Reproductive Adaptations
In the animal world, there are several ways of reproduction – asexual, when genetically identical organisms are formed, and sexual, when male and female gametes unite outside the body during external fertilization or inside the female’s body during internal fertilization. For humans, only sexual reproduction is possible, when haploid reproductive cells of two individuals combine to form diploid offspring (Phelan, 2021). Sexual reproduction is characterized by the process of gene combination, which is important for adaptation in unstable, unpredictable environments. Gender is determined genetically by the presence of X- and Y-chromosomes when homozygous XX individuals are female, and heterozygous XY individuals are male.
Menopause in humans and animals also has specific distinctive features. At the age of 45-55, women go through hormonal changes and stop reproducing offspring. Compared to animals, humans live quite a long time after the loss of reproductive function, whereas in most animals, reproduction continues almost until death (Gruen, 2018). Killer whales are an exception to this rule, they live 8-13 years after menopause, which is evolutionarily explained by the need for older females to support the offspring of younger females.
Development of a Human Embryo
Human embryonic development takes place in several stages, starting with fertilization and the formation of a single-celled zygote. The embryonic stage of development lasts eight weeks, after which the embryo is called a fetus. The process of embryogenesis includes changes in gene expression and cellular differentiation. During the embryonic stage, the zygote divides in the process of cleavage, and a blastocyst is formed, which is implanted in the uterus. Next, the stage of gastrulation begins, and the formation of three embryonic germ layers in the process of histogenesis: endoderm (inner layer), ectoderm (outer layer), and mesoderm (middle layer).
Cells derived from the endoderm form the inner linings of the body, including the gastrointestinal tract, lungs, liver, pancreas, upper urinary tract, and female vagina. The ectoderm eventually forms the outer coverings of the body, such as the epidermis and hair, and is the precursor of the mammary glands and the central and peripheral nervous systems (Ellenbogen, 2021). Finally, the mesoderm gives rise to the dermis of the skin, the tissues of the heart, the muscular system, the genitourinary system, the bones, bone marrow, and therefore the blood.
References
Gruen, L. (2018). Dismantling oppression: An analysis of the connection between women and animals. In Living with contradictions (pp. 537-548). Routledge.
Ellenbogen, E. (2021). Embryology of the Skin. In Skin and the Heart (pp. 3-9). Springer.
Phelan, J. (2021). What is life? A Guide to biology with physiology (5th ed.) W.W.H. Freeman Company.