The English word “woman” is derived from the word “wifman,” which was formed by adding the word “wife” onto the word “man.” However, it has been discovered that at the genetic level, it is the female organism that becomes the male organism, by means of the “male” gene. The origins of the two sexes (male and female) are now becoming clear.

• 1. The Creation of Adam
• 2. Tumbling fast and furiously
• 3. Before the division into male and female
• 4. The chivalrous gene

Almost all plants and living creatures are divided into two clearly defined sexes—male and female. Consequently, most people would expect that the very start of life on Earth began with two distinct sexes. In fact, the first living organisms were almost identical in every way.

One species related to the Volvox—Chlamydomonas—has two gametes that join together and are of the same shape. The two gametes are indistinguishable from one another and therefore it is impossible to distinguish between male and female. Reproducing in this way—with two gametes of the same shape—is known as isogamy.

In contrast, Pleodorina, more evolved than Chlamydomonas, is a single organism consisting of a collection of cells (see diagram below). Pleodorina’s male gamete (sperm) and female gamete (egg) differ in shape. This is known as anisogamy—the opposite of isogamy. Male Pleodorina produces sperm packets, which it squeezes out of its round, gelatinous body. The sperm enters the body of the female and this leads to fertilization of the female’s reproductive cells.

The evolution of the male gamete into sperm and of the female gamete into an egg is believed to have been for the purpose of ensuring the birth of the next generation. In other words, the female gamete became an egg in order to ensure the provision of nourishment, while the male gamete—the sperm—acquired mobility, since the egg is large and immobile.

Where are the genes that determine male and female? Seemingly basic research to answer this question took off with the 2006 discovery of the male gene otokogi. The word “otokogi” (chivalry) denotes an inclination towards helping the weak. Professor Nozaki, who discovered and named this gene, retained deep and abiding memories of a University of Tokyo Komaba festival poster that was produced in 1968—at the height of the student riots. The pattern on the poster was the university’s logo in the form of a tattoo on a man’s back, in imitation of contemporary gangster movies about chivalry. Inspired by this, he named the male gene discovered at the same university “otokogi.”

The otokogi gene was successfully extracted from Pleodorina. Using a new species of Pleodorina that Professor Nozaki and his team had discovered independently, they manufactured large amounts of male Pleodorina and identified the chromosomes that determine maleness. This gene was known to operate within the nucleus of Pleodorina sperm and to be characteristic of maleness. This represented the discovery of the gene that undoubtedly determines the maleness of an organism.

Incidentally, in the case of isogamous Chlamydomonas, with its identically shaped gametes, one is, for descriptive purposes, termed the “plus” and the other, which joins with the plus gamete to produce offspring, is termed the “minus.” Later, the sex-determining gene MID (minus dominance), which is only present in the latter gamete, was discovered. However, it is not known which of the two (the plus or minus) evolves into the male and which into the female.

Even before the otokogi gene in Pleodorina was discovered, Professor Nozaki’s team predicted that it would closely resemble the MID gene in Chlamydomonas. This was intuitive in a way that only researchers who had long focused on genes would understand. As expected, the otokogi gene and the MID gene had evolved from a common progenitor. This made it clear that the isogamous minus gamete evolved into a male and the plus gamete into a female.

It had already been discovered that in the case of isogamous Chlamydomonas, a minus gamete with the MID gene removed becomes a plus gamete. However, it is impossible to change a plus gamete to a minus gamete. In other words, the manipulation of genes can change a male (minus) to a female (plus), but not a female to a male. This means that the original sex is female, from which male derives.

Subsequently, in 2008, a German team discovered the female-specific gene, using species related to none other than Volvox. It is anticipated that sex determination will be better understood in the near future.

The process of understanding sex determination has only just begun.