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

The first reptiles and the amniotic egg

Hatching a turtle from an egg

One of the greatest evolutionary innovations of the Carboniferous (360 - 268 million years ago) was the amniotic egg, which allowed early reptiles to move away from coastal habitats and colonize dry areas. The amniotic egg allowed the ancestors of birds, mammals and reptiles to breed on land and prevent the embryo inside from drying out, so water could be dispensed with. It also meant that, unlike amphibians, reptiles were able to produce fewer eggs at any given time, as the risks of hatchlings were reduced.

The earliest date for the development of an amniotic egg is about 320 million years ago. However, reptiles were not exposed to any significant adaptive radiation for about 20 million years. The current thinking is that these early amniotes still spent time in the water and came ashore mainly to lay their eggs rather than feed. Only after the evolution of herbivores did new groups of reptiles emerge that could exploit the abundant floristic diversity of the Carboniferous.

Hylonomus

The early reptiles belonged to an order called the captorhinids. Gilonomus were representatives of this detachment. They were small, lizard-sized animals with amphibian skulls, shoulders, pelvis, 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 lineage of reptiles. This transition began during the Permian period (286 - 248 million years ago), when a group of reptiles that included the Dimetrodons gave birth to the "terrible" therapsids. (Other large branches, sauropsids, gave rise to birds and modern reptiles.) These reptilian mammals in turn gave birth 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 mammalian feature, the presence of a single bone in the lower jaw (compared to several in reptiles), can be traced in 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 ones. Other new features found in this lineage include the development of different types of teeth (a feature known as heterodontia), the formation of a secondary palate, and an increase in dentary bone in the lower jaw. The legs are located directly below the body, an evolutionary advance that occurred in the ancestors of the dinosaurs.

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

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

Dinosaur classification

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

Triceratops

Dinosaurs, or "terrible lizards", are divided into two groups based on the structure of the hip joint: lizards and ornithischians. Ornithischians include Triceratops, Iguanodon, Hadrosaurus, and Stegosaurus). The lizards are further subdivided into theropods (eg Coelophys and Tyrannosaurus Rex) and sauropods (eg Apatosaurus). Most scientists agree that from theropod dinosaurs.

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

Further development of early mammals

Mammals are highly developed synapsids. Synapsids are one of the two great branches of the amniote family tree. Amniotes are a group of animals that are characterized by having embryonic membranes, including reptiles, birds, and mammals. Another large amniotic group, the Diapsid, 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 according to the number of openings in the temporal region of the skull.

Dimetrodon

Synapsids are characterized by the presence of a pair of accessory openings in the skull behind the eyes. This discovery gave synapsids (and similarly diapsids, which have two pairs of holes) 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 shared many of the characteristics of mammals, including a reduced number or complete absence lumbar ribs, suggesting the presence of a diaphragm; well developed fangs and secondary palate; increased size of the dentition; openings for nerves and blood vessels in the lower jaw, indicating the presence of whiskers.

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 opossums) were also present. Until recently, placental mammals (the group to which most living mammals belong) were thought to be of a later evolutionary origin. However, recent discovered fossils and DNA evidence suggest that placental mammals are much older, and may have evolved over 105 million years ago.

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

Plesiosaurs

However, despite the fact that mammals had what many consider "advanced", they 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, in the sea and in the 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 pterosaurs populated the air.

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 seemed to combine features of both birds and reptiles: a reptilian skeleton accompanied by a clear imprint of feathers.

While Archeopteryx was originally described as a feathered reptile, its for a long time considered a transitional form between birds and reptiles, making this animal one of the most important fossils ever discovered. Until recently, it was the earliest known bird. Recently, scientists have realized that Archeopteryx bears more resemblance to the maniraptors, a group of dinosaurs that includes the infamous Jurassic Park velociraptors, than to modern birds. Thus, Archeopteryx provides a strong phylogenetic relationship between the two groups. Fossil birds have been found in China that are even older than Archeopteryx, and other feathered dinosaur discoveries support the theory that theropods evolved feathers for insulation and thermoregulation 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 bird lineage is erratic and many "experimental" forms appear. Not everyone achieved the ability to fly, and some looked nothing like modern birds. For example, Microraptor gui, which appears to have been a flying animal with 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 enanciornis birds ( Enantiornithes).

End of the Dinosaur Age

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

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

The mass extinction at the end of the Cretaceous, 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 are about 60 million years old. 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 were well suited to this habitat: gripping hands, rotating shoulder joints, and stereoscopic vision. They also had relatively big size brain and claws on the fingers.

The earliest known fossils of most modern orders of mammals appear in a short period during the early Eocene (55.5-37.7 million years ago). Both groups of modern ungulates - artiodactyls (a detachment to which cows and pigs belong) and equids (including horses, rhinos and tapirs) have become widespread throughout North America and Europe.

Ambulocetus

At the same time that mammals were diversifying on land, they were also returning to the sea. The evolutionary transitions that led to whales have been carefully studied in last years with extensive fossil finds from India, Pakistan and the Middle East. These fossils point to a change from terrestrial 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) contributed to the emergence of grasses, which were to spread to vast 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 high content silica in 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 during 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 cycles of cold ice age and warm interglacial periods at intervals of about 100,000 years. During the Ice Age, glaciers dominated the landscape, snow and ice spread in the lowlands and transported great amount breeds. Because a lot of water was locked up on 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 plunged under the water again. These repeated episodes of environmental fragmentation resulted in 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 had already been created long before the beginning of the era. The Holocene epoch began 11.7 thousand years ago and continues to the present day.

When warming came on Earth, she gave way. As the climate changes, very large mammals that have adapted to extreme cold, such as woolly rhinoceros, died out. Humans, once dependent on these "mega-mammals" as their main source of food, have switched to smaller animals and started harvesting 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 there were few animals and plants. As temperatures began to recover, animal populations grew and new 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.

Following the intake and absorption of feed, the stage of splitting complex polymer structures to monomers follows. This happens under the influence of hydrolytic enzymes.

The resulting monomers are absorbed into internal environment organism. The initial stage of food assimilation, i.e., the transformation of the original food structures into components devoid of species specificity and suitable for absorption and participation in intermediate metabolism, is referred to as the process of digestion.

Ability different types animals to digest feed of a certain quality was formed in the course of evolution. Due different character food and different conditions the life of animals in the process of phylogenesis, the digestive apparatus also develops in different ways. Vegetable food is less nutritious than animal food, and therefore herbivores are forced to absorb much more food than carnivores. So, a cow weighing 600-700 kg eats about 100 kg of feed per day. In this regard, herbivores have a much longer digestive tract than carnivores. Here are some data showing differences in the ratio of the length of the trunk to the length of the intestines in various animal species.

Bat - 1:2

Rabbit - 1:10

Ermine - 1:4

Horse - 1:12

Dog - 1:5

Cow - 1:20

As can be seen, nutrition with indigestible substances, especially those rich in fiber, in all groups of animals leads to an elongation of the alimentary canal and is accompanied by the development of its additional sections. Especially indicative in this regard is the digestive tract of ruminants, in the digestion of which symbionts (bacteria and protozoa) play a huge role. A similar complication of the digestive system is noted in small ruminants - the posterior sections of the intestine in small herbivorous mammals are well developed and are intended for protozoal and bacterial hydrolysis of cellulose.

Based on the study of the digestive system, it can be concluded that the main direction of the evolution of herbivorous species, starting from the early Miocene (4th period cenozoic era) there was a transition from protein-lipoid to fiber nutrition. This process was greatly accelerated due to the process of great steppe stepping of the land that took place during the Pliocene period. Changing the protein type of food to fiber means a transition from eating high-calorie, but hard-to-get food to the consumption of low-calorie, but easy-to-get food. This transition led to a reduction in the individual area and, consequently, to a decrease in the mobility of animals, to an increase in the total amount of feed consumed and to a corresponding adaptive morphophysiological change in the gastrointestinal tract.

With the simplification of finding food, the locomotor organs are simplified, the sense organs are reduced: smell, sight, taste. The digestive system is also changing. So, with an increase in the amount of food consumed, the chewing muscles increase. At the same time, the nature of the movement of the jaws (grinding) changes, and in connection with this, the dental apparatus is also transformed (tuberosity is replaced by folding). There is a reduction of the sensitive papillae of the tongue, the size of the digestive tube increases, the length of the small and large intestines increases. The liver changes, and since bile must be continuously secreted when eating low-calorie foods, this in some cases leads to a reduction in the gallbladder (in horses, elks, deer, camels). The adaptation of the digestive apparatus occurred in full accordance with the classical ideas coming from Cuvier (1812), who believed that when the conditions of existence change, the organs of one system are transformed more or less synchronously in the same direction. This situation is well illustrated by the example of adaptive rearrangements of the digestive tract.

Thus, part of the mammals in the process of historical development switched to eating plant foods, which differ sharply from foods of animal origin.

1. Vegetable foods are easily available for consumption, but are not so beneficial for digestion and assimilation.

2. Vegetable feeds are significantly inferior to feeds of animal origin in terms of nutritional value.

3. Vegetable feeds, unlike animal feeds, contain a large percentage of carbohydrates, including indigestible ones (cellulose, hemicellulose, etc.).

4. The main structural component of the body of plants - cellulose (fiber) in most animals is not split due to the absence of the cellulase enzyme in the digestive juices. This enzyme is synthesized only by bacteria, protozoa and some invertebrates. As for mammals, they are not able to synthesize cellulase. Therefore, the use of plants as food by mammals can be realized only with the help of symbiote microorganisms capable of synthesizing and secreting cellulase.

5. Vegetable feeds are characterized by a low content of protein and lipids and, which is especially important, a poor amino acid composition. Feeding with rough plant foods led to the emergence of many anatomical and physiological transformations of the digestive organs: a change in the dental system, an increase in the volume of the digestive tract, and the formation of special chambers (fore-stomachs and caecum).

Terrestrial community vertebrates in the Cenozoic developed independently in three separate territories, faunal contacts between which were practically absent. Australia (with its marsupials and monotremes) is isolated to this day, and South America retained its isolation from the rest of the land until the Pliocene, when the Isthmus of Panama arose; hence the modern division of the world into three zoogeographic regions: Notogaea (Australia), Neogaea (South America) and Arctogea (Eurasia, Africa and North America). So, according to Zherikhin (1993), in all these three areas, grass biomes arose independently, on the basis of completely different complexes. large mammals; Strictly speaking, there are serious grounds for believing that mammals have truly entered the large size class only in grass biomes.

The earliest (in the middle Eocene) this process began in South America. There, among the originally leaf-eating "South American ungulates", the first herbivorous forms appear, and also giant herbivorous glyptodont armadillos appear, resembling a small tank (54, a). In the Middle Eocene in South America, for the first time, pollen spectra with a high content of grass pollen, paleosols of the steppe type, as well as fossilized dung balls belonging to dung beetles, were also found. Later, in the Oligocene and especially in the Miocene, it appears here in the highest degree a peculiar complex of pasture herbivores. It included edentulous (glyptodonts and ground sloths), “South American ungulates” (various litopterns show a strong convergent similarity, partly with horses, partly with camels, pyrotheres have much in common with elephants, and among notoungulata there were forms similar to both rhinos and with hippos, and with rabbits (54, b-d), as well as giant caviomorphic rodents (some of these relatives guinea pig reached the size of a rhinoceros) and existed until the establishment of a land connection with North America in the Pliocene.

As for predators, they were always in short supply in the ancient South American fauna. None of the local orders of placentals, for reasons that are not entirely clear, gave rise to carnivorous forms - this role was played exclusively by marsupials. The rather diverse borghienids somewhat resembled dogs (but even more so than the thylacine, the Tasmanian marsupial wolf), and the thylacosmylus well deserves the name "marsupial saber-toothed tiger" and is a striking example of convergence with saber-toothed cats Northern Hemisphere (54, e-f). The shortage of mammal predators (A.S. Rautian and N.N. Kalandadze, 1987 drew attention to the “imbalance” of the local fauna) led to the fact that this niche was filled by the most unexpected characters.

So, from the Paleocene to the Miocene, there were sebecosuchians here - land crocodiles with a high and narrow muzzle (it is assumed that their way of life resembled modern Komodo monitor lizards), and in the Eocene there appeared fororakos that survived until the Pleistocene - giant (up to 3 m tall) flightless birds of prey, belonging to the cranes

In Australia (Notogaea), the formation of the grass biome began much later, in the Neogene; here, the drift of this continent in the direction from the pole to the equator clearly played a role - as a result, a significant part of its territory fell into an arid climate.

The basis of the local community of pasture mammals was made up of large herbivorous marsupials - kangaroos and diprotodonts, which have become extinct in human memory (they are sometimes, due to two large incisors, not very well called "rabbits the size of a rhinoceros"). As in the ancient South American fauna, there is a distinct shortage of predators here: only two large-sized predatory mammals are known - the thylacine (Tasmanian marsupial wolf) and the arboreal thyla-coleo, which by analogy can be called the "marsupial leopard". The lack of mammal predators was compensated (again, as in South America) at the expense of reptiles - gigantic megalanium monitor lizards up to 7 m long and land crocodiles, similar in lifestyle to secosuchia; flightless birds of prey did not arise here, but some of the Australian ostriches, apparently, served as scavengers.

The third case of grass biome formation is Arktogeya. Here the situation is complicated by the fact that it is formed on a single taxonomic basis (condylarthrian), but, apparently, independently in Eurasia and North America. The community of grazing mammals is originally made up of odd-toed ungulates (tapirs, rhinos in the broad sense and chalicotheres) and non-ruminant artiodactyls (sviniformes and camels); a little later, primitive three-toed horses and ruminant artiodactyls (deer) are added to them (55). In addition to the descendants of condylarthras, only dinocerates, specialized descendants of some extremely primitive therian mammals, tried to master the niche of large herbivores (55, b), but this group completely died out already in the Eocene.

The unity of the "northern" ungulate complex is quite high; the most interesting thing is that although almost all of these groups have American origin(they penetrated into Eurasia through Beringia - the area around the Bering Strait, where vast areas of the shelf then dried up), grass biomes with their participation in Asia begin to take shape much earlier than in America. In Central Asia, savannas appear already at the end of the Eocene (giant hornless rhinos like indricotherium appeared at that time - “a hybrid of an elephant and a giraffe”, the largest land mammal 6 m high at the withers ~ clearly lived in an open landscape, and not in a forest), while in America this occurs in the Oligocene. In Africa, however, grass biomes do not appear to have existed before the Miocene; artiodactyls and equids penetrated here from Eurasia relatively late, and proboscideans endemic to this continent (elephants and mastodons) were at that time small and purely leaf-eating and, apparently, could not maintain succession at the treeless stage.

As for predatory mammals, in the north they, in contrast to southern continents, were only placental: marsupials generally existed here for a very short time and never managed to leave the niche of small insectivores. Before specialized carnivorous forms of creodonts (56, a) and modern carnivores (Carnivora) appeared in these parts, peculiar ungulates, mesonychids, labored in this role (56, b-c). Mesonychids were omnivores (thought to be "more carnivorous than the boar, but less carnivorous than the bear"); they often reached the size of a hyena, and the Andrewsarchus from the Paleocene of Inner Mongolia was the largest terrestrial predatory mammal- its skull reaches a length of 85 cm. Surprisingly, it is from mesonychids that cetaceans originate.

Before the Oligocene, the situation in the grass biomes of Arctogea and South America developed in parallel. Both there and there, the main herbivores were ungulates, descendants of various condylartrs (in the North, unpaired and artiodactyls, in the South - "South American ungulates"). Both there and there, predators were clearly more primitive than their victims (marsupials in the South, archaic omnivorous ungulates, mesonychids in the North): an alignment that strikingly distinguishes the Paleogene from the Mesozoic. Both there and there, the shortage of mammal predators was compensated by reptiles and birds: land crocodiles (Sebecosuchia in the South, Baurusuchia in the North) and giant flightless crane-like creatures (Phororakos in the South, Diatryma in the North). This situation changed radically when modern carnivores (order Carnivora) entered the evolutionary scene, and this is best seen just by the immediate disappearance in the Oligocene of all these "ersatz predators" - omnivorous mesonychids, land crocodiles and diatryms, as well as creodonts. (ancestral carnivore). Interestingly, at the same time, archaic hoofed herbivores - dinocerates - also disappear.

In the Miocene, the unity of the territories of the Northern Hemisphere increases: there is a direct trans-Mediterranean contact between Europe and Africa, the disappearance of the Turgai Sea in place Western Siberia facilitates migration between Europe and Central Asia. The main thing is that open landscapes appear in the hitherto purely forested Beringia, and this territory turns for the steppe faunas of Asia and America from a “filter” into a “corridor”. Since that time, the grass biome has become virtually unified throughout Arktogea, and each of the territories contributes to the formation of its fauna, which is already acquiring quite modern features.

From America come herbivorous (in the sense - not leaf-eating) horses, from Asia - bovids (bulls and antelopes), from Africa - proboscis (elephants and mastodons); together with some other groups of ungulates, both “new” (giraffes and hippos) and “old” (rhinos), they form the so-called hipparion fauna (hipparion is one of the three-toed horses). The same picture is with the carnivores that are part of the hipparion fauna: cats originated in America, canines - initially too, but flocking social organization(which has become a key success factor for this group) was acquired already in Asia, hyenas (then among them were not only scavengers, but also active predators such as the cheetah) - in Africa. Interestingly, cats were originally saber-toothed; later, in the Miocene, cats of the modern type arose, however, a return to saber-toothedness (which, obviously, gives advantages when hunting large prey with a strong skin) occurred in felines repeatedly and independently.

At the beginning of the Pliocene (7-8 million years ago), nature itself staged a grandiose evolutionary experiment: land communications between North and South America through the Isthmus of Panama, and their faunas - North American (which is part of Arctogea) and South American (which stayed in island isolation throughout the Cenozoic) - came into direct contact with each other. There was a mixing of faunas: in the North, marsupials, edentulous (armadillos, glyptodonts and ground sloths), kavimorphic rodents and fororakos appeared, in the South - higher rodents (hamsters), equids (horses and tapirs), artiodactyls (sviniformes, camels and deer), proboscis (mastodons) and carnivores (raccoons, mustelids, canines, bears and cats).

The final results of the Great American exchange(as these events were called by J. Simpeon, 1983) turned out to be, however, very different for the North and the South. The North American fauna was simply enriched by three exotic "immigrants" (opossum, nine-banded armadillo and tree porcupine), while in the South there was a real catastrophe, cleaner than any asteroid impact: here the entire pasture complex of "South American ungulates", giant kavimorphic rodents, completely died out, carnivorous marsupials and fororakos, which could not withstand competition with higher ungulates and carnivorous predators (57). It must be assumed that the fate of the Australian marsupials and monotremes, if this continent had direct land contact with Asia, would have been just as unenviable ... In general, it is easy to see direct (and sad) analogies with human history in the history of the Great American Interchange: let's remember what turned out "contact" with European civilization for the ancient original cultures of pre-Columbian America and Black Africa.

The offspring of living beings are very similar to their parents. However, if the habitat of living organisms changes, they too can change significantly. For example, if the climate gradually becomes colder, then some species may acquire more and more dense wool from generation to generation. This process is called evolution. Over millions of years of evolution, small changes, accumulating, can lead to the emergence of new plant and animal species that differ sharply from their ancestors.

How does evolution take place?

At the heart of evolution is natural selection. It happens like this. All animals or plants belonging to the same species are still slightly different from each other. Some of these differences allow their owners to better adapt to the conditions of life than their relatives. For example, a certain deer has especially fast legs, and every time he manages to escape from a predator. Such a deer is more likely to survive and have offspring, and the ability to run fast can be passed on to its cubs, or, as they say, be inherited by them.

Evolution has created countless ways to adapt to the difficulties and dangers of life on Earth. For example, horse chestnut seeds eventually acquired a shell covered with sharp thorns. The thorns protect the seed as it falls from the tree to the ground.

What is the rate of evolution?

Previously, these butterflies had light wings. They hid from enemies on tree trunks with the same light bark. However, about 1% of these butterflies had dark wings. Naturally, the birds immediately noticed them and, as a rule, ate them before others.

Usually evolution proceeds very slowly. But there are times when a species of animal undergoes rapid changes and spends not thousands and millions of years on it, but much less. For example, some butterflies have changed their color over the past two hundred years in order to adapt to new living conditions in those parts of Europe where many industrial enterprises have arisen.

About two hundred years ago in Western Europe began to build coal-fired factories. The smoke from the factory chimneys contained soot, which settled on the trunks of the trees, and they turned black. Now the bright butterflies are more noticeable. And a few previously dark-winged butterflies survived, because the birds no longer noticed them. From them came other butterflies with the same dark wings. And now most of the butterflies of this species that live in industrial areas have dark wings.

Why are some animal species going extinct?

Some living beings are unable to evolve when their environment changes drastically and die out as a result. For example, huge hairy animals that look like elephants - mammoths, most likely died out because the climate on Earth at that time became more contrasting: it was too hot in summer and too cold in winter. In addition, their numbers have declined due to increased hunting for them by primitive man. And after the mammoths died out and saber-toothed tigers- after all, their huge fangs were adapted to hunt only large animals like mammoths. Smaller animals were saber-toothed tigers inaccessible, and, left without prey, they disappeared from the face of our planet.

How do we know that man also evolved?

Most scientists believe that man evolved from animals that lived on trees, similar to modern monkeys. The proof of this theory are some features of the structure of our bodies, which, in particular, allow us to assume that once our ancestors were vegetarians and ate only the fruits, roots and stems of plants.

At the base of your spine there is a bone formation called the coccyx. This is all that's left of the tail. Most of the hair that covers your body is just soft fluff, but our ancestors had much thicker hair. Each hair is equipped with a special muscle and stands on end when you get cold. So it is with all mammals with hairy skin: it retains air, which does not allow the heat of the animal to escape.

Many adults have wide outer teeth - they are called "wisdom teeth". Now there is no need for these teeth, but at one time our ancestors chewed with them the hard plant food they ate. The appendix is ​​a small tube attached to the intestines. Our distant ancestors, with its help, digested plant foods that were poorly absorbed by the body. Now it is no longer needed and is gradually getting smaller and smaller. In many herbivores - for example, rabbits - the appendix is ​​​​very well developed.

Can humans control evolution?

Humans drive evolution some animals are over 10,000 years old. For example, many modern breeds of dogs, in all likelihood, descended from wolves, packs of which roamed near the camps of ancient people. Gradually, those of them that began to live with people evolved into a new kind of animal, that is, they became dogs. Then people began to specially raise dogs for certain purposes. This is called selection. As a result, there are over 150 different dog breeds in the world today.

• Dogs that could be taught different commands, like this English Sheepdog, were bred to herd cattle.
• Dogs that could run fast were used to chase game. This greyhound has powerful legs and runs with huge leaps.
• Dogs with a good sense of smell were bred specifically for tracking down game. This smooth-coated dachshund can rip rabbit holes.

Through natural selection, as a rule, proceeds very slowly. Selective selection allows you to dramatically accelerate it.

What is genetic engineering?

In the 70s. 20th century scientists have invented a way to change the properties of living organisms by interfering with their genetic code. This technology is called genetic engineering. Genes carry a kind of biological cipher contained in every living cell. It determines the size and appearance every living being. With the help of genetic engineering, it is possible to breed plants and animals that, say, grow faster or are less susceptible to any disease.

Digestive system

Digestion This is the process of mechanical processing of food in the alimentary canal and the chemical breakdown of nutrients by enzymes into their constituent parts:

• food proteins are broken down into amino acids;

• dietary fats break down into glycerol and fatty acids;

• Complex dietary carbohydrates are broken down into glucose and other simple sugars.

In addition to proteins, fats and carbohydrates, other important compounds come to us with food, both organic, such as vitamins and other biologically active substances, and inorganic, such as water, mineral salts.

• First stage of digestion. Oral cavity

• Chewing food is the first stage in the process of digestion. Saliva starts digestion and turns food into a soft mass. This mass becomes slippery, making it easier to swallow and pass food further down the esophagus. Before entering the stomach, food passes through the esophageal sphincter.

• Third stage of digestion. Small intestine

Diluted food passes through the pyloric sphincter and enters the first section of the small intestine - the duodenum. Here, enzymes from the pancreas, liver, and gallbladder break down food into elements that are easily absorbed and can be used by the body. The small intestine is lined with folded mucosal tissue and finger-like villi. With the help of villi nutrients enter the bloodstream. It is in the small intestine that vitamins and nutrients are absorbed.

• Fourth stage of digestion. Colon

Liquefied food travels 20 feet in the small intestine before passing through the colonic valve into the large intestine. Here digestion practically does not occur. Undigested masses that have entered the large intestine are unprocessed waste. They become harder and harder as they pass through the intestines as fluid is constantly being absorbed from them.