Module 106. Human Biology –Evolution, Anatomy, Immunobiology & Physiology

Human biology is a comprehensive field that examines the structure, function, and evolution of the human body. It integrates principles from anatomy, physiology, genetics, and immunobiology to understand the mechanisms that sustain life and the evolutionary processes that shaped humankind. The discipline bridges biological science with medicine, anthropology, and biochemistry, offering insights into both the normal functioning and the pathological states of the human organism.

Human Evolution

The evolution of humans traces the gradual transformation of early primates into modern Homo sapiens over millions of years. Fossil and genetic evidence suggest that human evolution began approximately six to seven million years ago in Africa, diverging from a common ancestor shared with chimpanzees.
Early hominins, such as Sahelanthropus tchadensis and Australopithecus afarensis (notably the fossil “Lucy”), exhibited bipedalism — walking upright on two legs — which is considered a defining feature of human lineage. This adaptation freed the hands for tool use and allowed greater mobility across diverse environments.
The genus Homo emerged around 2.5 million years ago with Homo habilis, the first known toolmaker, followed by Homo erectus, who mastered fire and expanded into Asia and Europe. Anatomically modern humans (Homo sapiens) appeared roughly 300,000 years ago, distinguished by larger brain capacity, complex language, and cultural sophistication.
Genetic studies, including mitochondrial DNA analysis, confirm that all modern humans share a common African ancestry. Over time, evolutionary forces such as natural selection, genetic drift, and migration have led to the diversity of human populations.

Human Anatomy

Human anatomy is the scientific study of the body’s structure and organisation. It can be divided into gross anatomy, which deals with structures visible to the naked eye, and microscopic anatomy, which examines cells and tissues through microscopy. The human body consists of several major systems that work together to maintain life:

• Skeletal System: Comprising 206 bones, it provides support, protection, and movement. The axial skeleton includes the skull, vertebral column, and rib cage, while the appendicular skeleton includes limbs and girdles.
• Muscular System: Made up of skeletal, smooth, and cardiac muscles, responsible for movement, posture, and circulation.
• Nervous System: Consisting of the brain, spinal cord, and peripheral nerves, it coordinates bodily functions through electrical and chemical signals.
• Circulatory System: Includes the heart, blood, and vessels, transporting oxygen, nutrients, and hormones while removing waste products.
• Respiratory System: Involves the lungs and airways, facilitating gas exchange — oxygen intake and carbon dioxide expulsion.
• Digestive System: Converts food into energy and nutrients via organs such as the stomach, intestines, liver, and pancreas.
• Excretory System: Comprising kidneys and urinary tract, responsible for filtering blood and removing metabolic waste.
• Endocrine System: Produces hormones through glands such as the pituitary, thyroid, and adrenal glands, regulating metabolism, growth, and reproduction.
• Reproductive System: Ensures continuation of the species through gamete production and fertilisation.
• Integumentary System: The skin, hair, and nails protect the body and regulate temperature.
• Lymphatic System: A network of vessels and nodes aiding in immune defence and fluid balance.

Anatomical knowledge is essential for medical practice, surgery, and diagnostics, providing the foundation for understanding physiology and pathology.

Human Physiology

Physiology focuses on the dynamic functions of the human body — how organs and systems work together to maintain homeostasis, the stable internal environment necessary for survival.

• Circulatory Physiology: The heart functions as a muscular pump driving blood through systemic and pulmonary circuits. Oxygen and nutrients are delivered to tissues while carbon dioxide and wastes are removed.
• Respiratory Physiology: Gas exchange occurs in the alveoli of the lungs through diffusion, regulated by breathing mechanisms controlled by the medulla oblongata.
• Digestive Physiology: Food is broken down mechanically and chemically into absorbable molecules. Enzymes such as amylase, pepsin, and lipase facilitate digestion.
• Nervous Control: The central nervous system processes sensory input and initiates motor responses, while the autonomic system regulates involuntary functions such as heartbeat and digestion.
• Endocrine Regulation: Hormones act as chemical messengers influencing metabolism, growth, and reproduction. The feedback loops between hypothalamus, pituitary, and target glands maintain hormonal balance.
• Renal Function: Kidneys filter approximately 180 litres of blood daily, reabsorbing essential substances and excreting urea and other wastes through urine.
• Thermoregulation: Controlled by the hypothalamus, the body maintains optimal temperature through mechanisms like sweating and vasodilation.

Physiological processes are intricately interconnected; disruption in one system often affects others, leading to disease.

Immunobiology

Immunobiology, or immunology, is the study of the body’s defence mechanisms against pathogens such as bacteria, viruses, and fungi. The immune system consists of innate (non-specific) and adaptive (specific) components that work together to detect and neutralise harmful agents.

• Innate Immunity: The body’s first line of defence, including physical barriers (skin, mucous membranes) and cellular responses (phagocytes, natural killer cells). It provides immediate but non-specific protection.
• Adaptive Immunity: Develops after exposure to antigens and involves lymphocytes — B cells (which produce antibodies) and T cells (which mediate cellular responses). This system provides memory, allowing faster responses upon re-exposure to the same pathogen.
• Antigens and Antibodies: Antigens are foreign molecules that trigger immune responses. Antibodies, or immunoglobulins, are specific proteins that bind to antigens, marking them for destruction.
• Vaccination: A preventive application of immunobiology, vaccines introduce harmless antigenic material to stimulate immunity without causing disease.
• Autoimmunity and Immunodeficiency: Disorders such as rheumatoid arthritis and HIV/AIDS arise from malfunction of immune regulation, either through overactivity or deficiency.

Immunobiology is fundamental in medicine, guiding developments in vaccines, immunotherapies, and organ transplantation.

Integration of Systems and Homeostasis

The human body functions as an integrated whole, with organ systems interacting continuously to maintain equilibrium. Homeostasis regulates parameters such as temperature, pH, and blood glucose through feedback mechanisms. For example, insulin and glucagon maintain glucose balance, while the kidneys adjust water and salt levels to regulate blood pressure.
The nervous and endocrine systems serve as primary control centres, ensuring coordination across the body. Sensory feedback and hormonal signals enable the organism to adapt to environmental changes while maintaining internal stability.

Evolutionary Physiology and Adaptation

Human physiology has evolved in response to environmental pressures, diet, and lifestyle. Adaptations such as upright posture, opposable thumbs, and thermoregulation mechanisms illustrate evolutionary refinement for survival. Genetic evolution also influences physiological traits — for example, lactose tolerance in populations with a history of dairy consumption or sickle-cell trait conferring malaria resistance.
Modern challenges, such as sedentary behaviour and pollution, test these evolved systems, often leading to metabolic and cardiovascular diseases. Understanding evolutionary physiology helps contextualise such health issues in terms of adaptation and mismatch with contemporary environments.

Significance and Applications

The study of human biology underpins medicine, public health, and biotechnology. It enables understanding of disease mechanisms, drug action, and human development. Advances in immunology have led to breakthroughs in vaccine design and cancer immunotherapy, while knowledge of anatomy and physiology is central to clinical practice.
Human evolution offers insight into our origins and biological diversity, while modern research in genetics and molecular biology continues to refine understanding of human function and adaptation.