Evolutionary Physiology

Evidence of evolution and divergence

To confirm divergent speciation, science uses the method of triple parallelism, which combines data from three independent disciplines:

• Comparative anatomy reveals structural similarities. A classic example is homologous organs. The forelimbs of vertebrates (from salamanders to humans) share a single skeletal structural plan, although they perform completely different functions (swimming, flying, grasping). This proves the divergence of traits from a common ancestor.
• Embryology is based on the laws of embryonic similarity and recapitulation.
• Paleontology studies transitional forms and constructs phylogenetic series based on fossil remains.

Embryological laws

Organism development is subject to strict rules formulated by Karl Ernst von Baer. In embryogenesis, general features of a large taxon (phylum, class) are laid down first, followed by highly specialized species traits. An embryo never copies an adult low-organized animal; it only resembles its embryo.

This principle is complemented by the biogenetic law (E. Haeckel, F. Müller, refined by A.N. Severtsov): individual development (ontogeny) is a brief repetition of the historical development of a species (phylogeny). In a human embryo, traits of distant ancestors can be observed:

• Neula stage: formation of the notochord, neural tube, and gill slits (characteristic of fish).
• Heart development: sequential transition from a two-chambered to a three-chambered structure.
• Temporary presence of a tail and a smooth surface of the cerebral hemispheres in the early stages.

Vestiges and atavisms

Specific morphological phenomena of the musculoskeletal system and integument serve as anatomical evidence of animal origin.

Vestiges are underdeveloped structures that have lost their original function but are invariably present in all members of the species. Examples in humans include:

• Coccyx bone (remnant of the tail region);
• Appendix and wisdom teeth;
• Darwin's tubercle on the auricle;
• Webbing between fingers and the third eyelid.

Atavisms are the sporadic appearance of traits of distant ancestors in individual people. They occur when the normal reduction of ancient embryonic rudiments is disrupted during embryogenesis. Examples include:

• True tail;
• Excessive hair growth (hypertrichosis, "dog-boy syndrome");
• Polymastia, polythelia.

Evolutionary principle of nervous system maturation

Evolutionary patterns are clearly manifested in the maturation of a child's nervous system. The process of myelination of nerve fibers proceeds heterochronously (non-simultaneously) and obeys the evolutionary principle: phylogenetically older structures become myelinated first.

Motor fibers ensuring vital functions (swallowing, breathing, sucking) mature first, followed by sensory fibers. The direction of myelination is always centrifugal — from the neuron body to the periphery. The spinal cord begins to myelinate in the 4th month of intrauterine development. Cranial nerves actively myelinate in the first months of life (the process is completed by one year, except for the vagus nerve — n. vagus). The pyramidal tracts fully mature only by 4 years of age, which clinically manifests as increased precision of voluntary movements.

By the time of birth, the spinal cord is morphologically much better developed than the brain. This explains the presence of basic spinal automatisms in the newborn. An infant's movements are chaotic due to the immaturity of the cerebellum and neostriatum, and motor development is at the pyramidal-striatal stage with a physiological predominance of flexor muscle tone.

Features of brain meninges and topography

In newborns, the dura mater (dura mater) is relatively thin and has extensive adhesions to the bones of the skull base. The arachnoid (arachnoidea) and pia mater (pia mater) are also thinned, with the latter being richly vascularized and cell-rich. The subarachnoid space is small.

Due to the difference in the growth rates of the vertebral column and nervous tissue, a displacement occurs: in an infant, the lower end of the spinal cord is located at the level of the third lumbar vertebra, whereas in an adult it ascends to the second. The final syntopic ratio is established by 5–6 years of age.

Class and exam questions

What else to study on this topic

• Timing of completion of brain structure differentiation (hypothalamus, cortex)
• Stages of development of visual and cervical motor coordination in the first weeks of life
• Molecular and ontogenetic mechanisms of the formation of atavistic malformations
• Indirect evidence of evolution: biochemical and genetic data
• Functional principle of nerve fiber myelination

Other topics in the "Additional Topics" section

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