Human Anatomy Part 1 - K. A. Dyubenko, A. K. Kolomiysev, Yu. B. Chaykovsky 2002

General Part
Man and the Environment

The Emergence of humans is a natural outcome of the evolution of living organisms on our planet. This complex process, which, according to Academician V. L. Vernadsky, has spanned over 2 billion years, involves A number of stages. If we exclude chemical stages—those preceding the appearance of living organisms—the initial period of life's development includes the emergence of prokaryotic organisms, Eukaryotic Cells, and subsequently, Multicellular Organisms. The appearance of Multicellular animals, which were already part of Earth's fauna about 700 million years ago, dramatically accelerated the evolution of living beings. To a large extent, this was driven by The Development of epithelial, connective, muscular, and nervous Tissues specialized for various Functions. In turn, the emergence of tissues led to The formation of Organs composed of different tissue groups. The development of organ complexes performing similar functions (systems or apparatuses) drove a further elevation in the organizational level of multicellular animals. About 500 million years ago, the species diversity of multicellular structures increased significantly, and 65 million years ago, the Age of Mammals began.

Mammals, representing the highest Class of vertebrates, belong to the phylum Chordata, since an Axial Skeleton (the notochord) appears during Embryogenesis and later undergoes reduction. Driven by the Progressive development of the Central Nervous System, the appearance of Hair, the evolution of warm-bloodedness (homeothermy), the transition to intrauterine embryonic development, and the nursing of offspring with milk—which significantly increases the survival rate of descendants—mammals gained the upper hand in the competitive struggle with other vertebrates. In addition to traits shared with other vertebrates, mammals possess specific structural features that determine their higher position on the "evolutionary ladder." In particular, these include:

1. The presence of a more complexly organized Integumentary System which, in addition to Connective Tissue (dermis) and epithelium (epidermis), includes a number of Skin Appendages (hair, sebaceous, and Sweat Glands). Mammary Glands are also derivatives of the skin.

2. A more distinct division of the spinal Column into cervical, thoracic, lumbar, sacral, and caudal regions.

3. Significant development of striated Muscles and the presence of a thoracoabdominal partition (the Diaphragm), which takes an active part in respiration. Mammals have a more developed subcutaneous Muscle layer, which is secondarily reduced in apes and humans over most of the body, but highly developed on the face, where it forms mimic (facial) muscles.

4. A higher level of development of digestive glands and the alimentary canal, with a clear division into distinct sections. Through evolution, mammals developed complexly structured Teeth.

5. Intensive Development of the Respiratory system, performing the function of gas exchange (external respiration).

6. Structural reorganization of the excretory system, manifesting as the loss of connection between the Urinary Bladder and the cloaca, and its opening directly into the external environment via the Urethra.

7. The presence of a four-chambered Heart, whereas among mammalian ancestors—reptiles—only crocodiles possess such a heart.

8. A high level of development of Sensory Organs and the higher centers of the central nervous system, specifically the cerebral hemispheres and the Cerebellum.

9. The presence of a specialized organ, the Uterus, in which the INTRAUTERINE DEVELOPMENT OF the embryo takes place.

Paleontological evidence indicates that anthropoid apes appeared about 35 million years ago, and only 5–10 million years ago did the divergence of the evolutionary lines of apes and humans occur. The most pronounced processes characterizing anthropogenesis took place within the last million years, while the anatomical emergence of modern humans (Homo sapiens) was completed merely 40 thousand years ago.

When evaluating the material factors that led to THE ORIGIN OF humans, domestic literature long discussed Charles Darwin's theory, which argued for the existence of an ape-like human ancestor whose evolution was governed by natural Selection, and Friedrich Engels' labor theory, which identified labor as the primary driving force of anthropogenesis (The Essence of this theory is revealed in his work "The Part Played by Labour in the Transition from Ape to Man"). While Darwin's theory attributed decisive importance to the biological factors of anthropogenesis (natural selection), Engels' theory emphasized social factors.

Today, when defining The Role of biological factors in anthropogenesis, scientists rely on the principles of genetics. Apparently, significant Mutations affecting various organs and systems arose in certain populations of anthropoid apes, consistently securing the fixation of traits in the genotype that brought the anatomy of these animals closer to that of humans. This is supported by paleontological data showing that many Structural and functional indicators characteristic of humans arose during anthropogenesis long before the appearance of humans themselves. For example, fossils of prehistoric higher primates have been discovered whose Skeletal Structure indicates bipedal locomotion. Alongside these, skeletons belonging to animals whose Skull and hand Morphology brought them closer to humans have also been unearthed. Numerous data on the skeletal structure of higher primates suggest that during the Initial Stages of anthropogenesis, a substantial reorganization of hormonal regulation took place, which, by influencing the genotype, stimulated the Formation of the physical type of modern humans.

Examination of fossil human remains has revealed skull features indicating a general increase in Brain volume and the emergence of new areas in the Cerebral Cortex associated with cognitive, linguistic, and labor processes in humans. The result of these changes was the emergence of hominid family structures with characteristic traits that defined the rise of humanity: 1) bipedalism; 2) an upper limb free from locomotor support, featuring a developed hand; and 3) a highly developed brain. Brain development was accompanied by the remodeling of the facial skeleton and facial muscles, as well as the formation of the brain's speech centers.

The capacity of hominids for verbal communication, the refinement of upper limb motor skills (especially the hand), and the Increasing complexity of the brain gradually led to the development of labor skills, an active impact on the external environment, and progressive forms of communication, which drove the formation of human communities. The increasing complexity of the hominid brain and the development of social relations became the primary conditions for a new stage in Human Evolution—anthroposociogenesis (B. A. Nikityuk, A. I. Kozlov)—which was accompanied by selection for sociality.

Humanizing selection for sociality transformed humans into a part of the "human-society" system, subordinating them to the demands of this system. These factors ensured the stabilization (fixation) of human species-specific traits, whose physical appearance has remained practically unchanged over the past 40 thousand years.

Academician V. L. Vernadsky made one of the most profound Conclusions: that the emergence of life led to the formation of the biosphere on Earth—a system uniting all living things. Humans are the most complex component of the biosphere, as their emergence is the natural result of the evolution of life. At the same time, the appearance of humans on Earth, and particularly of human communities, exerted a feedback effect on nature, becoming a powerful ecological factor. The expansion of agricultural activity, the establishment of settlements (including megacities), the growth of mineral extraction, the development of nuclear energy, and anthropogenic interventions in our planet's geography have led to environmental changes, elevating the biosphere into a new state—the noosphere.

Thus, under METABOLISM/18.html">The Influence of scientific and technological progress in the 20th century, the entire biosphere became the object of transformative human activity. In turn, the noosphere does not always exert a favorable or minor influence on the human Organism. Growing environmental pollution and the accumulation of substances with mutagenic properties not only affect human health but also lead to an increased frequency of congenital anomalies. Studying these aspects of Environmental Impact on the human organism constitutes one of the tasks of anatomy.

Because living organisms, including humans, are open systems that exchange matter and energy with their external environment, changes in environmental conditions trigger a complex of reactions in the body aimed at preserving Homeostasis (the relative constancy of the internal environment), which is vital for the survival of cells, tissues, and organs under new living conditions. Such reactions are collectively termed adaptation (from Latin *adaptatio*—adjustment). A distinction is made between phenotypic adaptation—as a defense reaction to the action of a damaging factor—and genotypic adaptation, which results from the selection of organisms with new traits best suited to altered environmental conditions. For instance, the colonization of new ecological regions by hominids contributed to the emergence of new physical traits. Fixed genetically, these traits led to the formation of different races. Groups of humans finding themselves in different Regions of the Earth with similar environmental conditions often acquired similar physical features As a result of genotypic adaptation.

For humans, alongside the biological habitat, the social environment has acquired essential significance. The Nature of labor activity, professional specialization, living conditions, dietary habits, and other factors significantly influence bodily anatomy. The adaptive capacities of the organism are limited by its degree of physiological reliability. Exposure to excessively strong stimuli or the intense action of ordinary factors can lead to a breakdown of adaptive processes and the onset of disease.