Here’s the platypus. You could almost hear David Attenborough’s soothing voice describing such a strange creature, the mystery of how it appeared gave too many scientists a headache.
Platypus could just as well be extraterrestrial life forms on Earth. I lay eggs. Sweat milk. They have bright biofluorescent fur, venomous tips on the backs of their legs and 10 sex chromosomes when mammals should have two. These are one of only five species of monotremes in existence, creatures that appeared millions of years before modern mammals ever appeared on stage. Now, what is arguably the most bizarre mammal in the world, has had its genome completely mapped by scientists. The exposure of his genes eventually explained how and why he evolved some of his more extreme features.
“Mammals that lay eggs (monotremes) are the only existing group of mammals for terrestrials (marsupial and eutrean animals) and provide essential information on mammalian evolution,” said biologist Guojie Zhang of the University of Copenhagen, who recently co-authored a study published in The nature.
Monotremes are technically mammals – “technical” being the operative word here. What they really are is a mashup of mammalian, reptilian, and bird genes that somehow worked to help the platypus and four species of echidna (which look like some kind of alien hedgehog) survive that long. Eutherian mammals, like humans, give birth to young life. Metaterian mammals or marsupials carry their young in a pouch in which they grow until they are ready to roam the wilderness alone. Monotremes, also known as prototerians, lay eggs, but still produce milk for their offspring. This milk is secreted by the sweat glands.
How did this develop into something that is a mammal or at least adjacent to the mammal? Vitellogenin genes are proteins in the blood from which an egg yolk is formed. They can be found in anything 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 will become the yolk. Humans and marsupials have lost these genes. As it evolved, the platypus managed to cling to one, which explains why it lays eggs. It can get rid of this, because the vitellogenin gene it has makes young people less dependent on egg yolk proteins, because it also produces milk for them.
What vitellogenin revealed in platypus genes is that mammalian milk production was passed down from a common ancestor who shared the planet with dinosaurs more than 170 million years ago. Its genome is also eliminated when it has lost its teeth: when half of the eight genes needed for teeth disappeared only 50 million years later. Instead, he uses horn plates inside his duck bill to crush the small crustaceans that are usually on the menu. Another question that Zhang and his colleagues finally managed to answer was how the platypus managed to keep the 10 sex chromosomes of their ancestors. Eutherians and marsupials have a single X chromosome and a Y chromosome, while the platypus has five of each.
What the team’s research suggested was that the monotreme ancestors had all 10 X and Y in a ring until they broke into smaller pieces. This is so far from eutherians as we are that the sex chromosomes of a platypus are actually closer to those of chickens, but they still show that we are related to birds in some way.
The strongest feature of the platypus may be its shiny fur in the dark. Biofluorescence occurs when wavelengths of light that are too short for human eyes to see are absorbed, then re-emitted as longer, visible wavelengths that make this glow happen. Do you often see this phenomenon in deep-sea fish, but a (kind of) mammal? Platypuses are nocturnal creatures that usually sneak in when the sun just sets and swims with their eyes closed. This explains the electrical receivers on his bill that help him look for prey. What it doesn’t explain is why they need it when they can’t even see, but the absorption of UV light can make it less visible to UV-sensitive predators with almost preternatural night vision.
Although we will always keep an eye on the aliens, it’s kind of amazing how aliens can get some creatures that appeared and evolved right here on Earth.