Properly speaking, the evolutionary branch that contains humans is known as the great apes, and contain orangutans, gorillas, humans, chimps and bonobos. The last four species are called primates.
By both fossil and genotype estimates, chimps and bonobos had a common ancestor 2.5 million years ago, humans, chimps and bonobos had a common ancestor 5.5 million years ago, and all four primates had a common ancestor seven million years ago.
Although each species has continued to evolve in the millions of years it has been separate, the path of its evolution is still constrained by the qualities of the common ancestors. So looking at our primate cousins tells us a great deal about the basic layout and behavior of humans.
Chimpanzees and bonobos diverged when the Congo River created an uncrossable barrier over two million years ago. The common ancestor that lived north of the river in mixed savannah and forest became the chimp. To the south, what became the bonobo lived in dense, lush forest rich with fruit and monkeys. Chimps had a more competitive environment and relied more on meat for food.
Adult male chimps can be as large as 150 pounds (68kg) and 5 foot 6 inches (1.68m) when standing erect. Bonobos are a similar size, but they are more slender and have longer arms and legs. Bonobos are also known for their flatter, expressive and individual faces.
Chimps are on the aggressive end of the primate scale, dominated by male power politics. The top chimp gets the most food and the most females, and stays on top until another chimp topples him, often in bloody fashion.
Bonobos, by contrast, are matriarchies, where the females create status hierarchies through family ties and erotic partnerships. Food and sex are shared throughout the community, and males acquire status through relationships with important females.
Frans de Waal in Our Inner Ape tells stories of chimp and bonobo emotion, planning, and action that are startling for their parallels to human patterns of reciprocity, conflict, and reconciliation. De Waal makes it clear that this is not just instinctual behavior. He says in Our Inner Ape,
What’s perhaps most significant about this research is not what apes reveal about our instinctual side. With their slow development (they reach adulthood at about sixteen) and ample learning opportunities, apes are really not that much more instinctual then we are. Apes make lots of decisions in their lives, such as whether to threaten a newborn or defend it or whether to save a bird or abuse it. What we compare, therefore, are the ways in which humans and apes handle problems through a combination of natural tendencies, intelligence, and experience. It’s impossible to extract from this mixture what is inborn and what is not.
What if human instincts are not what we think they are? What if they are only tools for our use, as they seem to be for our closest primate cousins. Chimps and bonobos demonstrate affection, empathy, rage, despair, ferocity, desire and loyalty. These behaviors seem simpler than the behaviors of humans, but it is clear that our cousins make distinctions as to when to display different behaviors, just as we do.
Bonobos and chimps are fascinating mirrors for humans in everything from violence to social status to sex. De Waal notes:
Human social organization is characterized by a unique combination of (1) male bonding, (2) female bonding, and (3) nuclear families. We share the first with chimpanzees, the second with bonobos, and the third is ours alone.
Understanding the primate roots of bonding and social behavior gives us reference points for how we are similar and how we are different from other primates. And helps us understand why we do what we do.