When the platypus lost its stomach

Platypuses show an amazing combination of reptile and mammalian traits. They have hair, mammary glands, and three auditory ossicles, characteristics unique to mammals, as well as a duck bill, flattened beaver tail, and webbed feet.

Males have a claw on their hind legs endowed with a venom similar to that of reptiles. Females not only lay eggs, but the segmentation of their ovules is typically reptilian (meroblastic) rather than holoblastic as in other groups of mammals. Like echidnas, they are monotreme, meaning “one hole” due to their common external opening for the genitourinary and digestive systems.

All this is visible from the outside, but if you look inside the platypus, you will see another strange feature: the throat connects directly to the intestines. There is no intermediate chamber that secretes stomach acids and digestive enzymes.

That is, the platypus does not have a stomach.

If it’s a vertebrate, why doesn’t it have a stomach? The answer is in the genes.

Origin and evolution of the stomach

The stomach is an enlarged part of the intestine, a hallmark of the evolution of jawed vertebrates (gnathostomy). It was an anatomical innovation characterized by the presence of acid- and pepsin-secreting glands and first appeared about 450 million years ago.

The stomach allowed our ancestors to digest larger proteins, as powerful stomach acids break down macromolecules and enhance the action of peptidases that break them down. Now many animals do just fine without a stomach.

Since Cuvier first noticed in 1805 that some groups of fish do not have stomachs, over the past 200 years it has been shown that in at least eighteen cases many vertebrates have lost their stomachs. These include, in addition to platypuses, the rare lungfish and agnathuses, as well as a quarter of the nearly 30,000 bony fish species, this group includes most fish.

In platypuses and jawed fish, agastria is accompanied by the disappearance of genes (Atp4A and Atp4B) encoding the production of the gastric proton pump, an ATPase enzyme that activates the production of hydrochloric acid in the stomach. They also lost many of the Pg genes that code for pepsinogens, precursors of pepsins, stomach enzymes that break down proteins.

In conclusion, as in many other cases, the loss of a gene leads to a change 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 during acid-peptic digestion.

Why lose your stomach?

At the moment, science has no answer. If we take into account that avoiding the stomach affects the process of digestion, we can assume that part of the answer lies in the diet.

The first hypothesis is that the disappearance or inhibition of genes encoding the production of pepsinogens will lead to the elimination of gastric pumps from the primitive chambers of the stomach.

Animals produce very different sets of pepsinogen-coding genes for digesting proteins in their specific diet. It is possible that the ancestors of the stomachless species switched to a different diet that rendered these enzymes physiologically useless. Over time, the debilitating mutations accumulated until they eventually disappeared.

Evidence of this process can be seen in animals with a stomach. To digest milk proteins, many newborn mammals use the Cym gene, encoding the gastric enzyme chymosin, designed to break the chain of casein phosphoprotein. In humans, this gene occurs as a pseudogene, meaning it is never expressed because our milk is relatively poor in protein.

Pepsinogens work best in an acidic environment, so if they disappear, the acid chamber is no longer needed. Since the gastric pumps require a large amount of energy to maintain the acidity of the stomach, if they are no longer needed, they will be lost, because this represents an evolutionary advantage.

The second alternative hypothesis is that the disappearance of the need to maintain an acidic environment in the digestive tract would make it unnecessary to develop gastric chambers and, as a result, the disappearance or inhibition of genes encoding pepsinogen.

All species of gastropods live in water (or, like echidnas, have aquatic ancestors). Many of these animals eat a lot of crustaceans and corals, whose shells are rich in calcium carbonate, a substance that neutralizes the acidity of the stomach, and other arthropods with an exoskeleton made of chitin, a highly acid-resistant polymer.

Why acidify the stomach if the diet immediately cancels this process? Energy-expensive gastric pumps would be redundant and would soon be lost. And without an acidic environment, the pepsinogen genes are also useless, so they would do the same.

Aside from the assumptions, one thing is clear: many animals do just fine without a stomach, the loss of which, according to Dollo’s law, is an irreversible evolutionary process, and even more so when you consider that there are alternative solutions. The intestines have their own enzymes that break down proteins. Some fish, such as the remora, suckling, catfish, or the goldfish popular in home aquariums, have an extra set of pharyngeal teeth in their throats to help break down what they eat.

One of the first Latin names for the platypus was Ornithorhynchus paradoxical. In the half century since the first description of the platypus in 1799, Origin of Species Since Darwin, the platypus has withstood countless attempts to deny or downplay its true combination of features associated with various vertebrate groups. Nature demanded clear categories defined by divine wisdom. An animal could not simultaneously lay eggs and feed its offspring with mammary glands.

They were wrong. The platypus is not a mistake of divine creation: it is a miracle of evolution.

Manuel Peinado Lorca, University Professor. Director of the Royal Botanic Gardens of the University of Alcala, University of Alcala

This article was originally published on The Conversation. Read the original.

We recommend you METADATA, an RPP technology podcast. News, analytics, reviews, recommendations and everything you need to know about the tech world.