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.