Platypuses exhibit a fascinating combination of reptile and mammalian characters. They have hair, mammary glands, and three auditory ossicles, characteristics unique to mammals, but also a duck-like beak, a beaver’s flattened tail, and webbed feet.
The males have on their hind legs a claw endowed with a venom similar to that of reptiles. Females not only lay eggs, but the segmentation of their ovules is typically reptilian (meroblastic), not holoblastic as in other groups of mammals. Like echidnas, they are monotreme, which means “single hole” due to their common external opening for the urogenital and digestive systems.
All of this is visible on the outside, but if you look inside a platypus, you will see another strange feature: the throat connects directly to the intestines. There is no intermediate chamber that secretes gastric acids and digestive enzymes.
That is, the platypus has no stomach.
If it’s a vertebrate mammal, why doesn’t it have a stomach? The answer is in the genes.
Origin and evolution of the stomach
The stomach is an enlarged portion of the intestine, a hallmark of jawed vertebrate (gnathostome) evolution. It represented an anatomical innovation characterized by the presence of acid- and pepsin-secreting glands and first evolved around 450 million years ago.
The stomach allowed our ancestors to digest larger proteins, since powerful gastric acids break down macromolecules and enhance the actions of peptidases, which break them down. Now many animals do quite well without a stomach.
Since Cuvier first observed in 1805 that some groups of fish lacked stomachs, in the last 200 years it has been shown that, on at least eighteen separate occasions, many vertebrates have lost their stomachs. They include, in addition to platypuses, the rare lungfish and agnathus, but also a quarter of the nearly 30,000 species of teleosts, the group that includes most fish.
In platypuses and jawed fish, agastria is accompanied by the disappearance of the genes (Atp4A and Atp4B) that code for the production of the gastric proton pump, the ATPase enzyme that activates the production of hydrochloric acid in the stomach. They have also lost many of the Pg genes that code for pepsinogens, the precursors to pepsins, the stomach enzymes that break down proteins.
In conclusion, as in many other cases, gene loss leads to changes in phenotypic variability. Simplification of the intestine by loss of the gastric chamber has occurred several times in vertebrate evolution as a result of the loss of key genes in the acid-peptic digestion process.
Why lose your stomach?
At the moment science has no answer. If we take into account that stomach avoidance affects the digestive process, it is possible to speculate that part of the answer lies in the diet.
A first hypothesis is that the disappearance or inhibition of the genes that encode the production of pepsinogens would result in the elimination of the gastric pumps from the primitive stomach chambers.
Animals develop very different sets of pepsinogen-coding genes to digest the proteins in their specific diets. Perhaps the ancestors of the stomachless species switched to a different diet that rendered these enzymes physiologically useless. Over time, debilitating mutations accumulated until they were eventually lost.
Evidence of this process can be seen in animals with stomachs. To digest milk proteins, many newborn mammals use the Cym gene that encodes a gastric enzyme, chymosin, designed to break the casein phosphoprotein chain. In humans, the gene is found as a pseudogene, that is, it is never expressed because our milk is relatively poor in protein.
Pepsinogens work best in acidic environments, so if they were to disappear, an acid chamber would no longer be needed. Since gastric pumps need a large amount of energy to maintain stomach acidity, if they are no longer needed they will be lost because that represents an evolutionary advantage.
A second alternative hypothesis is that the disappearance of the need to maintain an acidic environment in the digestive tract would make the development of gastric chambers superfluous and, as a consequence, the disappearance or inhibition of pepsinogen-encoding genes.
All agastric species live in water (or, like the echidna, have aquatic ancestors). Many of these animals eat many crustaceans and corals, whose shells are rich in calcium carbonate, a substance that neutralizes stomach acidity, and other arthropods with an exoskeleton made of chitin, a highly acid-stable polymer.
Why acidify the stomach if the diet immediately undoes the process? The energetically expensive gastric pumps would be superfluous and would soon be lost. And without an acidic environment, the pepsinogen genes are also useless, so they would do the same.
Beyond speculation, one thing is clear: many animals get by just fine without a stomach, the loss of which, according to Dollo’s law, is an irreversible evolutionary process, and even more so if we take into account that there are alternative solutions. The intestine has its own enzymes that break down proteins. The throats of some fish such as remoras, sucking catfish, catfish, or the popular goldfish in home aquariums have an extra set of pharyngeal teeth that help break down what they eat.
One of the earliest Latin names for the platypus was Ornithorhynchus paradoxus. During the half century since the first description of the platypus in 1799 and the publication of The origin of species After Darwin, the platypus endured innumerable attempts to deny or mitigate its true mix of characters associated with different groups of vertebrates. Nature required clear categories defined by divine wisdom. An animal could not both lay eggs and feed its offspring with mammary glands.
They were wrong. The platypus is not a mistake of divine creation: it is a marvel of evolution.