The roamin' Roman Empire, and Charles Darwin's love life

Plus, a look into the world of animal hybrids
27 February 2024


The Colosseum


In this edition of Naked Genetics: The surprising findings of a roman empire diversity study; Are hybrid animals the way out of the biodiversity crisis? And, was charles darwin as romantic as he was scientifically brilliant?

In this episode

Ancient Rome

00:55 - Surprising genetic diversity of Roman Empire revealed

And why the different backgrounds might not have mixed as much as anticipated...

Surprising genetic diversity of Roman Empire revealed
Shivani Shukla & Aylwyn Scally

An international team of researchers led by Stanford University have used ancient DNA to map the migration of people during the Roman Empire. This new study published in the Journal eLife, took thousands of samples and discovered that at least 7% of the historic individuals carry ancestry uncommon in the region where they were sampled, implying that they were buried somewhere different to where they originated.

Will - Shivani, like last month's story on the origins of multiple sclerosis, is this another classic case of bone sampling?

Shivani - Yes, it is. Essentially, they took thousands of skeletons, took samples and sequence the DNA from them. These skeletons are between the first and seventh century, which is during the Roman Empire.

Will - And Aylwyn, to bring you in. What were the surprise findings of the paper? Was it just that we didn't realise how much people moved such a long time ago?

Aylwyn -
Well, that seems to be something that they found and, in a previous paper where they looked at the city of Rome, they found a lot of people there who clearly had come, or whose ancestry was from lots of different parts of Europe, of the Roman world. That's maybe not so surprising because Rome was the capital of the empire. People coming or being brought there from all over the world. But this paper then shows that actually that was true when you go out into the regions, into the provinces, and into the countryside and elsewhere around the Roman Empire, that you see similar levels of diversity. And you're right, maybe it's not unexpected because we know that empires do that they move people around. But you know, we can't assume these things are going to necessarily be found in the data and sometimes what we found in the past.

Shivani - What was actually quite interesting about the paper was, despite all of this migration, even today a lot of the populations are still quite genetically distinct, which is almost the opposite of what you would expect if there was such mass migration during the Roman Empire. The paper hypothesises there's two key reasons for that. The first is this idea of transient migration, which basically says that someone might be born in X and they might travel to Y for employment or due to military reasons. But they will only move to Y for a few months or a few years and actually never properly settle down there or reproduce there and returned back to their original place of birth. And that's something that haven't really happened in the history of human migration because generally when people moved, they moved there for good. And you know, part of the Roman Empire was creating huge networks of roads and waterways and that kind of allowed for this unique concept of transient migration. The second reason that they hypothesise might be why populations are still so genetically distinct is that after the fall of the Roman Empire, all these kind of global cities and transport links crumbled and sort of fell away. And so people generally then returned back to where they came from because there was no employment or military recruitment. So I think in the paper those were the two main reasons they attributed it to.

Will - Perhaps me and my jaded brain thought that migration is a fairly modern affectation because it's so simple now. And as you say, you'd assume back then if you moved it was once and permanently, but people were so much more transient than we realised and this genetics backs that up.

Aylwyn - Yeah, I mean it's very interesting how early this signal seems to set in. The signal being one where the level of diversity stops decreasing. Put it that way. That's what we see, as Shivani said, if you look at earlier periods, as we come forwards in time gradually people are, around Europe anyway, are becoming genetically more similar. There's mixing of all the different ancestries that we have at the very earliest prehistoric period. And then from the round about the Bronze Age onwards and into Roman time and up to the present day, the sort of the level of genetic diversity of heterogeneity, that stops decreasing. And certainly, one of the theories that this paper puts forward is that yes, we're finding people in other parts of the world, but they haven't necessarily produced and therefore their ancestry hasn't started to merge into the local population there. There is another possibility of course, which is that it's also possible you could imagine a society where it's just incredibly stratified and caste ridden, say, and therefore people just don't mate with people from different ancestral backgrounds which then subsequently become just different groups of people living in the same place. That would also have a similar effect of maintaining this kind of diversity of ancestry over a long period of time in different parts of the empire.

Will - Yes, I suppose that does bring the kind of uncomfortable nature of asking the question, why were these people moving in the first place? And a lot of it will be for jobs or for stuff like that, but many of them will have been moved against their will and perhaps the mating opportunities for said people were severely limited.

Aylwyn - Exactly. And I think maybe in the future we'll start to really explore the genetic and ancestral consequences of things like that. And you know, with the idea being then that we can turn around and say, well actually maybe this factor has been more prevalent than this other one. Or maybe they are both important, that it's not just transient movement and people moving back, but there are also these kinds of segregating factors involved as well.

Will - That's fascinating that genetics can be used to understand the social proclivities of such an old civilization.

Aylwyn - Yeah so they made a prediction, which I always like when papers do that. They said, if that's the case, what we might expect then following the fall of the Roman Empire, if we do the same study over the next sort of the subsequent several hundred years, is that actually then we do see a sort of a decrease in this heterogeneity, a reduction diversity in people start to stay put, be unable to travel and therefore we would see things starting to merge together. And at the moment we don't have sufficient amounts of data from the following period, but that's going to come along very soon. And so it's going to be a very exciting time in the relationship between genetics and early and mediaeval history.

Blue whale

Blue whales and fin whales have been 'hybridising'
Mark Engstrom, Royal Ontario Museum

So, here’s one for you. What does Ancient Roman civilisation and a blue whale have in common? Both are remarkably large? Both have seen a catastrophic decline in numbers? Yes to both of those. But, now we can also say that both have a much higher genetic diversity coming in from elsewhere than we previously realised. You see where I’m going with this? A study just out in Conservation Genomics has shown that the north atlantic blue whale, the largest organism on the planet, is getting up to some interesting activities with its smaller counterpart the fin whale, which leads us to ask some questions about what these two species are getting up to in the depths, if you catch my drift. Mark Engstrom is an author on the paper...

Mark - Hybridisation is, in the context that we're discussing it here, integrating between two distinct species.

Will - How common is that in the wild?

Mark - Hybridisation? It depends on which group you're talking about. It's fairly uncommon in mammals, but it certainly occurs on occasion. And it depends on the species, how closely related they are and probably their history of previous contact.

Will - Not everything can hybridise. There's got to be some form of common ground for them to have a successful attempt.

Mark - Yes, you know it's interesting because it used to be that the old definition of a species was two groups of organisms that had their own evolutionary history and could not interbreed. But however, when you look at interbreeding, it's an acquired characteristic. It's something that occurs during evolution to prevent mating between groups that are incompatible already, genetically incompatible. And so it's something that's highly selected against. In groups that haven't been in contact for which it's not so deleterious, it may never evolve. So in the case of the blue whale, one of the things that's really interesting is that blue whales and fin whales are not each other's nearest relatives. The nearest relative of a blue is probably a Sei whale or a Rice's whale, but they do not cross breed with those as far as we know.

Will - And how much of the fin whale's DNA could you detect in the blue whales?

Mark - Well, that's interesting because fin whales and blue whales are again, not that closely related in the sense that they've been separated. The ancestors have been separated for at least probably 8 million years. But we're detecting about a leakage of DNA from fin whale to blue whale of about 3-3.5%. So about 3% of the North Atlantic blue whale's genome is derived from fin whales on average.

Will - When I spoke to an expert in Neanderthal DNA a few episodes ago, he said we've got shared Neanderthal DNA and we may well have got that because we bred with them. Is this a similar situation?

Mark - It's very similar. In fact if you look at the Neanderthals, the percentage of DNA in humans north of Africa is about three to 3.5%. It's very similar to the situation we're seeing between blue and fin whales.

Will - Like with Neandertals, the amount that each one of us alive has today, in terms of Neanderthal DNA, varies. Is it the same with blue and fin whales? Is there sort of a variance across the board?

Mark - There is. So we had some individuals in our historic samples of blue whales. We looked at not only modern blue and fin whales, but we went back to museum specimens and looked at some blue whales from over a hundred years ago. We tried to map them through the process of whaling, which occurred from about 1900 to about 1970 was when the blue whale populations were decimated. We wanted to get a snapshot of their genetic variation prior to whaling, during whaling, and now after whaling. And we had two individuals that did not have any fin whale DNA from our historic samples. And all of our modern samples had at least some, but in our historic sample we had an F1 hybrid, so we had a first generation hybrid between the two. So hybridisation has been going on for quite a while. We don't know how far back in time it goes, but it's been an ongoing phenomenon. And so we get anything from zero to 50% fin whale in our blue whales samples.

Will - As you said, they're not the closest related thing. So I suppose I have to ask a fairly indelicate question here, <laugh>, which is, when a blue whale in a fin whale meet and they decide to hybridise, for lack of a better term. How does that really work? Do they both know what's happening?

Mark - Well, we haven't witnessed a hybridisation event, so we don't know exactly how this works. Blue whales are a huge animal. The reason I work on small mammals is that they're easy to observe and you can get large sample sizes in the small space. If you work on large mammals, especially a blue whale, they occur across a huge area and there are few of them. So there tends to be a lot of facts abating and so on that we don't have a lot of information for because it hasn't been observed very often. However, we have observed and we know how courtship works with blue whales. What happens is the female and the male pair up. Sort of in the fall and into the early winter, you'll get pairs that are travelling together. And then at some point if another male joins so that they'll try to sneak in on the pair, then you wind up getting this wild swimming behaviour where the female just takes off and the males take off with her and the males will fight with one another. You'll have collisions, you'll have breaching and all kinds of things with the males. And then one of the males will typically leave. So the fin whales obviously have managed, at least on some occasions, to either be the primary pair with a blue whale or to win one of these battles with a male blue whale. So when they're mating with female blues. So presumably when you have one male fin whale and one female blue whale, he can successfully mate with her.

Will - It's extraordinary that given the intricacies of the courtship, that increasingly sounds less and less like it's by accident that this is happening.

Mark - Certainly not accidental on the part of the male fin whale.

Will - What a sight to behold that would be...

Mark - <laugh>. Yeah, exactly. We know that when they mate, they do actually do a deep dive. The pair does a deep dive and then they mate on the way back up. So it's not, it's not something that's been observed very often. At least.

Will - One can dream, I suppose.

Mark - <laugh>.


Should we hybridise endangered species?
Andrew Whitehead, University of California Davis

With the ecosystems of the world up a certain creek without a proverbial paddle, and extinctions occurring faster than any time since the dinosaurs were wiped out, could we preserve some of the genetic material of endangered species by hybridising it with a close relative. In a world on fire, is that not worth a go? To talk through the pros and cons, the University of California Davis’ Andrew Whitehead.

Andrew - I think what's really cool to think about is that we, ourselves, as a species are a product of hybridisation. When modern humans migrated out of Africa, we encountered a number of other Hominid species and hybridised and interbred with them. And we can see that through the archaeological record and we can actually see it in our DNA. So it's cool to think that we ourselves are a product of hybridisation that just really emphasises that hybridisation is a natural process. Happens all the time. We as a species have also used hybridisation as agriculture. For example, domestication of various plants has involved hybridisation. For example, strawberries. Bananas are also a product of hybridisation of different cultivars. And we've also used it sparingly in conservation so far. Usually when a species is totally at the brink of extinction and is facing inbreeding depression, that's when individuals meet with close relatives and that can be really problematic genetically speaking. So probably the most well-known example is the rescue of Florida panthers by deliberately mating them with a different subspecies of panther from Texas. And that injected enough genetic variation into the Florida species so that, you know, they started doing better, at least brought them out of the death spiral of inbreeding depression.

Will - That sounds like a pretty potent argument in favour of hybridisation. Is that the only reason that we would do it, if they were in such dire straits that this was kind of the only way out?

Andrew - Our thinking about the use of hybridisation and conservation has really changed over the decades, and very, very quickly I might add. And part of that is that things are just getting more and more dire. And there are lots of really good reasons to avoid hybridisation because it can break down species barriers. And we're trying to deal with the biodiversity crisis. We're trying to maintain species. And through hybridisation we're sort of deliberately breaking those species barriers and homogenising species. And that seems to go antithetical to this idea of maintaining species diversity. But when certain important species are facing extinction, it's either no species or some blending of species is sort of the option that conservationists are faced with. And in those situations, I think over the last few decades, more of those situations have arisen and will continue to arise as species face the brink. And we're now facing these decisions about maintaining some of that genetic variation represented by that species, by blending that genetic variation with other species to at least maintain that ecosystem function. So maintaining isolated, intact original species is becoming harder and harder to do for some species. I think that should be the goal to maintain that diversity of ecosystems. But sometimes that's not possible.

Will - So to pick an example then, if there was a very fragile ecosystem that needed a large organism acting in the same way that an elephant is an ecosystem driver and that big mammal is dying out, would a hybridisation therefore be kind of useful in the way of keeping that ecosystem up and running?

Andrew - A thing to keep in mind is most ecosystems aren't a product of a single species. And usually when an ecosystem is in trouble, it's because many species are in trouble and fixing one species, trying to engineer the success of one species, is usually not going to cut it. And the problem is why is this ecosystem in decline in the first place. And it's usually because of factors that aren't going to be fixed by hybridisation. So hybridisation is in many instances a bandaid solution. It's a sideline solution. It might be important because what goes on on the sidelines, as we all know, can be important. But the main show is on the field, and that's the integrity of ecosystems. That's the integrity of the evolutionary process itself. And many argue that that's where conservation efforts should be focused, not on individual species, but on maintaining and conserving landscapes in a way that enables continuance of the evolutionary process.

Will - Yes, I suppose saving an organism or preserving as much of it as you can is all well and good, but if the place that it lives is on fire, that's not really going to help in the long run, is it?

Andrew - And it's important to recognise, if you look at the species that we work really, really hard to preserve, they're usually species that kind of look like us <laugh> that are big creatures with backbones that are cute and furry or that we like to eat. And those aren't necessarily the most ecologically important species in every instance. So we need to recognise our own bias in the species that we choose to conserve, that those efforts might be misdirected in terms of eyes on the prize of conserving ecosystems and evolutionary processes.

Will - So almost to step away from the genetics for just a second, and you've alluded to it throughout this entire thing, but rather than focusing the finite amount of time and effort that we have as conservationists on trying to hybridise individual species, what would you recommend instead for the overall health of the ecosystem?

Andrew - Part of what I think is an important part of the solution requires us to swallow our ego as a species <laugh>, and that's hard for us to do, and thinking that we are smarter than the natural sorting of genetic variation in enabling sort of long-term persistence of species. So conservation is often an intervention to preserve individual species. It's also often thought of as an intervention to preserve landscapes so that nature can take care of itself through other evolutionary processes. So to think that we can genetically engineer species in a more durable way than nature can is entirely delusional. So I think the meagre resources that we often have to deal with as a society to put towards conservation should be less towards these highly micromanaging intervention kind of things where we try to save individual species and more towards preserving landscapes and evolutionary processes so that nature can take care of itself, which is going to be much more efficient than a single species, us, thinking we know what the solution is at the genetic level.

Charles Darwin

Was Charles Darwin lucky in love?
Rebecca Coffey

A couple of weeks back was Valentine’s day, a day when the typical animal scientists walk past restaurants and wonder why they’re so busy. But was one of the most influential scientists of all time, Charles Darwin, as prolific in love as he was at coming up with world changing theories? Author of ‘Beyond Primates: essays on Darwin and evolution’, Rebecca Coffey.

Rebecca - Well, I think he was very successful in love. He was gobsmacked by love. In fact, when he got back from his five year voyage throughout the southern hemisphere on the HMS Beagle. On that voyage, they had gone throughout most of the southern hemisphere to some quite cold and some quite warm climates. Every time they hit a port, he would get off and take an amazing adventure. And when he came back, he began to visit with a young woman named Emma Wedgwood. He was gobsmacked by love for her. He was terribly nervous about his own appearance because evidently he had a kind of flabby nose that the captain didn't like at all. And in fact had almost refused to let him be the scientist on board because the captain was an adherent of a fake science called physiognomy, in which you thought you could decipher character and temperament traits from the shape of certain aspects of a person's face. But Darwin walked away from that feeling terribly self-conscious of his nose. He courted Emma Wedgwood. He proposed to her because in those days, you couldn't court a woman without proposing. And she accepted. But there was one huge problem with the fact that he had fallen in love with Emma, and it was that she was his first cousin, and he should have known better than to do that because he was the scientist who had written about the fact that inbreeding caused problems for animals and that great apes avoided it entirely, went to great lengths to avoid it entirely. So what made him think that it was okay for him to do that?

Will - As you say, the irony really should not have been lost on him. Did it have any effect?

Rebecca - Well, it did. Now let me, in his defence, say that both he and Emma were from long lines of interbreeding <laugh>, as were most aristocrats in that day. Because British aristocrats wanted to protect land holdings. Charles Darwin should have avoided it. We can't really say that cause made an effect. In the case of the Darwin's, they had 10 children, three of them died young. The rest of them always seemed very frail to their parents. And in fact, of the surviving Darwin children, three or four of them couldn't procreate, which indicates that their genetic line by that point was critically weak.

Will - They did manage to have children. And how was the state of the relationship between the two of them? Because it seems like Darwin was perhaps a little bit apprehensive of marrying her in the first place.

Rebecca - Right. He didn't feel that he was well established enough in life. He thought that he was ugly. He had lots of ideas about how he wanted to spend his time and having a wife didn't fit into them at all. And he made two lists. One was called marry and one was called not marry. For marriage, he says - 'children, constant companions and friends in old age, it's better than a dog.' Anyhow, 'someone to take care of the house, charms of music and female chit chat.' To not marry, 'freedom to go wherever one likes conversation of clever men at clubs, not forced to visit relatives, loss of time, no fatness and idleness.' So he had very severe qualms about getting married in the first place. But he did it because he was lonely. He thought he was ugly. She was level headed pretty enough, an excellent pianist who had studied under Chopin briefly, and she really liked the guy and they were happy for the rest of their lives.

Will - There is truly hope for us all then, isn't there? But is there anything to the idea that Darwin's work and outlook changed after this marriage had happened?

Rebecca - Well, Darwin was a very precise observational scientist, his descriptions of the organisms that he collected, thousands of living and dead plants and animals in his five year journey through the southern hemisphere, his descriptions were impeccable. So he was very reliable and astute as an observer. Darwin worked at home and while he worked at home, three of his children sickened and died. Imagine the sadness in a home when three of the children die. Imagine the children's love of the mother and father and the father and mother's love of the children. So Darwin worked at home in an intense atmosphere of love. Now since then, lots of research has been done showing that emotion influences memories as they're experienced and as they are recalled. So Darwin writing from deep within a sea of love and appreciation for how critical love is to life began to describe love in animals, even in insects. He said it is certain that animals love each other. Even today, some scientists will back up that point of view. Most scientists are much more clinical about the whole thing, saying that love, certainly romantic love, requires a human to have a sense of imagination and be able to form hope for the future. That it's more than a physiological response, that it depends on a certain cognitive ability to see beyond the moment and the physical impulse and into the future. Darwin didn't seem to think so. He thought animals loved each other. He described their love dances, their love ardour. And in that way, he seemed to have made a fundamental scientific error of confusing his own impulses with those of the species that he was studying. Yes, those are mistakes, but Darwin is one of the stars of science in the world. And so on the veneer of his reputation, these are not more than slight scratches. The man was brilliant and his contributions will last forever.


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