See the platypus. You can almost hear the soothing voice of David Attenborough describing such a peculiar creature, that the mystery of how it came to be has given too many scientists headaches.
Platypuses may as well be alien life forms on Earth. They lay eggs. They sweat milk. They have glowing biofluorescent fur, poisonous spines on the back of their legs, and 10 sex chromosomes when mammals should have two. They are one of only five extant species of monotremes, creatures that emerged millions of years before modern mammals ever appeared on the scene. Well, what is arguably the world’s most bizarre mammal has its entire genome mapped by scientists. Uncovering its genes finally explained how and why it evolved some of its more extreme traits.
“Egg-laying mammals (monotremes) are the only extant mammalian outgroup of therians (marsupials and euthrean animals) and provide important insights into mammalian evolution,” said University of Copenhagen biologist Guojie Zhang, who recently co-authored a study published in Nature.
Monotremes are technically mammals – with “technical” being the operative word here. What they really are is a hodgepodge of mammal, reptile, and bird genes that have somehow helped the platypus and four species of echidna (which looks like some sort of alien hedgehog) can survive that long. Eutheric mammals, like humans, give birth to live young. Metatherian mammals, or marsupials, carry their young in a pouch in which they continue to develop until they are ready to roam the wild on their own. Monotremes, also called prototherians, lay eggs but still produce milk for their offspring. That milk is excreted through their sweat glands.
How did this even evolve in something that is a mammal or at least adjacent to a mammal? Vitellogenin genes are proteins in the blood from which an egg yolk is formed. They can be found in everything that lays eggs. Estrogen helps them form in the liver, where they are modified and then sent to the ovaries to be processed into what becomes the yolk. Humans and marsupials have lost these genes. As it evolved, the platypus managed to hold on to a bird, which explains why it lays eggs. It can get away with this because the only vitellogenin gene it has makes its young less dependent on yolk proteins as it also produces milk for them.
What the vitellogenin in platypus genes has revealed is that mammalian milk production was passed down from a common ancestor that shared the planet with dinosaurs more than 170 million years ago. Its genome also reveals when it lost its teeth: when half of the eight genes needed for teeth disappeared just 50 million years later. It instead uses horn plates on the inside of its duck-like beak to crush small crustaceans that are usually on the menu. Another question that Zhang and his colleagues were finally able to answer was how platypus managed to preserve their ancestors’ 10 sex chromosomes. Eutherians and marsupials have only one X and one Y chromosome, while the platypus has five.
What the team’s research suggested was that monotreme ancestors had all 10 X’s and Y’s in a ring until they broke into smaller pieces. This is so far removed 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 somehow related to birds.
Perhaps the platypus’s coolest feature is its glow-in-the-dark coat. Biofluorescence occurs when wavelengths of light that are too short for human eyes are absorbed and then re-emitted as longer, visible wavelengths, creating that glow. You often see this phenomenon in deep sea fish, but a (kind of) mammal? Platypuses are nocturnal animals that typically crawl out when the sun is just setting and swim with their eyes closed. This explains the electrical receptors on its beak that help it search for 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 near-supernatural night vision.
While we’ll always keep an eye out for aliens, it is quite amazing how aliens can become some of the creatures that originated and evolved here on Earth.