Look at the platypus. One can almost hear the calming voice of David Attenborough describing such a peculiar creature, that the mystery of how it arose has caused a headache to too many scientists.
Platypus could also be life forms alien to Earth. Put eggs. They are milk. They have bright biofluorescent skin, poisonous spikes on the back of their legs, and 10 sex chromosomes when mammals are supposed to have two. They are one of five existing species of monotremes, creatures that emerged millions of years before modern mammals appeared on the scene. Now, what is possibly the strangest mammal in the world has been mapped to its entire genome by scientists. Exposure of its genes has finally explained how and why some of its most extreme features evolved.
“Egg-laying mammals (monotremes) are the only group of mammals in terians (marsupials and eutraus) and provide key information about the evolution of mammals,” said biologist Guojie Zhang of the University of Copenhagen. who recently co-authored a study published in Nature.
Monotremes are technically mammals, being “technically” the operative word here. What they really are is a mashup of genes from mammals, reptiles, and birds that somehow worked to help the platypus and four species of equidna (which looks like a species of alien hedgehog) survive for so long. Eutherian mammals, like humans, give birth to young. Metaterian mammals, or marsupials, carry their young in a bag where they develop until they are ready to roam wildlife on their own. Monotremes, also known as prototerians, lay eggs, but still produce milk for their offspring. This milk is secreted through the sweat glands.
How did this even develop into 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 anything that makes eggs. Estrogens help form them in the liver, where they are modified and then sent to the ovaries to transform them into what will become the yolk. Humans and marsupials lost these genes. As it evolved, the platypus managed to hang on to one, which explains why it lays eggs. It can get away with it because the vitellogenin gene it has makes its young less dependent on yolk proteins, as it also produces milk for them.
What vitellogenin has revealed from platypus genes is that milk production in mammals was passed on to a common ancestor who shared the planet with dinosaurs more than 170 million years ago. Its genome also emerges when it lost its teeth: when half of the eight genes needed for teeth disappeared just 50 million years later. Instead, he uses horn dishes on the inside of his duckbill to crush small crustaceans that usually appear on the menu. Another question that Zhang and his colleagues were finally able to answer was how platypus managed to maintain the 10 sex chromosomes of their ancestors. Eutherians and marsupials have only one X chromosome and one Y chromosome, while the platypus has had five.
What the team’s research suggested was that monotreme ancestors had the 10 X and Y in a ring until they broke into smaller pieces. This is as far away from eutherians as we are that the sex chromosomes of a platypus are actually closer to those of chickens, but it still shows that we are related to birds in some way.
The coolest feature of the platypus may be its glowing skin in the dark. Biofluorescence occurs when wavelengths of light that are too short for human eyes to see are absorbed and re-emitted as longer, more visible wavelengths, which cause this glow to pass. You often see this phenomenon in tall fish, but a (kind of) mammal? Platypus are nocturnal creatures that usually escape when the sun has just set and swim with their eyes closed. This explains the electric receivers on your bill that help you look for prey. What it doesn’t explain is why they need it when they don’t even see it, but absorbing UV light can make it less visible to UV-sensitive predators with near-preternatural night vision.
While we will always be on the lookout for extraterrestrials, it is puzzling to see how alien some creatures that appeared and evolved right here on Earth may be.