Fundamentals of Evolution - O.P. Korzh - 2006

Part III. PATHS OF LIFE EVOLUTION

Chapter 20. Main Stages in the Evolution of Multicellular Animals

20.8. Progressive Development of Terrestrial Features

The first true terrestrial animals can be considered only the reptiles, which emerged in the Carboniferous period and became the dominant forms in the Mesozoic era.

First and foremost, this was driven by the appearance of the amniotic egg, which allowed reptiles to completely free themselves from dependence on the aquatic environment at all Stages of Ontogeny. The amniotic egg possesses special embryonic membranes that protect the embryo from drying out (primarily the amnion). The next major achievement of reptiles during ontogeny was the disappearance of metamorphosis, with all transformation processes being shifted to the embryonic Selection/3.html">Stages of development (The process of Embryonization of ontogeny). In addition, the amniotic egg reduces the NEGATIVE IMPACT OF the external environment on the overall course of embryonic development, thereby increasing the autonomization of reptile ontogeny.

A whole series of significant morphological and physiological transformations in reptiles allowed them to follow a path of broad adaptive radiation and group progress. Horny scales appear in the integument, densely covering the entire body and protecting it from mechanical damage and moisture loss (the scarcity of Skin glands also significantly reduces moisture loss). Such skin Structure allows adult reptiles to become independent of humidity and occupy quite arid landscapes, notably deserts. Furthermore, a substantial reorganization of Water-Salt METABOLISM takes place overall: a true (pelvic) Kidney is formed, producing primarily urea and uric acid, which significantly reduces water excretion from the body. From then on, the function of Water and Salt Metabolism is almost entirely taken over by the Kidneys, resulting in a more precise regulation of excess moisture and metabolic waste excretion.

Amphibians expend a significant amount of energy on keeping their torso elevated above the ground surface, which is why reptiles undergo an active restructuring of their free limbs: in most forms, locomotion occurs on semi-extended limbs, while in the most advanced ones, the limbs shift beneath the torso (moving as close as possible to the center of gravity, which reduces the energy cost of maintaining the body in this posture).

The need to strengthen the support for the free limbs leads to The formation of the chest (ribcage), consisting of Ribs connected to the Sternum. Strengthening the support of the Pelvic Girdle occurs by increasing the number of sacral vertebrae to two. With the increased agility of animals, the need arises to process a greater volume of information received by the Organism, which is achieved by enhancing HEAD mobility. This process occurs in two ways:

1) increasing the number of cervical vertebrae to 7-10;

2) substantial transformation of the first two vertebrae: the atlas loses its centrum (acquiring a ring-like shape), and the axis articulates with the centrum of the atlas, forming a tooth-like process (odontoid process) around which the atlas rotates.

Since the skin ceases to act as an oxygen supplier, there arises a necessity to intensify respiratory movements and increase the relative surface area of the respiratory exchange. The latter occurs in almost all amniotes due to the Increasing complexity of the internal lung structure: the pulmonary saccules of amphibians gradually transform into multi-chambered spongy Lungs, to which air is delivered via the Trachea and Bronchi. Inhalation and exhalation are accomplished through the Formation of the rib cage, in the movements of which intercostal and Abdominal Muscles participate.

The structure of the Circulatory system is directly related to the efficiency of gas exchange. The loss of cutaneous Respiration and the increased efficiency of pulmonary respiration create conditions for the final Separation of arterial and venous Blood. To achieve this, a partial septum develops in The Heart, dividing the ventricle into arterial and venous chambers, along with special Valves that regulate the sequential flow of blood through the aortic arches. Such a mechanism ensures a fairly efficient distribution of blood flows. It is believed that already some highly developed dinosaur forms possessed a four-chambered heart and true endothermy (warm-bloodedness).

All these adaptations did not arise simultaneously. For instance, cotylosaurs (the earliest reptiles) differed from their ancestors practically only in their reproductive features (the amniotic egg). At the end of the Permian, therapsids emerged—active, predatory animals that were quite widespread at the time. Their elbow and knee joints were positioned close to the lateral surface of the torso (parallel adaptations arose independently in several archosaur groups). However, the appearance in the Triassic of other large groups of more advanced reptiles, notably dinosaurs, led to the extinction of the majority of therapsids (only small forms survived). Throughout the Triassic, Jurassic, and Cretaceous periods, mammal-like reptiles, and subsequently the small mammals that evolved from them in the Triassic, remained under heavy ecological pressure from dinosaurs.

Among saurischian dinosaurs, large long-legged predators prevailed, hunting primarily herbivorous dinosaurs (mammals were very small at the time). In the aquatic environment, forms such as plesiosaurs and ichthyosaurs dominated, while pterosaurs ruled the skies. Nevertheless, precisely because of the pressure exerted by dinosaurs, mammals developed such highly efficient adaptations that after the disappearance of the latter in the Cenozoic era, they became the dominant terrestrial forms.

Among the significant achievements of mammals, their intellectual capacities should be noted first and foremost, in which they significantly surpass reptiles.

The Development of intelligence became possible only after the combination of several key aromorphoses that had previously arisen independently in various reptile representatives, but individually could not produce such an elevation in the overall level of Organization.

In mammals, metabolic processes are accelerated thanks to the final separation of the SYSTEMIC AND PULMONARY circuits in the heart (two ventricles are formed), which significantly increases the efficiency of tissue gas exchange. An increase in the total volume of the lungs, which occupy almost the entire thoracic cavity, and their internal differentiation (The Emergence of alveoli) facilitate better oxygen uptake, while the formation of a Diaphragm improves the inhalation-exhalation mechanisms. In the kidneys, the renal portal system undergoes reduction, which improves metabolism and allows for a more consistent maintenance of internal Homeostasis. Through the development of Hair and skin glands, mammals acquire an efficient thermoregulation mechanism that normally allows them to maintain a constant body Temperature regardless of environmental conditions (homeothermy). This, in turn, requires substantial energy expenditures, and consequently, enhanced Nutrition. A differentiated dental system enables mammals to pursue broad adaptive radiation regarding the food they consume.

Fundamental changes also occur in mammalian reproduction: the embryo begins to develop within the female reproductive tract (in the Uterus), where the most advanced forms develop a special adaptation—the Placenta. This makes it possible to significantly increase reproductive success, since the embryo receives necessary nutrients from the female's body, excretes metabolic wastes, and becomes inaccessible to predators. The newborn offspring is fed with milk and cared for by the female for a considerable time, which ensures better survival of the animals and allows young individuals to learn—acquiring the skills necessary for subsequent independent existence.

Aside from those listed, there are several other adaptations that allowed mammals to undergo broad adaptive radiation and ensure survival in the most diverse environments. However, most of these adaptive transformations are significant only for individual mammalian groups in accordance with their specific living conditions (allogenic transformations).

A large number of traits characteristic of mammals also appear in birds, which branched off from reptiles during the Jurassic period. However, owing to the fact that birds are adapted primarily to Life in the aerial environment, they acquire many specialized features. This group of vertebrate animals originated not from pterosaurs—which were capable of flight via a skin membrane stretched between the elongated fourth finger of the forelimbs and the body (complex flight)—but from another group, the only known representative of which today is Archaeopteryx.

The primary evolutionary acquisition of birds is feathers, which arise, much like hair in mammals, through the transformation of reptile scales. Feathers provide effective thermal insulation and the capacity for flight. This ability demands a very intensive metabolism, which is why birds also undergo a separation of the systemic and pulmonary circuits, though unlike mammals, they retain the right aortic arch. The efficient terrestrial respiratory mechanism in birds is similar to that of mammals, except for the absence of a diaphragm, but in flight, it becomes insufficient. To compensate, birds develop air sacs in which gas exchange does not take place; they serve as reservoirs. These sacs are inflated and deflated with the raising and lowering of the wings, respectively, ensuring that Gas Exchange in the lungs occurs during both inhalation and exhalation. A special valvar mechanism ensures the continuous unidirectional flow of air through the avian respiratory pathways. The countercurrent oxygen uptake mechanism enables birds to soar high into the sky, where oxygen pressure drops significantly. A high demand for nutrients is supported by active Digestion and constant foraging.

All this leads to the fact that birds, like mammals, are warm-blooded animals with well-developed mechanisms for maintaining homeostasis, while the heightened intensity of metabolic processes ensures a higher body temperature than in mammals.

Unlike mammals, birds reproduce by laying relatively large eggs (in small species, the total clutch weight is almost equal to the weight of the female). Established parental care for the offspring makes their reproduction no less efficient than that of mammals.