The researchers have attempted to construct a tidy evolutionary history:
Lays eggs, sweats milk and has no teeth
One of the platypus’ most unusual characteristics is that, while it lays eggs, it also has mammary glands used to feed its babies, not through nipples, but by milk — which is sweat from its body.
During our own evolution, we humans lost all three so-called vitellogenin genes, each of which is important for the production of egg yolks. Chickens on the other hand, continue to have all three. The study demonstrates that platypuses still carry one of these three vitellogenin genes, despite having lost the other two roughly 130 million years ago. The platypus continues to lay eggs by virtue of this one remaining gene. This is probably because it is not as dependent on creating yolk proteins as birds and reptiles are, as platypuses produce milk for their young.
In all other mammals, vitellogenin genes have been replaced with casein genes, which are responsible for our ability to produce casein protein, a major component in mammalian milk. The new research demonstrates that the platypus carries casein genes as well, and that the composition of their milk is thereby quite similar to that of cows, humans and other mammals.
“It informs us that milk production in all extant mammal species has been developed through the same set of genes derived from a common ancestor which lived more than 170 million years ago — alongside the early dinosaurs in the Jurassic period,” says Guojie Zhang.
Another trait that makes the platypus so unique is that, unlike the vast majority of mammals, it is toothless. Although this monotremes’ nearest ancestors were toothed, the modern platypus is equipped with two horn plates that are used to mash food. The study reveals that the platypus lost its teeth roughly 120 million years ago, when four of the eight genes responsible for tooth development disappeared.
Only animal with 10 sex chromosomes
Yet another platypus oddity investigated by the researchers was how their sex is determined. Both humans and every other mammal on Earth have two sex chromosomes that determine sex — the X and Y chromosome system in which XX is female and XY is male. The monotremes, however, including our duck-billed friends from Down Under, have 10 sex chromosomes, with five Y and five X chromosomes.
Thanks to the near-complete chromosomal level genomes, researchers can now suggest that these 10 sex chromosomes in the ancestors of the monotremes were organized in a ring form which was later broken away into many small pieces of X and Y chromosomes. At the same time, the genome mapping reveals that the majority of monotreme sex chromosomes have more in common with chickens than with humans. But what it shows, is an evolutionary link between mammals and birds.Faculty of Science – University of Copenhagen, “How Earth’s oddest mammal got to be so bizarre” at ScienceDaily
All of this attempted reconstruction depends on the assumption that the platypus is not just an oddity of nature but a manuscript of genetic changes. Well, we shall see.
The paper is open access. We were mistaken in earlier assuming that it was paywalled.