In most living things, fertilization occurs when the gametes of two compatible creatures are brought together. In mammals, these gametes are the (female) egg and the (male) sperm. When they meet, the egg and sperm will fuse into one body, allowing genetic information from the two parents to combine. As the offspring grows, it displays genetic traits similar to each of its parents, as well as traits uniquely its own. In this way, genetic diversity is preserved. However, there exists in nature a phenomenon known as parthenogenesis ("virgin birth"), in which an egg cell can develop into an offspring without being activated by a sperm. In such cases, the offspring, as well as its siblings, would be genetically identical to the mother. Parthenogenetic offspring can be male or female, and haploid (chromosomally normal) or diploid (chromosomally doubled), depending upon the species.
Natural parthenogenesis was discovered by the Swiss naturalist Charles Bonnet in the early 1740s, who noted the phenomenon in spindle-tree aphids. Still a very young man, he announced his discovery and quickly went on to other research. It was not until much later in life that Bonnet, his eyesight failing, turned his attentions toward the philosophical ramifications of his discovery. Since a sperm was not necessary for reproduction, he surmised, then the egg must be the key to the preservation of the species. He went on to hypothesize that within every egg there existed a daughter creature, perfectly formed, that contained within it another egg containing another daughter, ad infinitum.
While this theory explained the phenomenon of parthenogenesis, it seemed to indicate that every species was genetically fixed, without the possibility for evolutionary change. This was highly disputed by archaeologists, who had found fossil remains of creatures unlike anything seen in our time. Bonnet explained that these creatures had perished in world-wide catastrophes; these catastrophes happened periodically, wiping out all life on the planet. After each catastrophe, life on earth would "step up" a notch on the evolutionary ladder, remaining at that level until the next catastrophe. Bonnet continued by asserting that further catastrophes would advance apes into men and men into angels. This evolutionary theory became known as catastrophism.
In addition to its philosophical impact, the discovery of parthenogenesis created a stir among biologists who feared that, since eggs could reproduce alone, the male gender might be superfluous. As scientists began to learn more about genetics and heredity, however, it became clear that the parthenogenetic offspring were genetically inferior to those produced through fertilization.
Research on parthenogenesis continued for many years, and in the early years of the twentieth century, two American scientists, working separately, discovered that parthenogenesis could be triggered artificially. Working with near-transparent sea urchins, Jacques Loeb and Ernest Just each used a variety of methods to activate an egg cell without the presence of a sperm. They found that temperature changes, seawater solutions, diluted acids, and even thin needles could be used to stimulate parthenogenetic reproduction.
By observing the offspring of this artificial parthenogenesis, scientists were able to study the importance of the egg and sperm in the transfer of genetic information. For example, it is now known that the sperm plays little part in the formation of the embryo--its only essential task is to contribute chromosomes to the offspring.
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