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SYFY WIRE evolution

The platypus almost shouldn’t exist, but its genes are revealing how it got so weird

By Elizabeth Rayne

Behold the platypus. You can almost hear David Attenborough’s soothing voice describing a creature so peculiar, the mystery of how it came into being has given too many scientists a headache.

Platypuses might as well be alien life-forms on Earth. They lay eggs. They sweat milk. They have glowing biofluorescent fur, venomous spikes on the backs of their legs, and 10 sex chromosomes when mammals are supposed to have two. They are one of only five extant species of monotremes, creatures that came into being millions of years before modern mammals ever appeared on the scene. Now, what is arguably the world’s most bizarre mammal has had its complete genome mapped by scientists. Exposing its genes has finally explained how and why it evolved some of its more extreme features.

“Egg-laying mammals (monotremes) are the only extant mammalian outgroup to therians (marsupial and euthreian animals) and provide key insights into mammalian evolution,” said biologist Guojie Zhang of the University of Copenhagen, who recently co-authored a study published in Nature.

Monotremes are technically mammals — with “technically” being the operative word here. What they really are is a mashup of mammalian, reptilian, and bird genes that somehow worked out to help the platypus and four species of echidna (which looks like some sort of extraterrestrial hedgehog) survive for so long. Eutherian mammals, like humans, give birth to live young. Metatherian mammals, or marsupials, carry their young around in a pouch where they keep developing until they are ready to roam the wild on their own. Monotremes, also known as prototherians, lay eggs, but still produce milk for their offspring. That milk is secreted through their sweat glands.

How did this even develop in something that is a mammal or at least mammal-adjacent? Vitellogenin genes are proteins in the blood from which an egg yolk forms. They can be found in anything that lays eggs. Estrogen helps form them in the liver, where they are modified and then sent to the ovaries to be processed into what will become the yolk. Humans and marsupials lost these genes. As it evolved, the platypus managed to hang onto one, which explains why it lays eggs. It can get away with this because the one vitellogenin gene it has makes its young less dependent on yolk proteins since it also produces milk for them.

What the vitellogenin in platypus genes has revealed is that milk production in mammals was passed down from a common ancestor that shared the planet with dinosaurs over 170 million years ago. Its genome also gives away when it lost its teeth: when half of the eight genes required for teeth vanished only 50 million years later. It instead uses horn plates on the inside of its ducklike bill to crush small crustaceans which are usually on the menu. Another question that Zhang and his colleagues were finally able to answer was how platypuses managed to keep the 10 sex chromosomes of their ancestors. Eutherians and marsupials have only one X and one Y chromosome, while the platypus has five of each.

What the team’s research suggested was that monotreme ancestors had all 10 Xs and Ys in a ring until they broke into smaller pieces. This is so far off from eutherians like us that the sex chromosomes of a platypus are actually closer to those of chickens, but it still proves that we are related to birds in some way.

The coolest feature of the platypus may be its glow-in-the-dark fur. Biofluorescence happens when wavelengths of light that are too short for human eyes to see are absorbed then re-emitted as longer, visible wavelengths, which make that glow happen. You often see this phenomenon in deep-sea fish, but a (sort of) mammal? Platypuses are nocturnal creatures that typically creep out when the sun is just setting and swim with their eyes closed. This explains the electric receptors on its bill that help it seek out prey. What it doesn’t explain is why they need it when they don’t even see each other, but absorbing UV light can make it less visible to UV-sensitive predators with almost preternatural night vision.

While we will always keep an eye out for aliens, it is kind of mind-blowing how alien some creatures that spawned and evolved right here on Earth can get.