The most primitive life forms on our planet, such as bacteria and yeast, reproduce by simple cell division or by budding. More complex, multicellular organisms have developed specialized organs which release male and female gametes; these cells, each containing a single copy of the parent's genome, combine through the process of fertilization to produce a new individual. This process, known as sexual reproduction, has been occurring for more than a billion years and has played a crucial role in the evolution of life on Earth.
Most marine and freshwater animals release their gametes (sperm and eggs) directly into the water, where fertilization occurs and the developing embryos are at the mercy of the elements (including natural predators). Most terrestrial plants, on the other hand, retain the female gamete on the parent plant (within the flower or cone) while the male gamete (pollen) is spread by wind, rain or wildlife (insects, birds, bats). Higher life forms, including reptiles, birds and mammals, transfer the male gamete (sperm) via mating and the embryo develops within the female's uterus or within eggs protected by shells.
In most life forms, sexual reproduction is timed to offer the best chance of survival for the developing offspring (and/or for the mother that will nurse it). Spring, providing milder conditions and plentiful food, is thus a common season for births and hatchings and, depending upon the gestation period, fertilization is timed accordingly. Humans, governed by a complex neuro-chemical system and capable of adaptation, are not as tied to the seasons; while the human sex drive may wax and wane, it is present throughout the year. Many industries have taken advantage of this fact while others, including puritanical religions, have attempted to deny its persistence and have suffered the consequences.
