aquatic, if they exclusively live in water; and semi-aquatic if they spend part of their life cycle in water and part on land.

Birds that can fly and build nests on hard surfaces are considered a special group of land animals. As in many fields of science, continuous study and research may add additional categories or refine the parameters of existing ones in the future.

Instruction

According to abiogenic hypotheses about the origin of life on Earth, the first step towards the origin of living things was the synthesis of organic biopolymers. Through chemical evolution, biopolymers passed to the first living organisms, which further developed according to the principles of biological evolution. During this historical development and complication appeared many forms of life.

The history of the Earth is divided into long time intervals - eras: Katarchean, Archean, Proterozoic, Paleozoic, Mesozoic and Cenozoic. Paleontology, the science of ancient organisms of past geological eras, helps scientists to obtain data on the development of life on Earth. Fossil remains - mollusk shells, fish teeth and scales, egg shells, skeletons and other hard parts - are used to study organisms that lived tens, hundreds of millions of years ago.

It is believed that in the Archean (“earliest”) era, bacteria dominated the planet, the result of their vital activity was marble, graphite, limestone, etc. Archean deposits also found the remains of cyanobacteria capable of oxygen-free photosynthesis. In the end ancient era living organisms, according to assumptions, were divided into prokaryotes and eukaryotes.

In the Proterozoic era early life- living organisms continued to become more complex, and their methods of nutrition and reproduction improved. All life was concentrated in the aquatic environment and along the banks of water bodies. Among animals, a wide variety of coelenterates and sponges appeared. Toward the end of the Proterozoic era, all types of invertebrates and the first chordates arose. Remains of worms, mollusks and arthropods are also found in the sediments. The only descendant of the early life era that has survived to this day is considered the lancelet.

The Paleozoic is the era ancient life". It distinguishes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian periods. At the beginning of the Paleozoic, Cambrian, invertebrates appeared, covered with a solid skeleton built from chitin, calcium carbonate and phosphate, silica. Animal world was predominantly represented by benthic organisms - coral polyps, sponges, worms, archeciates, echinoderms and arthropods. Trilobites - the oldest arthropods - have reached their greatest prosperity.

The Ordovician is characterized by the strongest flooding of the Earth and the appearance of a multitude. Arthropods and mollusks were especially widespread during this period, but the first jawless vertebrates also appeared.

In the Silurian, animals and plants came to land. The first were arachnids and centipedes, apparently descended from trilobites. In the Devonian period, primitive jawed fish arose, having a cartilaginous skeleton and covered with a shell. Sharks and lobe-finned fish originated from them, and lobe-finned fish that can already breathe atmospheric air, - the first amphibians (ichthyostegs, stegocephals).

In the Carboniferous period, the period of swamps and vast swamp forests, amphibians flourished and the first insects appeared - cockroaches, dragonflies, beetles. Primitive reptiles also appeared, inhabiting drier places. In Perm, the climate became drier and cooler, which led to the extinction of trilobites, large mollusks, large fish, large insects and arachnids. Reptiles were the most numerous at this time. The ancestors of mammals appeared - therapsids.

In the Mesozoic, the Triassic, Jurassic and Cretaceous periods are distinguished. In the Triassic, many reptiles (turtles, ichthyosaurs, crocodiles, dinosaurs, plesiosaurs) and insects arose. At the end of the period, the first representatives of warm-blooded animals appeared. AT jurassic dinosaurs reached their peak, the first birds similar to reptiles appeared.

AT Cretaceous marsupials and placental mammals arose. At the end of the chalk happened mass extinction many types of animals - dinosaurs, large reptiles, etc. Scientists attribute this to climate change and general cooling. Advantages in the struggle for survival were received by warm-blooded animals - birds and mammals, which flourished in the Cenozoic - the era of new life, consisting of periods of the Paleogene, Neogene and Anthropogen.

This strange multi-legged creature was one of the first animals to walk on land.

Pioneers of life on land

The oldest known terrestrial invertebrate fossils are 400 million years old. They look like scorpions and belong to a group of arthropods that have an articulated body covered with a shell, which is well preserved in the fossil state. It can be assumed that in that era worms and some mollusks lived on land, but their soft body is poorly preserved, so they did not leave any traces. Paleontologists believe that the true pioneers of land conquest appeared several million years earlier and that they are probably very close to modern tardigrades. These tiny animals, not exceeding 1 mm in length, teem with thin films of water that cover mosses and lichens. They are able to survive even if their habitat dries up: their body is almost completely dehydrated and remains viable for many years. Such an ability could allow these animals, regularly deprived of water, to be the first to gradually conquer the land.

First insects

During the Devonian period (400-360 million years ago) and the following Carboniferous period, arthropods spread rapidly on land. Appeared different groups animals: centipedes, soft-footed (animals similar to slugs, but with primitive legs), arachnids (resembling modern ticks). Some species reached considerable sizes, such as, for example, the giant centipede Arthropleura (1.8 m), the largest arthropod that has ever existed on land. Despite its impressive appearance, this harmless animal ate only plants.
Before him, the first microscopic insects, springtails, were a few tenths of a millimeter in size. These primitive wingless animals exist everywhere: they live under the bark of trees, under stones, in moss. Since the Carboniferous period, cockroaches and mayflies have been no different from their modern descendants. Scorpions, although they were among the first animals that went to conquer land, still for a long time(up to the Carboniferous period) remained amphibians and lived without moving away from the water.

Wings to fly

All of these terrestrial arthropods were vegetarian and wingless until predators appeared, mainly spiders and scorpions, fully adapted to life on land. It was with the advent of these predators that insects acquired an excellent means of escaping from them - flight. For 50 million years, insects were the only animals able to fly. Meganeura, a huge dragonfly with a wingspan of more than 70 cm, flew in the forests of the Carboniferous period.

Carapace and limbs

Arthropleura is a giant centipede (1.8 m long). Despite its intimidating appearance, it ate plants.

It is difficult to explain what caused the first invertebrates to leave the water. Perhaps persecuted marine predators they had no choice if they wanted to survive. However, life on land is hostile and difficult: the lack of water threatens animals with dehydration, gravity presses them to the ground, and one must be able to breathe oxygen from the air, not from water. But, according to scientists, the first terrestrial invertebrates had a double advantage: a shell that protected them from the effects of gravity, and limbs for movement. Muscles that control the limbs are attached to the shell. This shell (called the cuticle in insects) is waterproof and therefore prevents the animal from dehydration. Thus, the shell allows animals to survive in arid places. Worms that do not have a shell are forced to burrow into the damp earth.

Insects, masters of the earth

It is believed that the Earth is most densely populated by mammals, to which man also belongs. But the development of insect diversity has been even more successful. AT mesozoic era(245-65 million years ago) insects were already very numerous, and in Cenozoic era the number of insect species has increased especially. Today, while mammals represent 3,600 species, the insect class numbers about 800,000, and it is estimated that 5 times as many more are yet to be discovered. The bronze beetle (see photo) belongs to the group of beetles, which includes 280,000 species (scarabs, ladybugs etc.).

How do insects breathe?

On the abdomen of this locust, a series of small holes can be distinguished: these are breathing holes called spiracles. Air enters through them and is sent to the cells of the body through the smallest, very thin tubes - the trachea. This system is effective only for small animals. She allowed insects to master the land, but she also limited their growth. Spiders, scorpions, snails do not have tracheae, they breathe very simple lungs. Insect blood serves only to carry nutrients and waste. Their blood is transparent because it does not contain the red pigment hemoglobin, which carries oxygen in the blood of vertebrates.
Some time after the plants, the first animals left the water to settle on land. These were invertebrate animals belonging to arthropods. Their body was covered with a shell, they moved underwater thanks to their jointed limbs. Gradually, these animals emerged from the water and learned to move on land. At first they lived in damp places, near ponds and rivers. Then they improved and began to dare to go farther and farther from the water. Finally, they grew, reaching impressive sizes.