Genetic study furthers dispute of ecocide on Rapa Nui

First up, let us delve into a couple of new pieces of genetics research, and get our in-house genetics to give us their say on it. It’s time to link back up with my genetics experts, Shivani Shukla and Aylwyn Scally...

Will - Let's kick off with the first one which asks the question of how much of your personality is decided by the composition of your genes? A new study in the journal Nature Human Behaviour has shone a light on the relationships between the genes that code for what scientists refer to as the big five personality traits. In doing so, they have looked at their interrelationship of personality traits and the relationship with other complex human traits to see how these genes affect how we think and behave. Shivani, to kick us off then, what are the big five personality traits?

Shivani - So the big five is based off of the FFM, which is an accepted psychological framework. So under that comes extroversion, which is risk taking, openness, agreeability, neuroticism and conscientiousness.

Will - And prior to this, had any of them been more studied than others?

Shivani - Neuroticism, which is basically how likely someone is to engage in negative behaviour traits such as anxiety and depression and those sort of things, was very heavily focused on. So most of the previous literature had linked genetic loci with that trait, but this was the first study and is actually the biggest study of personality traits and have focused on the other four a lot more.

Will - It seems like an absolutely vast endeavour and undertaking to try and map so much of the human psyche. What did this study do to try and do that?

Shivani - It was a huge data set. There were 70,000 individuals, and it's another example of a GWAS study. So you take all the genetic information from these people and you try to map loci, which are variations in the genes. So all these 70,000 people had answered personality quiz questions and had self-reported the degree to which each of the five personality traits. And then once their DNA was sequenced, they mapped the traits to certain loci and then that's how they kind of associated X with Y and found what loci are associated with each of the five.

Will - How did it find that these five traits interacted with one another?

Shivani - They kind of found for each trait, what does it increase the person's likelihood of doing certain things. So for example, people who had loci for openness were more likely to be artistic and pursue educational opportunities. People who were conscientious were more likely to engage in day-to-day physical activities such as DIY and being risk-taking is linked to displaying neurotic behaviour, but being agreeable actually is a negative predictor for displaying neurotic behaviour. So it was quite an interesting mix of what you are more likely to do in your day-to-day life. And then how each trait links to the other

Will - Aylwyn, to bring you in. Given how much we love the discussion of nature versus nurture, does this provide any new revelations as to how much of your personality is hardwired into your DNA?

Aylwyn - On the face of it, that seems like a very unsurprising result to say that um, people who are more agreeable are less likely to be neurotic. What one certainly can't say. I mean, I think it's really important to reiterate again, with all of these studies, how much of the variation in these traits is actually being accounted for by genetics or is correlated with the genetics. It turns out it's about 5% in this particular study.

Will - Yeah, that figure of 5% really does hammer home just how much of people's personalities has to be down to their life experiences and their nurture. I mean, the genes encode for a rough map of brain wiring and then everything you are and everything you go through kind of moulds the map into the final specimen. I think that's fair to say. And it would be extraordinary for me to be able to look at someone's genes and go, 'ah, they've got one gene here and they've got one loci here. They must be an agreeable person.' That just doesn't sound possible to me.

Aylwyn - Indeed. And that's definitely not what we've discovered here.

Will - What stands out to me is that given that this is a study that is almost entirely self-reported, it seems a very subjective way of trying to quantify how different individuals think and act

Shivani - Without doubt. And there's obviously the reporter bias, there's the bias of veterans perhaps not being representative of the general population. This dataset was taken from veterans. So personality is definitely subject to change over a lifetime. And since it was self-reported, veterans are perhaps not an accurate representation of the entire population.

Aylwyn - Yeah, I mean we should also add, you end up sounding pretty negative if you say, well really you are showing a very small amount of variation in this trait, explained by genetics or accounted for by genetics. But of course the interesting thing in many cases in many such studies is which bits of the genome are implicated? Are there proteins or genes that we can potentially target for, for drugs in psychiatric care or in other circumstances? Does this help us when we're trying to understand complicated social or psychological interactions or disorders in other ways we can use these genetic correlates as a sort of a quantitative handle on those traits in other research? And I think that's the real purpose of doing these things. It's not really to show yes, genes can affect some traits in your body or in your mind.

Will - Yes, I think as you well put out there, it will be far more interesting for me to be able to look at someone's genetic makeup and go, ah, they have, however small it is, a predisposition towards neuroticism due to the presence of this gene. Can we help them treat this?

Shivani - Yeah. And actually from this study, they found a certain gene which is linked with the hypothalamus pituitary adrenal gland pathway and by kind of linking it's a receptor of a hormone. So by linking that to neurotic behaviour, it's kind of been identified as a good target for psychiatric treatment. So I think it's absolutely right. The application in terms of what we can do with it from a medical or treatment sense is probably more important than a very expensive and detailed personality quiz.

Will - Thank you very much, Shivani. We look forward to seeing what you bring to the table next month.

Will - But now it is over to you Aylwyn. Rapa Nui, also known as Easter Island, is one of the most isolated islands in the world. 3,700 kilometres away from the nearest continental landmass of South America. The story of the inhabitants of Rapa Nui has long been presented as a cautionary tale of environmental exploitation as it was believed that their rampant use of the local fauna and flora led to a resource shortage, followed by conflict and cannibalism, and subsequent genetic collapse sometime around the year 1600. But a new study just out in the journal Nature disputes that theory saying there was genetic evidence pointing towards a healthy population throughout their history. So I guess the first question is why was there ever an ecocide theory in the first place?

Aylwyn - In this case, I certainly think even as a child hearing stories about Easter Island, as it was called then, and how there had been this terrible collapse due to them using up all the resources of their island and leading to wars and even cannibalism and a massive population collapse. I think most probably it was the subject of a book by Jared Diamond about 20 years ago where he probably did a lot to repopularise or to disseminate this idea, this ecocide story. It's important to say that we're at risk of propagating another sort of mini myth here, which is that genetics is coming along and constantly disproving the archaeologists and confounding their myths. Actually, there's been lots of evidence presented before this study involving archeological evidence, not genetic evidence, which has cast a lot of doubt on this theory. So I would say that actually already there were many reasons to think that it wasn't true, but genetic evidence has provided another very important piece of evidence against it.

Will - How was this helped to be further disproved with genetic studies?

Aylwyn - So there's two things really that you can look at with a set of genetic evidence and they looked at some, what they're calling, ancient Rapa Nui individuals. But actually this data probably came from individuals who may have lived as late as early 19th century or mid 19th century. Some of them in fact are from the 20th century. And what they did essentially was they looked at two things. One was could they see any evidence of the impact of a population crash on these, in the timeframe that had been proposed? Could they see any evidence of that in their ancient samples? And they also looked at another big question about Rapa Nui, and about Polynesia in general, which was contacts with South America. That's another interesting story. Taking the second one first, they were able to see signals of ancestry that was similar to the ancestry that you find in native American populations in South America, not just in South America, but on the west coast of South America. They find ancestry that is similar to that in these Rapa Nui individuals. And you can actually try and date, by looking at the distribution of those ancestry segments, you can actually try and date when they appear in that population. And that contact maybe goes back about a thousand years. So long before the arrival of Europeans in Polynesia, the ancestors of these individuals had actually managed to get all the way across the Pacific Ocean and indeed have contact such that people were coming back from South America to Rapa Nui and other islands. So that's really fascinating. The other side of the story about the ecocide, well that's a little bit harder to actually to analyse because there are various types of things that you might call a population crash that may or may not have a strong signal in the genetic evidence. And they looked for evidence of a certain particularly strong population crash along the lines that people had argued that population fell to only 10% of its previous value, within a couple of generations they looked for evidence of that and they couldn't find any of their models that fitted that data very well. What their data fitted much better was just a steady, slow increase in population since from a sort of a founding population maybe about a thousand years ago, right up to the time of the 18th century. Just a slow growth, no particular evidence for any sort of massive crash. And that's not to say that there weren't lots of other things that did happen, there clearly was deforestation. I mean, we can see that and there may well have been wars and there may well have been famine and all kinds of other things, but then none of those things necessarily lead to the kind of emptying out of the island that this ecocide story seems to presuppose.

Will - So not only has this disproven the previous ecocide theory, it almost goes the other way because there were far more unexpected visitors in their genetic lineage than perhaps was anticipated.

Aylwyn - Yeah, I would argue that there's no particular evidence to assume that this was a population in decline. There may well have been lots of terrible things happening. So there are some aspects of that story which we should be careful about what we're saying is getting disproved here. Just in narrow sort of demographic terms, there was not some sort of terrible collapse happening. But again, just to come back to this point, you could have things which would be perceived as quite severe demographic events in the sort of oral history, or in our historical memory or historical record, such as 30% of a population being wiped out within a generation. Something like that, which would actually be quite a dramatic event in terms of our understanding, actually wouldn't necessarily have a big impact on the genetics of a population. So there are things which one, you know, there might well have been events like that which could have happened. I'm not saying that there's any evidence at all for those things actually in this particular case, but there are other cases around the world where it's not so straightforward to match up genetic evidence with historical records or accounts that might exist about such events.