Yes that’s a picture of a puppy, with a little bit of its rear end showing. And no, we couldn’t actually illustrate this story with an anus, now could we?

And before we discuss the origin of the anus; let’s back up a little. It’s a subject surrounded by, how should we put it, a bit of cheek. A topic right for puns, or a touch of verbal diarrhoea as we can’t but help see the innuendo.

See what I mean? So we try to get serious, to focus, and ask why has no one gotten to the bottom of this particular mystery before? Is it a crappy research topic, or by not addressing it, have other scientists fallen behind? Is even reporting such a subject, well, a little anal?

Perhaps, if jokes and innuendo are all we care about. But if we’re interested in some of the most fundamental questions about how animals evolved and function, then read on.

Because scientists have just published a review into the origin of the anus, and in doing so, they articulate how the subject is far more important than it at first appears.

That’s because the anus is one of the most important parts of many animals; an essential structure that changes how an organism’s digestive system works. Our puppy would struggle to eat and grow without it. But intriguingly, not all animals have one. Some have simple versions, others have many, while a few organisms uniquely appear to have transient anuses, which come and go. Others have anuses that are… how shall we put it… multifunctional?

While several organ systems have been investigated, such as the nervous system, the anal opening has been largely neglected

But those that do have an anus, an organ we can’t help but smirk and joke about, have vastly improved digestion. They can eat and grow more effectively, and reach much larger sizes. And the story of the origin of the anus is actually a story about how animals evolved, diverged from one another, and became sophisticated creatures.

The review into the evolutionary origins and development of the anus is published in the journal Zoologischer Anzeiger - A Journal of Comparative Zoology.

Molecular biologist Dr Andreas Hejnol and Dr Chema Martín-Durán, of the University of Bergen, Norway undertook the research in part, because no one else had. Over recent years, scientists have been able to study how genes affect the development of a range of species.

There are a select few animals that appear to have a ‘transient’ anus

“That revived the interest in the origin of our organ systems. Where did brains come from? How did blood evolve?” Dr Hejnol told BBC Earth. “But while several organ systems have been investigated, such as the nervous system, the anal opening has been largely neglected.”

But through evolution, the anus has appeared in many forms, occasionally disappearing again. “It is a fascinating subject to investigate how changes on the molecular level during evolution led to the shaping of this part of the gut,” says Dr Hejnol.

Digesting the facts

The appearance of the anus is of course inextricably linked to the evolution of the whole digestive system.

The digestive tract is one of the most important organs in any animal; it allows the efficient digestion of food and uptake of nutrients to maintain growth and sustain the body.

Despite this, some types of animal cope even though they lack a digestive tract, including sea sponges (Porifera) and tapeworms, for example. These species, perhaps obviously, also lack an anus.

Others have a very simple gut, a single sac with a single opening.

These have an anus of sorts; as their single gut opening acts as both mouth and anus – being used to both ingest food and expel excrement.

Many gelatinous-type creatures that live or float in the open ocean, including sea anemones, corals and jellyfish (types of animals known as Cnidaria) and comb jellies (Ctenophora), process food this way. As do many types of flatworm (Platyhelminthes).

Sessile animals often possess a U-shaped gut with the gut and anus in close proximity. "They manage," says Dr Hejnol.

Then there are a select few animals that appear to have what Hejnol and Martín-Durán describe as a ‘transient’ anus.

The tiny jaw worm called Haplognathia, which is just a few millimetres long, has a temporary opening to its gut, through which researchers speculate it may occasionally defecate, although the creature has never been caught in the act.

A similar transient anus is found in one of the smallest animals known, a microscopic creature called Limnognathia, which was only discovered in 1994, living in the hot springs of Disko Island, Greenland.

Trying to understand when and why the anus evolved is also complicated because it appears and disappears again within many groups of animals.

While most flatworms lack an anus, some species have independently evolved one. Some, such as the polyclad flatworm (Thysanozoon nigropapillosum) have multiple anuses, situated on their back.

Most deuterostomes, a superphlum of animals including sea stars, sea cucumbers, and all vertebrates such as birds, fish and mammals such as humans, have an anal opening (in some groups, such as reptiles and birds, the anal opening is fused with the reproductive opening, forming a structure called a cloaca). But one group of deuterostomes, the sea stars, now lack the organ.

Just imagine if we have to wait for lunch, because we still have not gotten rid of the remnants of the breakfast

And the history and function of the anus gets even more complicated. Some animals grow up with an anus, but then lose it during their own lifetime.

A rare group of scorpions, for example, will occasionally break off their tails in order to escape an attack by a potential predator, a process called autotomy.

While the benefits of doing so are clear, as the scorpions avoid being eaten, the act has consequences for the animals’ own eating habits. For when shedding their tails, they also shed their own anuses.

Since the tail never grows back, and the wound scars over, the scorpion (Ananteris balzani) can never defecate again, their abdomens swelling with the build up of poo.

The tale of the scorpion’s tail, so to speak, serves to reinforce how important anuses are in animals that have a gut that opens at two ends.

This basic digestive body plan is used by most major animal groups. Insects, birds, mammals, fish and amphibians, for example, all have an alimentary canal, or continuous through-gut, with an opening through which they eat, the mouth, and another opening for defecation.

Having a through-gut with two openings, instead a simple sac, with just one opening, has two main advantages, explains Dr Martín-Durán.

“First, an animal can take up new food items while it is still digesting a former meal,” he says, because the food flows in one direction through the body. Animals with a sac-like gut, however, have to wait to finish their first meal, and defecate out its remains, before eating again.

“Just imagine if we have to wait for lunch - up to nine hours - because we still have not gotten rid of the remnants of the breakfast,” he says.

Some animals are also the wrong shape to cope with a sac-like gut. Ribbon worms, reputedly the longest animals in the world, can reach 60 metres long. “A sac-like gut would make the sorting and digestion of food difficult,” says Dr Hejnol. Or put another way, 50 metres or more is a long way for the waste to come back up. Much easier to have another opening, i.e. an anus, at the far end, to excrete it from.

Second, a through gut, with mouth and anus, can be subdivided into different, specialised sections, each performing a different role in digestion. Animals have evolved weird and wonderful mouths; think how different the mouths of a worm, bee, bird or fish are, for example.

Many have evolved intricate guts, including fore, mid and hind-guts, each with different types of cells and functions. Cows are a classic example of a ruminating mammal, which have many gut chambers to progressively break down hard plants and grasses.

We, for example, chew with our mouths, digest proteins in our stomachs, secrete bile to help digest fats, and finally absorb most nutrients via the small intestine.

“It is hard to imagine processing food as efficiently with just a sack-like gut that lacks an anal opening,” says Dr Martín-Durán.

So the advantages of a through-gut, facilitated by the appearance of the anus, seem clear.

Less clear is when and where the anus originated in evolutionary history.

“The pattern of presence and absence of the anus in animals is quite fascinating,” says Dr Hejnol.

The shape of any organ, and the specialised cells it is made from, depend on a network of genes that contain the information used to build it. These genes are inherited down the generations, and the same genes are present within many very different types of animal. That means the early common ancestors of these various species also had these genes, and likely the same organ.

Hejnol and Martín-Durán’s review confirms that two sets of genes in particular, known as brachyury and ParaHox genes, which are present in nearly all animals, play a key-role in the formation of the anal orifice.

Animals that have an anus, almost uniformly express these genes in the tissues surrounding the organ. Those animals that do not have an anus, do not.

Intriguingly, and perhaps confusingly, despite the clear advantages of having an anus, some species have undergone a second wave of evolution, losing the anus that appeared in their ancestors. They often lose too the genes that code for the organ when it disappears, raising questions as to why it was beneficial to lose something so vital.

Dr Hejnol and Martín-Durán are conducting further research in a bid to resolve these mysteries.

They are sure that the anus has evolved independently more than once among animals. “But its deep origin remains unclear,” says Dr Hejnol.

Our own hypotheses is that the anal opening has some evolutionary connection to the male gonopore

Their studies do provide tantalising evidence, however, that the evolution of the anus is linked to another structure that animals use for sex.

They are researching a group of animals called Acoela, which, in evolutionary terms, are extremely primitive. Resembling tiny flattened worms, no more than two millimetres long, these creatures live free in the sea. They have no gut at all, nor anus and even lack circulatory or respiratory systems. In essence they do not have any body cavities; their bodies are solid.

But they do create sperm, which is released through an opening in the body called a gonopore. Many animals that lack back-bones use gonopores to release sperm and eggs, and genetic and molecular studies by Hejnol and Martín-Durán and colleagues suggest the development of the gonopore might be linked to the origin of another opening – the anus.

It is very difficult to have a serious conversation about this subject

“Our own hypotheses is that the anal opening has some evolutionary connection to the male gonopore,” Dr Hejnol told BBC Earth.

“This of course makes the whole subject even more delicate. But this is how nature is - nature does not care about taboos in human society.”

“It is very difficult to have a serious conversation about this subject,” he admits. “It is much easier to talk about the evolution of sex and sexuality than about the evolution of the anus.”

“This indicates that the daily business we have to do on the toilet is clearly a taboo in industrialised societies.”

“Maybe our research can contribute to change the discourse about the subject and communicate about it more openly.”

And with that, perhaps we should all find a new, less humorous, respect for what many still see as our most embarrassing organ.

The sea cucumber (Parastichopus tremulus), for example, has a wonderfully multifunctional anus. Not only does it defecate through its anal opening, it breathes through it too.

And if that fails to impress, and innuendo and puns is what you’re still looking for, then the phrases ‘stick it’ and ‘where the sun don’t shine’ come to mind. Sorry, I couldn’t help myself.

Or if you’re still reeling with the shock of it all, here’s another cute picture for you, this time of a little kitten.