In the list of "strange creatures" inhabiting Earth, the duck-billed platypus occupies a niche all its own.
With the bill of a duck, the tail of a beaver, and venomous spurs on its hind legs, it would appear to be an egg-laying mammal scrambled together from all of the left over pieces of evolution; a failed experiment on the tree of life.
Nothing is further from the truth.
Yes, the platypus occupies the very tip of a single branch in the tree but it is eminently successful organism having evolved to take advantage of its environment. Indeed, its singularity is a sign of its success as there is little drive for improvement by the formation of other, similar species. It has existed on this planet for over 150 million years predating even the dinosaurs.
Consider the tree of life - the main stem runs from single celled organisms to colonies to insects to vertebrates to tetrapods to mammals to primates. At least from an anthropocentric point of view, this is the main branch as it ultimately leads to our existence.
If your focus was other life forms - say, plants - then the tree would appear different as animals would split off from the main stem at some relatively early point.
Fleshing out how each of our modern species fits into and onto the tree of life continues to be a very long and arduous task. When it was based on external and internal characteristics such as the shape of certain bones or the presence of limbs or position of organs finding a platypus's place in the scheme of things was difficult to say the least. Where did it fit on the tree?
Indeed, when the platypus was first described to the scientists at the London Museum of Natural History, they refused to believe such a creature existed. They thought it must be a joke. Someone was trying to pull the wool over their eyes. A duck-billed, egg laying mammal? Must be a hoax.
Even when presented with specimens, there was a great deal of consternation and disbelief. It was only through careful examination biologists began to accept the evidence before their eyes. The creature is an oddity.
With genetic sequencing of an increasing number of creatures, the tree of life is being re-cast along biomolecular lines. The decoding of the platypus genome was a high-priority project. Determining how this mixture of creatures arises could tell us a great deal about where and when various genes split from the main stem - about how fish, reptiles, birds, and mammals came to be.
For example, mammals are generally defined by certain characteristics such as the presence of hair on the body and the production of milk to feed their young from mammary glands. Although the female platypus doesn't have nipples, it does produce true milk filled with a nutritious blend of fats, sugars and proteins. The young suckle from a glandular patch on their mother's skin.
Genetic analysis shows that the platypus has essentially the same genes for the production of a family of milk proteins, called caseins, found in other mammals, including humans. This would indicate that one of the genetic innovations leading to the development of milk occurred more than 160 million years ago before the platypus branched off on its own from the other mammals and after the break with the lizard-like sauropsids which gave rise to modern reptiles and birds.
On the other hand, the genes associated with a platypus's egg laying ability offer other insights. The embryos develop in the maternal uterus for 21 days before being expelled in thumbnail-size leathery eggs. After a further 11 days of incubation, the young hatchlings emerge and like other marsupials, they finish developing while nursing.
The platypus shares four genes with other mammals that are associated with the zona pellucida, a gel-like coating facilitating fertilization of the egg. But the platypus also has two genes only found in birds, amphibians, and fish for the development of eggs and the gene for an egg yolk protein called vitellogenin found in chicken. Clearly, the ability to lay eggs pre-dates the split from the sauropsids along with mammals and other animals.
In contrast, the poisonous spurs of the platypus - which contain a venom capable of killing a full grown dog - is similar to the poison found in various types of snakes. Like snake venom, it is a cocktail of at least three kinds of peptides. However, the variation arose from different genes sequences suggesting a case of convergent evolution.
On the whole, understanding the DNA of the platypus has played an integral part in unravelling the evolution of the mammalian genome and allowing science to better understand our place on the tree of life.
