Humans seat the top of the evolutional hierarchy of animals. The largest brain per body size among mammals has enabled us to conquer other species. We reproduce our offspring without fear of being interfered by other animals.
On the other hand, humans have longer penis than most other apes. At the same time, human's testicles are relatively small. This gap has been a mistery for many primatologists.
In a recent hypothesis, the size of the testicle is associated with the mating system in each species. For example, chimpanzees have polygamy system. Any male chimpanzees have sex with any females. Thus, the sperm of some male chimpanzees is mixed in a uterus of a female. Competition among males to get combined to the egg occurs. For getting the egg, a male chimpanzee has to produce much sperm. It is the reason for larger testicles they have got. On the contrary, gorillas have monogamy marriage system. Any female gorillas have only one male mate. And, a gorilla has smaller testicle.
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So, what is attributed to human's large penis? Darren Curnoe, a professor in New South Wales University, explained that it is a means of sex appeal. A person can stand up while walking. It means women can see directly the penis of the men. A larger penis imagines women the healthy and vigorous gene, even if it is a myth in real. Therefore, human's penis became much bigger through the history of evolution.
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However, this hypothesis ignited some objections. Alan Dixson, a primatologist, said that the idea that human penis is the largest among mammal is incorrect, suggesting hamadryas baboon with a bigger penis.
After all, it is not easy to make a rational explanation of our nature and society. The community style of human may have changed through long centuries. But the evolution from ape to human has a much longer history. It is difficult to track the line precisely. Nonetheless, investigating our ancestors is an exciting challenge.