As science evolved, people started asking questions and developing theories about how evolution on the biological scale occurred, and how organisms that apparently have nothing in common may have been related million of years ago. And even though in the past the methods to prove such theories were mostly intuitive and rudimentary, modern genetics can now support them, and provide the necessary evidence to link different mammals on the evolution scale.

Scientists have linked birds and reptiles as descendants of a common ancestor, and what better example to prove such a connection that the platypus (Ornithorhynchus anatinus). The creature, which is endemic to Australia, may hold a lot of answers in the evolution theory, and it’s genetic code is a true manuscript on the animal kingdom.

A team of scientists led by Wesley C. Warren, a geneticist at Washington University School of Medicine in St. Louis, unveiled in the study published online in the May 7 issue of the journal Nature that the enigmatic creature, which is in fact a semi-aquatic bird-reptile-beaver-like mammal, holds the key to a series of genetic events in the evolution of mammals.

The platypus is not only a bizarre-looking mammal, but its ability to lay eggs, produce milk, or defend (through a painful, sometimes deadly venom) links it to almost all other groups of vertebrates.

“The analysis is beginning to align these strange features with genetic innovation,” Wesley C. Warren, lead author of the study, said.

The structure of the platypus genome unveiled that although this venomous mammal doesn’t have any nipples, it is still able to produce milk (containing fats, sugars and proteins). Among the proteins produced, the caseins match a cluster found in humans, which led scientists to the conclusion that the development of milk happened 166 million years ago, after mammals split from the sauropsids (birds and reptiles’ common ancestor).

Another strange feature that links the platypus to other groups of vertebrates is its capacity to lay eggs, which is a unique feature among mammals (who give birth to live young). After the platypus lays the egg, it takes 11 more days for the hatchlings to emerge, and even more to fully develop their organs (behavior similar to that of marsupials).

What links it even further to birds, as well as amphibians and fish, is the presence of the ZPAX genes, as well as vitellogenin, an egg-yolk type of protein found in chickens and fish, which suggests that the gene was inherited from the sauropsids, despite the difference today (the chicken has three vitellogenin genes, whereas the platypus has only one).

The presence of its venomous spur on the hind foot, which is part of its defense system, suggests a connection with the reptiles: the venom contains a variation of more than three kinds of peptides, just like in reptiles.

“There is nothing quite as enigmatic as a platypus,” Richard Gibbs, who directs the Human Genome Sequencing Center at Baylor College of Medicine in Houston, Texas, noted. “You have got these reptilian repeat patterns and these more recently evolved milk genes and independent evolution of the venom. It all points to how idiosyncratic evolution is.”

Unlike other mammals or birds, the platypus also has a distinct feature: its sex is determined by a set of ten chromosomes, which during meiosis (when the number of chromosomes in cells is cut in half and the gametes result) designate a set of all Xs or all Ys for every sperm, scientists explained.

And while the Xs are very different from those found in other mammals, they do have striking similarities with the birds’ Z sex chromosome, which was a clear indication that the sex chromosomes have clearly evolved after the monotremes diverged from mammals.

Dr. Chris Ponting, of the MRC Functional Genetics Unit at the University of Oxford, who also took part in the study, told BBC: “It’s wonderful to see all of the different mishmash of features that the platypus exhibits, to see those features reflected in the DNA, in the genes of this creature, which has held the mysteries for the scientists and the general population ever since it was discovered 200 years ago.”