Comments on the Bonobo

Over the weekend, a number of biological anthropologists took a critical lens to some of my articles and blogposts. One of them found a pretty important mistake, but I figure this might be a good time to talk about some thoughts I’ve had regarding bonobos and human evolution.

Note: this post contains a number of academic articles, which can be accessed by following these instructions on how to use Sci-hub to get articles for free

Answering Fan Mail

Among the reviewers of my numerous essays and blogposts was the very esteemed Bill Jungers from Stony Brook University who correctly pointed out an error I had made in my Areo article, The Selfish Nature of Human Cooperation. The error stated:

“Recent evidence has supported the hypothesis that bonobos may be more closely related to the last common ancestor (LCA) of chimps and bonobos than chimpanzees are…”

To many biologists, the error is obvious, but for some reason I never put a second thought to it after writing the essay, and my editors were not possibly going to catch this technical error. The problem is that I said that bonobos, which are the sister species to chimpanzees, may be more closely related to the two species’ last common ancestor (LCA), but this is impossible. Well…it’s “technically possible” for complex reasons involving highly unlikely non-monophyletic hybridization that I won’t bore you hypothesizing about, but the point is that Koolakambas aside, this probably wasn’t the case with bonobos and chimpanzees. Because the last common ancestor of bonobos and chimps was their last common ancestor, it follows that the two are equally related to this ancestor!

The article has been changed to state that, “Recent evidence has supported the hypothesis that bonobos may be more similar to the last common ancestor of chimps and bonobos,” and Areo’s editor Helen Pluckrose was kind enough to even leave an editor’s note on the bottom of the article letting everyone know that the mistake has been corrected.

Last Common Ancestors

Just as a background, there are two species of chimpanzee: the bonobo (Pan paniscus) and the common chimpanzee (Pan troglodytes). The two split from one another roughly roughly 2 million years ago and both of them split from us roughly 5-7 million years ago.¹ The two species are separated by the Congo River, with the bonobo living on the south side and the chimpanzee living on the north side. Both species express different behavioral patterns, with the general idea being that bonobos are more peaceful than chimpanzees (more on this later). What’s interesting about bonobos is that the lack of gorillas in their range and larger patch sizes in terms of food availability has produced a very interesting pattern for them, to the point of bonobos possibly being peer-domesticated!²

What is this evidence that bonobos are closer to the last common ancestor of them and chimpanzees to begin with, anyway? In the essay, I am referring to this 2017 paper co-authored by Rui Diogo, Julia Molnar, and Bernard Wood.³ Taking a soft-tissue approach, the co-authors (as part of the Bonobo Morphology Initiative) dissected seven bonobos from the Antwerp Zoo in Belgium and compared findings from both the hard and soft-tissue of all the other ape taxa to build a phylogeny, or tree of relationships.

This is the phylogeny from Diogo et al. 2017. Each of the 166 squares represents a different trait, or character state in the forelimb and heads of these different species. A blank square indicates loss of an ancestral state while a shaded one indicates addition of a trait. Bonobos are Pan paniscus.

The researchers found that in most off their trees, it was the bonobos (Pan paniscus) which had undergone the smallest number of evolutionary changes since splitting from the last common ancestor, not chimpanzees. Highlighting this, the authors state, “…within all the 124 HN (head-neck) and FL (forelimb) muscles of bonobos there is not a single minor change – even including a simple site of origin or attachment, or a fusion, of a muscle – in the 2 [million years] of evolution of the bonobo lineage. This is a striking example of evolutionary stasis.” Italics are theirs.

Implications for Behavior

There are a lot of technicalities in the paper, but I accept the conclusions: bonobos have changed a lot less in the head/neck and forelimbs than chimpanzees have, and thus it’s possible that bonobos are more similar to the last common ancestor of chimps and bonobos. The question is what this actually means. The paper’s title states that their findings support, “bonobos as the most appropriate extant model for the common ancestor of chimpanzees and humans,” but I disagree with the way others have chosen to interpret their results (I’d like to note that I incorrectly stated that the authors of this paper made a comment about behavioral reconstructions, one of them reached out in the comments below for a correction). From Frans de Waal to Adrien Zihlman, biological anthropologists often like to point out that we shouldn’t be using chimpanzees as behavioral models for our last common ancestor, we should be using bonobos; and the authors of this study choose to add to this group by repeating this view at the end of their paper.

The fact that we more commonly invoke the more aggressive chimpanzee to understand human evolution, over the less aggressive, sexy-time loving bonobo, says more about how we naturalize violence than it does about violence's evolutionary origins. They are equally our relatives.

— Rick W. A. Smith (@rickwasmith) September 17, 2018

Proponents of this view have been argued that our model of violence in the human past is partially invalid because while we are closely related to the violent chimpanzee, we are equally as related to the bonobo, which is not known to perform inter-group raids and uses sex, rather than violence, to deal with upset individuals and to release social tension. For this reason, it’s likely that our chimp models are invalid and we should look to a bonobo model, instead.

I think neither are “ideal models” and disagree with a pro-bonobo argument for serveral reasons:

1. Phylogenetic Inertia: Given the stark behavioral, vocal, and now soft-tissue differences between chimpanzees and bonobos who split from one another just two million years ago, all I can really infer is that phylogenetic inertia (which refers to how strongly controlled the outcomes of a species are by its ancestry) is not very strong at all in this lineage! This indicates it’s equally likely that neither are ideal reconstructions because the lack of influence of their LCA in producing a similar species implies it.
2. Bonobo Uniqueness: I want to refer everyone to my friend Will’s blog where he discusses some unique features of bonobo behavior, among them being female dominance in an ape species and female coalitionary behaviors. Many of these characteristics aren’t in humans or chimps. Does this really mean to say bonobos are the best option we have for these reconstructions? Even over chimpanzees?

   

   A map showing the geographic ranges for all subspecies of chimpanzee (with the inclusion of the bonobo, in red).

3. Variation: A picture we really don’t get from bonobos that we get from chimpanzees is that of variation. Virtually every site you go to has chimps acting in different ways from chimps at other sites. There are four sub-species of chimps and one sub-species of bonobos. If anything, chimpanzees show us an LCA model based on what human origins looked like in variation. We are virtually at a point where we are conducting chimpanzee ethnologies, but have nothing yet of the sort for bonobos. For this reason, we should be including bonobos as further on the range of the behavioral variation in the chimpanzee genus.
4. Single-species models are foolish to begin with: One of the most brutal attempts at tearing down behavioral reconstructions of the last common ancestor of chimpanzees and humans was in a paper published 11 years ago by Ken Sayers and Owen Lovejoy titled “The Chimpanzee Has No Clothes,” where they fought tooth and nail to destroy every single possible argument for using chimpanzees at all for behavioral reconstructions.⁴ I remember reading this for the first time and thinking primatology as a discipline was done. I disagree with many of the arguments in the article, but if you have a couple of hours to put aside, read it. It doesn’t dismiss comparative primatology as a discipline entirely, but really does a great job of pointing out why single species models are silly to begin with. I will do a blogpost on this paper in the future because there’s a lot to talk about.
5. Selection Pressures: Leading off of single-species models, we have to remember that the real utility of these two species for behavioral reconstructions of our own last common ancestor is two-fold: we can look at what behaviors appear in all three of us, but in looking at which ones are shared only between one or the other we can understand why specific trends might arise. By understanding peacefulness in bonobos, we might move further to understanding homosexuality or peer-domestication in our species. In looking at raiding in chimpanzees, we might see answers to the origins of warfare.

When it comes to behavioral reconstructions, it’s really not a one-or-the-other type of situation. The fact of the matter is that the the last common ancestor of chimpanzees and bonobos was something in the middle of the two. The last common ancestor of humans and chimps…that’s even harder to answer, but it surely wasn’t anything like a bonobo and probably wasn’t like a chimp, either. Not to mention that this idea of bonobos being some ever-peaceful creature with an amazing heart and capacity for nothing but love is, and has always been, a meme. While it’s true they are maybe less violent than chimps, don’t believe for a second it’s all love and sex when it comes to bonobos. These are awesome animals, but as this article reminds you, people who work with them are having trouble dealing with this myth already.

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References

¹Prüfer, K., Munch, K., Hellmann, I., Akagi, K., Miller, J.R., Walenz, B., Koren, S., Sutton, G., Kodira, C., Winer, R. and Knight, J.R., 2012. The bonobo genome compared with the chimpanzee and human genomes. Nature, 486(7404), p.527.

²Hare, B., Wobber, V. and Wrangham, R., 2012. The self-domestication hypothesis: evolution of bonobo psychology is due to selection against aggression. Animal Behaviour, 83(3), pp.573-585.

³Diogo, R., Molnar, J.L. and Wood, B., 2017. Bonobo anatomy reveals stasis and mosaicism in chimpanzee evolution, and supports bonobos as the most appropriate extant mode l for the common ancestor of chimpanzees and humans. Scientific reports, 7(1), p.608.

⁴Sayers, K., Lovejoy, C.O., Emery, N.J., Clayton, N.S., Hunt, K.D., Laland, K.N., Brown, G.R., McGrew, W.C., Marchant, L.F., Stanford, C.B. and Strier, K.B., 2008. The chimpanzee has no clothes: a critical examination of Pan troglodytes in models of human evolution. Current Anthropology, 49(1), pp.87-114.