who fail to care for their young, cads who have extramarital affairs, and trigger-happy murderers wh

who fail to care for their young, cads who have extramarital affairs, and trigger-happy murderers who take the lives of innocent people. What changes in the brain enabled us, but no other species, to engage in promiscuous thinking? To understand what changed in the brain, it is useful to paint a few broad-stroke comparisons, and then narrow in on the details. We know, for example, that brain size changed dramatically over the course of our evolutionary history, ultimately reaching three times the size of a chimpanzee’s brain with the appearance of the first modern humans some 100-200,000 years ago. From the archaeological evidence, we can infer that some aspect of the internal workings of the brain — not simply size — must have changed at about the same time in order to explain the appearance of a new material culture of tools with multiple parts and functions, musical instruments, symbolically decorated burial grounds, and cave paintings. Before this period, the material culture of our ancestors was rather uncreative, with simple tools and no symbolism. The new material culture was heralded by a mind unlike any other animal. No other animal spontaneously creates symbols, though chimpanzees and bonobos can be trained to acquire those we invent and attempt to pass on. No other animal creates musical instruments or even uses their own voice for pure pleasure. No other animal buries its dead, no less memorializes them with decorations; ants drag dead members out of their colony area and deposit them in a heap, though this is driven by hygiene as opposed to ceremonial remembrance and respect. Only a species with the capacity to combine and recombine different evolved specializations of the brain could create these archaeological remains. This period in our evolutionary history marks the birth of our promiscuous brain. The brain sciences have helped us see the fine details of this new species of mind. The comparative anatomists Ralph Holloway, James Rilling, and Kristina Aldridge have analyzed brain scans and skull casts of humans and all of the apes: chimpanzees, bonobos, gorillas, orangutans, and gibbons. This sample represents approximately 15 million years of evolution, and includes considerable diversity in mating systems, dietary preferences, use of tools, group size, life span, locomotion style, communication system, aggressiveness, and capacity for cooperation. Thus, gibbons are monogamously pair bonded, live in small family groups in the upper canopies, swinging and singing to defend their territories, never use or create tools, are omnivorous, restrict cooperation to within the family group, and show little aggression. Gorillas are folivores or leaf eaters, live in harem societies, knuckle walk on the ground, rarely use or make tools in the wild, show aggression primarily between harems, communicate with a diversity of sounds, and show limited cooperation even under captive conditions. Chimpanzees are promiscuous, omnivores who hunt for meat on the ground and in the tree tops, create a diversity of tools that are culturally distinctive between regions, communicate with a diversity of sounds, are lethal killers when they confront individuals from a neighboring community, and are cooperative especially in competitive situations. Despite this diversity, nonhuman ape brains are much Hauser Chapter 1. Nature’s secrets 29 HOUSE_OVERSIGHT_012775