Humans boast much bigger penises than our primate ancestors, and an Australian scientist might have finally figured out why.
According to University of New South Wales' evolutionary biologist Darren Curnoe, the size of a primate's genitalia comes from the level of competition involved in impregnating females.
Chimpanzees have medium-sized penises compared to humans, but their testicles are larger.
Female chimps desire the best DNA for their children, and having sex with as many males as possible increases their chances of obtaining this.
The testicles of chimps are therefore large because they have to contain a lot of sperm to compete with other males.
Male gorillas, on the other hand, don't have to worry about this.
Their social groups designate one alpha male who gets to have sex with as many females as he wants.
The lack of competition and diminished need for sperm has resulted in small testicles and penises for gorillas.
Curnoe believes the size of the human penis can partially be explained by our upright posture.
Walking on two feet brings the penis out in the open, where it can attract females.
Human females also don't willingly make themselves available like female chimps do, so the male penis must advertise itself to solicit sex.
It seems that because presentation is a factor in obtaining mates, the male penis must be large and appealing.
Another reason is the different environments humans live in compared to primates.
Early humans were subjected to freezing temperatures that were made more bearable thanks to large penises, which release heat.
Curnoe suggests the human penis is large partially because it's needed to keep the body warm.
Here's the biologist giving a more thorough explanation in his exclusive web series "How Did We Get Here?":