With 6.8 billion humans on Earth, humans have evolved a successful reproductive system. Three concepts define human reproduction: sexual reproduction, internal fertilization and the use of a placenta to support internal development of offspring. Each reproductive option has advantages and disadvantages for continuation of the species.
Sexual Reproduction
Species reproduce their genes by sexual or asexual reproduction. Asexual reproduction or cloning has advantages because the species can produce a large number of identical genetic clones quickly from one individual. Human sexual reproduction is more complicated in that two individuals must cooperate to combine their reproductive cells. Individuals of opposite sex must be able to find each other for mating and have complementary organs to combine their reproductive cells. Sexual reproduction produces fewer offspring than asexual reproduction.
The advantage of sexual reproduction is that it creates genetic diversity. Combining the genes of two individuals creates new genetic combinations, which may be better suited to changing environments and have future advantages for survival of the individual. Asexual reproduction or cloning consistently reproduces the same genes in succeeding generations which may become suboptimal in a changing environment.
Internal Fertilization
Humans use internal fertilization in which fertilization of the male and female gametes--sperm and eggs--occurs within the body of the female. Some fish use external fertilization in which the female lays her eggs and the male swims over the eggs, releasing his sperm. Human fertilization occurs internally, in the fallopian tube, in a more protected environment.
Humans evolved specialized reproductive organs in the male and female to allow combination of gametes internally. The male's testicles produce sperm and hormones, which regulate semen production and male sexual function. The male has a penis to deliver sperm to the female so internal fertilization can occur. The female supports internal fertilization by having a vagina to receive the penis and fallopian tubes where fertilization can occur. The early development of the embryo happens in the fallopian tube as the embryo travels to the uterus for implantation.
Placental Mammal
Placental mammals, like humans, support the development of their offspring internally until birth. The entire development of the human fetus occurs within the mother, which is possible because humans have a placenta. In contrast, marsupials have internal fertilization but then their offspring are born at a very immature stage and must crawl into a maternal pouch where they attach to a teat for nutrition and continued growth until they are able to live independently.
In humans, the early embryo implants into the uterus where a specialized organ called the placenta forms. The placenta develops from a combination of embryonic trophoblasts and uterine cells. The embryo produces a hormone called human chorionic gonadotropin, hCG, which signals the maternal ovary to continue producing progesterone to support the pregnancy and prevents shedding of the pregnancy with menses. The placenta supports the developing fetus by providing maternal nutrients and oxygen and removing fetal waste. The placenta also protects the fetus from the immune system of the mother, which would otherwise react against the fetus because half of its genes are from the father and so produces "foreign" proteins. The placenta also produces hormones near term that help mature fetal organs for life after birth.
The advantage of using a placenta for reproduction is that the human fetus is protected within the mother throughout its development. The disadvantage is that the mother is placed at greater risk of complications from a longer more invasive pregnancy.
