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WHEN news reached London of a mole-like animal with webbed feet and a duck’s bill, many people thought it was a hoax. It was the late 18th century, Britain had just begun colonising Australia and the strange creature had been spotted by no less a figure than David Collins, founder of New South Wales. However, when zoologist George Shaw at the British Museum examined sketches and specimens of the animal, he was sceptical. “It naturally excites the idea of some deceptive preparation by artificial means,” he wrote.

Attitudes changed as more specimens arrived. In 1799, Shaw was the first to scientifically describe the creature, giving it the name Platypus anatinus, meaning “flat-footed duck”. It was later referred to as the “paradoxical bird-snout” before being officially renamed Ornithorhynchus anatinus, meaning “duck-like bird snout”. Today, most people just call it the platypus.

It took more than 80 years just to work out how this animal fits into the tree of life. Since then, biologists have gone even further and found that it possesses a range of features that mean it is among the most unusual creatures on Earth. But it isn’t simply an oddity. As a mammal that shares many characteristics with birds and reptiles, the platypus holds the key to unlocking some fundamental evolutionary mysteries.

Now, geneticists have mapped its entire genome and are starting to understand how it came to be so strange – and what it can tell us about the origins of all mammals, including us. Even today, it turns out, the platypus has the ability to surprise.

The platypus is one of just five remaining species from an ancient group of mammals called monotremes that lay eggs rather than giving birth to live young, as other mammals do. The other extant monotremes, the four species of echidna, are equally strange. Found only in waterways across Tasmania and eastern Australia, platypuses are nocturnal and grow to about half a metre long. They may look like a mash-up of various animals, but, ecologically, they make sense. “They are exquisitely adapted to what they do,” says biologist Jonathan Losos at Washington University in St Louis, Missouri. Their webbed feet, sprawled body and dense, waterproof fur are perfect for their semi-aquatic lifestyle. Their claws make them efficient diggers: they excavate tunnels around 5 metres long in riverbanks in which to live. And platypuses’ distinctive, duck-like bills allow them to search for crustaceans and insects while swimming underwater with their eyes, ears and noses shut. “It has this amazing electroreception sense in its bill that can detect the muscular activity of its prey, even slight muscle twitches,” says Losos.

These leathery bills are made of hardened gum tissues and, lacking teeth, platypuses use them to mash their food – sometimes employing stones to help crush harder items. Once swallowed, food travels straight from the oesophagus to the intestine: platypuses, together with their echidna cousins, are the only mammals without a stomach. These organs evolved some 450 million years ago, well before platypuses came along, and why these animals have lost theirs is unclear. One suggestion is that the shells in their diet are rich in calcium carbonate and would neutralise any stomach acids, making the organ redundant.

When they aren’t hunting, platypuses rest. They can spend around 14 hours a day sleeping, , which is more than any other animal. That’s odd, given that – in humans at least – REM sleep is associated with brain development and processing memories. Its abundance in platypuses suggests this type of sleep originated earlier in mammalian evolution than previously thought, says Losos, and that its functions may have evolved separately to meet the needs of different species.

A mysterious glow

A recently discovered platypus quirk is even more puzzling. Last year, researchers at the Field Museum of Natural History in Chicago, Illinois, reported finding that the animals . They had been trying to confirm this phenomenon, known as biofluorescence, in another species in their collection when they accidentally shone ultraviolet light onto a platypus specimen nearby. To their surprise, the fur emitted a blue-green glow. Biofluorescence is typically used for communication or camouflage, but that makes no sense in a nocturnal species. “Scientists are completely baffled by what this is all about,” says Losos.

“The leathery bill, made of hardened gum tissue, is used to mash food”

Then there is the platypus’ peculiar reproductive behaviour. Like birds and reptiles, monotremes have a cloaca, a single opening used for reproduction and excretion – which is where the name “monotreme” comes from. During the mating season, male platypuses fight for females, using poisonous spurs on their hind feet that deliver enough venom to kill a small animal or cause excruciating pain in humans. Females, meanwhile, use their claws to build a nursery. They dig a 30-metre-deep burrow, which they line with wet leaves for bedding. Once mated, they seal themselves inside, lay two eggs and hug these tightly underneath their tails to incubate them. In just 10 days, the butter-bean-sized babies, known as puggles, hatch. Their mothers don’t have teats. “They kind of ooze milk out of their belly for the puggles to lap up,” says Losos. The nursing young have teeth, but they lose these at around 4 months old when they leave their natal burrows and become independent.

Given that they lay eggs and produce milk, it is tempting to see monotremes as an evolutionary link between mammals and reptiles. In reality, it isn’t that simple. To understand why, and to set the scene for the latest genetic discoveries, we need to go back to the first animals to conquer land: the tetrapods.

People often think that mammals evolved from reptiles, but this couldn’t be further from the truth, says palaeontologist Elsa Panciroli at Oxford University Museum of Natural History. Synapsids (mammals) and sauropsids (reptiles and birds) evolved separately after splitting from a single lineage around 310 million years ago. “Up to that point, they were neither reptiles nor mammals; they were tetrapods,” says Panciroli.

Early mammals lived alongside the dinosaurs, so faced stiff competition. “They hit on this absolutely fantastic way of life,” says Panciroli. Being small, nocturnal, shrew-like insectivores allowed them to thrive. These first true mammals belonged to an extinct order called morganucodonta. Later, other mammal groups evolved, but ultimately only one survived. Today, it consists of three lineages: monotremes, marsupials and, by far the largest lineage, the placentals, which are distinguished by carrying the fetus in the mother’s uterus into later stages of development. Marsupials and placentals jointly form a group called therians, which split from a common ancestor around 130 million years ago. Monotremes are older, originating some 180 million years ago (see diagram). “They’re this ancient group of mammals and they have such an amazing story to tell us,” says evolutionary biologist Vera Weisbecker at Flinders University, Australia.

Toothy ancestors

All this suggests that the first monotremes didn’t resemble those that are alive today, but instead looked similar to the shrew-like morganucodonts. Unfortunately, we have no fossil evidence of them, say Panciroli. Nor do we have fossils of intermediate forms. The oldest evidence of platypuses in Australia consists of fossilised skulls dating from just 15 million years ago. These are from an extinct species, which had teeth and grew to a metre long. A tooth from another extinct species has been dated to 62 million years ago, but it was living in what is now Argentina. That isn’t so surprising, given that South America and Australia were connected as part of the supercontinent Gondwana when monotremes first emerged. Clearly, they died out in South America following the break up of Gondwana, which began around 140 million years ago. Why they persisted in Australia, and why only one platypus species survives today, is still unknown. Panciroli predicts that the answer lies in yet-to-be discovered fossils of monotreme ancestors in South America. But Weisbecker thinks that the single piece of evidence from South America indicates that platypuses were rare in that region. “They may have just always sat in Australia,” she says.

Looking further back, the lack of fossil evidence is even more problematic. The first mammal was thought to have produced milk, laid eggs and possessed a cloaca. But there isn’t definitive evidence to confirm any of these assumptions. “It would be the holy grail if we could find a fossil mammal egg,” says Panciroli.

What we do have, however, is genetic evidence inside every cell of living monotremes – and now this evidence can be scrutinised. Earlier this year, Guojie Zhang at the University of Copenhagen, Denmark, and his team of the platypus, alongside the first fully sequenced echidna genome. “They hold a lot of the clues to understanding how the earliest mammals evolved,” says Zhang.

For a start, the researchers were able to put dates on key branches in the mammalian family tree. By comparing the monotreme genomes with those of other extant mammals, they were able to date the therian-monotreme split to around 187 million years ago – slightly earlier than previously estimated. The analysis also reveals that platypuses and echidnas went their separate ways around 55 million years ago. The two monotremes have since adapted to their own unique environments, yet are genetically very similar. For example, monotremes lost half of the eight genes related to tooth development just before the platypus and echidna lineages split. And many of the genes involved with digestion disappeared around this time too, suggesting this is when both echidnas and platypuses lost their stomachs as well as their teeth.

“Other mammals generally have two sex chromosomes. Platypuses have 10”

The genetic evidence also sheds light on the transition from egg laying to live births. The platypus genome contains genes for a protein called casein, which is seen only in mammals and is crucial for the production of mammalian milk. “So platypuses produce similar components to the milk we produce,” says Zhang. This suggests milk production must have developed in the common ancestors of all modern mammals more than 187 million years ago, he adds. Platypus milk has a unique ingredient, however. The genome reveals that it contains antibacterial proteins, perhaps to help protect puggles from pathogens while they are in their natal burrows.

The end of eggs

The ability to produce milk would have meant that early mammals didn’t need to lay eggs capable of entirely sustaining their developing offspring, as other egg-laying animals do. The platypus genome reflects this. Reptiles and birds have three genes for proteins called vitellogenins, which are instrumental in the production of the egg yolk. Humans and all other therian mammals lack these genes. But platypuses and echidnas still have one of them, and continue to lay eggs by virtue of the vitellogenin this gene produces. The other two genes seem to have been lost around 130 million years ago, says Zhang. In which case, the last gene must have been lost later in therian mammals and the practice of giving birth to live young must have evolved after they split from the monotremes.

Platypus sex chromosomes tell another intriguing story. Other mammals, including humans, generally have two of these, X and Y – paired as XX in females and XY in males. Platypuses have 10 – occurring as X₁X₁X₂X₂X₃X₃X₄X₄X₅X₅ in females and X₁X₁X₂X₂X₃X₃X₄X₄X₅X₅ in males. Looking more closely at the sex chromosomes in a male, the researchers found that the X₁ chromosome had similar genetic sequences to those of the Y₁ chromosome. Y₁ was similar to X₂, and so on until Y₅ matched back to the X₁ chromosome. “We can predict they formed a ring in the monotreme ancestor,” says Zhang. What this says about other early mammals is unclear, but such chromosomes have never been found to occur normally in a living animal. This ring must have broken into smaller pieces of X and Y chromosomes over the course of mammalian evolution, and the system of two sex chromosomes that we have later evolved.

That isn’t all. After comparing the platypus sex chromosomes with those of other animals, the researchers concluded that they show more parallels with those of chickens than humans. “It doesn’t have any hallmarks with our human XY chromosomes,” says Zhang. “The five pairs of XY chromosomes are more like the ZW chromosomes of birds.” Platypuses have several genetic sequences that match those found in birds, whereas the sex chromosomes of therian mammals and birds have no genes in common. This overlap with birds could mean that the platypus may still contain genetic sequences inherited from the early tetrapod species before birds and mammals diverged from them.

Zhang’s genome analysis has expanded our understanding of platypus and mammalian evolution. “Monotremes are that magical third coordinate that gives you the proper location to find your way through the evolution of mammals,” says Panciroli. The challenge, however, is to work out which aspects of platypus biology are really representative of the past. Although they possess ancient characteristics, we shouldn’t think of them as living fossils, says Panciroli. “They are evolutionarily distinct.”

Sadly, platypus numbers are declining as their habitats become increasingly damaged. They are now classed as by the International Union for Conservation of Nature. Echidnas and the platypus are the only monotremes left, the end of an evolutionary line that goes back to before the dinosaurs. “It would be a shame if we lost them,” says Losos. “There’s nothing else like them out there.”