This is an extraordinarily complex system of eukaryotic cell division which makes sexual reproduction possible. In multicellular organisms (and some algae) each cell may contain an entire set of chromosomes from each parent. For sexual reproduction to occur, the number of chromosomes in an adult cell has to be reduced to only one set in each gamete (eggs or sperm). Through an elaborate process of two successive divisions (called meiosis), sex cells are produced which differ in their collection of traits from the collection in either parent. Scientists have no idea what kind of process could have led to the development of such a system, especially as there is nothing like it in prokaryotic organisms (which lack an organized nucleus).