A trove of ancient fish fossils helps trace the origin of jaws

Host: Benjamin Thompson

Welcome back to the Nature Podcast. This week: what a trove of fossil fish reveals about the evolution of jawed vertebrates, and why policy affecting trans people’s lives needs to take a more evidence-based approach. I’m Benjamin Thompson.

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Host: Benjamin Thompson

Humans are – according to Nature editor Henry Gee – a particular kind of specialised bony fish. Go back far enough, and our distant ancestors were swimming the seas, having successfully evolved bones, while another group were exploring the options of a cartilaginous skeleton – the ancestors of today’s sharks and rays. The period in which a lot of this evolution and diversification was happening is called the Silurian, which began 448.3 million years ago. But exactly what was going on with fish during the Silurian has never quite been clear. Reporter Shamini Bundell spoke to Henry Gee to find out about some new fossils that could shed light on the matter.

Interviewer: Shamini Bundell

Thanks for chatting to me, Henry. I wanted to get you on because we have four papers coming through in Nature this week about new fossils from the Silurian period.

Interviewee: Henry Gee

That's correct, yeah. Not one, not two, not three, but four papers.

Interviewer: Shamini Bundell

And so, the Silurian is pretty key to this story, I think. So, this was a period when, on land, there were sort of plants and arthropods and insects kind of taking over, and the sea was full of weird and wonderful fish creatures, which is mostly what we're going to be talking about. But I wondered if you could sort of set the scene a bit for this sort of Silurian story and maybe introduce us to what was going on at that time and some of the kinds of characters we might be talking about?

Interviewee: Henry Gee

Cast your mind back, if you will, to the Devonian period, which was after the Silurian period, which began about 490 million years ago. That's generally known as the age of fishes. But the Silurian period just before that was when a lot of fishes originated, but we haven't got many really good fossils of them. They tend to be a bit scruffy and scrappy and fragmentary, which is inconvenient because a lot of the major evolution in early fishes was happening about then.

Interviewer: Shamini Bundell

And a particularly successful evolutionary invention of this kind of time is jaws, basically, so jawed fish.

Interviewee: Henry Gee

The earliest vertebrates didn't have jaws. Their mouths were kind of suckers. And only two kinds of jawless vertebrates survive today – they're the lamprey and the hagfish.

Interviewer: Shamini Bundell

So, we want to know what was going on in the Silurian that led to all the varieties of jawed fish and jawed creatures now, which includes us, that we see today.

Interviewee: Henry Gee

This is what these papers are all about. Now, these days, there are two kinds of jawed vertebrates. There are the bony fishes, so cod, halibut, sturgeons, seahorses. They're all bony fishes. And then there are the cartilaginous fishes, the sharks and rays. But back in the day, there were two other extinct groups of jawed vertebrates. There were these little tiddly fish called acanthodians, or spiny sharks. They have no internal bony skeleton so they're quite hard to get a grasp on. And the other group are placoderms. These were armoured jawed fishes, and it is likely that all the other groups of jawed vertebrates arose from somewhere in the radiation of placoderms.

Interviewer: Shamini Bundell

So, let's get on to these four papers then. So, this is a team of palaeontologists in China who have discovered several new fossils. How did this come about?

Interviewee: Henry Gee

Yeah, Min Zhu and his crew have hit on a fantastic fish bed – a layer of bones dating from the early Silurian near Chongqing in South China. And rather than just the usual scrappy mess, they found entire articulated skeletons of fishes that really allow us to get a much better view of early fish life.

Interviewer: Shamini Bundell

What's your favourite finding of the four fossil papers?

Interviewee: Henry Gee

Well, one of the most intriguing is going to be a fish called Xiushanosteus, which is a placoderm – one of those early jawed vertebrates probably from which all the others sprang somehow. This one was only a tiddler, about 3 centimetres long.

Interviewer: Shamini Bundell

Tiny.

Interviewee: Henry Gee

Yeah, this one was quite small, but there are lots of different kinds of placoderms, and the relationships between all of them is very kind of contested. But this one, it seems to combine in one body a lot of features of several otherwise disparate groups of placoderms, and also shows signs of bony fishness, and this is going to cause a certain amount of head scratching in the fossil fish community.

Interviewer: Shamini Bundell

So, that's the first of the papers. Tell me a bit about the other fossils.

Interviewee: Henry Gee

There’s Shenacanthus – another little fish that looks like a very early representative of the cartilaginous fish, but it has big dorsal fin plates like you don't see in sharks. This is a placoderm thing. And then we got Qianodus, which is basically a load of teeth. These are the earliest jawed vertebrate teeth anywhere in the fossil record. And then there's another one called Fanjingshania, which looks like one of these spiny sharks or acanthodians, but there are no teeth associated with it. So, when this was going through review, the referee said, ‘Hang about, maybe Qianodus is the teeth that Fanjingshania hasn't got,’ but Zhu et al. convinced everyone that actually that couldn't be the case. So, Fanjingshania, is a kind of toothless acanthodian. And then there's another great thing which is not a jawed fish at all. It's a jawless fish called a galeaspid. Now, the thing about galeaspids is they were only known from their head shields. No other soft part has ever been known. But this one has got the whole fish. And on each side, on the kind of bottom edge of each side, is a fold, like a fold of fins on each side like go faster stripes, and this is kind of interesting because it looks like a precursor to what happened in jawed vertebrates, which is the evolution of paired fins – two at the front and two at the back – which turned into our arms and legs. And the fun thing is they made some reconstructions of this fish and tested it aerodynamically, and it would have generated lift. It would have been a useful thing. And this is the first time from this unique fish bed that we have a whole one.

Interviewer: Shamini Bundell

Does it confuse at all the story of their relations because our sort of modern understanding, we have a very clear tree of evolutionary relationships, where you've got first they invented the jaws, then you've got the placoderms and then bony fish and cartilaginous fish. Has that picture changed at all?

Interviewee: Henry Gee

Oh, very, yes, Shamini. This picture is always changing. Many of these early fishes, they're not clearly definable into modern bony fish, modern cartilaginous fish and so on. They tend to have mixtures of features. So, this early tiny shark thing, Shenacanthus, had bony plates like a placoderm. And this early placoderm had characteristics of bony fish. So, you tend to see that evolution was kind of experimenting. All these groups hadn't quite parted ways yet. They hadn't become as distinct as they are now. And these new finds, to use that well-worn phrase, will raise more questions than they answer.

Interviewer: Shamini Bundell

So, this Silurian story is still being rewritten, with new characters popping up all the time.

Interviewee: Henry Gee

This discovery of this new locality is going to produce lots more, I'm sure, in the future, and it's an entirely new, refreshing window into the past.

Host: Benjamin Thompson

That was Henry Gee, a senior editor at the Nature journal, talking with Shamini Bundell. You can find links to all four of the papers they discussed in the show notes. Next up on the podcast, Dan Fox is here with this week’s Research Highlights.

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Dan Fox

Some people with a rare genetic condition have heightened musical and verbal abilities. And now, thanks to studies in mice, we might know why. Williams-Beuren syndrome is a condition caused by the absence of a specific chunk of genome. This can erase one copy of up to 27 genes and lead to cognitive deficits. But it can also enhance music skills. Researchers studied the auditory cortex – the brain’s sound-processing centre – in mice missing the equivalent genes. They found that the loss of one copy of one gene enhanced the rodents’ ability to distinguish different sound frequencies. The team suggest that losing this gene reduces the levels of a specific protein, which changes the function of some auditory cortex neurons. This change made the mice highly sensitive to small shifts in the frequency of a tone. Read that research in full in Cell.

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Dan Fox

High-resolution imaging has revealed the secrets of how honeybees build their honeycombs. Honeycombs are one of nature's best engineered structures, offering strength for minimal material use. They weigh less than a sheet of paper when empty, but can hold several kilograms of bees’ honey and nectar. To watch a comb’s evolution, researchers used high-energy X-rays to create 3D images with micrometre-scale resolution. These revealed that bees first create a corrugated vertical structure that acts as the comb’s foundation. Its bumps and depressions form a pattern of hexagons on which bees deposit bulbs of wax, and then stretch the wax like pizza dough to build the honeycomb cells’ walls. Construction goes from top to bottom, with the bees reinforcing the foundation with more wax as the comb grows. The team say that these insights could lead to improvements in the structural design of synthetic materials. You don't need to comb the web for that research. It’s in Advanced Materials.

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Host: Benjamin Thompson

Next up, reporter Adam Levy is looking at the lack of evidence in transgender policy.

Interviewer: Adam Levy

What does good policy look like? Of course, any policy has to take into account a host of considerations, from the ethical to the social. But for many, good policies should also be backed up by evidence. What are the impacts of a policy, and are the justifications for a policy supported by the data? And this question is particularly relevant right now for trans people. Transgender people are people whose gender doesn’t line up with the gender they were assigned at birth. So, for example, a trans woman is a woman who was assigned male at birth. That’s as compared to a cisgender person, whose gender is the same as they were assigned at birth. Around the world, many laws are being proposed and passed on the rights of transgender people to participate in various aspects of society. But how well does the available evidence back up all these new policies? Well, not so well, according to Paisley Currah, researcher of political science and women's and gender studies. He's got a World View out in this week's Nature, arguing that policy over trans people's lives needs to take a more evidence-based approach. I caught up with him, and we started by discussing what kinds of policies we're talking about, particularly in the US.

Interviewee: Paisley Currah

In the United States, we've seen, for example, a raft of bills and state legislatures – almost 20 have passed – that have said trans girls can't play on women’s’ and girls’ teams in women's and girls’ sports. Another example would be some of the legislatures and more local libraries and school boards banning the teaching of anything related to transgender people. So, we have quite a lot of stuff happening on that end.

Interviewer: Adam Levy

Now, for sports, for example, just how big a thing even is this in the first place, the idea of trans people and especially trans girls playing sports with other children of their gender, other girls?

Interviewee: Paisley Currah

Right, well, in terms of the political legislative response, it's really like a solution in search of a problem. So, for example, in Utah, they have 75,000 students in Utah who play high school sports. They have one transgender girl, and there were no issues raised about that transgender girl, yet the Utah legislature passed a bill banning trans girls from playing girls’ sports. So, when the governor of Utah, who is a Republican, he vetoed the bill, he said, never had so much ire been directed at so few. And unfortunately, the legislator overrode his veto and it became law. The political response is out of whack with what's going on, on the ground.

Interviewer: Adam Levy

And recently, actually, in August, a judge reversed this law, and so there's been a lot of back and forth here. But regardless, a lot of laws governing the lives of trans people are being pushed forward and in many cases are passing. In your World View, you argue that these policies are running counter to the research, counter to the data. Take a particular spate of policies around which people can use which bathrooms. What does the data say here?

Interviewee: Paisley Currah

Right, well, there's no good solid data saying that allowing trans people to use the bathroom associated with their gender identity poses a problem. For example, there was a study that looked at jurisdictions in Massachusetts that had laws that ensured that people could use the bathroom associated with their gender identity, and they compared those jurisdictions with jurisdictions that didn't have that. And they found no evidence that these laws put anyone, including women, at risk, and that the fears of safety were completely unfounded.

Interviewer: Adam Levy

The other issue we mentioned was which people can play which sports. This question is often asked in a very blanket way: should trans people, especially trans women, be able to compete in sports with people of their own gender? What kind of research is there in this area?

Interviewee: Paisley Currah

Unfortunately, the discussion sometimes isn't about comparing cisgender women to transgender women and what advantages transgender women might have or what disadvantages they might have. Some of the policymaking actually focuses on comparing cisgender men with cisgender women, and that's like a different question. Trans women are not the same as cis men.

Interviewer: Adam Levy

Now, what about something really fundamental – healthcare and healthcare in particular for transgender people. For example, hormone therapies, which are designed to alter an individual's hormones to levels that better reflect their gender identity. How is policy now aligning, or perhaps I should say misaligning, with research?

Interviewee: Paisley Currah

I think maybe that's the most extreme in the bans on transforming care we see, especially in the United States. For example, policymakers will describe hormone therapy as experimental and not proven and unsafe. And like 22 major medical associations have said, ‘No, we've been doing this for a long time. It's not experimental. It's safe. It leads to good outcomes.’ But like with other issues where science is involved, it becomes a he said she said thing. And what's interesting is that conservative judges, sometimes appointed by President Trump even, when they look at these issues in a court and they're faced with the evidence, they've ruled against the conservative lawmakers again and again. So, it's really kind of a sign that the lawmakers are kind of turning transgender issues into a political issue that’s got very little to do with the evidence.

Interviewer: Adam Levy

Now, some people will say that this is purely a political issue and perhaps question why we're talking about it on the Nature Podcast and talking about it in the pages of Nature. Why do you think these questions are questions that the research community ought to think about?

Interviewee: Paisley Currah

I think the research community is concerned with people and the harms that people suffer. So, I do think it's a mistake and it doesn't really help anyone. We get into these abstract fights about what is gender and what is sex, and that becomes politicised, it becomes all about culture. I think it's so important for us to focus on the harms that actual people face when they're denied gender-affirming care, or they're like some seventh-grade volleyball player who's told that they can't play in the girls’ volleyball team. And then when you bring it down to that level, we can see the importance of research to show that these policies are really not needed.

Interviewer: Adam Levy

Do you think that research and data are the only things that should be informing these kinds of policies?

Interviewee: Paisley Currah

No, because I think ultimately, it's also a human rights issue. So, I think we have to always kind of keep that as a backdrop to make sure that every individual has the right to affirm their gender identity and to express their gender. So, for example, with bathrooms, the biggest victims of harassment in bathrooms are often gender non-conforming cisgender women. They're the ones who are threatened, assaulted, chased out of bathrooms, and we don't need research to say that gender non-conforming cisgender women should be allowed to use the women’s bathroom. That's just a human rights point.

Host: Benjamin Thompson

That was Paisley Currah from the City University of New York. To check out his World View, look for a link in the show notes. And that's all we've got time for this week. But just before we go, time to mention a new video on our YouTube channel about research trying to crack the nature of consciousness by dosing volunteers with psychedelic drugs and scanning their brains. Look out for a link to that in the show notes. Don't forget, you can keep in touch with us over on Twitter – we're @NaturePodcast – or you can send us an email to podcast@nature.com I'm Benjamin Thompson. See you all next time.