Male and female embryos share tissue origins
The tissues that become male and female genitalia are similar in many ways
The clitoris is an extension of the same tissue that becomes the penis
Why do men have nipples?
The labia majora and the scrotum come from the same fetal tissue
Some of the nerves in the female reproductive system also control erections
In human development, all fetuses start out looking like each other. The embryonic cells form into the structures necessary for survival, but they don't always differentiate into specific male or female parts until much later on. In fact, the embryo itself isn't even considered to be one sex until about week six of gestation, when testosterone levels rise enough to cause the formation of male genitals. Before then, it could go either way, depending on hormonal influences. This means that there are several homologous sexual structures between males and females – body parts that look different, but actually come from the same basic starting place. Let's take a closer look at some of these.
The clitoris is often called "the female penis" because it looks so similar to its counterpart. It's made up of the same spongy tissue as the shaft and head of the penis, with the clitoral glans being a small bud of this tissue surrounded by protective skin. Like the penis, it can swell during arousal, and some women have reported experiencing orgasms through clitoral stimulation alone. However, unlike the penis, it doesn't serve any reproductive purpose. So why does the clitoris exist? Some scientists believe that it may be an evolutionary holdover from when humans were more promiscuous and needed a reliable means of pleasure without risking pregnancy. Others think that it simply developed along with the rest of our sexuality.
Men also have nipples, though they're less prominent than those on women. They start out as the same tissues as female breasts, but hormones prevent them from growing as large or protruding as much. Nipples are thought to play a role in breastfeeding, which both genders have been capable of doing for millennia. In fact, many male mammals have nipples and lactate (produce milk) as well. The reason for this is still unclear, but some theories suggest that males who could provide nourishment for offspring would be more attractive to females looking for good genes.
The labia majora and scrotum come from the same fetal tissue. During development, these areas become enlarged and grow hair, but in men, testosterone causes them to remain smooth while the body uses them to create something else entirely. Similarly, the uterus, vagina, and clitoris all develop from the same tissues, though only the former two are necessary for reproduction. This suggests that human embryos begin life with a sort of "default" gender, which can then be modified by hormonal influences later on.
Some nerves in the female reproductive system actually control erections. These nerves extend into the spinal cord and are activated when the brain sends signals for arousal. They affect the vagina, clitoris, and even the anus – meaning that some women can achieve orgasm through anal stimulation alone. Interestingly, there's no corresponding set of nerves in the male reproductive tract, so it's not clear why they exist at all. It may just be a side effect of the way our sexuality developed over time.
In summary, male and female embryos share tissue origins for several body parts. The clitoris resembles a penis in many ways, and both sexes have nipples. The labia majora and scrotum start out as the same tissue before becoming distinct. Finally, some nerves in the female reproductive system help cause arousal in both genders. While we don't know exactly why this is the case, it provides fascinating insights into human development and evolutionary history.