a bonobo humanity?

The Hapsburg, and sickly, Charles II of Spain (1665-1700). All in the family.

Here’s a simple thought experiment, if perhaps an unlikely scenario. Imagine two children born to the same parents, a male and a female. For some reason they’re both separated from those parents at a very early age, before they know each other as siblings. For the next twenty years or so they’re brought up in separate households, unaware of each others’ existence.

Guess what happens next – they meet, they ‘fall in love’, because after all they’re each as good-looking as the other, and they share interests, politics and ambitions, and they each want to start a family. What could go wrong?

Or more to the point of these explorations, how would these two individuals have any sense that they might be entering into dodgy territory? Of course they might start comparing backgrounds and entertaining suspicions, but what if they both share a tendency to bullshit about their past? Obviously this is a ridiculously rare scenario, but hopefully it makes a point. 

So, when I try to learn about inbreeding I’m quickly taken into the inherent dangers, and the complexities of autosomal inheritance, whether dominant or recessive – all of which is fascinating in itself, but of little interest to bonobos, bats or budgerigars – or bodgies and widgies – when mating season arrives. Inbreeding avoidance is all very well if you know who to avoid. But a lot of the info I’m gathering tells me that we do know who to avoid, and I don’t just mean we humans. According to Wikipedia’s article on inbreeding avoidance, ‘there have been numerous documented examples of instances in which individuals are shown to find closely related conspecifics unattractive’. But this bald statement strikes me as totally unconvincing. I’m sure we could find plenty of ‘documented examples’ of the exact opposite too. 

However there does seem to be something, beyond choice or preference or awareness, that promotes inbreeding avoidance, whether it’s pheromones, MHC (major histocompatibility complex) genes, or other evolved mechanisms. We generally manage it – and by ‘we’ I mean just about all breeding entities – but not in a foolproof way, it seems. What interests me is the kind of set process for inbreeding avoidance that we find in chimps, bonobos, Tiwi Islanders and other human societies, which rises to the point of incest taboos in humans. With humans there is knowledge. We know that breeding with siblings and other close kin is problematic, yet we probably didn’t always know that, and our taboos are more about it being ‘icky’ and ‘creepy’ than about giving birth to unhealthy children. What gave rise to those feelings, which seem to be shared by other apes, and perhaps further down the evolutionary bush? 

Let’s look at MHC genes, though I don’t think they’re going to provide an answer that satisfies me. Here are the opening lines from a 2008 PubMed article entitled ‘Major histocompatibility complex alleles, sexual responsivity, and unfaithfulness in romantic couples’:

Preferences for mates that possess genes dissimilar to one’s own at the major histocompatibility complex (MHC), a polymorphic group of loci associated with the immune system, have been found in mice, birds, fish, and humans. These preferences may help individuals choose genetically compatible mates and may adaptively function to prevent inbreeding or to increase heterozygosity and thereby immunocompetence of offspring.

So, yes, I find nothing wrong with this finding, but maybe it’s a bit of a correlation-causation problem for me. I’d like a bit more info on the causal. And obviously all these creatures have no idea whether a potential mate has similar/dissimilar MHC genes, so why mention preference, which is about choice, or intention? But then, doesn’t love, or desire, strike us below the level of awareness? Ain’t love blind?  Not to our MHC genes, apparently. And then again, is our preference for sweet rather than sour, or vice versa, really a personal choice or something we find out about ourselves? Do I contradict myself? Very well…

Anyway I’m glad to note the phrases ‘may help individuals choose…’ and ‘may adaptively function to prevent inbreeding…’ here, an acknowledgement, methinks, of the fact that they’ve observed a correlation which hasn’t yet been found as determinative – though probably everyone thinks it is. Meanwhile, we – human and myriad other species – seem pretty good at avoiding inbreeding, mostly, so why worry…

References

https://pubmed.ncbi.nlm.nih.gov/17100780/#:~:text=Abstract,HLA%2DA%20Antigens%20/%20genetics