Current scientific evidence suggests that life first appeared in Earth's oceans about 3.6 billion years ago. These initial organisms were unicellular, bacteria-like species that reproduced by simple division, leading to two identical individuals. Over time and countless generations, natural genetic mutations produced diversity and, about 2.5 years ago, intracellular organelles began to appear; these are thought to have developed when smaller cells and viral-like particles invaded larger cells. It is also believed that unicellular organisms began to exchange DNA-containing plasmids with one another, providing a means to repair lethal genetic mutations.
Nucleated cells (eukaryotes) were present by 1 billion years ago and multicellular organisms inhabited shallow seas near the end of the Precambrian, some 600-700 million years ago. It was also during this period that the process of meiosis became more widespread, resulting in cells with one half of the adult organism's genetic material (i.e. a single set of chromosomes). These cells, known as gametes, combined through fertilization to produce a new individual with a full compliment of paired chromosomes; sexual reproduction has since become the dominant means of procreation throughout the plant and animal kingdoms.
The rise of sexual reproduction, initially an adaptation to combat lethal mutations, has slowed the rate of population growth but has dramatically increased the rate of evolutionary diversification. Acted upon by natural selection, the gene mixing that occurs with sexual reproduction helps to insure that life-sustaining traits are retained and that life-threatening defects are removed from the population. The resulting diversification of life forms is readily apparent; while life evolved from bacteria to shell-bearing marine invertebrates during the first 3 billion years of its history, the rise of sexual reproduction has produced a vast diversity of species, plant and animal, marine and terrestrial, extinct and living, within the last 600 million years. We humans, of course, owe our existence to this turn of events.
