NPR logo Our Family Tree: Chimps, Bonobos And Our Commonality


Our Family Tree: Chimps, Bonobos And Our Commonality

This figure is adapted from The Demonic Male (Mariner Books, 1997) by Richard Wrangham. Mariner Books hide caption

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Mariner Books

This figure is adapted from The Demonic Male (Mariner Books, 1997) by Richard Wrangham.

Mariner Books

As anticipated last week, I’m here at the Great Ape Trust in Iowa, where I’ve spent a fantastic afternoon with Sue-Savage Rumbaugh and seven bonobos. Since I’ll also be here tomorrow, I’ll blog about the full experience next week. This week I’ll continue the great-ape theme by walking through the above image — Our Family Tree — and reflecting on some of its implications.

Charles Darwin is best known for his recognition that Variation and Natural Selection serve as the drumbeat of biological evolution. But fully as important, to my mind, was his articulation of the concept of Common Ancestry: all modern beings share ancestry, via countless convergences, with an original cellular creature (as expanded here). Darwin’s one published drawing offers a spindly sketch of this key insight, a sketch that looks for all the world like contemporary understandings of evolutionary relationships, like Our Family Tree.

A young Bonobo enjoys a blade of grass. Joachim S. Müller/via Flickr hide caption

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Joachim S. Müller/via Flickr

A young Bonobo enjoys a blade of grass.

Joachim S. Müller/via Flickr

An important spin on this idea, developed by Richard Dawkins in his marvelous book The Ancestor’s Tale, is the concept of the Most Recent Common Ancestor or MRCA. I pull my students into the MRCA concept with this image documenting that persons named Samuel Hinkley and Sarah Soole, of Barnstable MA in the mid-17th century, were the MRCAs of Barack Obama and George Bush. Obama and Bush for sure share common ancestry with fish and worms and amoebae, but their most recent common ancestors are Hinkley and Soole.

The MRCA of humans, chimpanzees, and bonobos is indicated by the dashed circle in Our Family Tree. Circa 6 million years ago — an eyeblink in evolutionary time — there occurred a bifurcation, with the Homo lineage moving along one path to present times and the Pan lineage moving along a second. More recently, some one to two million years ago, a second bifurcation occurred, yielding Pan troglodyte (common chimpanzee) and Pan paniscus (bonobo).

So here’s the six-million-dollar question: What was the human-chimp-bonobo MRCA like? A great ape for sure. But what about behavior? Was it Homo-like? Pan-like? And what do we mean by such distinctions?

Well, the short answer is that nobody knows the answer. But there are some interesting ways to frame speculation along these lines.

If we start with modern chimps and bonobos, they manifest some striking behavioral differences. 1) Chimp societies are characterized by strong male dominance hierarchies, whereas bonobo societies have strong female dominance hierarchies. 2) Chimp males have been documented to engage in warfare with neighboring troops and kill troop members, whereas such behavior has not been observed in bonobos. 3) Chimp males are known to engage in infanticide, again a behavior unreported in bonobos. 4) Chimps engage in sex only when females are in estrus (“heat”), at which times males make great efforts to monopolize females and hence guarantee paternity. By contrast, bonobos engage in sex often (ten times per day has been reported) and throughout the estrus cycle, and seem quite disinterested in keeping track of paternity. 5) Homosexual sex has not been observed with chimps, whereas it occurs frequently between female and often between male bonobos.

If we now take this list and apply it to humans, we come to an intriguing realization: depending on the culture and, indeed, on the individual, we can recognize both patterns. We find dominant males and dominant females; we find warlike and peaceable cultures and persons; we find restricted and laissez-faire sexual practices; and we find heterosexual, bisexual, and homosexual preferences.

Putting all this together, then, a case can be made that 1) the MRCA to the three lineages was endowed with both chimp-like and bonobo-like tendencies; 2) in the Pan radiation, these bifurcated into two distinctive sets of behaviors in troglodtye and paniscus; and 3) in the Homo radiation, the “mixed bag” persisted.

Speculation? For sure. But the next time you’re at a human gathering and find yourself bored, you might amuse yourself by engaging in a P. troglodyte vs. P. paniscus taxonomic exercise.

The humans at the gathering will, of course, be using syntactical symbolic language nonstop. This is our defining trait. The project at the Great Ape Trust is to ascertain the extent to which bonobos can learn elements of that trait when they are raised in a Pan/Homo culture. More next week!