Is vegetarianism genetic, and the penis-eating spider

And how much neanderthal is in you?
14 November 2023
Presented by Will Tingle
Production by Will Tingle.


this is a picture of a chopping board full of vegetables


This episode, we catch up with some of the more curious genetics news from the past month including how much Neanderthal is in you and how that affects your risk of disease. Also, we look at one of the most remarkable breeding strategies in the animal kingdom, a spider with two penises that eats one of them...

In this episode

A wicker basket filled with an assortment of different vegetables

Vegetarianism could be in your genes
Shivani Shukla & Aylwyn Scally

What has been going on in the headlines of genetics over the past month? But, whilst we can definitely cover that, could we also take it a step further? Because it’s all well and good to look at a genetic study and see that the presence of gene X leads to disease Y. But in something as complex as the human body, which has over 20,000 genes, how can anyone really say with confidence that a certain gene or gene cluster is solely responsible for something as huge as a vulnerability to a disease. So what are the questions that actual geneticists ask when they see the headlines?

Aylwyn - I'm Aylwyn Scally. I'm a researcher in human evolutionary genetics here in the Department of Genetics in Cambridge. And my work is based on using lots of genome sequence data, complicated mathematical models and computing to try and understand what happened in the past and how we got where we are today.

Will - What is it about genetics that really excites you? Why did you choose to walk this path?

Aylwyn - Well, interestingly enough, I initially didn't. I was trained as a physicist, so this is a field I came into somewhat later on. It's actually not so uncommon in the field that I work in because it's quite mathematical and the training that you get in physics is relevant for it. But I think something I've always been interested in because genetics is such a fundamental part of understanding what we are and what was the journey that we all went on to get here. So I think in order to understand that you have to understand genetics.

Will - And last but not least…

Shivani - I'm Shivani. I'm a final year medical student at Cambridge. My interest is primarily in oncology, clinical oncology. I did my bachelor's in genetics three years ago. Now, my interest in genetics comes from interest in science and understanding who we are. Why are we more likely to get certain diseases, what can we do about that? And obviously oncology is where it all comes together and we go deeper into the genes and why did things change, what can we do to prevent it and treat it?

Will - Far be it for me to drive a wedge, but why is clinical so much better than <laugh> evolutionary?

Shivani - I think with clinical it's applied. So we're living in the UK which is obviously an aging population and there's so many advances with genetics as well. So you can take a finding, you can apply it to human health and you can actually see an outcome in terms of people living longer and better quality of lives. And that's sort of why I chose medicine in the first place as opposed to pure science.

Will - Very noble. Any thoughts?

Aylwyn - No, absolutely. I can see that it's probably what drives a lot of people's interest in genetics actually, is that desire to kind of use it to have a real impact on people's lives and health. And often you meet biologists who are driven by surprising different motives, someone to understand how things work today in animals and other organisms and other people want to understand how it came to be. And it's quite interesting how those two questions are quite different and involve different ways of thinking about the same problems.

Will - I hope we are all united by an insatiable curiosity. Well let us jump straight into the news then. I think first up having just come off the back of a delightful lunch, we have the story that a new Northwestern University medicine study has found that a person's genetic makeup plays a role in determining whether they can stick to a strict vegetarian diet. Now Shivani, before we get into genetics, why do you think a study into vegetarianism is so pertinent now?

Shivani - That's a very relevant question. I think one should always ask, what impact does this have on society, on people? There's a couple of reasons I believe so one is the environment. Obviously thinking about climate change, we know that methane produced by raising cattle and beef is a large percent, I think at least 20% of greenhouse gases. There's obviously ethics. People are not happy with how cattle and other animals are being raised. There's a question of health. So red meat's been linked to colon cancer and I think people are generally more aware about what's in their food, what percentage of saturated versus unsaturated fats. So there's probably quite a lot of factors driving towards a more vegetarian based diet.

Will - When a study like this comes out, when it says there's a genetic disposition, when you're looking at something like that and there's X amount of genes are dictating whether or not you have a predisposition towards vegetarianism is. What does that mean?

Shivani - So that's a very fair question and before I read these sort of articles it's something that I asked too. So speaking directly about this is quite interesting. So the genes themselves affect two main things, metabolism and taste and the interplay between metabolism, taste and preference towards a certain diet are genetically controlled. So that's where that comes in.

Will - That sounds like a lot of things to factor in when talking about genetic disposition. Is it a lot of genes that they've looked at that account for this?

Shivani - A total of 11

Will - Is that a lot <laugh>? That doesn't sound like a lot.

Shivani - So the thing with GWAS studies is I think as we were speaking over lunch there's a lot to do with maths and kind of saying is this statistically significant versus is it not? And you have to be careful when drawing a conclusion when things are associated versus to 100% say this causes that. And that will probably never be the case because genes and environment there's a lot of interplay. So we can say there's an association that has been found.

Will - Hmm. So you're saying that people who have a gene that makes vegetables taste better <laugh>?

Shivani - Well there are definitely genes that control metabolism. Your metabolism then controls how you taste certain things and then that controls your preferences. So it's more of a network of very complex factors.

Will - You touched on it briefly for a second there, but there's also this huge environmental thing, upbringing, what you have access to eating as well in terms of this genetic predisposition. Does it have an equal weight? What's the ratio between environmental and genetic in this instance?

Shivani - So I'll give the example of coffee so people who metabolise coffee quicker find that the taste is better and will therefore consume more coffee. That's been shown in the previous study. Now if that person who enjoys coffee is living in a society where there's not a lot of coffee consumption versus if you're living somewhere in South America where it's a staple part of your diet, I think that would definitely influence how often you are consuming coffee.

Will - So I hate coffee so does that mean I can't metabolise it properly?

Shivani - Perhaps that could be a problem with your gene <laugh>. Yes.

Will - Coming back to the whole 'why is this relevant?' meat alternatives are better for the planet that's provable in terms of emissions but aren't necessarily currently better for the person eating them. This study therefore is presumably useful for those seeking meat-free alternatives.

Shivani - I think that really varies. So in terms of plant-based foods, it depends on how it's processed, it depends on how much salts are added. Preservative plants definitely have a better unsaturated to saturated fat ratio than animal meats and red meats have. And in terms of this kind of vegetarian thing is actually to do with fatty acid metabolism and the type of fatty acids which are complex sphingolipids found in meats have been linked with certain neurodegenerative diseases such as Alzheimer's, autism, et cetera. So again, it's one thing to say this causes this, it's another to say there's an association, but there's definitely a role in terms of the types of lipids found in meats versus vegetables and how is that prepared?

Will - Are either of you vegetarian?

Aylwyn -
Not me, no. Although increasingly these days thanks to my daughter being vegetarian, that's sort of had an impact on all our diets at home. There's a lot of stories like this and we're probably going to talk about quite a few of them and I think the context for all of any study like this is just how complicated the biology of any given trait is. And particularly something like this, which is a behavioural trait, human beings are complicated enough just in their physiology and their biology in that sense. But then when you add into things that are going on in their brains that make them do one thing rather than another, it suddenly becomes almost impossibly complicated to imagine how does one study this? And traditionally in science we approach things from the perspective of, you know, trying to put together a causal chain of this causes that, this causes that, and we end up with this outcome here and now we understand this phenomenon. And, Those, you might call that sort of an understanding of mechanism. But it's just too complicated. And so instead we do these association studies, which is really just a statistical correlation, but as soon as you've done that, the complexities don't go away, they're all still in there. So the people who you've identified as being more likely to be vegetarian or drink coffee or whatever, there's a whole bunch of factors in their culture and how that impacts their diet and the environment that they live in and all kinds of other things that you can think of. Every single one of those is potentially going to be a factor and trying to tease those things apart is very difficult.

Will - The more you read into it, it seems from an outside perspective, the sheer nebulous nature of genes, even in just one human being means that, how could you possibly tell that a certain amount can lead to a concrete evidence that you are more or less likely to be predisposed to anything?

Aylwyn - Yeah, I mean there are a few cases, rare cases where we have the typical example of Mendelian diseases where, if you have this particular genetic variant then you will have the disease. Most traits do not behave like that. It'll be lots of tiny little effects from all over the genome. You know, one gene maybe has an effect on a protein which is involved in some kind of minor way and another one will do something else to some other protein somewhere else. And after a huge complicated interaction of all these things, there'll be some sort of 5%, 10% effect on the trait. You know, that's what we're trying to deal with. So most things we're trying, we're teasing apart these tiny effects and trying to look at them in aggregate and maybe if you have a large enough number of people they start to become clear.

Will - And are you a vegetarian <laugh>?

Shivani - No. But definitely something to consider.

Will - Having said all of that, we must now turn to a study which is probably going to make us all tear our hair out again. We've already been through enough pain. However, a study out of the French National Institute for Agriculture Food and Environment has concluded that having certain Neanderthal genes can raise your sensitivity to pain. But I guess before we get into that, how on earth do you measure or quantify pain to begin with?

Aylwyn - I'm fairly sure it involves inflicting pain on people in rooms and seeing when they say 'ow' or some equivalent, you know, that's, I think, the nature of it. It's one of those physiological responses that people have devised. I'd say the ethics are probably quite unpleasant to try and get through the experiment.

Will - I saw the methodology of that paper and it's extraordinary because it involves rubbing mustard oil onto people's arms until it burns. But I guess the question is, which, if we could even put a location on it, which genes would code for pain reception?

Aylwyn - Well, I know that there's a couple of genes that people have identified and I think the interesting thing about it to my mind is that different types of pain seem to have different genetic mechanisms that are involved or that have been associated with them. So pain due to temperature has a different physiological, potentially genetic, mechanism from pain due to pinpricks in the skin or insults to the body in that way. So I think that the actual details of it, in most cases are going to involve genes that are involved with signalling because pain is one of these signals that goes through the body and therefore cell signalling mechanisms like channels that are there in the cell membranes that let ions through. That's kind of how cells communicate with each other in the body. Those kinds of genes, I think, are typically what's implicated in these mechanisms. But again, those are the ones that we've seen so far. These are going to be complicated, probably not going to be a very simple thing. And the ones that we've seen, I think we know them because something's gone wrong and the person with some unfortunate person with this variant has a hypersensitivity to pain or equally bad and no sensitivity to pain. You know, both of those are bad conditions to have.

Will - It's interesting that you say that there's different areas for different kinds of pain because I think there might be studies that say the sort of people who are less sensitive to pressure based pains are more sensitive to use say temperature based pains. So there could be some kind of even evolutionary trade off between the amount of the types of pain you would have to endure. Say your ancestors walked a lot in the cold but didn't have to carry a lot, so maybe you don't need as much pressure pain, but they do need more temperature pain. I don't know if there's anything in your mind.

Aylwyn - Well, it's a good example of when you're trying to address evolutionary questions. I mean we've been talking about how hard it's to come up with explanations for just how the body works today, trying to come up with explanations for how something might have evolved is if anything even harder. And there is a tendency, it's actually quite easy to come up with stories sometimes, many of which are very plausible. I mean the one you've said is quite plausible, but how on Earth would go about actually establishing that that was how it evolved? Sometimes there are signals that we can see, and particularly with ancient DNA sometimes we can see that yes, actually in the past this gene was more prevalent and this one hasn't, or this variant I should say. But by and large we have to be pretty careful about leaping to a story about why something may have happened or whether or not there was a trade off. It's kind of opening up another whole can of worms. I think that the interest for me in this particular story actually was not so much about, you know, what does it tell us about pain? I don't know if it does, but the context being the prevalence of Neanderthal allele variants in everybody's genome are nearly everyone's genome. So they're in a few percent of people's genomes outside of Africa. People with African ancestry have a lot fewer or known Neanderthal ancestry in their genome. And there's been a lot of studies in the last decade or so about this and this was an interesting example of how people are very keen to find meaningful impacts of Neanderthal ancestry in us. You know, it's not enough just to say, 'well this is what's happened in the past and this interesting event,' but it also affects, it also makes us more or less likely to have some kind of condition. So there's some sort of deep requirement for us to be able to explain things in those terms.

Will - Exactly on point. There was a study a few months ago, maybe a year ago, that having a certain amount of Neanderthal DNA in you meant you were more susceptible to COVID and long COVID.

Aylwyn - Yeah. And there's been a bunch of others. A related study looking at cold adaptation in various people. There's been effects that have been injected about effects on diabetes. I mean there is some interesting genetics behind this, some interesting evolutionary genetics about the, that what you would expect to happen when a population like Neandertals who have been in a different environment and maybe in quite a small population size, what happens when their genes come into a much larger population, which is what we think happened about 50 to a hundred thousand years ago. And we expect actually by and large there to be those kinds of variants to not do very well in the new population and to be responsible for deleterious effects, you know, things that generally have a bad impact. Every now and then, there might be a few good things that help you adapt to environments, but that's the actual expectation here. And so it's totally plausible that they are having some, you know, they're involved in maybe certain diseases and things, but again, it's quite hard to establish it with certainty. So one has to be a little bit careful about some of these things. Because the numbers are often quite small in some of these studies.

Will - We set this up as explaining the news segment, but now it's let's trash the news. It's all impossible to say.

Aylwyn - I think that's where we're going, right.

A portrait of a neanderthal in a museum.

How did we get all of our Neanderthal genes?
Pontus Skoglund, The Francis Crick Institute

Neanderthal DNA has been very prominent in the headlines for the past couple of years. This came to a crescendo when winner of the 2022 Nobel Prize for Physiology and Medicine, Svante Paabo, got said award for sequencing the genome of the Neanderthal. We’ve learned that we share around 2% of our genome with our now extinct relatives. But how did it get there, and what does that mean for us in our day to day lives? The Francis Crick Institute’s Pontus Skogland…

Pontus - That's a great question. When people have looked at DNA and tried to sort of estimate using the known rate that we get mutations when Neaderthals and humans separated or diverged as we call it in the past, they've gotten something like half a million years ago. But I think there's really fascinating hints that there were more complex things going on. Some of it is more than hints. We know that on our mitochondrial DNA and our Y chromosomes, we have a common ancestor with Neanderthals more on the order of 400,000 years ago. Furthermore, there's some indications from people who look at fossil morphology, you know, how they look, the fossils that make it seem like sort of neanderthal traits and what is to become Homo sapiens traits. Might be diverging much earlier as well, perhaps, you know, almost 1 million years ago or maybe a bit less than that. And so I think what this is hinting at might be that it was sort of a two step process where perhaps our ancestors initially became diverged and then there was a point of contact 300,000 years ago or something like that. And that's sort of the average of that number comes out in our calculations for half a million.

Will - Ah, so this is much less of a clean split, like if some birds flew off on an island and they never saw their original counterparts ever again. This is a bit of interspecies breeding in the meantime.

Pontus - Exactly. So yeah, the equivalent would be that, you know, maybe the sea levels went down at some point and there was a bridge between the two islands, but then perhaps they became isolated again. And yet there's really been a bunch of different studies looked, have looked at this phenomenon, all publishing hints that this is what's going on.

Will - So assuming then that this 2% figure of sharing Neandertal DNA is an average, let's say. What affects the sort of exact amount from person to person or groups of people?

Pontus - The main difference is that Neanderthal ancestry is found in people that have recent non-African ancestry. So in the past few thousand years, ancestors that were from outside Africa. And that's really the line of evidence for that. This is something that happened during this expansion in our species from living only in Africa and its vicinity to expanding into far-flung areas like Europe, Asia, Oceania, and the Americas. People with strictly, for example, west African ancestry will have negligible levels of Neanderthal ancestry. People with only some sort of recent Asian or Oceanian or European ancestry will have about 2%. There's actually very little variation between people. Some of the ancestry companies kind of report information as to what your rank is among customers.

Will - And we've touched on it before in the program, but it seems the presence of Neanderthal DNA in an individual's genome does affect their susceptibility to diseases such as, for instance, covid. There's one theory that Neanderthals were exposed to certain illnesses before homo sapiens and it gave them more time to build up a resistance. So the presence of Neanderthal genes gives our immune system a bit more to play with. I was wondering if you had an opinion on that. Where do you think that's a worthy theory or if you have one of your own?

Pontus - There's the COVID question and then there's sort of the general fact that yes, since many people have up to 2% in Neandertal ancestry, that will of course affect the variation they have in their genomes and the mutations that they have. And those will inevitably affect biology in some way. There's a certain degree of variation within humans and 0.1% of that type of variation on the average trait would be explained by Neanderthal ancestry. And so of course it will impact biology. I think it's important when you see in the news of Neanderthal variants having an effect on particular physical traits or other traits, is that that's kind of the expectation. They are mutations among many others in our genome. They will influence them. Usually they do at a sort of smaller proportion of the trade. But it's really fascinating and in several examples it might indeed have been adaptive for various reasons, might have been things that were adaptive in Neanderthals that were adapted later on. COVID is of course a fascinating example. I think it could just be to chance as far as I know that Neanderthals had this variation and that it was sort of transmitted to people today. Maybe it was selected later on in Homo sapiens. That seems to be a reasonable thing to ask.

Will - If we were so genetically similar to Neanderthals to the point that we could interbred with them and they are part of our genome. Why is it that we are still here and Neanderthals seemingly are not?

Pontus - Yeah, that's another fascinating question. Neandertals mostly disappeared around 40,000 years ago. One possibility that people of course like Svante Pääbo, who's been the main researcher by far on Neanderthal DNA and awarded the Nobel Prize last year, has brought up is that it may be that they were just sort of lower in numbers and they were kind of absorbed in expanding populations. And that's sort of where this 2% number comes from. But I think it's also important to remember that this kind of ebb and flow of ancestry groups that don't contribute much to later generations. When you bring the clock forward of thousands of years, that's quite common. Neandertals disappeared 40 or so thousand years ago. The first people in Europe don't seem to have contributed much to populations just a few thousand years later. And then of course when you get even to more recent times, the last hunter gatherers didn't contribute much. You know, they lived 10,000 years ago. They didn't contribute much to later forming populations, later forming populations in turn didn't contribute much to later populations. This kind of ebb and flow I think is really quite common in our history, but that doesn't mean that human ancestry is shallow in time. In fact, our big family tree connecting all of us goes back several hundreds of thousands of years.

Will - I do like the idea that the Neanderthals aren't gone. They're just within us. Even only slightly.

Pontus - Yeah, absolutely. They're still around.

A spider's web

23:33 - The spider that eats its own penis

The lengths some organisms will go to mate successfully...

The spider that eats its own penis
Rebecca Coffey

On the southwestern coast of India lives Nephilengys malabarensis, the Malabar Spider. It’s an orb weaving spider. The females can grow up to 3cm long, which is 3-6 times larger than the male. All pretty standard stuff for a spider. But when it comes to the act of courtship, things get very strange indeed. Rebecca Coffey is the author of Beyond Primates: Every Sperm is Sacred...

Rebecca - For the Malabar spider. Oddly enough, he has two penises. They're called palps and they dangle from either side of his face. Though the female has two receptacles or vaginas, they're actually called epigynum or epigyna and they are on the abdomen. So what he generally does is climb up on the web, jostle it a little so that she knows he's there, go over to her, stroke her, and then mount her from the underside and he takes one of his palps and puts it into one epigynum and starts ejaculating. And that's where the odd stuff begins. His palp is detachable, it breaks off. So he puts it into the epigynum and then he breaks it off and leaves it in the epigynum to enjoy the sex, right? And he comes back out on the web and stands ready to defend her from any other potential mate. This is an area of evolutionary theory called sperm competition. What sperm competition is, is the male trying to guarantee that if his female has many mates, his are the sperm that get to the egg. So the male Malabar leaves his ejaculating palp inside the epigynum and comes out to defend her from other males. And then if one approaches, things get really weird. You would think that he would stick his other palp into her other epigynum, break it off, let it ejaculate, and then he would have two plugs in her two epigyna and the whole battle would be over. But no, what he does is he eats his other palp, he eats his penis.

Will - You could've given me a hundred guesses and I wouldn't have been able to guess that that was what happened.

Rebecca - Right? You wouldn't go there at all. So no one knows why because no one has ever asked a male Malabar why he eats his own penis at that point. But a team of scientists from Asia and Europe built basically little boxing rings for male Malabar spiders and paired male eunuchs with fully bodied males. And they paired male partial eunuchs with fully bodied males. And the name of the paper says it all - eunuchs are better fighters.

Will - The mind boggles, but is there anything to be said that eating your own palp might give you that burst of energy needed to defeat whoever you are fighting?

Rebecca - Certainly that's one interpretation. Another is that it just pisses you off so much that you become a monster. But after he has left one in an epigynum and eaten the other, he has no more reproductive potential in his life. Those things aren't going to grow back. And Charles Darwin defined procreation as the whole point of life. It's why sex feels good, right? <laugh>, we just keep going out and doing it. And the result is that we send our genes into the next generation. Well, he can't do that anymore. And so his next act, to me, is almost as astonishing as eating his own palp. And it is that he offers himself as dinner to the female. Why? <Laugh> Because he's just inseminated maybe 200 eggs. She has a huge nutritional need and he is there to satisfy it.

Will - That is absolutely amazing and almost it's kind of a one shot deal. If nothing's growing back, then you've really got to hope that this is a successful mating attempt.

Rebecca - That's right. And you know, male spiders mate very rarely. They're not always eaten and they're not always threatened by another male. But you only get so many shots in life if you're a male Malabar spider and they're willing to give it their all.

Will - And nothing but respect to them. And people thought that male praying mantis had it bad, just having their head ripped off. Imagine having to rip off your own organ and then be eaten as well.

Rebecca - And you know, across species there are sperm competition strategies. They can be biological, they can be physiological, and they can be behavioural. With canines like coyotes and dogs, there's a bulb at the base of the penis that inflates, sticking the female onto the male for long enough for his sperm to get to know her eggs really well before another canine approaches. For humans mate guarding, jealousy on the part of the male, is possibly just a sperm competition strategy. It keeps other males away from her so that your sperm gets to her eggs.Across species, sperm competition is absolutely fascinating.


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