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How the Sexes Evolved

The distinction between males and females is one of the oldest facts of biology—but how did it come to affect our social identity?
(Photo: Crystal Eye Studio/Shutterstock)

(Photo: Crystal Eye Studio/Shutterstock)

Every Friday this month we’re taking a look at the relationship between the social and the biological—specifically, how and why the former becomes the latter. Check back next week for another installment.

One way to understand how and why the social becomes biological is to flip the question: How does biology become social? One of the best ways to see how biology drives social behavior is to look at one of the oldest social relationships in the living world: sex.

Sex and its central role is biology is the main reason why men and women are different, but it's hard to tease apart biology from culture. Anatomical differences are obviously biological, while a quick survey of various human societies is enough to see that many differences between men and women are cultural and liable to change. The distinction between males and females, however, is one of the oldest facts of biology. Except for bacteria, nearly all life reproduces sexually at least some of the time, and two distinct sexes are generally necessary to do this.

Sex and its central role is biology is the main reason why men and women are different, but it's hard to tease apart biology from culture.

Just why sex is such a popular method for transferring genes to the next generation is still a bit of an enigma, but it's clear that distinct male and female sexes emerged multiple times throughout evolutionary history. It happened in the ancestors of plants and animals more than a billion years ago. That's too far back for us to recover even sketchy details about how the original distinction between males and females evolved. Fortunately for us, we have an "evolutionary time machine" of sorts: the volvocine green algae. It's a family of present-day species that nicely illustrates the evolutionary transition from a single-celled organism without very distinct sexes, to a multi-cellular one with genuine males and females.

Volvocine algae are like a time machine because, in the relatively brief period of 200 million years, this group evolved from a free-living, single-cell ancestor into a variety of multi-cellular life forms. To study this evolutionary process, biologists can compare a present-day single-cell species, Chlamydomonas reinhardtii, with its much larger, multi-cellular relative, Volvox carteri. Volvox has males and females, while Chlamydomonas doesn't; by comparing the two, we can learn about how sexes evolve.

Chlamydomonas sex is typical for a single-cell sexual organism: primitive. These algae come in two different "mating types," called "plus" and "minus." Instead of issuing specialized male and female reproductive cells, this organism transforms its entire one-celled self into a gamete and seeks out a partner of the opposite mating type. Like two poles of a magnet, "plus" and "minus" organisms aren't that different from each other. The differences that do exist serve mainly to allow one mating type to recognize and fuse with the other.

Volvox sex, on the other hand, is surprisingly familiar. These algae, made up of about 2,000 cells, exist as distinct males and females. The males produce sperm that fertilize the eggs of the females, producing a zygote that develops into an embryo. Sound familiar? But here's the stunning fact about it: This whole reproductive process evolved from scratch in algae within the last 200 million years, during a time when reptiles, amphibians, mammals—descended from a long line of sexually reproducing ancestors—were roaming the Earth. The two sexes of Volvox are an evolutionary rerun.

SO HOW DO YOU get from mating types to males and females? That's the question asked by a trio of researchers, at the Danforth Plant Science Center in St. Louis and the Salk Institute in La Jolla, in a study published this summer. The scientists began from the observation that distinct sexes have evolved repeatedly in life's history, and the result is nearly always the same: females who produce relatively few large, immobile eggs, and males who produce many small, mobile sperm. To understand the genetic basis of Volvox males and females, the researchers tried swapping genes between the sexes.

They were interested in one gene in particular, called MID. Male Volvox have this gene, while the females don't. This gene exists in single-celled algae as well, where it acts as a master mating type switch that prevents "minus" from turning into "plus." The scientists reasoned that MID might do something similar in Volvox. They transferred a copy of this male-specific gene into females, which became pseudo-males and started making sperm. And when they deleted MID from the males, the algae became pseudo-females and started making eggs. Their results show that this master switch gene "control[s] two very different manifestations of sexual reproduction." This gene was reprogrammed from a simple regulator of primitive mating types into one that controls the major differences between females and males.

The oldest and most fundamental way that males and females differ is in their reproductive cells. Once you have sex-specific reproductive cells, new sex-specific evolutionary strategies come into play. These drive the development of even more physical and behavioral differences between males and females. New genes fall under the control of the master switch to become sex-specific genes. In Volvox, about a dozen genes are found only in males or females. In humans, there are about 80 protein-coding genes on the Y chromosome that are specific to men. The human master switch is called SRY, and like MID in Volvox, its primary job is to switch on the genetic program to produce sperm.

There is no doubt that the trend in human societies is to exaggerate the differences between men and women—typically by emphasizing the supposedly lesser capabilities of women. Then society changes, and sex differences that many people took for granted as biological turn out to be social after all. Certainly compared to many species, the differences between human males and females are small. But those biological differences drive so much of our social behavior, for better and for worse, and the case of Volvox algae show how the differences between men and women probably began.