Human Reproduction: Conception, Development, and Birth

The Nature and Evolutionary Logic of Sexual Reproduction

Humans have many unique features as sexually reproductive beings. Unlike single-celled organisms, humans do not divide into identical clones. Instead, the species relies on the ability of two sets of genetic material to be combined and passed on to the resulting offspring. As in reptiles and all mammals, fertilization takes place inside the body of the female. "This technique increases the chances of successful sexual reproduction" (Freudenrich 2010, p. 1). It is also a far more complex process than mitosis, or simple cell division.

Despite the protections provided by human internal reproduction, there are many inefficient aspects of sexual distinction. Sexual reproduction, from an evolutionary standpoint, has disadvantages because of its inherent instability. If the goal is maximizing the spread of one's offspring in the interests of the survival of the species, it is "costly" in terms of time and energy. "Special reproductive cells and structures must be constructed," and our sexual organs are specialized for this purpose in a manner that leaves the male sexual organs vulnerable to injury and the female vulnerable throughout her pregnancy during a long gestation period (Human reproduction and development, 2010).

However, sexual reproduction could be argued to have generated a rich human culture centering on the process of love and courtship. In all sexually reproducing organisms, "hormones, colors, scents must be developed. Reproductive timing, finding and recognizing mates, sperm and egg available at the same time must occur," and "the developing embryo and young must be cared for, often for years or decades" (Human reproduction and development, 2010). "From the child's viewpoint he or she is about as dependent as any in the placental mammals. This dependency is extraordinarily prolonged and the child requires parental care for many years" (Johnson 2010). That is assuming reproduction takes place at all: males and females may reject one another and fail to reproduce, or the sperm and egg may not connect. The embryo and developing child is highly vulnerable to external and internal threats throughout the mother's pregnancy.

So why has sexual reproduction continued to survive in multi-cellular organisms? The genetic diversity that results from sexual reproduction can be highly beneficial for the hardiness of a particular species. Having a greater variety of genetic combinations circulating in the gene pool allows more potentially adaptive responses to the environment to manifest. If there is a sudden change in the environment, genetically varied offspring may survive better. For example, during the Industrial Revolution, a particular species of light-colored moth with dark mutations was able to survive because dark-colored mutated moths within its gene pool could camouflage themselves from predators against soot-darkened surfaces (Miller 1999). This demonstrates how genetic mutations can give certain species a comparative survival advantage over others within the same area. Sexual reproduction also mitigates the ability of potentially negative traits to be passed on to offspring uniformly — though, of course, mutations can also be negative for the species, which is yet another downside of sexual reproduction.

Fertilization: Sperm, Egg, and the Formation of the Zygote

Most humans think of sexual intercourse as beginning with a mysterious emotion like love or lust that has nothing to do with the hard facts of science. But from a biological point of view, this sensation has a very practical purpose: to ensure that the male uses his exterior sex organs to impregnate the female. "From the outside, the male has two visible sex organs, the testes in a sac called the scrotum and penis. When engorged with blood during sexual excitation and intercourse, the spongy tissue stiffens and causes the penis to become erect, which is important for the penis's main function — to place the sperm inside the female" (Freudenrich 2010, p. 2). The sperm and egg then meet to create a fertilized embryo.

"Eggs develop inside the ovary and are released upon ovulation into a tube (the oviduct or Fallopian tube) lined with finger-like projections. The egg travels through the Fallopian tube, where fertilization can take place, to a muscular chamber called the uterus" (Freudenrich 2010, p. 3). If the egg is not fertilized, the uterus will gradually shed its lining through the process of menstruation. "Reproduction is not a continuous activity and is subject to certain patterns and cycles. Oftentimes these patterns and cycles may be linked to environmental conditions which allow organisms to reproduce effectively" (Bailey 2010, Sexual reproduction).

The male's sperm are far more numerous than the female's eggs, and far smaller. The number of sperm is designed to improve the chances that fertilization will occur. When "a single sperm penetrates the mother's egg cell, the resulting cell is called a zygote. The zygote contains all of the genetic information (DNA) necessary to become a child. Half of the genetic information comes from the mother's egg and half from the father's sperm. The zygote spends the next few days traveling down the fallopian tube and divides to form a ball of cells" (Vorvick 2009). The zygote resembles both the mother's and the father's genetic profile, but is a combination of both organisms and thus exhibits unique features — unlike an organism that is the result of cell division or cloning.

Early Development: Blastocyst, Implantation, and the First Trimester

As the zygote divides, it creates an inner group of cells with an outer shell. "This stage is called a blastocyst. The inner group of cells will become the embryo, while the outer group of cells will become the membranes that nourish and protect it. The blastocyst reaches the womb (uterus) around day 5, and implants into the uterine wall on about day 6. At this point in the mother's menstrual cycle, the lining of the uterus has grown and is ready to support a baby. The blastocyst sticks tightly to the lining, where it receives nourishment via the mother's bloodstream" (Vorvick 2009).

The blastocyst is distinct from an embryo because it "represents such an early stage of embryonic development that the cells it contains have not yet differentiated, or taken on the properties of particular organs or tissues — kidneys, muscles, spinal cord, and so on. This is why the stem cells that are extracted from the blastocyst hold the promise of developing, with proper coaxing in the lab, into any kind of cell the researcher wants to study or repair" (Sandel 2009). The blastocyst is effectively a blank slate, although it possesses a full copy of the resulting organism's potential DNA.

The next three weeks are some of the most critical weeks of the developing embryo's life. The process of differentiation occurs at a rapid pace, including the formation of the organism's blood cells, kidney cells, and cells that will eventually form the body's nervous system. The first trimester is the period when development is most likely to be negatively impacted by maternal nutritional deficiencies, alcohol or drug use, or infections. The organs most essential for human life are taking shape. During week 3 of gestation (week 5 of pregnancy), the brain, spinal cord, and heart begin to develop along with the gastrointestinal tract (Human reproduction and development, 2010). The embryo develops so rapidly that this is one reason it is so important for the woman to find out as early as possible if she is pregnant, in order to alter her diet and lifestyle accordingly.

During weeks 4 to 5 of gestation (weeks 6 to 7 of pregnancy), arm, leg, eye, and ear structures begin to form; the brain develops into five distinct areas; and heart and blood circulation develops. By week 7, all essential organs have formed, and by week 8 of gestation (week 10 of pregnancy), external features such as facial characteristics are evident (Human reproduction and development, 2010).