Why Is Sex Fun__ The Evolution of Human Sexuality - Jared M. Diamond [4]
Thus, even a non-mammalo-centric view reinforces our dog’s interpretation: we are the ones who are bizarre. We marvel at what seems to us the weird behavior of peacocks and big-bang marsupial mice, but those species actually fall securely within the range of animal variation, and in fact we are the weirdest of them all. Species-ist zoologists theorize about why hammer-headed fruit bats evolved their lek mating system, yet the mating system that cries out for explanation is our own. Why did we evolve to be so different?
This question becomes even more acute when we compare ourselves with our closest relatives among the world’s mammal species, the great apes (as distinguished from the gibbons or little apes). Closest of all are Africa’s chimpanzee and bonobo, from which we differ in only about 1.6 percent of our nuclear genetic material (DNA). Nearly as close are the gorilla (2.3 percent genetic difference from us) and the orangutan of Southeast Asia (3.6 percent different). Our ancestors diverged “only” about seven million years ago from the ancestors of chimpanzees and bonobos, nine million years ago from the ancestors of gorillas, and fourteen million years ago from the ancestors of orangutans.
That sounds like an enormous amount of time in comparison to an individual human lifetime, but it’s a mere eye-blink on the evolutionary time scale. Life has existed on Earth for more than three billion years, and hardshelled, complex large animals exploded in diversity more than half a billion years ago. Within that relatively short period during which our ancestors and the ancestors of our great ape relatives have been evolving separately, we have diverged in only a few significant respects and to a modest degree, even though some of those modest differences—especially our upright posture and larger brains—have had enormous consequences for our behavioral differences.
Along with posture and brain size, sexuality completes the trinity of the decisive respects in which the ancestors of humans and great apes diverged. Orangutans are often solitary, males and females associate just to copulate, and males provide no paternal care; a gorilla male gathers a harem of a few females, with each of which he has sex at intervals of several years (after the female weans her most recent offspring and resumes menstrual cycling and before she becomes pregnant again); and chimpanzees and bonobos live in troops with no lasting male-female pair bonds or specific father-offspring bonds. It is clear how our large brain and upright posture played a decisive role in what is termed our humanity—in the fact that we now use language, read books, watch TV, buy or grow most of our food, occupy all continents and oceans, keep members of our own and other species in cages, and are exterminating most other animal and plant species, while the great apes still speechlessly gather wild fruit in the jungle, occupy small ranges in the Old World tropics, cage no animal, and threaten the existence of no other species. What role did our weird sexuality play in our achieving these hallmarks of humanity?
Could our sexual distinctiveness be related to our other distinctions from the great apes? In addition to (and probably ultimately as a product of) our upright posture and large brains, those distinctions include our relative hairlessness, dependence