Whether you’re a couple of inches more or less than the average penis size, chances are you’re a lot larger than your primate cousins. In the latest “How Did We Get Here?” video series for the University of New South Wales, Australia, evolutionary biologist Darren Curnoe explains how evolution has shaped the human penis to be larger than its primates, the chimpanzee or gorilla, and why there exists such a discrepancy.

Variations in penis size boil down to the evolutionary pressures associated with reproduction. These pressures vary from species to species. According to Curnoe, the size of a primate’s genitalia comes from the level of competition involved in impregnating females.

“Amazingly the size of the reproductive organs, especially the testicles, are a reflection of how the apes are organized socially,” said Curnoe in the video. For example, chimpanzees have medium-sized penises and large testicles — much larger than humans — because there is a lot of sperm competition between males. If a female is “receptive to multiple males in each reproductive episode,” it maximizes her chances of getting high-quality DNA. Therefore, this encourages chimpanzee males to fight to fertilize the egg.

Unlike chimpanzees, gorillas’ social groups assign one alpha male who gets to have sex with as many females as he wants. Therefore, gorillas are more likely to have small penises and small testicles. This is due to a lack of competition and the diminished need for sperm.

Now, Curnoe asks, “Why is ours the longest?” He believes the size of the human male penis can be partially explained by the upright posture. Walking on two feet makes leaves the penis out in the open with the possibility of attracting females. Since human females don’t make themselves readily available like female chimpanzees, the human male penis must advertise itself, making it large and appealing.

The environment in which humans live in also factors in penis size. Early humans had to endure freezing temperatures that were more tolerable due to large penises, which release heat. The human penis helped regulate body temperature keeping it warm and cool.

Science still has a lot of uncover when it comes to the penis, but studying primates can help lead to a better understanding about why it has evolved the way it has. While evolution can help explain the human male penis size, Curnoe admits, “There is still much more to the penis that meets the eye.”