The first land animals. What were the first animals to come out of the water? The most ancient people - Omo

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

Pioneers of life on land

The oldest known fossils of terrestrial invertebrates are 400 million years old. They are similar to scorpions and belong to a group of arthropods that have an articulated body covered with a shell that is well preserved in fossil form. It can be assumed that worms and some mollusks lived on land at that time, but their soft bodies are 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 were probably very close to modern tardigrades. These tiny animals, no more than 1 mm in length, swarm in thin films of water covering mosses and lichens. They are able to survive even if their habitat dries out: 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 land.

The first insects

During the Devonian period (400-360 million years ago) and the following Carboniferous period arthropods quickly spread on land. Different groups of animals appeared: centipedes, soft-footed animals (animals similar to slugs, but having primitive legs), arachnids (resembling modern ticks). Some species reached significant sizes, such as giant centipede Arthropleura (1.8 m), the largest arthropod ever to exist on land. Despite its impressive appearance, this harmless animal ate only plants.
Before him, the first microscopic insects, collembolas, 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 to set out to conquer land, for a long time(until the Carboniferous period) remained amphibians and lived without moving away from water.

Wings to fly

All these terrestrial arthropods were vegetarians and were deprived of wings until predators appeared, mainly spiders and scorpions, which were 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 capable of flight. Meganeura, a huge dragonfly with a wingspan of more than 70 cm, flew in the forests of the Carboniferous period.

Shell and limbs

Arthropleura is a giant centipede (1.8 m in length). Despite its terrifying appearance, it ate plants.

It is difficult to explain what caused the first invertebrates to leave the water. Perhaps, pursued by sea 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, the force of gravity presses them to the ground, and one must be able to breathe oxygen from the air, and 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. The muscles that control the limbs are attached to the shell. This shell (which insects call the cuticle) is waterproof and therefore prevents the animal from dehydrating. Thus, the shell allows animals to survive in dry places. Worms without shells are forced to burrow into damp soil.

Insects, masters of the Earth

It is believed that the Earth is most densely populated by mammals, which include humans. But the development of insect diversity has been even more successful. IN Mesozoic era(245-65 million years ago) insects were already very numerous, and in Cenozoic era the number of insect species has especially increased. Today, while mammals represent 3,600 species, the insect class numbers about 800,000, and it is estimated that 5 times more remain to be discovered. The bronze beetle (see photo) belongs to the group of Coleoptera insects, 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 directed to the cells of the body through tiny, very thin tubes - tracheas. This system is only effective for small animals. It allowed insects to colonize the land, but it also limited their growth. Spiders, scorpions, snails do not have tracheas, they breathe very simple lungs. The blood of insects serves only to transport 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, and they moved under water thanks to their jointed limbs. Gradually these animals came out of 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 venture further and further from the water. Finally, they grew, reaching impressive sizes.

Which animals were the first to inhabit land? You will find the answer to this question in this article.

Which animals were the first to inhabit land?

Today it is not known for certain how animals managed to conquer land, and who was the first to conquer land. But scientists are sure that these were invertebrates. However, unfortunately, very few remains of these animals, which were the first to set foot on land, have been found.

The fact that plants began to develop and reproduce on land created the very precondition for the emergence of terrestrial fauna. While the land was empty, it was covered only by sands and rocky mountains, and there was no life on it. However, as soon as the land areas began to be covered with greenery, food could already be found there various types animals.

Scientific research gives us facts that The first animals to come onto land were centipedes and scorpions. Based on the findings, wingless insects can be added to these animals. All invertebrate animals began to live on land immediately after the first plants appeared on the banks of reservoirs.

The first inhabitants of the land resembled modern scorpions in appearance. And if the first amphibians appeared in the Devonian, then the active conquest of land by vertebrates began in the Carboniferous. The first vertebrates to fully adapt to life on land were the reptiles. Reptile eggs were covered with a hard shell, were not afraid of drying out, and were provided with food and oxygen for the embryo. The first reptiles were small animals, and resembled living lizards. Insects reach significant development in the Carboniferous.

Amphibians - the first terrestrial vertebrates - represent an intermediate link between the typical aquatic organisms– fish and truly land forms – reptiles. The origin of amphibians is associated with a number of aromorphoses: the appearance of a five-fingered limb, the development of the lungs, the division of the atrium into two chambers and the appearance of two circulation circles, the progressive development of the central nervous system and sense organs.

1. Amphibians are adapted to live both in water and on land 2. Appeared on Earth about 350 million years ago, from ancient lobe-finned fish 3. Move on land using paired terrestrial limbs 4. Breathe using lungs and skin 5. Body consists of a head, torso and limbs 6. Eyes have eyelids 7. Skin is bare, moist, with a large number of glands 8. Two circles of blood circulation, three-chambered heart 9. Cold-blooded animals 10. Amphibians are dioecious animals 11. Fertilization is external, rarely internal 12. Development indirect (larval) 13. Amphibians are the smallest class of vertebrates (about species)

1. The body is slightly flattened and is divided into a head, a trunk and two pairs of five-fingered limbs. A small group of amphibians have a tail. 2. The skin is thin, bare, moist, rich in mucous glands. 3. The skull is movably connected to the spine, which consists of four sections: cervical, trunk, sacral and caudal. The shoulder and pelvic girdles provide support for the limbs. The skeleton of the limbs is built like a system of movable levers, allowing the animal to move on a hard surface. There is a lot of cartilage in the skeleton. 4. Muscular system consists of individual differentiated muscles. Movements different parts bodies are more varied than those of fish. 5. Amphibious predators. They have developed salivary glands, the secretion of which moisturizes the oral cavity, tongue and food. Actively captured prey is digested in the stomach. The last section of the digestive canal is the dilated cloaca.

6. The respiratory organs of adult animals are the skin and lungs; in larvae there are gills. 7. The heart is three-chambered. There are two circles of blood circulation: large (trunk) and small (pulmonary). Mixed blood flows through the arteries of the systemic circulation, and only the brain is supplied with arterial blood. 8. Excretory organs: paired trunk kidneys. Urine flows through two ureters into the cloaca, and from it into the bladder. The excreted end product of nitrogen metabolism is urea. 9. The forebrain of amphibians, compared to that of fish, is larger and is divided into two hemispheres. The cerebellum is less developed due to low mobility. The structure of the organs of hearing and vision is adapted to life on land. Amphibian larvae have a lateral line organ. 10. Fertilization is external, in water. Development with incomplete metamorphosis, with the stage of a fish-like larva.

The earliest amphibians lived about 370 - 350 million years ago. The ancestors of amphibians are freshwater lobe-finned fish of the Devonian period of the Paleozoic era. Three branches separated from the first primitive amphibians stegocephalians. One of them gave rise to modern tailed amphibians, the other gave rise to tailless amphibians, and from the third branch primitive reptiles were formed.

Amphibians are usually found in and near freshwater bodies. Here they feed on various invertebrates. In case of danger, they quickly jump into the water. In spring and early summer, newts live in shallow, stagnant bodies of water. The rest of the year they can be found in deciduous and mixed forests, parks, and gardens. Toads and grass frogs live mainly away from bodies of water. They live in water only during the breeding season. All amphibians are active only in the warm season. Frogs overwinter at the bottom of reservoirs, and toads and newts hibernate in shelters on land.

Body sections: head (flattened, the front part is wedge-shaped), trunk (slightly flattened in the dorso-ventral direction), paired limbs, tail (in tailless animals - reduction of the caudal section). The skin is thin and moist and contains many glands (among them there are poisonous ones). The glands produce mucus, which moisturizes and disinfects the skin. Skin respiration occurs through moist skin.

At the front end of the head of amphibians there is a large mouth. Higher up on the dais are two large bulging eyes and a pair of nostrils. The eyes have eyelids that protect and moisturize them. The upper eyelid is mobile, and the lower one has a translucent nictitating membrane. Rounded eardrums are visible on the sides of the head behind the eyes. They separate the first section of the hearing organ, the middle ear cavity, from the external environment. The inner ear of amphibians, like that of fish, is located in the bones of the skull.

In adult amphibians, the general structure of the limbs is the same as in other terrestrial vertebrates. Forelimb: shoulder, forearm, hand. Hind limb: thigh, lower leg, foot. In tailless amphibians, the hind legs are longer and stronger than the front ones, which allows these animals to move by jumping. Swimming membranes are developed between the toes of the hind legs of tailless amphibians.

The coloring of amphibians is varied, often camouflaging and hiding them well in thickets of aquatic or coastal plants. Some amphibians have very bright colors, warning that they are poisonous. These are salamanders, fire toads. The inhabitants of caves, as a rule, are completely devoid of color.

The pond frog feeds on insects, spiders, mollusks, and fish fry. She lies in wait for her prey. Main role vision plays. The frog reacts only to moving prey, instantly assesses the distance to it, sharply throws out a long sticky tongue and quickly puts the prey into its mouth. The frog has no teeth. Newt larva eats worm

All modern amphibians in the adult stage are predators, feed on small animals (mainly insects and invertebrates), and are prone to cannibalism. There are no herbivores among amphibians due to their extremely sluggish metabolism. The diet of aquatic species may include juvenile fish, and the largest ones may prey on chicks of waterfowl and small rodents caught in the water. The feeding pattern of the larvae of tailed amphibians is almost similar to the feeding of adult animals. The larvae of anurans are fundamentally different, feeding on plant food and detritus, switching to predation only at the end of the larval stage.

The spine contains nine vertebrae: cervical (1 vertebra), trunk (7 vertebrae), sacral (1 vertebra), urostyle (12 fused caudal vertebrae). There are no ribs. The skeleton of the free limbs is built according to the type of system of multi-membered levers, movably connected by spherical joints.

The muscular system of amphibians has undergone significant changes under the influence of a terrestrial lifestyle. Uniformly constructed muscle segments of fish are transformed into differentiated muscles of the limbs, head, and oral cavity, involved in the process of swallowing food and ventilation of the respiratory system.

There are conical teeth. Food in the oral cavity is moistened with saliva (fish do not), it does not contain enzymes. The eyes are involved in the act of swallowing. The liver and pancreas are well developed. Undigested food remains exit through the cloaca. The appearance of a real tongue in the oral cavity, the main organ for food production, is associated with a terrestrial lifestyle. In frogs, it is attached to the front of the floor of the mouth and is able to quickly move forward, gluing prey. Adult frogs, like all other amphibians, are carnivorous and feed on moving small animals, sometimes caviar, and young fish.

The breathing mechanism of amphibians The structure of the lungs The lungs are small elongated sacs with thin elastic walls. The lungs of amphibians are primitive, so the skin is important in gas exchange. Breathing occurs due to the lowering and raising of the oropharyngeal cavity. The respiratory organs of adults are lungs, and the larvae have gills.

In connection with the development of the lungs in amphibians, a second, small, or pulmonary, circulation appears. They are cold-blooded. The heart has three chambers: two atria and one ventricle. All organs receive mixed blood. Only the brain receives pure arterial blood.

Circulatory system amphibians are represented by a three-chambered heart, consisting of two atria and a ventricle, and two circles of blood circulation - large (trunk) and small (pulmonary). The pulmonary circulation begins in the ventricle, includes the vessels of the lungs and ends in the left atrium. The great circle also begins in the ventricle. The blood, having passed through the vessels of the entire body, returns to the right atrium. Thus, arterial blood from the lungs enters the left atrium, and venous blood from the entire body enters the right atrium. Arterial blood flowing from the skin also enters the right atrium. Thus, thanks to the appearance of the pulmonary circulation, arterial blood also enters the heart of amphibians. Despite the fact that arterial and venous blood enters the ventricle, complete mixing of the blood does not occur due to the presence of pockets and incomplete partitions. Thanks to them, when leaving the ventricle, arterial blood flows through the carotid arteries to the head, venous blood into the lungs and skin, and mixed into all other organs of the body. Thus, in amphibians there is no complete separation of blood in the ventricle, therefore the intensity of life processes is low, and body temperature is variable.

The brain consists of 5 sections; The forebrain is highly developed, which is divided into two hemispheres; The cerebellum is poorly developed due to the monotony of movements; The organ of hearing has 2 sections: the middle and inner ear; The eyes have eyelids, the cornea is convex; The organs of taste, fascination and touch are also developed.

The exit of amphibians to land influenced the development of the sensory organ. Thus, the eyes of amphibians are protected from drying out and clogging by movable upper and lower eyelids and the nictitating membrane. The cornea acquired a convex shape, and the lens became lens-shaped. Amphibians see mainly moving objects. A middle ear with one auditory ossicle (stapes) appeared in the organ of hearing. The middle ear cavity is separated from environment eardrum and is connected to the oral cavity through a narrow channel of the eustachian tube, due to which the internal and external pressure on the eardrum is balanced. The appearance of the middle ear is caused by the need to enhance the perceived sound vibrations, since the density of the air environment is less than that of water. The nostrils of amphibians, unlike fish, are continuous and lined with sensitive epithelium that perceives odors.

The reproduction of amphibians has its own characteristics. Gonads are paired. The paired oviducts flow into the cloaca, and the seminal ducts into the ureters. Frogs reproduce in the spring during their third year of life. Fertilization occurs in water. After 715 days, fish-like tadpole larvae develop in the fertilized eggs. The tadpole is a typical aquatic animal: it breathes with gills, has a two-chambered heart, one circulatory system and a lateral line organ, and swims with a tail bordered by a membrane. During metamorphosis, the larval organs are replaced by the organs of an adult animal.

Comparative characteristics structures of larvae and adult frogs Character Larva (tadpole)Adult animal Body shape Fish-like, with rudiments of limbs, tail with a swimming membrane Body shortened, two pairs of limbs developed, no tail Method of locomotion Swimming with the help of the tail Jumping, swimming with the help of the hind limbs Breathing Gill ( gills are first external, then internal) Pulmonary and cutaneous Circulatory system Two-chambered heart, one circle of blood circulation Three-chambered heart, two circles of blood circulation Sense organs Developed lateral line organs, no eyelids in the eyes No lateral line organs, developed eyelids in the eyes Jaws and method of feeding Horny plates jaws scrapes off algae along with unicellular and other small animals There are no horny plates on the jaws, with a sticky tongue it captures insects, mollusks, worms, fish fry Lifestyle AquaticTerrestrial, semi-aquatic

Amphibians play a great role in the natural community; they eat a variety of invertebrates, larvae and pupae of blood-sucking insects that spread dangerous human diseases (malaria), and are food for other animals. Toads eat vegetable pests - slugs. The lake frog eats 7 pests in a day, and more than in six months. In some countries, the meat of amphibians is used as food. Amphibians are important as laboratory animals. Most experiments in medicine and biology take place using frogs. In many countries around the world, most amphibians are protected. Prohibited: catching in nature, destruction and pollution of their habitats.

Salamander Triton They live north of the equator, in the temperate zone of the Eastern and Western hemispheres. The body is elongated, fusiform, imperceptibly turns into long tail In water they move with the help of a tail and limbs (there is a membrane between the fingers), on land - with the help of two pairs of underdeveloped limbs They breathe with the help of the lungs, skin, oral mucosa or external gills Fertilization is internal or external, development with transformation, the larva is similar in appearance and method of movement on fish larvae

The most numerous order, numbering about 3,000 species. Distributed throughout the globe, with the exception of Antarctica and the northern islands. The body is short, squat, without a tail; the head is wide without a neck The skin is bare, equipped with numerous glands Active during all hours of the day Adult animals lead a predatory lifestyle They breed in water in the spring, and in the summer they live on land in search of food Frog Toad

Ceylon fish snake Ringed caecilian Homeland of caecilians Tropical part of Africa, South America and South Asia They live in the soil at a depth of cm, except for South American caecilians - they live constantly in water. Caecilians have a worm-shaped, cylindrical body without legs, slightly flattened in the ventral direction. The skin is bare, mucous with poisonous secretions. On top of the body is divided into many transverse rings - resembling segments of earthworms Sense organs - vision and hearing are absent, the sense of smell and touch is well developed Feed on invertebrate animals
the body consists of a head, torso, tail and paired limbs; the skin is bare, rich in glands, the skeleton and muscles are more complex than those of fish, the skeleton of paired limbs is developed; mucus; secretes glands in the oropharyngeal cavity; paired salivary buds; excretory organs - intestines, ducts of the cloaca; excretory and reproductive systems open into a three-chamber heart in adults; 2 circles of blood circulation are formed: small (pulmonary) and large; respiratory organs in adults - lungs; in larvae - gills; the brain consists of 5 sections, the forebrain is developed, the cerebellum is not developed. The sense organs are adapted to life on land.

The generally accepted story of the origin of life on Earth is outdated. Two scientists, Peter Ward and Joseph Kirschvink, offer a book that brings together all the findings of the latest 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 leisurely, 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 thought in terms of the evolution of species. In fact, there has been an evolution of ecosystems - from underwater volcanoes to tropical forests, - which shaped the world as we know it. Drawing on their decades of experience in paleontology, biology, chemistry, and 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 ignore.

Book:

The first land animals

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The first land animals

The main problem for any first land animal was an acute lack of water. All living cells must have water, and water image life easily provided this need. Living on land, however, requires a dense outer covering to retain water within the body. The difficulty is that solutions to reduce water loss in the air conflict with the needs of skin respiration. Here's a problem 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 respiration system, in which oxygen penetrates through the outer covering, but the risk of moisture loss through the same system increases. This dilemma had to be resolved by all the discoverers of sushi. Apparently, 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 numerous and most common modern sea ​​creatures, apparently, were never able to conquer the land: there are no terrestrial sponges, cnidarians, brachiopods, bryozoans and echinoderms, and many others too.

The oldest fossils of land animals are probably small arthropods resembling modern spiders, scorpions, ticks, isopods and primitive insects. It is unclear 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 land animals inevitably had to be done from fossils, which was not guaranteed to be accurate, since these were small terrestrial arthropods that have very weakly hardened exoskeletons and are therefore rarely preserved in sediments. By the end of the Silurian period or the early Devonian, about 400 million years ago, the spread of plants on land allowed the vanguard of the animal kingdom to emerge from the water. It is absolutely 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 systems of modern spiders and scorpions explain 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 so much important changes like breathing. It also seems quite clear that the lungs of the first land arthropods were a transitional link in evolution, almost as effective 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 certainly 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, mollusks, 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 exoskeleton of arthropods required the evolution of large gills on almost all body segments to ensure survival in the Cambrian (this is when most highly developed fossil arthropods appeared) in the low oxygen content of the environment. But gills do not function in air. The first land arthropods - spiders and scorpions - developed new look respiratory system called the “pulmonary book” (the internal structure of such a lung resembles the pages of a book).

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

Some very small spiders are known 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 even in low-oxygen environments. However, representatives of aeroplankton are so small that their respiratory needs can be satisfied by the passive penetration of gas into the body. Larger spiders are very vulnerable because of their book-like lungs.

More effective compared to the respiratory system of insects, consisting of trachea, possibly book-shaped gills. The insect respiratory system is passive in the sense that it has no or very weak air forcing mechanism, although recent research shows that some forcing is still present, but with very weak pressure. The book-lung 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 onto land is very difficult to determine, since ancient spiders and scorpions were very small in size and left almost no fossils. Modern Scorpios have greater degree hardenings than spiders and are therefore more common in sediments.

The earliest evidence of land animals dates back to the Late Silurian (fossils in Wales) - about 420 million years ago - 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 sparse and show little diversity. However, they were recognized and classified as centipedes.

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

Paleoentomologists have discovered that insects remained a small group of land 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 period - 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 to 330 million years ago. Soon after their first flight, insects made an incredible evolutionary leap, giving rise to many new species, mostly flying. This is a classic case of evolutionary radiation, when a 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 the atmosphere was very high level oxygen, and, undoubtedly, it was precisely this state of the atmosphere that ensured the success of these processes.

Insects 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 water, and they probably fed on the remains of dead animals, such as fish, washed ashore by waves. The gills remained moist, and their very large surface area allowed for some kind of breathing. They definitely didn't have any lungs, just gills.

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

The latest methods for determining the level of oxygen in the atmosphere make it possible to determine that maximum level oxygen content in the atmosphere dates back to about 410 million years ago. This was followed by a sharp decline, 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, let us remind you, the oxygen content in the atmosphere is 21%. Rhine's Devil, in which numerous accumulations of insects and arachnids were first discovered, dates back to the period of the oxygen maximum in the Devonian. Then, according to reports from paleontologists who study insect diversity, insect diversity is rare in fossils. This situation persists until the oxygen level jumps to 20% in the interval between the Early and Late Carboniferous, 330–310 million years ago, during the period of the spread of winged insects.

The spread of vertebrates on land was made possible due, most likely, to an increase in oxygen content in the atmosphere during the Ordovician and Silurian periods. If not for this circumstance, perhaps both the history of the development of animals on land and the forms of land animals would have been 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 low oxygen levels in the atmosphere, the animals were very few in number.

There are three possible explanations for the distribution of fossils observed in rocks from these 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 survived were able to give rise to a huge variety of forms when oxygen levels rose 30 million years later.

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

Arthropods weren't the only ones to adapt to life on land. Gastropods also made an evolutionary leap to land, but not earlier than the Late Carboniferous, that is, they were part of the second stage of land exploration by animals, when oxygen levels became high enough. Another group of animals, horseshoe crabs, arrived on land at about the same time as mollusks. However, these were all small colonists compared to the group that interests us 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, let us imagine the Devonian period, which has long been called the Age of Fishes. An example is our favorite place- Devonian Canning Basin (Canning Basin) V Western Australia, where the authors of this book conducted much field research. The Canning Basin is one of the most beautiful (very hot!) places in the world, with the best preserved barrier reef fossils - as if the modern Great Barrier Reef suddenly turned to stone and the water suddenly disappeared. Although most of the work on the Canning Basin focuses on this giant Devonian reef, the rocks formed in the deeper sea ​​places period, contain particularly impressive fossils that certainly deserve to be included in the pages of any new history of the development of life on Earth.

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Pterosaurs

History of animal evolution

The skull of Ichthyostega was similar to the skull of a lobe-finned fish Eusthenopteron, but a pronounced neck separated the body from the head. While Ichthyostega had four strong limbs, the shape of its hind legs suggests that this animal did not spend all its time on land.

The first reptiles and the amniotic egg

Hatching of a turtle from an egg

One of the greatest evolutionary innovations of the Carboniferous period (360 - 268 million years ago) was the amniotic egg, which allowed early reptiles to move out of coastal habitats and colonize dry areas. The amniotic egg allowed the ancestors of birds, mammals and reptiles to reproduce on land and prevent the embryo inside from drying out, so they could survive without water. This also meant that, unlike amphibians, reptiles could produce fewer eggs at any given time as the risks of hatchlings dying were reduced.

The earliest date for the development of an amniotic egg is about 320 million years ago. However, reptiles did not experience any significant adaptive radiation for another 20 million years or so. Modern thinking is that these early amniotes still spent time in the water and came ashore primarily to lay their eggs rather than feed. Only after the evolution of herbivores did new groups of reptiles emerge capable of exploiting the abundant floristic diversity of the Carboniferous period.

Gilonomous

Early reptiles belonged to an order called captorhinids. Hylonomus were representatives of this order. They were small animals about the size of a lizard, with amphibian skulls, shoulders, pelvises and limbs, as well as intermediate teeth and vertebrae. The rest of the skeleton was reptilian. Many of these new "reptilian" features are also seen in small, modern amphibians.

First mammals

Dimetrodon

A major transition in the evolution of life occurred when mammals evolved from a single line of reptiles. This transition began during the Permian period (286 - 248 million years ago), when a group of reptiles that included Dimetrodon gave rise to the "terrible" therapsids. (Other major branches, sauropsids, gave rise to birds and modern reptiles). These mammalian reptiles in turn gave rise to cynodonts such as Thrinaxodon ( Thrinaxodon) during the Triassic period.

Trinaxodon

This evolutionary line provides an excellent series of transitional fossils. The development of a key feature of mammals, the presence of a single bone in the lower jaw (compared to several in reptiles), can be traced through the fossil history of this group. It includes excellent transitional fossils, Diarthrognathus And Morganucodon, whose lower jaws have both reptilian and mammalian articulations with the upper jaws. Other new features discovered in this lineage include the development various types teeth (a feature known as heterodontity), formation of a secondary palate, and enlargement of the dentary in the lower jaw. The legs were located directly below the body, an evolutionary advance that occurred in the ancestors of dinosaurs.

The end of the Permian period was marked by perhaps the greatest. According to some estimates, up to 90% of species have become extinct. (Recent studies have suggested that this event was caused by an asteroid impact, triggering climate change.) During the subsequent Triassic period (248 - 213 million years ago), surviving individuals after mass extinction began to occupy vacant ecological niches.

However, at the end of the Permian period it was dinosaurs, not reptilian mammals, that took advantage of the newly available ecological niches to diversify into dominant land vertebrates. In the sea, ray-finned fish began a process of adaptive radiation, which made their class the most species-rich of all vertebrate classes.

Classification of dinosaurs

One of the major changes in the group of reptiles that gave rise to dinosaurs was the posture of the animals. The location of the limbs has changed: previously they protruded on the sides, and then began to grow directly under the body. This had significant implications for locomotion, allowing for more energy-efficient movements.

Triceratops

Dinosaurs, or “terror lizards,” are divided into two orders based on the structure of the hip joint: lizard-hipped and ornithischian. Ornithischians include Triceratops, Iguanodon, Hadrosaurs and Stegosaurs). Lizards are further divided into theropods (such as Coelophysis and Tyrannosaurus rex) and sauropods (such as Apatosaurus). Most scientists agree that they are from theropod dinosaurs.

Although dinosaurs and their immediate ancestors dominated terrestrial world During the Triassic, mammals continued to evolve during this time.

Further development of early mammals

Mammals are advanced synapsids. Synapsids - one of the two great branches family tree amniote Amniotes are a group of animals that are characterized by the presence of embryonic membranes, including reptiles, birds and mammals. The other major amniotic group, the Diapsids, includes birds and all living and extinct reptiles except turtles. Turtles belong to the third group of amniotes - Anapsids. Members of these groups are classified by the number of openings in the temporal region of the skull.

Dimetrodon

Synapsids are characterized by having a pair of additional openings in the skull behind the eyes. This discovery gave synapsids (and similarly diapsids, which have two pairs of openings) stronger jaw muscles and better biting abilities than early animals. Pelycosaurs (such as Dimetrodon and Edaphosaurus) were early synapsids; they were reptilian mammals. Later synapsids included therapsids and cynodonts, which lived during the Triassic period.

Cynodont

Cynodonts had many characteristic mammalian features, including a reduced number or complete absence of lumbar ribs, suggesting the presence of a diaphragm; well developed canines and secondary palate; increased size of the dentition; openings for nerves and blood vessels in the lower jaw, indicating the presence of vibrissae.

By about 125 million years ago, mammals had already become a diverse group of organisms. Some of these would have been similar to today's monotremes (such as the platypus and echidna), but early marsupials (a group that includes modern kangaroos and possums) were also present. Until recently, placental mammals (the group to which most living mammals belong) were thought to have a later evolutionary origin. However, recently discovered fossils and DNA evidence suggest that placental mammals are much older, possibly evolving more than 105 million years ago.

Note that marsupials and placental mammals provide excellent examples of convergent evolution, where organisms that are not particularly closely related evolved similar body shapes in response to similar environmental influences.

Plesiosaurs

However, despite having what many consider to be "advanced" mammals were still minor players on the world stage. When the world entered the Jurassic period (213 - 145 million years ago), the dominant animals on land, sea and air were reptiles. Dinosaurs, more numerous and unusual than during the Triassic, were the main land animals; crocodiles, ichthyosaurs and plesiosaurs ruled the sea, and the air was inhabited by pterosaurs.

Archeopteryx and the evolution of birds

Archeopteryx

In 1861, an intriguing fossil was discovered in the Jurassic Solnhofen Limestone in southern Germany, a source of rare but exceptionally well-preserved fossils. The fossil appeared to combine features of both birds and reptiles: a reptilian skeleton accompanied by a clear impression of feathers.

While Archeopteryx was originally described as a feathered reptile, it has long been considered a transitional form between birds and reptiles, making the animal one of the most important fossils ever discovered. Until recently, it was the earliest known bird. Scientists recently realized that Archeopteryx bears more similarities to maniraptorians, a group of dinosaurs that includes the infamous Velociraptor from Jurassic Park, than to modern birds. Thus, Archeopteryx provides a strong phylogenetic link between these two groups. Fossil birds have been discovered in China that are even older than Archeopteryx, and other discoveries of feathered dinosaurs support the theory that theropods evolved feathers for insulation and temperature regulation before birds used them for flight.

A closer look at the early history of birds is good example the concept that evolution is neither linear nor progressive. The lineage of birds is disordered, and many "experimental" forms appear. Not all achieved the ability to fly, and some looked completely different from modern birds. For example, Microraptor gui, which appears to have been a flying animal and had asymmetrical flight feathers on all four limbs, was a dromaeosaurid. Archeopteryx itself did not belong to the lineage from which true birds evolved ( Neornithes), but was a member of the now extinct enantiornhis birds ( Enantiornithes).

The end of the dinosaur era

Dinosaurs spread throughout the world during the Jurassic period, but during the subsequent Cretaceous period(145 - 65 million years ago) their species diversity decreased. In fact, many of the typically Mesozoic organisms, such as ammonites, belemnites, ichthyosaurs, plesiosaurs and pterosaurs, were in decline during this time, even though they were still giving rise to new species.

Emergence flowering plants during the Early Cretaceous period caused a major adaptive radiation among insects: new groups appeared, such as butterflies, moths, ants and bees. These insects drank nectar from flowers and acted as pollinators.

The mass extinction at the end of the Cretaceous period, 65 million years ago, wiped out the dinosaurs along with any other land animal weighing more than 25 kg. This paved the way for the expansion of mammals on land. In the sea at this time, fish again became the dominant vertebrate taxon.

Modern mammals

At the beginning of the Paleocene (65 - 55.5 million years ago), the world was left without large land animals. This unique situation was the starting point for a great evolutionary diversification of mammals, which were previously nocturnal animals the size of small rodents. By the end of the era, these representatives of the fauna occupied many of the free ecological niches.

The oldest confirmed primate fossils date back about 60 million years. Early primates evolved from ancient nocturnal insectivores, something like shrews, and resembled lemurs or tarsiers. They were probably arboreal animals and lived in or subtropical forests. Many of them characteristic features well suited to this habitat: hands designed for gripping, rotating shoulder joints, and stereoscopic vision. They also had a relatively large brain size and clawed toes.

The earliest known fossils of most modern mammal orders appear in short period during the early Eocene (55.5-37.7 million years ago). Both groups of modern ungulates, the Artiodactyls (the order that includes cows and pigs) and the Odd-toed ungulates (including horses, rhinoceroses, and tapirs), became widespread throughout North America and Europe.

Ambulocetus

At the same time as mammals diversified on land, they also returned to the sea. The evolutionary transitions that led to whales have been extensively studied in recent years, with extensive fossil finds from India, Pakistan and the Middle East. These fossils indicate a change from the land-based Mesonychia, which are the likely ancestors of whales, to animals such as Ambulocetus and primitive whales called Archaeocetes.

The trend towards a cooler global climate that occurred during the Oligocene epoch (33.7 - 22.8 million years ago) favored the emergence of grasses, which were to spread to extensive grasslands during the subsequent Miocene (23.8 - 5.3 million years ago ). This change in vegetation led to the evolution of animals such as more modern horses, with teeth that could handle the high silica content of herbs. The cooling trend has also affected the oceans, reducing the abundance of marine plankton and invertebrates.

Although DNA evidence suggests that hominids evolved during the Oligocene, abundant fossils did not appear until the Miocene. Hominids, on the evolutionary line leading to humans, first appear in the fossil record in the Pliocene (5.3 - 2.6 million years ago).

During the entire Pleistocene (2.6 million - 11.7 thousand years ago) there were about twenty cold cycles ice age and warm interglacial periods at intervals of about 100,000 years. During the Ice Age, glaciers dominated the landscape, spreading snow and ice into the lowlands and transporting vast amounts of rock. Because a lot of water was trapped in the ice, the sea level dropped to 135 m than it is now. Wide land bridges allowed plants and animals to move. During warm periods large areas sank under water again. These repeated episodes of environmental fragmentation led to rapid adaptive radiation in many species.

The Holocene is the current epoch of geological time. Another term that is sometimes used is the Anthropocene because its main characteristic is global changes caused by human activities. However, this term can be misleading; modern people were already created long before the era began. The Holocene era began 11.7 thousand years ago and continues to this day.

Mammoths

When warming came on Earth, it gave way. As the climate changed, very large mammals that adapted to extreme cold, such as the woolly rhinoceros, became extinct. Humans, once dependent on these "mega mammals" as their main source of food, switched to smaller animals and began collecting plants to supplement their diet.

Evidence shows that around 10,800 years ago the climate underwent a sharp cold turn that lasted several years. The glaciers did not return, but animals and plants were few. As temperatures began to recover, animal populations grew and new fauna species emerged that still exist today.

Currently, the evolution of animals continues, as new factors arise that force representatives of the animal world to adapt to changes in their environment.