A good 840 years before the Sex Pistols sang about anarchy in the UK, one chunk of the country faced the real thing. A crisis in the royal succession led to almost 20 years of civil war and rebellion in both England and Normandy, which was then controlled by the English crown. But this time the trigger was not disenfranchised youth, but a dish of fish.

Rather than dying in battle, or peacefully from natural causes, King Henry I reportedly died by gorging on lampreys: apparently against medical advice. After his death, people fought for the throne.

The story is almost certainly apocryphal, says historian Giles Gasper of Durham University in the UK. The only account of Henry's death that mentions lampreys was written by a 12th-century historian who was no fan of the dead king. Nevertheless, many schoolchildren down the years have learned of the dangers posed by a surfeit of lampreys.

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King Henry's experience evidently did not deter future monarchs. Many of his successors were fans of lampreys, and the fish has been regarded as a regal food for thousands of years. This might amaze many today, given that the alarming-looking lamprey has fallen off the menu in many parts of the world.

What might also come as a surprise is the high regard in which scientists hold lampreys. Ecologists know that lampreys maintain the health of rivers. Medical researchers study lampreys, which have a remarkable ability to heal themselves even after severe nerve damage: an ability that could offer a way to heal spinal cord injuries.

Finally, evolutionary biologists have discovered that lampreys are crucial in the history of life. Lampreys were among the first backboned animals to evolve, so these fish carry important clues about our ultimate origins.

Lampreys look a little like eels. They have a long, flexible body with eyes, mouth and gills at one end, and a tail fin at the other.

People can have an almost morbid fascination with them as these blood-sucking parasites

But it's what they lack that makes them really unusual. Like some other primitive fish, lampreys don't have bones: their skeleton is cartilaginous. They also lack some of the more important fins that fish usually carry, including the two pectoral fins and two pelvic fins which, in our ancestors, evolved into legs.

Most notably of all, lampreys lack jaws. Their mouth is a permanently open ring, filled with a vicious-looking set of teeth. Many of them clamp onto the side of other fish, scrape away the scales and skin, and suck their blood.

"When you see a picture of them, it's often of that flat oral disc with the teeth," says John Hume at Michigan State University in East Lansing, US. "People can have an almost morbid fascination with them as these blood-sucking parasites."

Unsurprisingly, lampreys have starred in their own horror movie: 2014's Blood Lake.

Hundreds of millions of years ago, jawlessness was very much in vogue

Even the Romans seemed to have viewed them with a certain amount of fear. The orator Vedius Pollio reportedly kept ponds of huge killer lampreys, and kept them well-fed by tossing in the occasional slave. This Blofeldesque story is "probably all nonsense," says Hume.

Lampreys certainly can and do mistakenly attach themselves to people, and Hume says they can be surprisingly difficult to dislodge, but they rarely draw blood.

"Some people think that lampreys have razor-sharp teeth. They don't," he says. Their frightening fangs actually face backwards, and simply help the lamprey stay attached to its target. It is actually tiny sharp structures on the lamprey's tongue that scrape away fish scales, and they are not really tough enough to make much of an impression on human skin.

The traits lampreys lack, particularly jaws, makes them seem peculiar today. But hundreds of millions of years ago, jawlessness was very much in vogue.

All of the earliest vertebrates lacked jaws, and they thrived. Almost all of these "agnathans" are now extinct: lampreys, and another group of jawless fish called hagfish, provide us with our only chance to study our agnathan origins.

The direct ancestor was probably rather similar to a lamprey

These primitive fish have existed in a virtually unchanged form for an amazingly long time. Fossil lampreys that look essentially identical to modern species date back 360 million years.

But this does not mean that our ancestors were once lampreys. The common ancestor of lampreys and hagfish, whatever it looked like, diverged from our distant ancestors even earlier, perhaps half a billion years ago.

This means that, when it comes to understanding our evolution, lampreys are a little bit like modern chimpanzees. Both animals can tell us a bit about our ancestors, but neither lampreys nor chimpanzees are on our direct evolutionary line.

The direct ancestor was probably rather similar to a lamprey, though. It seems to have evolved the forerunners of features like jaws, legs and – less obviously – our adaptive immune system, which allows us to remember pathogens that have made us unwell so we can fight them off more efficiently next time around.

Even Medieval Britons recognised that lampreys were unusual.

According to the Aberdeen Bestiary, a 12th-century compendium of animals, "lampreys… are of the female sex only and conceive from intercourse with snakes; as a result, fishermen catch it by calling it with a snake's hiss".

Lampreys were often cooked in their own blood, with copious amounts of wine

The bestiary also warns would-be lamprey hunters that the fish are tricky to kill. "You need to beat it repeatedly with a stick. It is a fact that the life-spirit of the lamprey is its tail, for when it is beaten on the head, it is difficult to kill; but when it is beaten on the tail, it dies at once."

Henry I's penchant for lamprey suggests the effort was worth it. The fish is highly calorific, says Hume, and was valued for its taste, which is unusually meaty. This feature would have made lampreys a useful addition to the Medieval dining table, as religious fasting often restricted the consumption of meats other than fish. "Fast days were about one-third of the year," says Gasper.

We don't know how Henry I ate his lampreys, as there are no cookery books from his reign. However, recipes survive from a slightly later date, and they suggest that the typical Medieval lamprey dish might be a bit much for 21st-century tastes. Lampreys were often cooked in their own blood, with copious amounts of wine.

Medieval cooks had good reason to use these recipes, though – or at least, they thought they did.

At the time, the Ancient Greek system of medicine based around the four humours was still considered valid. Foods had their own humours, and a "balanced" meal was one in which, for instance, phlegmatic (cold and wet) fish was cooked in choleric (warm and dry) wine.

Queen Elizabeth II received a lamprey pie from Gloucester to mark her coronation

It has even been suggested that this explains the story of Henry I's death. Phlegm was also the humour of extreme old age, so medics would have considered it dangerous for a man so advanced in years as Henry to eat anything that would further "cool" his body.

However, this is just a suggestion. "The humours are clearly part of the way in which Medieval food was understood, and we have plenty of evidence for this from the 13th and 14th centuries," says Gasper. "Whether this applies to Henry's enjoyment of lamprey is more difficult to tell."

Despite the dangers lampreys were believed to pose to the elderly, they remained a royal favourite.

King John, who famously signed Magna Carta 800 years ago this year, was characteristically ruthless when his subjects failed to provide enough lampreys for the royal table. The city of Gloucester faced a 40-mark fine, worth about £250,000 today, for this perceived slight.

Even a low weir can prevent a lamprey from migrating upriver to spawn

Even today's monarch has lamprey links. Queen Elizabeth II received a lamprey pie from Gloucester to mark her coronation, and two more on her Silver and Golden Jubilees.

However, by the time the Queen marked her Diamond Jubilee in 2012, the lampreys in Gloucester's rivers were in short supply. Lampreys had to be imported from abroad to fill the pie.

The disappearance of once-plentiful lamprey from British rivers is probably tied to the Industrial Revolution, says Hume.

The problem was not so much that pollution found its way into the rivers, but more that people built obstacles between lampreys and their breeding grounds. Even a low weir can prevent a lamprey from migrating upriver to spawn.

The UK is home to three species of lamprey: the European river and European brook lampreys, and the sea lamprey.

All three spend their early years as larvae in the muds and silts of rivers. "They filter-feed, like sponges," says Hume. Then, a little like butterflies, the larvae metamorphose.

Brook lampreys do not become parasitic as adults: they never even develop a gut. They simply mate and die soon after metamorphosis.

River lampreys do develop into parasites, and migrate out to coastal environments to feed on other fish.

Sea lampreys also do so, but they move out further and spend several years sucking blood from marine fish before returning to rivers – almost like salmon.

Lampreys were returning to catchments on certain rivers that had been lamprey-free for decades

However, things get a bit less clear-cut when you look at lamprey DNA. The brook and river lampreys may look and behave differently, but they are almost indistinguishable genetically, says Hume. They can even interbreed. "They don't seem to constitute two 'good' species," he says.

His latest work offers an explanation. The river and brook lamprey genes might be mixed, but those responsible for important traits like growth rates or the release of particular key hormones are expressed differently in each species. These small differences lead to markedly different life histories.

September 2015 brought good news for British lampreys. The UK Environment Agency reported that the fish were returning to English rivers after centuries of absence.

In truth, lampreys were never really gone from most rivers, says Hume, who was not involved with the Environment Agency work. The key shift was that lampreys were returning to catchments on certain rivers that had been lamprey-free for decades, thanks to efforts to install structures that make it easier for them to navigate obstacles like weirs.

The news was presented as an unambiguous good thing, but some people from North America expressed astonishment that the British would welcome the return of lampreys. Sea lampreys invaded the Great Lakes and have decimated a valuable fishery and harmed the ecosystem, including the loss of native fishes, altering the food web.

They might be famous for killing a king, but studying lampreys offers enormous potential health benefits

Consequently, the population of sea lamprey in the Great Lakes is considered a huge problem by locals and authorities alike. They are not even particularly safe to eat, because their tissue contains high levels of mercury – although that didn't stop Gloucester importing them to make the Queen's Diamond Jubilee pie.

But even in the Great Lakes, lesser-known native lampreys are more ecologically beneficial than many people realise, says Hume. "Some transport nutrients from the lakes into rivers, which ultimately provides the invertebrates that feed game species like Atlantic salmon. They clean the gravel when they spawn. And their filter-feeding larvae accumulate the waste we put into the water. The natives are overwhelmingly beneficial to the ecosystem, unlike the sea lamprey which has been extremely harmful."

But even the most anti-lamprey anglers might change their mind about the fish in light of their value for medical research. They might be famous for killing a king, but studying lampreys offers enormous potential health benefits.

Proteins in their saliva act as anticoagulants and widen blood vessels, all of which helps the lampreys suck blood from other fish. Both properties could prove useful.

Lampreys are also adept at dealing with excessive amounts of iron, a key component of blood. This might make them useful for researchers looking for treatments for haemochromatosis, a condition that leaves people unable to control the amount of iron they absorb from their food.

Is there an overlapping set of genes that some species can use to promote regeneration?

Finally, lampreys have an extraordinary power of regeneration. A lamprey can make a near-full recovery after having its spinal cord completely severed – something paralysed humans can only dream of.

"These animals are doing something we'd love to know how to achieve," says Ona Bloom at the Feinstein Institute for Medical Research in Manhasset, New York. "Our first step is to try to understand how they do it."

Lampreys are one of several groups that can regenerate parts of their bodies, alongside salamanders and some lizards, says Bloom. "Do these animals all do this by turning on some of the same molecular programs? Is there an overlapping set of genes that some species can use to promote regeneration?"

If there is, it could point the way to a technology for triggering regeneration in mammals, including humans.

This could be a completely unrealistic goal. After all, humans are only distantly related to lampreys.

They are scientifically important but they've really slipped under the public's radar recently

But we might be related enough. The sea lamprey genome was sequenced in 2013, and it revealed many surprising similarities between their genes and ours. "Lots of the same families of genes are present in lampreys and in humans," says Bloom. "One area that contains many conserved genes is the nervous system."

As a result, Bloom is hopeful that research into lamprey regeneration might provide some insights into how to promote regeneration in other animals – although she stresses that any such treatment is many years away.

It's one more piece of evidence that there is more to lampreys than meets the eye. "They are scientifically important but they've really slipped under the public's radar recently," says Hume. "No one wants to buy a giant plush toy of a lamprey."