Evolutionary Quirks
Excerpt from “Why Knot?” Buy the book at Amazon
Consider the evolutionary value of the beer-belly gut. Your metabolism changes as you get older, such that you tend to get fatter even if you eat less and exercise more than you did before. That makes you less physically attractive, and hence less likely to stray from your mate. In other words, your physical deterioration makes it easier for you to shift your focus from seeking new sexual partners to caring for your family, with obvious survival benefits for your off-spring.
Also, consider the survival value of physical discomfort. Your arms are short, and their flexibility is limited, in contrast to other primates. You can’t scratch your own back. You need someone to scratch it for you. Depending on others to help you builds social relationships which are important for survival.
And, of course, human babies are born helpless. The more helpless the better, because that builds a stronger parent-child bond.