The goblin shark is rarely seen, but when it does show up it makes headlines.
That's partly because of its unusual looks. Its pink flesh gives it the appearance of having been skinned, and a flattened, dagger-like snout protrudes from its head. No wonder it's been called the "alien of the deep".
But the goblin shark also evokes our imagination because of its special history. The family it belongs to, the Mitsukurinidae, seems to have barely changed in 125 million years. That means the goblin shark is a "living fossil", an animal that has survived seemingly unchanged for a huge span of time.
A living fossil will look just like a fossilised animal from millions of years ago. This seems to imply that, for these few species, evolution has stopped entirely – as if they evolved to such a peak of perfection that they just don't need to improve any more. But appearances can be deceiving, and there is more to these extreme survivors than meets the eye.
The term "living fossil" was coined by Charles Darwin in On the Origin of Species in 1859, the book in which he first spelled out the theory of evolution. In one section Darwin discussed the platypus and the lungfish, two modern species that belong to an ancient lineage, and still have some of the key features of their fossilised ancestors.
The fish belonged to a group that was thought to have gone extinct 65 million years ago
Darwin wrote that: "these anomalous forms may almost be called living fossils; they have endured to the present day, from having inhabited a confined area, and from having thus been exposed to less severe competition."
At the time the most famous living fossils had not yet been discovered. That would happen in 1938 in South Africa. A natural history curator called Marjorie Courtenay-Latimer realised that a fish she was examining should not have existed.
The fish belonged to a group that was thought to have gone extinct 65 million years ago, during the same cataclysm that wiped out the dinosaurs. It was a coelacanth.
Coelacanths have roots that stretch back 390 million years. They are large, bottom-dwelling fish that can grow up to 2m long. Their fleshy, limb-like fins and dappled scales look as if they've been flecked with blobs of white paint.
Everyone thought it had died with the dinosaurs
There are two known species: the African coelacanth and the Indonesian coelacanth. Together they are the only survivors of the lobe-finned fishes, a group that once dominated the oceans.
"The discovery of the coelacanth gave the term 'living fossil' a lot of currency," says palaeontologist Richard Fortey. "It was a dramatic discovery, as everyone thought it had died with the dinosaurs."
But the real importance of the coelacanth lies in what it can tell us about the evolution of land animals.
Around 400 million years ago, some fish began to walk on land, using their fins as legs. These explorers gave rise to all the 4-limbed land animals, from lizards and frogs to birds and bears.
Coelacanths living 400 million years ago were not identical to the fish that live on in 2015
In 2013 scientists sequenced the genome of the African coelacanth. They found that it is the closest living relative of those first land animals.
But that doesn't make it a true living fossil. A second study, also published in 2013, examined coelacanth fossils and DNA. It found that the two living species are significantly different to their dinosaur-era ancestors, both in their genes and in the design of their bodies.
"The phrase [living fossil] implies that evolution has not acted on the organism over these long timescales," say Chris Amemiya and Mark Robinson of the Benaroya Research Institute in Seattle, Washington, who worked on the coelacanth genome project. "That is clearly shown not to be true for coelacanths."
Quite simply, their skeletons have changed. A second dorsal fin has transformed from spiny to lobed, and they have lost bones around the rim of the mouth and around their scales. Coelacanths living 400 million years ago were not identical to the fish that live on in 2015. So are there other animals that really haven't changed their bodies?
Tadpole shrimps look even more prehistoric than coelacanths. Each one has a carapace that resembles a sequin. This protects a long tail-like abdomen ending in two long, thin appendages that look like antennae.
It seems the key to the tadpole shrimps' survival may be how they reproduce
Tadpole shrimps are found as far apart as China and Scotland, and have survived for 300 million years. That means they survived the Permian extinction, often known as the Great Dying, which wiped out almost every other animal species.
Given that, you might think tadpole shrimps have evolution all figured out. But genetics says otherwise. According to a 2013 analysis, tadpole shrimps have evolved and diversified significantly over millions of years. "There is clear evidence of evolution," says study leader Africa Gómez of the University of Hull in the UK.
In fact it seems the key to the tadpole shrimps' survival may be how they reproduce. A single tadpole shrimp can reproduce without a partner, because they are both male and female.
Tadpole shrimps are self-fertilising hermaphrodites. They have sperm-producing lobes in their ovaries, so they can fertilise their own eggs.
20,000 years ago, northern Europe was covered in an ice cap
"Hermaphroditism might allow organisms to colonise habitats better," says Gómez. "You only require one egg, so it gives them an edge in regions where there has been recent habitat change."
That could have helped them at the end of the last ice age. "20,000 years ago, northern Europe was covered in an ice cap," says Gómez. When the ice melted, it exposed new flood plains, rivers and ponds. "If you're a hermaphrodite you can colonise that relatively quickly."
They are also evolving. Gómez has found that tadpole shrimps in the Sahara reproduce faster than those in Europe, perhaps so they can finish before their puddle dries out in the heat. What's more, "some of the Australian species seem to have evolved to endure higher salinity in the sea water, whereas that would instantly kill some of the European ones," says Gómez.
So it seems we have been misled into thinking that these animals are unchanged. Partly it's our nature. Humans are visual animals, and good at recognising shapes, says Gómez. It is "hard to look beyond that" and see that there might be something different going on 'under the hood'.
Why on earth are they called living fossils?
Some supposed living fossils aren't even as old as we previously thought. For instance, cycad plants are said to have lived alongside the dinosaurs. No doubt some cycads did, but the DNA of modern cycads shows that they only evolved 12 million years ago.
"They have been evolving non-stop and speciating and radiating, so why on earth are they called living fossils?" asks Gómez.
Still, the overall look of each living fossil has stayed more or less the same. So while they are clearly evolving, perhaps they are doing so more slowly than everything else.
Though it might seem that these species have stagnated, they are changing. "The mathematical reality behind evolution is that there has to be a mechanism to keep you the same," says David Polly of Indiana University in Bloomington.
There really is something special about living fossils
Genes are always mutating, and being reshuffled by sex, but that doesn't necessarily mean big changes to the animals carrying them. "Evolution does not move inevitably forwards towards new morphology and new designs," says Fortey.
Since most species do change, there really is something special about living fossils. "That they've stayed roughly the same means there's something quite active keeping them that way," says Polly. "The interesting question is what."
So what is it about coelacanths, ghost sharks and tuataras? Something has allowed their bodies to stay mostly unchanged for hundreds of millions of years.
It may be because they were in the right places at the right times.
Animals can only survive if they have somewhere to live. Mass extinctions destroy many of these habitats, but not all of them. "If the habitat in which these organisms lived came through one of these crises, that carried through the organisms themselves," says Fortey. "They were then free to evolve after the crisis, and so the line wasn't broken."
Cockroaches can live in many places
Habitats can also disappear slowly. "In the geological past there were certain environments that were widespread and common," says Polly. "As we come to the geological present they have become less common, and there are new environments." This explains why many species have been forced to change.
Some have survived by being adaptable. For instance, cockroaches can live in many places, such as crevices, holes, rocks or drains. "They can live on almost anything," says Fortey, and that probably explains why they have lasted so long.
For less adaptable species, it's a question of picking exactly the right spot.
Take the animals known as Lingula, which are found on the sea floor, near the coast, of the Indian Ocean. They look like mussels, but they actually belong to an ancient group called the brachiopods. Their fossil ancestors lived in the inter-tidal habitat, the area between low and high tide, says Fortey.
Some of these survivors were buoyed through these events because they lived in the right place
During the Permian extinction event, the seas became drained of oxygen. This meant creatures living in the deep sea were particularly vulnerable, which helps explain why around 95% of marine species were wiped out. Meanwhile, land animals were killed off in similar numbers by a drier climate and expanding deserts.
But Lingula's ancestors came through unscathed. In the intertidal zone, the water was continuously recycled so lack of oxygen wasn't a problem. "Some of these survivors were buoyed through these events because they lived in the right place," says Fortey.
Beyond where it lives, a species' attributes can help it survive.
"The fact coelacanths taste disgusting could well have helped them stay alive," says Fortey. They look as if they are covered in mucus, and are said to taste waxy and make those who eat them sick to their stomachs.
Horseshoe crabs are also great survivors. The earliest versions show up in the fossil record nearly half a billion years ago. Modern ones have a particularly colourful secret weapon.
Their bright blue blood coagulates when faced with nasty bacteria, preventing infections from going further. Hundreds of thousands of horseshoe crabs are harvested every year by the medical community, because the crucial chemical in their blood can detect contamination in any solution that might come into contact with blood.
The truth is, there is literally no such thing as a "living fossil". All species evolve, even if it's not obvious.
There is one other species that's been proposed to be a living fossil
Gómez thinks we should retire the term altogether. "Darwin never intended it to be used seriously. The term is over-simplifying and leads to people believing that some things haven't evolved, which is so wrong."
Fortey would rather call creatures like coelacanths 'extreme survivors of a lineage'. It's more accurate, but it's not as catchy.
Finally, there is one other species that's been proposed to be a living fossil. That species is the human race. Is it true, as some people have said, that humans have stopped evolving?
The idea is that technological and medical advances have removed the pressure on us to evolve. Modern societies can keep even the weakest alive, by building shelters and developing vaccines against deadly diseases. As a result, our environment is now much easier to survive in, so we may be just evolving culturally, as David Attenborough suggested in a Radio Times interview in 2013.
Even within the last 10,000 years, humans have changed
However, the genetics doesn't support this. Around 40,000 years ago, the human population exploded, and evolution sped up. In 2007, John Hawks of the University of Wisconsin, Madison and his colleagues studied the DNA from 270 individuals and found that human evolution "has recently accelerated by 100-fold".
Similarly, a 2014 study estimated that the most recent common ancestor of all living humans lived around 239,000 years ago. That is much more recent than some estimates, and again suggests that humans have been evolving rapidly.
Even within the last 10,000 years, humans have changed. The existence of blue eyes, and the ability of some adults to drink animal milk that contains lactose, are two examples of recent innovations.
It's harder to say what has happened in the last few hundred years, when technological progress has been fastest, because it's such a short span of time. But if the other living fossils have taught us anything, it's that it should be impossible for humans to stop evolving.