If all reproduction were accomplished through mitosis, life would be quite dull because mitosis produces only genetically identical progeny. With only mitosis, you, your children, your parents, your brothers and sisters, your cousins, and many people whom you don’t even know would be clones—copies of one another. Only the occasional mutation would introduce any genetic variation. All organisms reproduced in this way for the first 2 billion years of Earth’s existence (and some organisms still do today). Then, about 1.5 billion to 2 billion years ago, something remarkable evolved: cells that produce genetically variable offspring through sexual reproduction.

The evolution of sexual reproduction is among the most significant events in the history of life. By shuffling the genetic information from two parents, sexual reproduction greatly increases the amount of genetic variation and allows for accelerated evolution. Most of the tremendous diversity of life on Earth is a direct result of sexual reproduction.

Sexual reproduction consists of two processes. The first is meiosis, which leads to gametes in which the number of chromosomes is reduced by half. The second process is fertilization, in which two haploid gametes fuse and restore the number of chromosomes to its original diploid value.